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YDIH: Younger Dryas Impact Hypothesis

Posted on: September 10, 2019








THE YOUNGER DRYAS IMPACT HYPOTHESIS (YDIH) proposes that the Younger Dryas cooling event was caused by a comet impact in North America in the form of an air burst event. The theory holds that this impact was also responsible for the end of the Clovis Culture of American PaleoIndians and the extinction of the mammoths and other large mammals of the continent. This post is a literature review of research in this area and a critical evaluation of the YDIH based on the literature and in terms of the chaotic nature of the glaciation and deglaciation process shown in the video above and described in a related post [LINK] .


  1. In 2007, Richard Firestone of Lawrence Berkeley National Laboratories (and 25 co-authors that included Albert Goodyear of the University of South Carolina) published “Evidence for an Extraterrestrial Impact 12,900 Years Ago that Contributed to the Megafaunal Extinctions and the Younger Dryas Cooling“. The paper is listed in Paragraph#2 of the YDIH Bibliography below and is considered by researchers in the field to  be the origin of the YDIH and the the baseline reference paper for research in YDIH.
  2. A prior paper by Firestone in 2001, Terrestrial evidence of a nuclear catastrophe in Paleoindian times.” though not directly related to the YDIH, sets the stage and the context for it in terms of Firestone’s research interest in ancient American Paleoindian culture and the history of the peopling of America in terms of his expertise in nuclear physics at the Lawrence Berkeley Labs and his interest in comets as a member of the Comet Research Group. The 2001 paper, listed in Paragraph#1 of the YDIH bibliography below, provides the historical context and the research pathway that led Firestone from PaleoIndian archaeology to the now famous 2007 paper on YDIH.
  3. Not mentioned in the other papers in the YDIH bibliography below, and apparently not well known, is that Firestone co-authored a recent paper on YDIH in 2014 with lead author Charles R. Kinzie and 21 other co-authors. This paper is listed in paragraph#12 in the bibliography below and contains responses to some of the critical reviews of the YDIH that were published between 2007 and 2014.
  4. Paleo Indians: Sufficient archaeological evidence exists to support the hypothesis that there were a mysterious population of humans residing in North America as far back as 30,000 years ago who disappeared about 11,000 years ago or so but left behind the native Americans we know today as the Indians. These people are referred to as PaleoIndians and also as the Clovis Culture because many of the archaeological evidence were unearthed near Clovis, New Mexico. These people are assumed to have migrated from Siberia or Asia across the Beringia region during the Last Glacial Maximum (LGM), when the sea level was more than 100 meters lower than it is today. Siberia and North America constituted a continuous land mass with the Beringia region between them now inundated by sea level rise of the Holocene. The various Indian Tribes we know today as the Native Americans are thought to  descendants of the PaleoIndians. The history and the later disappearance of the PaleoIndians is an important subject of interest and of scholarly research in anthropology, archaeology, and history. The PaleoIndian culture, generally referred to as the Clovis Culture has been established in terms of their tools, both stone and bone, their well developed social organization, and their geographical spread throughout the continents.
  5. Contemporaneous with the PaleoIndians, the American continents were home to very large mammals that have become extinct except for the Bison. Other than the Bison, these, so called MegaFauna creatures (extremely large animals) included the elephant-like Mastodons and Mammoths and the Giant Sloth. These creatures were the primary source of food for the hunter-gatherer PaleoIndians and their bones served as raw materials for tool making in the Clovis Culture. The paleontology of these MegaFauna of the American continents is an important line of research usually connected to the anthropology of PaleoIndians. A bibliography of this line of research is listed in  the “Paleo-Indian Bibliography section below.
  6. The connection between Clovis culture research listed in the PaleoIndian bibliography and YDIH research listed in the YDIH bibliography is that at sometime toward the end of the Pleistocene and at the inception of the Holocene, both the Clovis Culture and the MegaFauna disappeared from the American continents. The primary research question is of course “what happened”? What caused the extinction of the megafauna and disappearance of the PaleoIndian Clovis culture of Pleistocene America? Many theories have been proposed, debated, and studied but that debate has no satisfactory conclusion and it continues oftentimes in an acrimonious way. For example, a popular theory for Megafaunal extinction is the overkill hypothesis. It holds that The PaleoIndian hunt rate was at an unsustainable rate such that at some point the megafauna had all been killed for consumption (for example the various works of  Stuart Fiedel and Gary Haynes). The overkill hypothesis has many skeptics and critics (for example Donald Grayson and David Meltzer). In this way, though the fate of the Clovis Culture and the American MegaFauna remain a high interest research area, no easy answers can be derived from this work as the defining story of this saga for dissemination to the public.
  7. The YDIH of Firestone (2007) is a product of this debate. It is the result of the unlikely combination of Firestone’s interest in PaleoIndian history and the ancient “peopling of America”, his background in astrophysics, nuclear physics, and radiocarbon dating, and the state of uncertainty in the field of PaleoIndian anthropology. The origin of YDIH research is therefore the relatively unknown Firestone (2001) paper on PaleoIndian anthropology and archaeology (see Paragraph#1 in the YDIH bibliography below.
  8. The state of uncertainty and acrimonious debate regarding the disappearance of the Clovis culture and the extinction of the American MegaFauna set the stage for Firestone, after six years of study following his 2001 paper, to propose the so called Younger Dryas Impact Hypothesis (YDIH) for these events. The Younger Dryas climate event [LINK] is a volatile and chaotic climate event that occurred at the end of the Last Glacial Period and the beginning of the current interglacial, the Holocene. The chart in Figure 3 shows that at the end of the last glaciation ≈14 KYBP (thousands of years before the present), glaciation had apparently ended and the world had warmed but within the next millennium, it cooled and moved the climate back to glaciation conditions. This event is called the Younger Dryas cooling event. It is presented and understood as a climate anomaly in the glaciation cycle that requires a causal explanation and in the absence of which remains an unexplained phenomenon in climate history.
  9. It was in this context that Firestone 2007 entered the scene and with one single mechanism, explained both the Younger Dryas (YD), the disappearance of the Clovis culture, and the extinction of the MegaFauna of the American continents. That mechanism is an extraterrestrial object or objects either asteroid or comet that struck North America ≈13 KYBP and caused the YD climate event and the extinction of the MegaFauna of the American continents and at the same time ended the Clovis culture. Since no impact crater is found, and no material evidence exists of an extraterrestrial impact on the ground or ocean, the extraterrestrial hypothesis is framed as an air burst.
  10. The causal mechanism the YD cooling effect is described in terms of an “impact winter”. An asteroid air burst can cause dust and aerosols of various descriptions to be injected into the air in such large quantities that it can cause significant cooling at decadal and even centennial time scales and the amount of cooling and its duration in these time scales are sufficient to cause extinctions and other harmful effects. From Firestone 2007:We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.” However, it is in this timescale that we find the greatest weakness in the YDIH. It is an explanation of a millennial scale cooling event in terms of a decadal and at most centennial scale causation mechanism.
  11. Another weakness pointed out by many researchers in the YDIH bibliography is that the theory is malleable. As new evidence is found or old evidence is discredited, the theory is altered to fit the data. This kind of empirical evidence for theory suffers from circular reasoning in the sense that the data used to construct the theory do not serve as empirical evidence for it. This issue is discussed by Pinter and others in the YDIH bibliography. Of particular note is that the collection of archaeological findings such as magnetic spherules and nanodiamonds are evidence of convenience because they were deemed as evidence only after they were found. The scientific method has been corrupted by the zeal of researchers.
  12.  Other arguments against the YDIH proposed in the literature listed in the YDIH and Paleo-Indian bibliographies below are discussed in the next few paragraphs. Most of these authors have found fault with the evidence of extraterrestrial impact presented in Firestone 2007 in terms of findings in archaeological digs of: “(i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka”
  13. Surovell (2009) carried out an “independent analysis of magnetic minerals and microspherules from seven sites of similar age, including two examined by Firestone but were unable to reproduce any of the results of the Firestone et al. study and find no support for YDIH. The Surovell paper is generally considered a weak response to Firestone. For example, LeCompte (2012) say they checked Surovell’s claims against the existence or the appropriate interpretation of microspherules as evidence of YDIH.
  14. Daulton (2010) examined carbon-rich materials isolated from sediments dated 15,818 cal yr B.P.and did not find nanodiamonds. Instead, graphene- and graphene/graphane-oxide aggregates were found to be ubiquitous in all specimens examined. They demonstrate that previous studies misidentified graphene/graphane-oxide aggregates as hexagonal diamond and likely misidentified graphene as cubic diamond. The authors reject YDIH on this basis.
  15. Pinter (2011) is one of the mainline critiques of the YDIH. The paper addresses a wide range of issues focusing on the so called “12 main signatures” and partitions the 12 main signatures into two groups. The first group (particle tracks in chert; magnetic nodules in bones; that the Carolina Bays were formed by the impact; and high levels of radioactivity, iridium, and fullerenes enriched in Helium3) has been mostly discredited in the literature. Pinter addresses the second group of the 12 main signatures (carbon spheres, magnetic grains, magnetic spherules, products of catastrophic wildfire, and nanodiamonds) and finds that (1) carbon spheres and elongates do not represent extraterrestrial carbon, (2) wildfires, even megafires, are a pervasive surface phenomenon and that one such event lines up with the YDIH does not serve as evidence. (3) magnetic dust and spherules (and many other meteoric remains, have a more banal explanation in terms of the constant shower of small objects that enter the atmosphere. (4) Nanodiamonds have a natural explanation although it is conceded that cubic nanodiamonds requires further analysis.
  16. Boslough 2012 points out that there is not just one single YDIH but several. Different versions of the YDIH conflict with one another regarding many significant details. Also, fragmentation and explosion mechanisms proposed in some of the versions do not conserve energy or momentum. The paper also claims that the a priori odds of the impact of a 4 km comet in the prescribed configuration on the Laurentide Ice Sheet during the specified time period are about one in a thousand. The further claim by Boslough 2012 that no impact craters of the appropriate size and age are known, and no unambiguously shocked material are found in YD sediments has been circumvented by YDIH theorists with the proposal that the “impact could have been an air burst” with a later revision stating that the impact WAS an air burst.
  17. Israde 2012: found nanodiamonds, microspherules, and other unusual materials in a black, carbon-rich, lacustrine layer of Lake Cuitzeo in central Mexico.  The layer dates to the early Younger Dryas. Therefore the finding is interpreted as a confirmation of the YDIH. The confirmation bias evident in the Israde study is found in other works as well.
  18. Pigati 2012: states the air burst hypothesis as the mainline YDIH from the outset and argues against the black mat line of evidence. The paper claims that black mats are found as far away as the Atacama Desert of northern Chile and many contain elevated concentrations of iridium, magnetic sediments, magnetic spherules, and magnetite grains regardless of their age or location, and that therefore these odd objects are generic properties of black mats and not evidence of a catastrophic extraterrestrial impact or air burst event.
