MIOCENE HOMINIDS

MIOCENE ANIMALS

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MIOCENE TEMPERATURES

1. SUMMARY: What makes the Mid Miocene interesting in the AGW context is that it shows that the earth’s own geological heat sources can control deep ocean temperature and change ocean currents such as to dramatically change the climate on the surface. The Mid Miocene warming event underscores the atmosphere bias of climate science where all changes including changes in deep ocean heat are ultimately ascribed to changes in atmospheric composition that are assumed to be driven by fossil fuel emissions. [LINK] [LINK] [LINK] [LINK] .
2. The Miocene Age extends over a period of ≈18 million years from ≈23Ma to ≈5Ma. As seen in the deep sea and air temperature charts above, over this period the earth continued the cooling and glaciation trend that had started in the Oligocene (warmer than the Miocene) and that continued into the Pliocene (cooler than the Miocene) in a series of ice ages.  Of interest in this post is the period of warming in the middle of the Miocene labeled in the chart above as the “Mid Miocene Climate Optimum”  that began ≈16 Ma and that returned to the glaciation cooling trend ≈14 Ma (Ma=millions of years ago) with a rapid expansion of the East Antarctic Ice Sheet.
3. This two million years of warming is thought to have parallels with and important lessons for the current 160-year warming trend attributed to CO2 emissions from the use of fossil fuels by the industrial economy, particularly so in terms of the greenhouse effect of atmospheric CO2 and in terms of the possible catastrophic consequences, such as ice sheet collapse and sea level rise, of a CO2 driven warming trend. 
4. What makes the comparison of the Mid Miocene Climate Optimum with the current warming event attractive is that temperatures and atmospheric CO2 concentrations of that time were comparable with what we see today in the AGW warming event. Estimates from paleo data show that global mean temperature during the Mid Miocene global warming event peaked at 4.2C, about 3C warmer than the present and equal to the projected temperature in the year 2100 under the RCP8.5 business as usual scenario. This convenient equivalence is the basis for the usual assumption that the horror of our future without climate action can be seen in the past in terms of Mid Miocene warming event.
5. When the world cooled from the warmer late Oligocene to the Miocene in a cooling trend, atmospheric CO2 dropped from 350 ppm to a much lower level in the range 190-260 ppm. This gradual decrease in atmospheric CO2 during a time of cooling is considered to be consistent with the greenhouse effect of atmospheric CO2. This interpretation of the data without further statistical tests is likely to be one of convenience and confirmation bias since the observed association can be interpreted in terms of causation in either direction or of causation of both by a third unobserved variable, or even as a spurious relationship with no causation information.
6. In fact, this interpretation is confounded by what happened in the Mid-Miocene warming event. As temperatures rose from ≈12C to ≈18C, atmospheric CO2 levels dropped to the low end of the (190-260 ppm) range. If these changes are to be interpreted in terms of the CO2 greenhouse effect, the the CO2 level should have been higher at 18C than at 12C. Climate models show that under prevailing conditions in the Mid Miocene, AGW theory predicts atmospheric CO2 concentrations rising from 300 ppm to 600 ppm as described in the You paper below. References in the literature to the atmosphere being “supercharged with carbon dioxide” (Levy and Meyers, 2019) at this time may be a reference to these high values of atmospheric CO2 derived from climate models. These CO2 values are “inferred” and not observed. Their interpretation as observed data involves circular reasoning.
7. A further difficulty in interpreting these changes in terms of the greenhouse effect of CO2 is the spectacular growth of the East Antarctic Ice Sheet during the MMGW event without an associated sharp decrease in atmospheric CO2. In fact, toward the end of the massive growth in the East Antarctic ice sheet, atmospheric CO2 levels were higher at around 280 ppm equivalent to “pre-industrial” levels in the current warming event. As seen in the bibliography below, the general consensus is that the MMGW event is not an analog to the AGW event and not a demonstration of the greenhouse effect of atmospheric CO2. The analogy involves serious anomalies and paradoxical events.
8. The general consensus in the bibliography below seems to be that the Mid Miocene warming event is best explained in terms of deep ocean circulation or the so called “oceanographic control of Miocene climate“. Many of these authors who are still in paleo climate research now tend to soft pedal these anomalies and discrepancies in public discourse to present the Mid Miocene warming in terms of the CO2 greenhouse effect although their new improved assessment appears to contradict what they had written twenty or more years ago. In many of the works below, particularly the later papers, it appears that the authors are struggling to relate grossly anomalous situations to the greenhouse effect of atmospheric CO2.
9. SUMMARY: Paleo data show that the earth’s own geological heat sources controlled deep ocean temperatures and changed ocean currents such as to change the climate on the surface. Rather than atmospheric control of climate by way of its CO2 concentration that also controls ocean heat content, it serves as a demonstration of the climate impacts of the earth geological forces in terms of both ocean heat content and the climate. The Mid Miocene warming event underscores the atmosphere bias of climate science that makes it impossible for them to take geological forces and geothermal heat into account in the interpretation of data such that all changes including changes in deep ocean heat are ultimately ascribed to changes in atmospheric composition that are assumed to be driven by fossil fuel emissions. [LINK] [LINK] [LINK] [LINK] .

YET ANOTHER PALEO EVENT OF EXTREME GEOLOGICAL OCEAN HEATING IS RECORDED IN THE PALEOCENE EOCENE THERMAL MAXIMUM [LINK] . THIS EVENT ALSO INCLUDED OCEAN ACIDIFICATION. IT IS EMPHASIZED THAT THESE EVENTS WERE NATURAL.