  19. Van Hoesel (2014): makes a 3-point argument against YDIH as: (1)There is an age discrepancy between different sites where proposed impact markers have been found. (2)There is no unambiguous and diagnostic evidence to support YDIH, and (3)The origin of the nanodiamonds, lechatelierite and magnetic spherules are assumed to fit the hypothesis. Yet, the YDIH has taken on a life of its own in the PNAS with new evidence, both in favor and against the hypothesis including magnetic microspherules, nanodiamonds, iridium, shocked quartz, scoria-like objects and lechatelierite. There is a problem with the timing of the YD event. There is an apparent age discrepancy of up to two centuries between different sites associated with the proposed impact event. We would like to stress that if the markers at different locations have been deposited at different points in time, they cannot be related to the same event. Van Hoesel concedes, however, that some evidence used to support the Younger Dryas impact hypothesis cannot fully be explained or disputed.
  20. Holliday (2014): Says that some of the claims made in YDIH violate the basic principles and laws of physics of asteroid impacts. No YD boundary crater or other direct indicators of an impact are known. Age control is weak at 26 of the 29 localities claimed to have evidence for the YDIH. Attempts to reproduce the results have failed. Many indicators are not unique to an impact nor to ∼12.9k cal a BP.  Geomorphic, stratigraphic and fire records show no evidence of catastrophic changes at that time. Late Pleistocene extinctions varied in time and across space. Archeological data provide no indication of population decline, demographic collapse or major adaptive shifts at or just after ∼12.9 ka. The data and the hypotheses generated by YDIH proponents are contradictory, inconsistent and incoherent.
  21. Firestone restates his case for YDIH in Kinzie & Firestone (2014) following thee critical reviews of his work since 2007 listed in the YDIH biblipgraphy. He says (1) A cosmic impact event occurred at the onset of the Younger Dryas (YD) cooling episode at ≈12,800 ± 150 years before present forming the YD Boundary layer, distributed over >50 million km2 on four continents. In 24 dated stratigraphic sections in 10 countries of the Northern Hemisphere, the YDB layer contains a clearly defined abundance maximum of 500ppm with a mean of 200ppb in nanodiamonds. Nanodiamonds are a cosmic-impact proxy. Observed nanodiamonds include cubic diamonds, lonsdaleite-like crystals, and diamond-like carbon nanoparticles. Up to 3700 ppb of carbon spherules. Nanodiamonds were produced from terrestrial carbon, as with other impact diamonds, and were not derived from the impactor itself. Other impact-related proxies include cosmic-impact spherules, carbon spherules, iridium, osmium, platinum, charcoal, aciniform carbon (soot), and high-temperature melt-glass. The nanodiamond evidence  is consistent with YDIH and it therefore proves YDIH.
  22. In the Younger Dryas climate bibliography below we find as follows: The Younger Dryas cooling event was not global but limited to the North Atlantic region. It began 12,800 years ago when the temperature in Greenland fell by 15C within a few decades.  The Younger Dryas ended 2,100 years later or 10,700 years ago when the temperature in Greenland warmed suddenly by 7C, also within decades. These abrupt warming and cooling phenomena are indeed dramatic and inexplicable in terms of the traditional context of our understanding of such events. The study of the extraordinary YD event is therefore guided by the principle that extraordinary events require extraordinary causes and extraordinary explanations.
  23. The sudden cooling and two thousand years later a sudden warming have been explained by climate science as “abrupt climate change” ascribed to changes to the Thermohaline Circulation due to fresh water discharge from de-glaciation. This theory is contested by Carl Wunsch [LINK]  [LINK] and also by Carlson (2010) and Melott (2010) listed in the Younger Dryas bibliography below. A geological explanation for the Younger Dryas is offered by Carlson (2010) [LINK] where he also attacks the Thermohaline circulation hypothesis.
  24. Carlson 2010 & Melott 2010: describe both the sudden cooling 12,800 years ago and the subsequent sudden warming 10,700 years ago as not unprecedented and with an explanation in terms of geological forces. In the matter of the impact in the YDIH, they present the most and most credible challenge to Firestone 2007. Firestone has not responded and this issue is not mentioned in Firestone 2014. Carlson and Melott hypothesize from theoretical considerations that an air burst extraterrestrial impact would increase the nitrate and ammonia concentration of the atmosphere temporarily. This hypothesis is tested against Tunguska and proven correct; but the test failed to verify the hypothesized air burst impact postulated in the YDIH.
  25. The glaciation cycle video above shows a section of the Northern Hemisphere that contains the location where the Laurentide ice sheet forms during glaciation cycles.  It is an animation of the most recent glaciation-deglaciaqtion sequence. It begins in the Eemian interglacial ≈120,000 years before the present (120KYBP) relatively free of ice except for Greenland and moves forward at 1,791 years per second to the present; thus beginning and ending in almost identical iceless interglacial states except for Greenland. In between these iceless interglacial states is seen the growth and decay of the last glaciation cycle. These changes are violent, non-linear, and chaotic. As seen in the video, both the growth in glaciation from 120KYBP to about 56KYBP and its decay back to interglacial conditions contain multiple cycles of growth and decay at millennial time scales. Glaciation cycles appear to exhibit properties of non-linear dynamics and deterministic chaos. Glaciation is not a linear and well behaved period of cooling and ice accumulation and deglaciation is not a linear and well behaved period of warming and ice dissipation. Rather, both glaciation and deglaciation are chaotic events consisting of both processes differentiated only by a slight advantage to ice accumulation in glaciation and a slight advantage to ice dissipation in interglacials. Viewed in this way, the Younger Dryas cooling event can be interpreted as yet another chaotic event in the deglaciation process and not a cause and effect phenomenon. The general state of confusion in the search for a cause and effect explanation for the Younger Dryas cooling may have an interpretation in terms of the nature of non-linear dynamics and chaos in nature. A further implication is that the proposal that the brief return to Pleistocene conditions in the Younger Dryas cooling event could not have been the cause of the demise of Pleistocene creatures and human cultures.
  26. SUMMARY AND CONCLUSION: This post describes the YDIH and presents a literature review and bibliography for and against the theory that the Younger Dryas cooling was initiated by an asteroid air burst strike that also caused the extinction of the megafauna of the American continents and ended the Clovis culture of the early Beringia settlers of the Americas. We find that the evidence presented for the YDIH have alternative explanations described by Carlson and Melott. Inconsistencies in the evidence and absence of unique signature of the evidence presented by Boslough 2012, Pigati 2012, and Holliday (2014) are convincing. In general, the data appear to be sought and interpreted with a bias driven by a passionate zeal for the YDIH hypothesis. It is also noted that nonlinear dynamics and chaos in deglaciation described above imply that the assumption of cause and effect in the Younger Dryas event must first be verified before a cause is sought. A damning issue in the empirical evidence for YDIH, pointed out by Pinter 2011 and others, is its malleability. As new evidence is found or old evidence is discredited, the theory mutates to fit the evidence. This kind of empirical evidence for theory suffers from circular reasoning in the sense that the data used to construct the theory do not serve as empirical evidence for it. Also, there is a logical weakness contained in the proposal that a one millennium return to Pleistocene conditions during the Younger Dryas cooling would cause the extinction of Pleistocene creatures such as the megafauna and end Pleistocene cultures such as the Clovis people. The YDIH does not appear credible in this light. It is noted that Richard Firestone is a member of the Comet Research Group [LINK] and that association may imply a bias for extraterrestrial cause of unexplained phenomena. 




  1. Firestone, Richard B., and William Topping. “Terrestrial evidence of a nuclear catastrophe in Paleoindian times.” (2001).  A common problem at paleoindian sites in the northeastern region of North America is the recovery of radiocarbon dates that are much younger than their western counterparts, sometimes by as much as 10,000 years. Other methods like thermoluminescence, geoarchaeology, and sedimentation suggest that the dates are incorrect. Evidence has been mounting that the peopling of the Americas occurred much earlier than 12,000 bp. The discovery of tracks and micrometeorite-like particles in paleoindian artifacts across North America demonstrates they were bombarded during a cosmic event. Measurements of Uranium 235 (235U), depleted by 17-77%, and enhanced concentrations of Plutonium 239 (239Pu), from neutron capture on Uranium 238 (238U), in artifacts, associated chert types, and sediments at depth indicates that the entire prehistoric North American landscape was bombarded by thermal neutrons. Radiocarbon dating assumes that there is no substantial change in isotopic composition over time. A large thermal neutron event would convert residual Nitrogen 14 (14N) in charcoal to Carbon 14 (14C) thus resetting the radiocarbon date to a younger value and pushing back the date that paleoindians occupied the Americas by thousands of years. Analysis of data from 11 locations across North America indicates there were episodes of cosmic ray bombardments of the prehistoric landscape in Late Glacial times. Examination of the radiocarbon record suggests these events were coupled with geomagnetic excursions at 41,000, 33,000, and 12,500 bp and irradiated the landscape with massive thermal neutron fluxes of the order of approximately1015 neutrons/cm2. These data provide a clear body of terrestrial evidence supporting either one of two longstanding hypotheses for catastrophe in paleoindian times: (1) a giant solar flare during a geomagnetic excursion as explored by Wolfendale and Zook, and (2) a supernova shockwave as forwarded by Brackenridge, Clarke, and Dar. The evidence is reviewed, and logical implications for Late Glacial mass extinctions and associated plant mutations are explored.
  2. Firestone, Richard B., et al. “Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling.” Proceedings of the National Academy of Sciences 104.41 (2007): 16016-16021.  A carbon-rich black layer, dating to ≈12.9 ka, has been previously identified at ≈50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ≅12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.
  3. Surovell, Todd A., et al. “An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis.” Proceedings of the National Academy of Sciences 106.43 (2009): 18155-18158.  Based on elevated concentrations of a set of “impact markers” at the onset of the Younger Dryas stadial from sedimentary contexts across North America, Firestone, Kennett, West, and others have argued that 12.9 ka the Earth experienced an impact by an extraterrestrial body, an event that had devastating ecological consequences for humans, plants, and animals in the New World [Firestone RB, et al. (2007) Proc. Natl. Acad. Sci. USA 104:16016–16021]. Herein, we report the results of an independent analysis of magnetic minerals and microspherules from seven sites of similar age, including two examined by Firestone et al. We were unable to reproduce any results of the Firestone et al. study and find no support for Younger Dryas extraterrestrial impact.
  4. Daulton, Tyrone L., Nicholas Pinter, and Andrew C. Scott. “No evidence of nanodiamonds in Younger–Dryas sediments to support an impact event.” Proceedings of the National Academy of Sciences 107.37 (2010): 16043-16047.  The causes of the late Pleistocene megafaunal extinctions in North America, disappearance of Clovis paleoindian lithic technology, and abrupt Younger–Dryas (YD) climate reversal of the last deglacial warming in the Northern Hemisphere remain an enigma. A controversial hypothesis proposes that one or more cometary airbursts/impacts barraged North America ≈12,900 cal yr B.P. and caused these events. Most evidence supporting this hypothesis has been discredited except for reports of nanodiamonds (including the rare hexagonal polytype) in Bølling–Ållerod-YD-boundary sediments. The hexagonal polytype of diamond, lonsdaleite, is of particular interest because it is often associated with shock pressures related to impacts where it has been found to occur naturally. Unfortunately, previous reports of YD-boundary nanodiamonds have left many unanswered questions regarding the nature and occurrence of the nanodiamonds. Therefore, we examined carbon-rich materials isolated from sediments dated 15,818 cal yr B.P. to present (including the Bølling–Ållerod-YD boundary). No nanodiamonds were found in our study. Instead, graphene- and graphene/graphane-oxide aggregates are ubiquitous in all specimens examined. We demonstrate that previous studies misidentified graphene/graphane-oxide aggregates as hexagonal diamond and likely misidentified graphene as cubic diamond. Our results cast doubt upon one of the last widely discussed pieces of evidence supporting the YD impact hypothesis.
  5. Pinter, Nicholas, et al. “The Younger Dryas impact hypothesis: A requiem.” Earth-Science Reviews 106.3-4 (2011): 247-264. The Younger Dryas (YD) impact hypothesis is a recent theory that suggests that a cometary or meteoritic body or bodies hit and/or exploded over North America 12,900 years ago, causing the YD climate episode, extinction of Pleistocene megafauna, demise of the Clovis archeological culture, and a range of other effects. Since gaining widespread attention in 2007, substantial research has focused on testing the 12 main signatures presented as evidence of a catastrophic extraterrestrial event 12,900 years ago. Here we present a review of the impact hypothesis, including its evolution and current variants, and of efforts to test and corroborate the hypothesis. The physical evidence interpreted as signatures of an impact event can be separated into two groups. The first group consists of evidence that has been largely rejected by the scientific community and is no longer in widespread discussion, including: particle tracks in archeological chert; magnetic nodules in Pleistocene bones; impact origin of the Carolina Bays; and elevated concentrations of radioactivity, iridium, and fullerenes enriched in 3He. The second group consists of evidence that has been active in recent research and discussions: carbon spheres and elongates, magnetic grains and magnetic spherules, byproducts of catastrophic wildfire, and nanodiamonds. Over time, however, these signatures have also seen contrary evidence rather than support. Recent studies have shown that carbon spheres and elongates do not represent extraterrestrial carbon nor impact-induced megafires, but are indistinguishable from fungal sclerotia and arthropod fecal material that are a small but common component of many terrestrial deposits. Magnetic grains and spherules are heterogeneously distributed in sediments, but reported measurements of unique peaks in concentrations at the YD onset have yet to be reproduced. The magnetic grains are certainly just iron-rich detrital grains, whereas reported YD magnetic spherules are consistent with the diffuse, non-catastrophic input of micrometeorite ablation fallout, probably augmented by anthropogenic and other terrestrial spherular grains. Results here also show considerable subjectivity in the reported sampling methods that may explain the purported YD spherule concentration peaks. Fire is a pervasive earth-surface process, and reanalyses of the original YD sites and of coeval records show episodic fire on the landscape through the latest Pleistocene, with no unique fire event at the onset of the YD. Lastly, with YD impact proponents increasingly retreating to nanodiamonds (cubic, hexagonal [lonsdaleite], and the proposed n-diamond) as evidence of impact, those data have been called into question. The presence of lonsdaleite was reported as proof of impact-related shock processes, but the evidence presented was inconsistent with lonsdaleite and consistent instead with polycrystalline aggregates of graphene and graphane mixtures that are ubiquitous in carbon forms isolated from sediments ranging from modern to pre-YD age. Important questions remain regarding the origins and distribution of other diamond forms (e.g., cubic nanodiamonds).In summary, none of the original YD impact signatures have been subsequently corroborated by independent tests. Of the 12 original lines of evidence, seven have so far proven to be non-reproducible. The remaining signatures instead seem to represent either (1) non-catastrophic mechanisms, and/or (2) terrestrial rather than extraterrestrial or impact-related sources. In all of these cases, sparse but ubiquitous materials seem to have been misreported and misinterpreted as singular peaks at the onset of the YD. Throughout the arc of this hypothesis, recognized and expected impact markers were not found, leading to proposed YD impactors and impact processes that were novel, self-contradictory, rapidly changing, and sometimes defying the laws of physics. The YD impact hypothesis provides a cautionary tale for researchers, the scientific community, the press, and the broader public.
  6. Boslough, M., et al. “Arguments and evidence against a Younger Dryas impact event.” Climates, landscapes, and civilizations. Vol. 198. American Geophysical Union Washington, DC, 2012. 13-26.  We present arguments and evidence against the hypothesis that a large impact or airburst caused a significant abrupt climate change, extinction event, and termination of the Clovis culture at 12.9 ka. It should be noted that there is not one single Younger Dryas (YD) impact hypothesis but several that conflict with one another regarding many significant details. Fragmentation and explosion mechanisms proposed for some of the versions do not conserve energy or momentum, no physics-based model has been presented to support the various concepts, and existing physical models contradict them. In addition, the a priori odds of the impact of a >4 km comet in the prescribed configuration on the Laurentide Ice Sheet during the specified time period are infinitesimal, about one in 1015. There are three broad classes of counter-arguments. First, evidence for an impact is lacking. No impact craters of the appropriate size and age are known, and no unambiguously shocked material or other features diagnostic of impact have been found in YD sediments. Second, the climatological, paleontological, and archeological events that the YD impact proponents are attempting to explain are not unique, are arguably misinterpreted by the proponents, have large chronological uncertainties, are not necessarily coupled, and do not require an impact. Third, we believe that proponents have misinterpreted some of the evidence used to argue for an impact, and several independent researchers have been unable to reproduce reported results. This is compounded by the observation of contamination in a purported YD sample with modern carbon. Sandia National Laboratories, Albuquerque, New Mexico, USA.
  7. Israde-Alcántara, Isabel, et al. “Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis.” Proceedings of the National Academy of Sciences 109.13 (2012): E738-E747.  We report the discovery in Lake Cuitzeo in central Mexico of a black, carbon-rich, lacustrine layer, containing nanodiamonds, microspherules, and other unusual materials that date to the early Younger Dryas and are interpreted to result from an extraterrestrial impact. These proxies were found in a 27-m-long core as part of an interdisciplinary effort to extract a paleoclimate record back through the previous interglacial. Our attention focused early on an anomalous, 10-cm-thick, carbon-rich layer at a depth of 2.8 m that dates to 12.9 ka and coincides with a suite of anomalous coeval environmental and biotic changes independently recognized in other regional lake sequences. Collectively, these changes have produced the most distinctive boundary layer in the late Quaternary record. This layer contains a diverse, abundant assemblage of impact-related markers, including nanodiamonds, carbon spherules, and magnetic spherules with rapid melting/quenching textures, all reaching synchronous peaks immediately beneath a layer containing the largest peak of charcoal in the core. Analyses by multiple methods demonstrate the presence of three allotropes of nanodiamond: n-diamond, i-carbon, and hexagonal nanodiamond (lonsdaleite), in order of estimated relative abundance. This nanodiamond-rich layer is consistent with the Younger Dryas boundary layer found at numerous sites across North America, Greenland, and Western Europe. We have examined multiple hypotheses to account for these observations and find the evidence cannot be explained by any known terrestrial mechanism. It is, however, consistent with the Younger Dryas boundary impact hypothesis postulating a major extraterrestrial impact involving multiple airburst(s) and and/or ground impact(s) at 12.9 ka.
  8. LeCompte, Malcolm A., et al. “Independent evaluation of conflicting microspherule results from different investigations of the Younger Dryas impact hypothesis.” Proceedings of the National Academy of Sciences 109.44 (2012): E2960-E2969.  Firestone et al. sampled sedimentary sequences at many sites across North America, Europe, and Asia [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 106:16016–16021]. In sediments dated to the Younger Dryas onset or Boundary (YDB) approximately 12,900 calendar years ago, Firestone et al. reported discovery of markers, including nanodiamonds, aciniform-soot, high-temperature melt-glass, and magnetic microspherules attributed to cosmic impacts/airbursts. The microspherules were explained as either cosmic material ablation or terrestrial ejecta from a hypothesized North American impact that initiated the abrupt Younger Dryas cooling, contributed to megafaunal extinctions, and triggered human cultural shifts and population declines. A number of independent groups have confirmed the presence of YDB spherules, but two have not. One of them [Surovell TA, et al. (2009) Proc Natl Acad Sci USA 104:18155–18158] collected and analyzed samples from seven YDB sites, purportedly using the same protocol as Firestone et al., but did not find a single spherule in YDB sediments at two previously reported sites. To examine this discrepancy, we conducted an independent blind investigation of two sites common to both studies, and a third site investigated only by Surovell et al. We found abundant YDB microspherules at all three widely separated sites consistent with the results of Firestone et al. and conclude that the analytical protocol employed by Surovell et al. deviated significantly from that of Firestone et al. Morphological and geochemical analyses of YDB spherules suggest they are not cosmic, volcanic, authigenic, or anthropogenic in origin. Instead, they appear to have formed from abrupt melting and quenching of terrestrial materials.
  9. Pigati, Jeffrey S., et al. “Accumulation of impact markers in desert wetlands and implications for the Younger Dryas impact hypothesis.” Proceedings of the National Academy of Sciences 109.19 (2012): 7208-7212.  The Younger Dryas impact hypothesis contends that an extraterrestrial object exploded over North America at 12.9 ka, initiating the Younger Dryas cold event, the extinction of many North American megafauna, and the demise of the Clovis archeological culture. Although the exact nature and location of the proposed impact or explosion remain unclear, alleged evidence for the fallout comes from multiple sites across North America and a site in Belgium. At 6 of the 10 original sites (excluding the Carolina Bays), elevated concentrations of various “impact markers” were found in association with black mats that date to the onset of the Younger Dryas. Black mats are common features in paleo-wetland deposits and typically represent shallow marsh environments. In this study, we investigated black mats ranging in age from approximately 6 to more than 40 ka in the southwestern United States and the Atacama Desert of northern Chile. At 10 of 13 sites, we found elevated concentrations of iridium in bulk and magnetic sediments, magnetic spherules, and/or titanomagnetite grains within or at the base of black mats, regardless of their age or location, suggesting that elevated concentrations of these markers arise from processes common to wetland systems, and not a catastrophic extraterrestrial impact event.
  10. Van Hoesel, Annelies, et al. “The Younger Dryas impact hypothesis: a critical review.” Quaternary Science Reviews 83 (2014): 95-114. Bullet points: 1. There is an age discrepancy between different sites where proposed impact markers have been found. 2. There is no unambiguous and diagnostic evidence to support the claim that there was a Younger Dryas impact event. 3. Questions remain regarding the origin of the nanodiamonds, lechatelierite and magnetic spherules. Abstract: The Younger Dryas impact hypothesis suggests that multiple extraterrestrial airbursts or impacts resulted in the Younger Dryas cooling, extensive wildfires, megafaunal extinctions and changes in human population. After the hypothesis was first published in 2007, it gained much criticism, as the evidence presented was either not indicative of an extraterrestrial impact or not reproducible by other groups. Only three years after the hypothesis had been presented, a requiem paper was published. Despite this, the controversy continues. New evidence, both in favour and against the hypothesis, continues to be published. In this review we briefly summarize the earlier debate and critically analyse the most recent reported evidence, including magnetic microspherules, nanodiamonds, and iridium, shocked quartz, scoria-like objects and lechatelierite. The subsequent events proposed to be triggered by the impact event, as well as the nature of the event itself, are also briefly discussed. In addition we address the timing of the Younger Dryas impact, a topic which, despite its importance, has not gained much attention thus far. We show that there are three challenges related to the timing of the event: accurate age control for some of the sites that are reported to provide evidence for the impact, linking these sites to the onset of the Younger Dryas and, most importantly, an apparent age discrepancy of up to two centuries between different sites associated with the proposed impact event. We would like to stress that if the markers at different locations have been deposited at different points in time, they cannot be related to the same event. Although convincing evidence for the hypothesis that multiple synchronous impacts resulted in massive environmental changes at ∼12,900 yrs ago remains debatable, we conclude that some evidence used to support the Younger Dryas impact hypothesis cannot fully be explained at this point in time.
  11. Holliday, Vance T., et al. “The Younger Dryas impact hypothesis: a cosmic catastrophe.” Journal of Quaternary Science 29.6 (2014): 515-530.  In this paper we review the evidence for the Younger Dryas impact hypothesis (YDIH), which proposes that at ∼12.9k cal a BP North America, South America, Europe and the Middle East were subjected to some sort of extraterrestrial event. This purported event is proposed as a catastrophic process responsible for: terminal Pleistocene environmental changes (onset of YD cooling, continent‐scale wildfires); extinction of late Pleistocene mammals; and demise of the Clovis ‘culture’ in North America, the earliest well‐documented, continent‐scale settlement of the region. The basic physics in the YDIH is not in accord with the physics of impacts nor the basic laws of physics. No YD boundary (YDB) crater, craters or other direct indicators of an impact are known. Age control is weak to non‐existent at 26 of the 29 localities claimed to have evidence for the YDIH. Attempts to reproduce the results of physical and geochemical analyses used to support the YDIH have failed or show that many indicators are not unique to an impact nor to ∼12.9k cal a BP. The depositional environments of purported indicators at most sites tend to concentrate particulate matter and probably created many ‘YDB zones’. Geomorphic, stratigraphic and fire records show no evidence of any sort of catastrophic changes in the environment at or immediately following the YDB. Late Pleistocene extinctions varied in time and across space. Archeological data provide no indication of population decline, demographic collapse or major adaptive shifts at or just after ∼12.9 ka. The data and the hypotheses generated by YDIH proponents are contradictory, inconsistent and incoherent.
  12. Kinzie & Firestone + 21 co-authors, “Nanodiamond-Rich Layer across Three Continents Consistent with Major Cosmic Impact at 12,800 Cal BP, ResearchGate, 2014.  A major cosmic-impact event has been proposed at the onset of the Younger Dryas (YD) cooling episode at ≈12,800 ± 150 years before present, forming the YD Boundary (YDB) layer, distributed over >50 million km2 on four continents. In 24 dated stratigraphic sections in 10 countries of the Northern Hemisphere, the YDB layer contains a clearly defined abundance peak in nanodiamonds (NDs), a major cosmic-impact proxy. Observed ND polytypes include cubic diamonds, lonsdaleite-like crystals, and diamond-like carbon nanoparticles, called n-diamond and i-carbon. The ND abundances in bulk YDB sediments ranged up to ≈500 ppb (mean: 200 ppb) and that in carbon spherules up to ≈3700 ppb (mean: ≈750 ppb); 138 of 205 sediment samples (67%) contained no detectable NDs. Isotopic evidence indicates that YDB NDs were produced from terrestrial carbon, as with other impact diamonds, and were not derived from the impactor itself. The YDB layer is also marked by abundance peaks in other impact-related proxies, including cosmic-impact spherules, carbon spherules (some containing NDs), iridium, osmium, platinum, charcoal, aciniform carbon (soot), and high-temperature melt-glass. This contribution reviews the debate about the presence, abundance, and origin of the concentration peak in YDB NDs. We describe an updated protocol for the extraction and concentration of NDs from sediment, carbon spherules, and ice, and we describe the basis for identification and classification of YDB ND polytypes, using nine analytical approaches. The large body of evidence now obtained about YDB NDs is strongly consistent with an origin by cosmic impact at ≈12,800 cal BP and is inconsistent with formation of YDB NDs by natural terrestrial processes, including wildfires, anthropogenesis, and/or influx of cosmic dust.




  1. Wheat, Joe Ben. “A Paleo-Indian bison kill.” Scientific American 216.1 (1967): 44-52.  Some 8,500 years ago a group of hunters on the Great Plains stampeded a herd of buffaloes into a gulch and butchered them. The bones of the animals reveal the event in remarkable detail. Full text pdf file online [LINK] .
  2. Guthrie, R. Dale. “Bison evolution and zoogeography in North America during the Pleistocene.” The Quarterly Review of Biology 45.1 (1970): 1-15.  The fossil record and information about contemporary forms provide evidence that the evolutionary pattern of bison cannot be interpreted as either a unidirectional decrease in horn size or as a series of successive invasions to the New World from the Old. Rather, some species have persisted and remained relatively unchanged for long periods of time, while elsewhere other contemporaneous species were changing quite rapidly. Although the trends in the evolution of bison horn size have been remarkably regular, major reversals have taken place. Bison arose in Eurasia and have had a much longer history there than is North America. In spite of this longer history in the Old World, bison have undergone greater evolutionary changes in North America. This can be explained by a different mode and intensity of competition in the New World. The major points presented are the following: (1) The giant-horned B. latifrons was a New World product. (2) B. priscus (= B. crassicornus) appeared early as a holarctic northern species and remained in that niche until the late Wisconsin (Wurm). (3) Most of the other bison species in the late Pleistocene were derived indirectly or directly from this widespread northern species. (4) Middle and Late Pleistocene bison can be place into four species: B. priscus, which can be dated at least as far back as early mid-Pleistocene; B. latifrons, which extends back at least to late Illonoian (Riss) time (it is possible that B. latifrons gave rise to B. antiquus; if so the species B. alleni should be maintained); B. antiquus, which originated during the early to middle part of the Wisconsin (Wurm) glaciation; and B. bison, which was a late Wisconsin product. (5) B. latifrons became extinct, at least over most of its range, in pre-Wisconsin time. B. priscus and B. antiquus became extinct in the late Wisconsin, and B. bison still exists in relict populations. (6) Two or more species of bison have not occurred sympatrically for extended periods of time. (7) Neither the “orthogenetic” nor the “wave” theory adequately accounts for the evolution of bison in North America; rather, the fossils can only be explained by a combination of invasions from Siberia and evolutionary changes that occurred in the new environment.
  3. Turner, Christy G., and Junius Bird. “Dentition of Chilean paleo-Indians and peopling of the Americas.” Science 212.4498 (1981): 1053-1055.  Teeth of 12 cremated paleo-Indians (11,000 years old) from caves in southern Chile have crown and root morphology like that of recent American Indians and north Asians, but unlike that of Europeans. This finding supports the view that American Indians originated in northeast Asia. This dental series also suggests that paleo-Indians could easily have been ancestral to most living Indians, that very little dental evolution has occurred, and that the founding paleo-Indian population was small, genetically homogeneous, and arrived late in the Pleistocene.
  4. Clark, Donald W., and A. McFadyen Clark. “Paleo-Indians and fluted points: Subarctic alternatives.” Plains Anthropologist 28.102 (1983): 283-292. For more than three decades a postulated northern (Alaskan) origin for Paleo-Indians bearing fluted projectile points has been based on sparse fluted point occurrences in the north and expectations engendered by the hypothesis of migration from northeastern Siberia. To reaffirm the northern hypothesis this article models northern data that have become available during the past 15 years. Major elements of this model are (a) that tentative dating indicates that some northern fluted points are only a few hundred years younger than the oldest of their southern equivalent (Clovis points), (b) that with future discoveries this preliminary dating of fluted points in the north will be extended to encompass a broader time range, and (c) when that occurs the earliest northern fluted points will be found to be older than southern fluted points, which (d) would indicate spread from north to south.We profess that there is sufficient uncertainty regarding southern origin as being the ultimate and immutable source of fluted points that alternatives merit continued consideration. Thus, it is feasible at this time to set forth specific conditions to be met in order to validate the subarctic alternative. A corollary of the northern development hypothesis is that Paleo-Indians were present in the northwestern corner of North America preceding the arrival there of an Asian-derived microblade industry about 11,000 years ago.
  5. Fisher, Daniel C. “Mastodon butchery by North American Paleo-Indians.” Nature 308.5956 (1984): 271.  It has often been argued that North American Paleo-Indians hunted both mammoths and mastodons1–3. However, while numerous archaeological sites involving mammoths (genus Mammuthus) are recognized3,4, very few sites demonstrate direct human association with mastodons5–8. I report here a taphonomic analysis of several late Pleistocene mastodon (Mammut americanum) skeletons excavated in southern Michigan which provides compelling evidence of mastodon butchery. Butchery practices involved the production and use of tools fashioned from bones of the animal being butchered. Evidence for butchery and bone tool use includes: patterns of bone distribution and disarticulation recorded from a primary depositional context, disarticulation marks and cutmarks on bones, green bone fracturing, use wear and impact features on bone fragments, and burned bone. Moreover, determinations of the season of death of butchered mastodons9 suggest that butchery was associated with hunting and killing, not simply scavenging of natural deaths. These findings provide new evidence of a well developed ‘bone technology10–13 used by Paleo-Indians in eastern North America. They also add to our perception of Paleo-Indian subsistence activities and their possible role in the late Pleistocene extinction of mastodons.
  6. Fisher2, Daniel C. “Taphonomic analysis of late Pleistocene mastodon occurrences: evidence of butchery by North American Paleo-Indians.” Paleobiology 10.3 (1984): 338-357.  Taphonomic analysis of several late Pleistocene mastodon (Mammut americanum) skeletons excavated in southern Michigan provides compelling evidence of mastodon butchery by Paleo-Indians. The occurrence of butchery and details of butchering technique are inferred primarily from patterns of bone modification. An important aspect of butchering practice was production and use of tools fashioned from bones of the animal being butchered. Evidence for butchery and bone tool use includes matching marks on the conarticular surfaces of disarticulated pairs of bones; cutmarks on bones; green bone fracturing; use wear, secondary flaking, and impact features on bone fragments; and burned bone. Interpretation of these features is facilitated by information on patterns of bone distribution and disarticulation preserved in a primary depositional context. Preliminary comparisons among nine sites indicate that putative butchering sites differ consistently and in a variety of ways from sites that appear to record no human involvement. Although based on a small sample of sites, the apparent frequency of butchered individuals relative to those that were not butchered is unexpectedly high. These findings provide new evidence of a well-developed “bone technology” employed by the late Pleistocene human inhabitants of eastern North America. In addition, these data offer circumstantial support for the hypothesis that human hunting was an important factor in the late Pleistocene extinction of mastodons.
  7. Spiess, Arthur E. “Arctic Garbage and New England Paleo-Indians: The Single Occupation Option.” Archaeology of Eastern North America (1984): 280-285. Recurrent regularities in activity area distribution pattern, size, activity area and lithic count at the three major New England Paleo-Indian sites call for explanation. An examination of caribou-hunter ethnography shows that very large seasonal gatherings of people can be supported primarily by caribou-hunting in certain circumstances. A consideration of the number of stone tools produced per man-day of occupation by certain “high-tech” arctic/sub-arctic hunting groups, coupled with the above considerations, indicates that each of the three major Paleo-Indian sites could represent single seasonal occupations by large groups of people, or a very limited number of reoccupations. (NOTE: EVIDENCE OF NOMADISM)
  8. Grayson, Donald K. “Late Pleistocene mammalian extinctions in North America: taxonomy, chronology, and explanations.” Journal of World Prehistory 5.3 (1991): 193-231.  Toward the end of the Pleistocene, North America lost some 35 genera of mammals. It has long been assumed that all or virtually all of the extinctions occurred between 12,000 and 10,000 years ago, but detailed analyses of the radiocarbon chronology provide little support for this assumption, which seems to have been widely accepted because of the kinds of explanations felt most likely to account for the extinctions in the first place. Approaches that attribute the losses to human predation depend almost entirely on the assumed synchroneity between the extinctions and the onset of large mammal hunting by North American peoples. The fact that only two of the extinct genera have been found in a convincing kill context presents an overwhelming problem for this approach. Climatic models, on the other hand, are becoming increasingly precise and account for a wide variety of apparently synchronous biogeographic events. While a role for human activities in the extinction of some taxa is fully possible, there can be little doubt that the underlying cause of the extinctions lies in massive climatic change.
  9. Williams, Robert C., and Joan E. McAuley. “HLA class I variation controlled for genetic admixture in the Gila River Indian Community of Arizona: a model for the Paleo-Indians.” Human immunology 33.1 (1992): 39-46.  The genetic distribution of the HLA class I loci is presented for 619 “full blooded” Pima and Tohono O’odham Native Americans (Pimans) in the Gila River Indian Community. Variation in the Pimans is highly restricted. There are only three polymorphic alleles at the HLA-A locus, A2, A24, and A31, and only 10 alleles with a frequency greater than 0.01 at HLA-B where Bw48 (0.187), B35 (0.173), and the new epitope BN21 (0.143) have the highest frequencies. Two and three locus disequilibria values and haplotype frequencies are presented. Ten three-locus haplotypes account for more than 50% of the class I variation, with A24 BN21 Cw3 (0.085) having the highest frequency. Gm allotypes demonstrate that little admixture from non-Indian populations has entered the Community since the 17th century when Europeans first came to this area. As a consequence many alleles commonly found in Europeans and European Americans are efficient markers for Caucasian admixture, while the “private” Indian alleles, BN21 and Bw48, can be used to measure Native American admixture in Caucasian populations. It is suggested that this distribution in “full blooded” Pimans approximates that of the Paleo-Indian migrants who first entered the Americas between 20,000 and 40,000 years ago.
  10. Grayson, Donald K., and David J. Meltzer. “A requiem for North American overkill.” Journal of Archaeological Science 30.5 (2003): 585-593.  The argument that human hunters were responsible for the extinction of a wide variety of large Pleistocene mammals emerged in western Europe during the 1860s, alongside the recognition that people had coexisted with those mammals. Today, the overkill position is rejected for western Europe but lives on in Australia and North America. The survival of this hypothesis is due almost entirely to Paul Martin, the architect of the first detailed version of it. In North America, archaeologists and paleontologists whose work focuses on the late Pleistocene routinely reject Martin’s position for two prime reasons: there is virtually no evidence that supports it, and there is a remarkably broad set of evidence that strongly suggests that it is wrong. In response, Martin asserts that the overkill model predicts a lack of supporting evidence, thus turning the absence of empirical support into support for his beliefs. We suggest that this feature of the overkill position removes the hypothesis from the realm of science and places it squarely in the realm of faith. One may or may not believe in the overkill position, but one should not confuse it with a scientific hypothesis about the nature of the North American past.
  11. Fiedel, Stuart, and Gary Haynes. “A premature burial: comments on Grayson and Meltzer’s “Requiem for overkill”.” Journal of Archaeological Science 31.1 (2004): 121-131. Although their critical assessment of the Late Pleistocene archaeological record is laudable, Grayson and Meltzer unfortunately make numerous mistakes, indulge in unwarranted ad hominem rhetoric, and thus grossly misrepresent the overkill debate. In this comment, we first briefly address those aspects of their papers that represent mere theatrical posturing, and then we turn our attention to their more serious errors of fact and interpretation. First, the theater. A phrase repeated or paraphrased in each of the articles is that overkill is “a faith-based policy statement rather than a scientific statement about the past, an overkill credo rather than an overkill hypothesis” ([39], p. 591). By thus denying the very scientific legitimacy of the overkill hypothesis, Grayson and Meltzer seek to preclude any further serious engagement. [FULL TEXT PDF] .
  12. ***Shapiro, Beth, et al. “Rise and fall of the Beringian steppe bison.” Science 306.5701 (2004): 1561-1565.  The widespread extinctions of large mammals at the end of the Pleistocene epoch have often been attributed to the depredations of humans; here we present genetic evidence that questions this assumption. We used ancient DNA and Bayesian techniques to reconstruct a detailed genetic history of bison throughout the late Pleistocene and Holocene epochs. Our analyses depict a large diverse population living throughout Beringia until around 37,000 years before the present, when the population’s genetic diversity began to decline dramatically. The timing of this decline correlates with environmental changes associated with the onset of the last glacial cycle, whereas archaeological evidence does not support the presence of large populations of humans in Eastern Beringia until more than 15,000 years later.
  13. Hoppe, Kathryn A. “Correlation between the oxygen isotope ratio of North American bison teeth and local waters: implication for paleoclimatic reconstructions.” Earth and Planetary Science Letters 244.1-2 (2006): 408-417.  The oxygen isotope ratios of tooth enamel carbonate from 64 North American bison (Bison bison) from eleven locations were measured. The mean enamel oxygen isotope ratios for bison populations ranged from 28.3 to 15.9 ‰ SMOW and correlated well with the annual mean oxygen isotope ratios of local surface waters and precipitation. The standard deviation of oxygen isotope values among different individuals within each bison population averaged 1.0 ‰, and ranged from 0.7 to 1.4 ‰. The variability of enamel oxygen isotope ratios was not significantly different among different populations and did not correlate with changes in temperature, precipitation, or relative humidity. These results demonstrate that the average oxygen isotope values of bison tooth enamel can be used for use as a quantitative proxy for reconstructing the values of surface waters, and therefore may provide valuable paleoclimatic information. This study provides a baseline comparison for analyses of the oxygen isotope ratios of bison and other large herbivores from across North America.
  14. Grayson, Donald K. “Deciphering North American Pleistocene extinctions.” Journal of Anthropological Research 63.2 (2007): 185-213.  The debate over the cause of North American Pleistocene extinctions may be further from resolution than it has ever been in its 200-year history and is certainly more heated than it has ever been before. Here, I suggest that the reason for this may lie in the fact that paleontologists have not heeded one of the key biogeographic concepts that they themselves helped to establish: that histories of assemblages of species can be understood only by deciphering the history of each individual species within that assemblage. This failure seems to result from assumptions first made about the nature of the North American extinctions during the 1960s. [FULL TEXT PDF] .
  15. Rivals, Florent, Nikos Solounias, and Matthew C. Mihlbachler. “Evidence for geographic variation in the diets of late Pleistocene and early Holocene Bison in North America, and differences from the diets of recent Bison.” Quaternary research 68.3 (2007): 338-346.  During the late Pleistocene and early Holocene, Bison was widely dispersed across North America and occupied most regions not covered by ice sheets. A dietary study on Bison paleopopulations from Alaska, New Mexico, Florida, and Texas was performed using two methods that relate dental wear patterns to diet, mesowear analysis and microwear analysis. These data were compared to a mixed sample of extant Bison from the North American central plains, extant wood Bison from Alberta (Canada) and a variety of other modern ungulates. Mesowear relates macroscopic molar facet shape to levels of dietary abrasion. The mesowear signature observed on fossil Bison differs significantly from the hyper-abrasive grazing diet of extant Bison. Tooth microwear examines wear on the surface of enamel at a microscopic scale. The microwear signal of fossil samples resembles to modern Bison, but the fossil samples show a greater diversity of features, suggesting that fossil Bison populations regularly consumed food items that are texturally inconsistent with the short-grass diet typical of modern plains Bison. Mesowear and microwear signals of fossil Bison samples most closely resemble a variety of typical mixed feeding ungulates, all with diets that are substantially less abrasive than what is typical for modern plains Bison. Furthermore, statistical tests suggest significant differences between the microwear signatures of the fossil samples, thus revealing geographic variability in Pleistocene Bison diets. This study reveals that fossils are of value in developing an understanding of the dietary breadth and ecological versatility of species that, in recent times, are rare, endangered, and occupy only a small remnant of their former ranges.
  16. Firestone, Richard B., et al. “Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling.” Proceedings of the National Academy of Sciences 104.41 (2007): 16016-16021.  A carbon-rich black layer, dating to ≈12.9 ka, has been previously identified at ≈50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ≅12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of (i) magnetic grains with iridium, (ii) magnetic microspherules, (iii) charcoal, (iv) soot, (v) carbon spherules, (vi) glass-like carbon containing nanodiamonds, and (vii) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.
  17. Fiedel, Stuart. “Sudden deaths: the chronology of terminal Pleistocene megafaunal extinction.” American megafaunal extinctions at the end of the Pleistocene. Springer, Dordrecht, 2009. 21-37. If we ever hope to ascertain the cause(s) of the extinction of North American megafauna at the end of the Pleistocene, a necessary first step is to establish the chronology of this occurrence. Was it an abrupt event, in which about 30 or more genera disappeared simultaneously within no more than several hundred years, or instead a long, drawn-out, gradual process, with each species dying out independently and asynchronously, over the course of millennia?

    Those who advocate a vague climatic/environmental cause favor the latter gradual scenario; they recognize that if the extinctions were shown instead to be abrupt and synchronous, it would compel them to “attribute to the extinction ‘event’ …speed and taxonomic breadth …Once that is done, explanations of the extinctions must be structured to account for these assumed properties, whether those explanations focus on people, cli-mate…or disease” (Grayson and Meltzer, 2002:347).

  18. Faith, J. Tyler, and Todd A. Surovell. “Synchronous extinction of North America’s Pleistocene mammals.” Proceedings of the National Academy of Sciences 106.49 (2009): 20641-20645.  The late Pleistocene witnessed the extinction of 35 genera of North American mammals. The last appearance dates of 16 of these genera securely fall between 12,000 and 10,000 radiocarbon years ago (≈13,800–11,400 calendar years B.P.), although whether the absence of fossil occurrences for the remaining 19 genera from this time interval is the result of sampling error or temporally staggered extinctions is unclear. Analysis of the chronology of extinctions suggests that sampling error can explain the absence of terminal Pleistocene last appearance dates for the remaining 19 genera. The extinction chronology of North American Pleistocene mammals therefore can be characterized as a synchronous event that took place 12,000–10,000 radiocarbon years B.P. Results favor an extinction mechanism that is capable of wiping out up to 35 genera across a continent in a geologic instant. [FULL TEXT PDF] . 
  19. Zazula, Grant D., et al. “A late Pleistocene steppe bison (Bison priscus) partial carcass from Tsiigehtchic, Northwest Territories, Canada.” Quaternary Science Reviews 28.25-26 (2009): 2734-2742.  Rivers, Northwest Territories, Canada in September of 2007. The carcass includes a complete cranium with horn cores and sheaths, several complete post-cranial elements (many of which have some mummified soft tissue), intestines and a large piece of hide. A piece of metacarpal bone was subsampled and yielded an AMS radiocarbon age of 11,830 ± 45 14C yr BP (OxA-18549). Mitochondrial DNA sequenced from a hair sample confirms that Tsiigehtchic steppe bison (Bison priscus) did not belong to the lineage that eventually gave rise to modern bison (Bison bison). This is the first radiocarbon dated Bison priscus in the Mackenzie River valley, and to our knowledge, the first reported Pleistocene mammal soft tissue remains from the glaciated regions of northern Canada. Investigation of the recovery site indicates that the steppe bison was released from the permafrost during a landslide within unconsolidated glacial outwash gravel. These data indicate that the lower Mackenzie River valley was ice free and inhabited by steppe bison by ∼11,800 14C years ago. This date is important for the deglacial chronology of the Laurentide Ice Sheet and the opening of the northern portal to the Ice Free Corridor. The presence of steppe bison raises further potential for the discovery of more late Pleistocene fauna, and possibly archaeological evidence, in the region.
  20. Feranec, Robert S., Elizabeth A. Hadly, and Adina Paytan. “Stable isotopes reveal seasonal competition for resources between late Pleistocene bison (Bison) and horse (Equus) from Rancho La Brea, southern California.” Palaeogeography, Palaeoclimatology, Palaeoecology 271.1-2 (2009): 153-160.  Determining how organisms partition or compete for resources within ecosystems can reveal how communities are assembled. The Late Pleistocene deposits at Rancho La Brea are exceptionally diverse in large mammalian carnivores and herbivores, and afford a unique opportunity to study resource use and partitioning among these megafauna. Resource use was examined in bison and horses by serially sampling the stable carbon and oxygen isotope values found within tooth enamel of individual teeth of seven bison and five horses. Oxygen isotope results for both species reveal a pattern of seasonal enamel growth, while carbon isotope values reveal a more subtle seasonal pattern of dietary preferences. Both species ate a diet dominated by C3 plants, but bison regularly incorporated C4 plants into their diets, while horses ate C4 plants only occasionally. Bison had greater total variation in carbon isotope values than did horses implying migration away from Rancho La Brea. Bison appear to incorporate more C4 plants into their diets during winter, which corresponds to previous studies suggesting that Rancho La Brea, primarily surrounded by C3 plants, was used by bison only during late spring. The examination of intra-tooth isotopic variation which reveals intra-seasonal resource use among bison and horse at Rancho La Brea highlights the utility of isotopic techniques for understanding the intricacies of ecology within and between ancient mammals.
  21. Perego, Ugo A., et al. “Distinctive Paleo-Indian migration routes from Beringia marked by two rare mtDNA haplogroups.” Current biology 19.1 (2009): 1-8.It is widely accepted that the ancestors of Native Americans arrived in the New World via Beringia approximately 10 to 30 thousand years ago (kya). However, the arrival time(s), number of expansion events, and migration routes into the Western Hemisphere remain controversial because linguistic, archaeological, and genetic evidence have not yet provided coherent answers. Notably, most of the genetic evidence has been acquired from the analysis of the common pan-American mitochondrial DNA (mtDNA) haplogroups. In this study, we have instead identified and analyzed mtDNAs belonging to two rare Native American haplogroups named D4h3 and X2a. Phylogeographic analyses at the highest level of molecular resolution (69 entire mitochondrial genomes) reveal that two almost concomitant paths of migration from Beringia led to the Paleo-Indian dispersal approximately 15–17 kya. Haplogroup D4h3 spread into the Americas along the Pacific coast, whereas X2a entered through the ice-free corridor between the Laurentide and Cordilleran ice sheets. The examination of an additional 276 entire mtDNA sequences provides similar entry times for all common Native American haplogroups, thus indicating at least a dual origin for Paleo-Indians. A dual origin for the first Americans is a striking novelty from the genetic point of view, and it makes plausible a scenario positing that within a rather short period of time, there may have been several entries into the Americas from a dynamically changing Beringian source. Moreover, this implies that most probably more than one language family was carried along with the Paleo-Indians. (MULTIPLE BERINGIA MIGRATIONS) 
  22. de Saint Pierre, Michelle, et al. “Arrival of Paleo-Indians to the southern cone of South America: new clues from mitogenomes.” PloS one 7.12 (2012): e51311.  With analyses of entire mitogenomes, studies of Native American mitochondrial DNA (mtDNA) variation have entered the final phase of phylogenetic refinement: the dissection of the founding haplogroups into clades that arose in America during and after human arrival and spread. Ages and geographic distributions of these clades could provide novel clues on the colonization processes of the different regions of the double continent. As for the Southern Cone of South America, this approach has recently allowed the identification of two local clades (D1g and D1j) whose age estimates agree with the dating of the earliest archaeological sites in South America, indicating that Paleo-Indians might have reached that region from Beringia in less than 2000 years. In this study, we sequenced 46 mitogenomes belonging to two additional clades, termed B2i2 (former B2l) and C1b13, which were recently identified on the basis of mtDNA control-region data and whose geographical distributions appear to be restricted to Chile and Argentina. We confirm that their mutational motifs most likely arose in the Southern Cone region. However, the age estimate for B2i2 and C1b13 (11–13,000 years) appears to be younger than those of other local clades. The difference could reflect the different evolutionary origins of the distinct South American-specific sub-haplogroups, with some being already present, at different times and locations, at the very front of the expansion wave in South America, and others originating later in situ, when the tribalization process had already begun. A delayed origin of a few thousand years in one of the locally derived populations, possibly in the central part of Chile, would have limited the geographical and ethnic diffusion of B2i2 and explain the present-day occurrence that appears to be mainly confined to the Tehuelche and Araucanian-speaking groups.
  23. Carlson, Kristen, and Leland Bement. “Organization of bison hunting at the Pleistocene/Holocene transition on the Plains of North America.” Quaternary International 297 (2013): 93-99.  This paper focuses on the development of large scale bison hunting across the North American Great Plains. Prehistoric hunters were not merely opportunistic. An understanding of topography, environment, bison behavior, and migration patterns was necessary to perform complex, large scale bison kills. In turn, these kills required the existence of social complexity whereby multiple groups of hunters worked in unison toward a successful kill event. On the southern Plains of North America, evidence suggests large scale bison hunting arose as mammoths and other megafauna became extinct 11,000 radiocarbon years ago. We review this evidence in light of new site discoveries.
  24. Dixon, E. James. “Late Pleistocene colonization of North America from Northeast Asia: New insights from large-scale paleogeographic reconstructions.” Mobility and ancient society in Asia and the Americas. Springer, Cham, 2015. 169-184.  Advances in large-scale paleogeographic reconstruction define physical and environmental constraints relevant to understanding the timing and character of the first colonization of the Americas during the Late Pleistocene. Diachronic mapping shows continental glaciers coalesced in central Canada during the Last Glacial Maximum (LGM) 20,000–14,000 years ago while unglaciated refugia existed along the Northwest Coast. The Bering Land Bridge connected Asia and North America until about 10,000 years ago when the two continents were separated by rising sea level. This visual analysis from large-scale synthesis of recent geological and environmental research establishes timelines for biotically viable colonization corridors connecting eastern Beringia to southern North America and provides insights into probable Paleoindian origins and subsistence strategies.
  25. Thackeray, Francis,Did a large meteorite hit the earth 12,800 years ago? Here’s new evidence?  [LINK]  by 2019.  Just less than 13,000 years ago, the climate cooled for a short while in many parts of the world, especially in the northern hemisphere. We know this because of what has been found in ice cores drilled in Greenland, as well as from oceans around the world. Grains of pollen from various plants can also tell us about this cooler period, which people who study climate prehistory call the Younger Dryas and which interrupted a warming trend after the last Ice Age. The term gets its name from a wildflower, Dryas octopetala. It can tolerate cold conditions and was common in parts of Europe 12,800 years ago. At about this time a number of animals became extinct. These included mammoths in Europe, large bison in North America, and giant sloths in South America. The cause of this cooling event has been debated a great deal. One possibility, for instance, is that it relates to changes in oceanic circulation systems. In 2007 Richard Firestone and other American scientists presented a new hypothesis: that the cause was a cosmic impact like an asteroid or comet. The impact could have injected a lot of dust into the air, which might have reduced the amount of sunlight getting through the earth’s atmosphere. This might have affected plant growth and animals in the food chain. Research we have just had published sheds new light on this Younger Dryas Impact Hypothesis. We focus on what platinum can tell us about it.




  1. 1988: Broecker, Wallace S., et al. “The chronology of the last deglaciation: Implications to the cause of the Younger Dryas event.” Paleoceanography and Paleoclimatology 3.1 (1988): 1-19.  It has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions. This extraordinary event, referred to by European palynologists as the Younger Dryas, was centered in the northern Atlantic basin. Evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern Atlantic Ocean. Further, it appears that these mode changes may have been triggered by diversions of glacial meltwater between the Mississippi River and the St. Lawrence River drainage systems. We report here Accelerator Mass Spectrometry (AMS) radiocarbon results on two strategically located deep‐sea cores. One provides a chronology for surface water temperatures in the northern Atlantic and the other for the meltwater discharge from the Mississippi River. Our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern Atlantic basin with the meltwater history for the Laurentian ice sheet.
  2. 1989: Dansgaard, W. H. I. T. E., J. W. C. White, and S. J. Johnsen. “The abrupt termination of the Younger Dryas climate event.” Nature 339.6225 (1989): 532.  PREVIOUS studies on two deep Greenland ice cores have shown that a long series of climate oscillations characterized the late Weichselian glaciation in the North Atlantic region1, and that the last glacial cold period, the Younger Dryas, ended abruptly 10,700 years ago2. Here we further focus on this epoch-defining event, and present detailed heavy-isotope and dust-concentration profiles which suggest that, in less than 20 years, the climate in the North Atlantic region turned into a milder and less stormy regime, as a consequence of a rapid retreat of the sea-ice cover. A warming of 7 °C in South Greenland was completed in about 50 years.
  3. 1989: Fairbanks, Richard G. “A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation.” Nature 342.6250 (1989): 637.  Coral reefs drilled offshore of Barbados provide the first continuous and detailed record of sea level change during the last deglaciation. The sea level was 121 ± 5 metres below present level during the last glacial maximum. The deglacial sea level rise was not monotonic; rather, it was marked by two intervals of rapid rise. Varying rates of melt-water discharge to the North Atlantic surface ocean dramatically affected North Atlantic deep-water production and oceanic oxygen isotope chemistry. A global oxygen isotope record for ocean water has been calculated from the Barbados sea level curve, allowing separation of the ice volume component common to all oxygen isotope records measured in deep-sea cores.
  4. 1990: Fairbanks, Richard G. “The age and origin of the “Younger Dryas climate event” in Greenland ice cores.” Paleoceanography and Paleoclimatology 5.6 (1990): 937-948.  230Th/234U and 14C dating of Barbados corals has extended the calibration of 14C years B.P. to calendar years B.P. beyond the 9200 year tree ring series (Bard et al., 1990). This now permits the conversion of 14C chronozones, which delimit major climate shifts in western Europe, to calendar years. The Younger Dryas chronozone, defined as 11,000 to 10,000 14C years B.P., corresponds to 13,000 to 11,700 calendar years B.P. This calibration affects the interpretation of an intensely studied example of the “Younger Dryas climate event,” the δ18O anomaly between 1785 and 1793 m in Dye 3 ice core. The end of the δ18O anomaly in Dye 3 ice core has been dated by measurements of 14C in air bubbles (Andree et al., 1984, 1986) and by annual layer counting (Hammer et al., 1986). The older 14C dates fall out of the range of the tree ring calibration series but can now be calibrated to calendar years using the Barbados 230Th/234U calibration. The 14Ccorrectedage for the end of the δ18O event is 10,300 ± 400 calendar years B.P. compared to the annual layer counting age of 10,720 ± 150 years B.P. Thus, the “Younger Dryas” event in the Dye 3 ice core ends in the Preboreal chronozone (11,700 to 10,000 calendar years B.P.) and is not correlative with the end of the Younger Dryas event identified in pollen records marking European vegetation changes. The end of the Dye 3 δ18O event is, however, correlative with the end of meltwater pulse IB (Fairbanks, 1989), marking a period of intense deglaciation with meltwater discharge rates exceeding 13,000 km³/yr.
  5. 1993: Alley, Richard B., et al. “Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event.” Nature362.6420 (1993): 527. THE warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate, the best-known of which is the Younger Dryas event1. Despite much study of the causes of this event and the mechanisms by which it ended, many questions remain unresolved1. Oxygen isotope data from Greenland ice cores2–4 suggest that the Younger Dryas ended abruptly, over a period of about 50 years; dust concentrations2,4 in these cores show an even more rapid transition (20 years). This extremely short timescale places severe constraints on the mechanisms underlying the transition. But dust concentrations can reflect subtle changes in atmospheric circulation, which need not be associated with a large change in climate. Here we present results from a new Greenland ice core (GISP2) showing that snow accumulation doubled rapidly from the Younger Dryas event to the subsequent Preboreal interval, possibly in one to three years. We also find that the accumulation-rate change from the Oldest Dryas to the Bø11ing/Allerød warm period was large and abrupt. The extreme rapidity of these changes in a variable that directly represents regional climate implies that thalleye events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the North Atlantic climate system.
  6. 1994: Bard, Edouard, et al. “The North Atlantic atmosphere-sea surface 14C gradient during the Younger Dryas climatic event.” Earth and Planetary Science Letters 126.4 (1994): 275-287. We attempt to quantify the 14C difference between the atmosphere and the North Atlantic surface during a prominent climatic period of the last deglaciation, the Younger Dryas event (YD). Our working hypothesis is that the North Atlantic may have experienced a measurable change in 14C reservoir age due to large changes of the polar front position and variations in the mode and rate of North Atlantic Deep Water (NADW) production. We dated contemporaneous samples of terrestrial plant remains and sea surface carbonates in order to evaluate the past atmosphere-sea surface 14C gradient. We selected terrestrial vegetal macrofossils and planktonic foraminifera (Neogloboquadrina pachyderma left coiling) mixed with the same volcanic tephra (the Vedde Ash Bed) which occurred during the YD and which can be recognized in North European lake sediments and North Atlantic deep-sea sediments. Based on AMS ages from two Norwegian sites, we obtained about 10,300 yr BP for the ‘atmospheric’ 14C age of the volcanic eruption. Foraminifera from four North Atlantic deep-sea cores selected for their high sedimentation rates ( > 10 cm kyr−1) were dated by AMS (21 samples). For each core the raw 14C ages assigned to the ash layer peak is significantly older than the 14C age obtained on land. Part of this discrepancy is due to bioturbation, which is shown by numerical modelling. Nevertheless, after correction of a bioturbation bias, the mean 14C age obtained on the planktonic foraminifera is still about 11,000–11,100 yr BP. The atmosphere-sea surface 14C difference was roughly 700–800 yr during the YD, whereas today it is 400–500 yr. A reduced advection of surface waters to the North Atlantic and the presence of sea ice are identified as potential causes of the high 14C reservoir age during the YD.
  7. ACC 1995: Rahmstorf, Stefan. “Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle.” Nature 378.6553 (1995): 145.  The sensitivity of the North Atlantic thermohaline circulation to the input of fresh water is studied using a global ocean circulation model coupled to a simplified model atmosphere. Owing to the nonlinearity of the system, moderate changes in freshwater input can induce transitions between different equilibrium states, leading to substantial changes in regional climate. As even local changes in freshwater flux are capable of triggering convective instability, quite small perturbations to the present hydrological cycle may lead to temperature changes of several degrees on timescales of only a few years.
  8. ACC 1995: Manabe, Syukuro, and Ronald J. Stouffer. “Simulation of abrupt climate change induced by freshwater input to the North Atlantic Ocean.” Nature 378.6553 (1995): 165.  Temperature records from Greenland ice cores1,2 suggest that large and abrupt changes of North Atlantic climate occurred frequently during both glacial and post glacial periods; one example is the Younger Dryas cold event. Broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the Atlantic Ocean, which were caused by the release of large amounts of melt water from continental ice sheets. Here we describe an attempt to explore this intriguing phenomenon using a coupled ocean–atmosphere model. In response to a massive surface flux of fresh water to the northern North Atlantic of the model, the thermohaline circulation weakens abruptly, intensifies and weakens again, followed by a gradual recovery, generating episodes that resemble the abrupt changes of the ocean–atmosphere system recorded in ice and deep-sea cores4. The associated change of surface air temperature is particularly large in the northern North Atlantic Ocean and its neighbourhood, but is relatively small in the rest of the world.
  9. 1997: Bond, Gerard, et al. “A pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates.” science278.5341 (1997): 1257-1266.  Evidence from North Atlantic deep sea cores reveals that abrupt shifts punctuated what is conventionally thought to have been a relatively stable Holocene climate. During each of these episodes, cool, ice-bearing waters from north of Iceland were advected as far south as the latitude of Britain. At about the same times, the atmospheric circulation above Greenland changed abruptly. Pacings of the Holocene events and of abrupt climate shifts during the last glaciation are statistically the same; together, they make up a series of climate shifts with a cyclicity close to 1470 ± 500 years. The Holocene events, therefore, appear to be the most recent manifestation of a pervasive millennial-scale climate cycle operating independently of the glacial-interglacial climate state. Amplification of the cycle during the last glaciation may have been linked to the North Atlantic’s thermohaline circulation.
  10. 1997: Alley, Richard B., et al. “Holocene climatic instability: A prominent, widespread event 8200 yr ago.” Geology 25.6 (1997): 483-486.  The most prominent Holocene climatic event in Greenland ice-core proxies, with approximately half the amplitude of the Younger Dryas, occurred ∼8000 to 8400 yr ago. This Holocene event affected regions well beyond the North Atlantic basin, as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane. Widespread proxy records from the tropics to the north polar regions show a short-lived cool, dry, or windy event of similar age. The spatial pattern of terrestrial and marine changes is similar to that of the Younger Dryas event, suggesting a role for North Atlantic thermohaline circulation. Possible forcings identified thus far for this Holocene event are small, consistent with recent model results indicating high sensitivity and strong linkages in the climatic system.
  11. 1998: Severinghaus, Jeffrey P., et al. “Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice.” Nature 391.6663 (1998): 141.  Rapid temperature change fractionates gas isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice. The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time. The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 ± 3 °C colder than today.
  12. 1997: Broecker, Wallace S. “Thermohaline circulation, the Achilles heel of our climate system: Will man-made CO2 upset the current balance?.” Science 278.5343 (1997): 1582-1588.  During the last glacial period, Earth’s climate underwent frequent large and abrupt global changes. This behavior appears to reflect the ability of the ocean’s thermohaline circulation to assume more than one mode of operation. The record in ancient sedimentary rocks suggests that similar abrupt changes plagued the Earth at other times. The trigger mechanism for these reorganizations may have been the antiphasing of polar insolation associated with orbital cycles. Were the ongoing increase in atmospheric CO2 levels to trigger another such reorganization, it would be bad news for a world striving to feed 11 to 16 billion people.
  13. 1999: Marchal, O., et al. “Modelling the concentration of atmospheric CO2 during the Younger Dryas climate event.” Climate Dynamics 15.5 (1999): 341-354.  The Younger Dryas (YD, dated between 12.7–11.6 ky BP in the GRIP ice core, Central Greenland) is a distinct cold period in the North Atlantic region during the last deglaciation. A popular, but controversial hypothesis to explain the cooling is a reduction of the Atlantic thermohaline circulation (THC) and associated northward heat flux as triggered by glacial meltwater. Recently, a CH4-based synchronization of GRIP δ18O and Byrd CO2 records (West Antarctica) indicated that the concentration of atmospheric CO2 (COatm2) rose steadily during the YD, suggesting a minor influence of the THC on COatm2 at that time. Here we show that the CO2atm change in a zonally averaged, circulation-biogeochemistry ocean model when THC is collapsed by freshwater flux anomaly is consistent with the Byrd record. Cooling in the North Atlantic has a small effect on CO2atm in this model, because it is spatially limited and compensated by far-field changes such as a warming in the Southern Ocean. The modelled Southern Ocean warming is in agreement with the anti-phase evolution of isotopic temperature records from GRIP (Northern Hemisphere) and from Byrd and Vostok (East Antarctica) during the YD. δ13C depletion and PO4 enrichment are predicted at depth in the North Atlantic, but not in the Southern Ocean. This could explain a part of the controversy about the intensity of the THC during the YD. Potential weaknesses in our interpretation of the Byrd CO2 record in terms of THC changes are discussed.
  14. ACC 2002: Clark, Peter U., et al. “The role of the thermohaline circulation in abrupt climate change.” Nature 415.6874 (2002): 863.  The possibility of a reduced Atlantic thermohaline circulation in response to increases in greenhouse-gas concentrations has been demonstrated in a number of simulations with general circulation models of the coupled ocean–atmosphere system. But it remains difficult to assess the likelihood of future changes in the thermohaline circulation, mainly owing to poorly constrained model parameterizations and uncertainties in the response of the climate system to greenhouse warming. Analyses of past abrupt climate changes help to solve these problems. Data and models both suggest that abrupt climate change during the last glaciation originated through changes in the Atlantic thermohaline circulation in response to small changes in the hydrological cycle. Atmospheric and oceanic responses to these changes were then transmitted globally through a number of feedbacks. The palaeoclimate data and the model results also indicate that the stability of the thermohaline circulation depends on the mean climate state.
  15. ACC 2002: Vellinga, Michael, and Richard A. Wood. “Global climatic impacts of a collapse of the Atlantic thermohaline circulation.” Climatic change 54.3 (2002): 251-267.  Part of the uncertainty in predictions by climate models results from limited knowledge of the stability of the thermohaline circulation of the ocean. Here we provide estimates of the response of pre-industrial surface climate variables should the thermohalinecirculation in the Atlantic Ocean collapse. For this we have used HadCM3, an ocean-atmosphere general circulation model that is run without flux adjustments. In this model a temporary collapse was forced by applying a strong initial freshening to the top layers of the NorthAtlantic. In the first five decades after the collapse surface air temperature response is dominated by cooling of much of the Northern Hemisphere (locally up to 8 °C, 1–2 °C on average) and weak warming of the Southern Hemisphere (locally up to 1 °C, 0.2 °C onaverage). Response is strongest around the North Atlantic but significant changes occur over the entire globe and highlight rapid connections. Precipitation is reduced over large parts of the Northern Hemisphere. A southward shift of the Intertropical Convergence Zone over the Atlantic and eastern Pacific creates changes in precipitation that are particularly large in South America and Africa. Colder and drier conditions in much of the Northern Hemisphere reduces oil moisture and net primary productivity of the terrestrial vegetation. This is only partly compensated by more productivity in the Southern Hemisphere.The total global net primary productivity by the vegetation decreases by 5%. It should be noted, however, that in this version of the model the vegetation distribution cannot change, and atmospheric carbon levels are also fixed. After about 100 years the model’s thermohaline circulation has largely recovered, and most climatic anomalies disappear.
  16. 2003: Alley, Richard B., et al. “Abrupt climate change.” science299.5615 (2003): 2005-2010.  Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.
  17. ACC 2004: McManus, Jerry F., et al. “Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes.” Nature 428.6985 (2004): 834. The Atlantic meridional overturning circulation is widely believed to affect climate. Changes in ocean circulation have been inferred from records of the deep water chemical composition derived from sedimentary nutrient proxies1, but their impact on climate is difficult to assess because such reconstructions provide insufficient constraints on the rate of overturning2. Here we report measurements of 231Pa/230Th, a kinematic proxy for the meridional overturning circulation, in a sediment core from the subtropical North Atlantic Ocean. We find that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500 yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago. Following these cold events, the 231Pa/230Th record indicates that rapid accelerations of the meridional overturning circulation were concurrent with the two strongest regional warming events during deglaciation. These results confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate changes.
  18. ACC 2005: Zhang, Rong, and Thomas L. Delworth. “Simulated tropical response to a substantial weakening of the Atlantic thermohaline circulation.” Journal of Climate 18.12 (2005): 1853-1860.  In this study, a mechanism is demonstrated whereby a large reduction in the Atlantic thermohaline circulation (THC) can induce global-scale changes in the Tropics that are consistent with paleoevidence of the global synchronization of millennial-scale abrupt climate change. Using GFDL’s newly developed global coupled ocean–atmosphere model (CM2.0), the global response to a sustained addition of freshwater to the model’s North Atlantic is simulated. This freshwater forcing substantially weakens the Atlantic THC, resulting in a southward shift of the intertropical convergence zone over the Atlantic and Pacific, an El Niño–like pattern in the southeastern tropical Pacific, and weakened Indian and Asian summer monsoons through air–sea interactions.
  19. ACC 2006:  Stouffer, Ronald J., et al. “Investigating the causes of the response of the thermohaline circulation to past and future climate changes.” Journal of climate 19.8 (2006): 1365-1387.  The Atlantic thermohaline circulation (THC) is an important part of the earth’s climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-Sv (1 Sv ≡ 106 m3 s−1) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.
  20. Melott, Adrian L., et al. “Cometary airbursts and atmospheric chemistry: Tunguska and a candidate Younger Dryas event.” Geology 38.4 (2010): 355-358.  We find agreement between models of atmospheric chemistry changes from ionization for the A.D. 1908 Tunguska (Siberia region, Russia) airburst event and nitrate enhancement in Greenland Ice Sheet Project 2 (GISP2H and GISP2) ice cores, plus an unexplained ammonium spike. We then consider a candidate cometary impact at the Younger Dryas onset (YD). The large estimated NOx production and O3 depletion are beyond accurate extrapolation, but the ice core peak is much lower, possibly because of insufficient sampling resolution. Ammonium and nitrate spikes in both Greenland Ice Core Project (GRIP) and GISP2 ice cores have been attributed to biomass burning at the onset of the YD. A similar result is well resolved in Tunguska ice core data, but that forest fire was far too small to account for this. Direct input of ammonia from a comet into the atmosphere is adequate for YD ice core data, but not for the Tunguska data. An analog of the Haber process with hydrogen contributed by cometary or surface water, atmospheric nitrogen, high pressures, and possibly catalytic iron from a comet could in principle produce ammonia, accounting for the peaks in both data sets.
  21. Carlson, Anders E. “What caused the Younger Dryas cold event?.” Geology 38.4 (2010): 383-384. The Younger Dryas Cold Event (ca. 12.9–11.6 ka) has long been viewed as the canonical abrupt climate event (Fig. 1). The North Atlantic region cooled during this interval with a weakening of Northern Hemisphere monsoon strength. The reduction in northward heat transport warmed the Southern Hemisphere due to a process commonly referred to as the bipolar-seesaw (e.g., Clark et al., 2002). Although it is generally accepted that the cold event resulted from a slowing Atlantic meridional overturning circulation (AMOC), the forcing of this AMOC reduction remains intensely debated. The most common means of slowing AMOC involves the reduction of oceanic surface water density via an increase in freshwater discharge to the North Atlantic. The originally hypothesized source of freshwater was the eastward routing of Glacial Lake Agassiz from the Mississippi River to the St. Lawrence River, as the Laurentide Ice Sheet retreated northward out of the Great Lakes (Johnson and McClure, 1976; Rooth, 1982; Broecker, 2006). A clear Younger Dryas freshwater signal in the St. Lawrence Estuary (Keigwin and Jones, 1995; deVernal et al., 1996) only becomes apparent after accounting for other competing effects on commonly used freshwater proxies, in agreement with three other independent runoff proxies (Carlson et al., 2007). Lake Agassiz’s eastern outlet history also presents an issue, as the most recent study suggested that the outlet remained closed until well after the start of the Younger Dryas, with the lake having no outlet for much of the Younger Dryas (Lowell et al., 2009). In contrast, a simple consideration of Lake Agassiz’s water budget requires an outlet for the lake during the Younger Dryas (Carlson et al., 2009). This ongoing debate over the ultimate cause of the Younger Dryas has led to a search for other potential forcing mechanisms, such as an abrupt discharge of meltwater to the Arctic Ocean (Tarasov and Peltier, 2005) and a bolide impact (Firestone et al., 2007).On page 355 of this issue of Geology, Melott et al. (2010) present a quantitative assessment of the effect a comet would have on atmospheric nitrate, as well as estimates of its consequence for atmospheric ammonium, providing a test for the occurrence of a bolide at the onset of the Younger Dryas. Accordingly, comets break down N2 in the atmosphere to nitrate (NOx), increasing nitrate concentration. The authors use a two-dimensional atmospheric model to simulate the nitrate and ozone changes associated with the A.D. 1908 Tunguska event where a bolide airburst occurred over Siberia, Russia. The model performs well for the Tunguska event, accurately simulating the nitrate increase of ∼160 ppb observed in the Greenland Ice Sheet Project 2 (GISP2) ice core record from Summit Greenland. Scaling the predicted nitrate changes upward by six orders of magnitude to the suggested Younger Dryas–size bolide implies a very large increase in nitrate concentration (i.e., 106 times larger than the Tunguska increase) that should be recorded in Greenland ice at the start of the Younger Dryas (Fig. 1A). Greenland ice cores also show ammonium (NH4+) increases during the Tunguska event and the Younger Dryas (Fig. 1B). While biomass burning is implicated for the Younger Dryas increase (e.g., Firestone et al., 2007), the amount of burning during the Tunguska event is too small to account for the ammonium increase of >200 ppb (Melott et al., 2010). Another alternative, involving direct ammonium deposition from the bolide, still fails to account for the observed Tunguska increase. The authors thus suggest a third mechanism called the Haber process that could account for both the Younger Dryas and Tunguska increases, in which, under high pressure, nitrogen and hydrogen can form ammonia. For the Tunguska increase, a potential impact with permafrost could provide the hydrogen, whereas the Laurentide Ice Sheet itself might be the hydrogen source for the Younger Dryas impact. The Melott et al. study thus lays out a test for the occurrence of a Younger Dryas bolide impact, constrained by observations of the recent Tunguska impact. Their estimates, however, for the increases in nitrate and ammonium associated with a Younger Dryas–size comet are orders of magnitude larger than observed in the Summit Greenland ice core records; the Younger Dryas nitrate and ammonium increases are at most just half of the Tunguska increase. Likewise, the anomalies noted at the start of the Younger Dryas appear to be non-unique in the highest-resolution records (Figs. 1A and 1B). This may be due to the ice core sample resolution. The GISP2 ∼3.5 yr sample resolution could potentially under-sample a nitrate or ammonium increase (Mayewski et al., 1997) because both compounds have atmospheric residence times of a few years. As Melott et al. note, higher-resolution sampling from the Greenland ice cores could determine if large (i.e., orders of magnitude larger than the Tunguska event) increases in nitrate and ammonium occurred at the start of the Younger Dryas. Several other issues still remain with the bolide-forcing hypothesis for the Younger Dryas. For instance, the original Firestone et al. (2007) impact-marker records have not proven reproducible in a subsequent study (Surovell et al., 2009). Similarly, a compilation of charcoal records do not indicate large-scale burning of ice-free North America at the onset of the Younger Dryas (Marlon et al., 2009) as put forward by Firestone et al. (2007). Another recent study showed that late Pleistocene megafauna extinctions, potentially attributable to a Younger Dryas impact (Firestone et al., 2007), significantly preceded the Younger Dryas (Gill et al., 2009). Furthermore, it has yet to be demonstrated how a short-lived event, such as a bolide impact (or abrupt Arctic meltwater discharge, i.e., Tarasov and Peltier, 2005), can force a millennia-long cold event when state-of-the-art climate models require a continuous freshwater forcing for the duration of the AMOC reduction (e.g., Liu et al., 2009). If the bolide impacted the southern Laurentide margin near the Great Lakes, it could have opened the eastern outlet of Lake Agassiz, but Great Lake till sequences are not disturbed (e.g., Mickelson et al., 1983). Ultimately, the bolide-forcing hypothesis predicts that the Younger Dryas is a unique deglacial event, as suggested by Broecker (2006). However, high-resolution proxy records sensitive to AMOC strength (Chinese speleothem δ18O and atmospheric methane) document a Younger Dryas–like event during termination III (the third to the last deglaciation) (Figs. 2B and 2C; Carlson, 2008; Cheng et al., 2009). The boreal summer insolation increase during termination III is similar to the last deglaciation, as is the timing of the event relative to the peak in insolation (Fig. 2D). While not as well constrained, both events occurred at approximately the same sea level (Fig. 2A), suggesting there may be a common forcing related to the size of the Laurentide Ice Sheet (Carlson, 2008). During terminations II and IV (Fig. 2), greater increases in boreal summer insolation driving faster ice retreat and attendant continuous reduction in AMOC strength can explain the lack of Younger Dryas–like events in these cases (e.g., Ruddiman et al., 1980; Carlson, 2008). Alternatively, a bolide could have forced the termination III event as well. The direct (if there was a bolide, then there will be a very large nitrate spike) approach presented by Melott et al. is testable through sub-annual sampling of the Greenland ice cores, providing a step forward in resolving the forcing of the Younger Dryas and our understanding of abrupt climate events.

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