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KATHERINE FREEMAN AND MARK PAGANI

MIOCENE CO2 BIBLIOGRAPHY

1. 1985: Barron, Eric J., and Warren M. Washington. “Warm Cretaceous climates: High atmospheric CO2 as a plausible mechanism.” The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present 32 (1985): 546-553.  Sensitivity experiments with a general circulation model of the atmosphere coupled to a simple ocean model are the basis for an investigation of whether changing geography is a sufficient mechanism to explain warm Cretaceous (≈100 Ma) climates or whether other mechanisms, such as a higher atmospheric CO₂ concentration, are required. Although Cretaceous geography results in a substantial warming in comparison with the present day, the warming is insufficient to explain the geologic data. Several lines of evidence suggest that an estimated two to tenfold increase in CO₂ with respect to present values is a plausible explanation of this problem. Higher values of CO₂ result in additional climate problems. These model experiments have implications for geochemical models with climate‐dependent weathering rates.
2. 1985: Vincent, Edith, and Wolfgang H. Berger. “Carbon dioxide and polar cooling in the Miocene: The Monterey hypothesis.” The carbon cycle and atmospheric CO2: Natural variations Archean to present 32 (1985): 455-468.A pronounced shift in the δ¹³C of foraminifera in the latest early Miocene has been proposed by various authors. Our data in the tropical Indian Ocean show an excursion of δ¹³C signals toward heavier values, lasting for about 4 million years. The excursion is documented for benthic foraminifera as well as for deep‐living and for shallow‐dwelling planktonic species. The initial δ¹³c shift occurs within Magnetic Chron 16, at about 17.5 Ma. It represents a change toward heavier δ¹³C values by about 10/00 in surface and bottom waters. The excursion terminates at approximately 13.5 Ma. The Chron 16 Carbon shift coincides with the cessation of an early Miocene warming trend, seen in the δ¹⁸O signals. The mid‐Miocene cooling step (presumably associated with Antarctic ice buildup, near 15 Ma) is centered on the carbon isotope excursion. We propose that the initial carbon shift was caused by rapid extraction of organic carbon from the ocean‐atmosphere system. Subsequently, the excursion toward heavy values was maintained by continued extraction of organic carbon, into ocean‐margin deposits. Beginning at the end of the early Miocene, fine‐grained diatomaceous sediments rich in organic matter were deposited all around the margins of the northern Pacific. In California, these sediments are known as the Monterey Formation. This formation is the result of coastal upwelling, which arose because of the development of strong zonal winds and a strong permanent thermocline. Zonal winds and thermocline evolution, in turn, depended on increasing temperature contrast between high and low latitudes. We hypothesize that a feedback loop was established, such that an initial increase in the planetary temperature gradient started thermocline development which led to organic carbon extraction at the ocean margins which resulted in a drop in atmospheric carbon dioxide concentration. Concomitant cooling (reverse greenhouse effect) strengthened thermocline development, leading to further cooling. The loop was broken when available nutrients were used up. The total amount of excess carbon buildup, according to the hypothesis, is between 40 and 80 atmospheric carbon masses for the duration of the Monterey carbon isotope excursion. This amount corresponds to that present in the ocean, that is, one ocean carbon mass.
3. 1992: Wright, James D., Kenneth G. Miller, and Richard G. Fairbanks. “Early and middle Miocene stable isotopes: implications for deepwater circulation and climate.” Paleoceanography 7.3 (1992): 357-389.  The middle Miocene δ¹⁸O increase represents a fundamental change in the ocean‐atmosphere system which, like late Pleistocene climates, may be related to deepwater circulation patterns. There has been some debate concerning the early to early middle Miocene deepwater circulation patterns. Specifically, recent discussions have focused on the relative roles of Northern Component Water (NCW) production and warm, saline deep water originating in the eastern Tethys. Our time series and time slice reconstructions indicate that NCW and Tethyan outflow water, two relatively warm deepwater masses, were produced from ∼20 to 16 Ma. NCW was produced again from 12.5 to 10.5 Ma. Another feature of the early and middle Miocene oceans was the presence of a high δ¹³C intermediate water mass in the southern hemisphere, which apparently originated in the Southern Ocean. Miocene climates appear to be related directly to deepwater circulation changes. Deep‐waters warmed in the early Miocene by ∼3°C (∼20 to 16 Ma) and cooled by a similar amount during the middle Miocene δ¹⁸O increase (14.8 to 12.6 Ma), corresponding to the increase (∼20 Ma) and subsequent decrease (∼16 Ma) in the production of NCW and Tethyan outflow water. Large (>0.6 ‰), relatively rapid (∼0.5 m.y.) δ¹⁸O increases in both benthic and planktonic foraminifera (i.e., the Mi zones of Miller et al. (1991a) and Wright and Miller (1992a)) were superimposed in the long‐term deepwater temperature changes; they are interpreted as reflecting continental ice growth events. Seven of these m.y. glacial/interglacial cycles have been recognized in the early to middle Miocene. Two of these glacial/interglacial cycles (Mi3 and Mi4) combined with a 2° to 3°C decrease in deepwater temperatures to produce the middle Miocene δ¹⁸O shift.
4. 1994: Flower, Benjamin P., and James P. Kennett. “The middle Miocene climatic transition: East Antarctic ice sheet development, deep ocean circulation and global carbon cycling.” Palaeogeography, palaeoclimatology, palaeoecology108.3-4 (1994): 537-555. The middle Miocene represents a major change in state in Cenozoic climatic evolution, following the climax of Neogene warmth in the late early Miocene at ∼16 Ma. The early stage of this climatic transition from ∼16 to 14.8 Ma was marked by major short term variations in global climates, East Antarctic Ice Sheet (EAIS) volume, sea level, and deep ocean circulation. In the later stage from ∼14.8 to 12.9 Ma, climatic developments included major growth of the EAIS and associated Antarctic cooling, a distinct increase in the meridional temperature gradient, large fluctuations in sea level followed by a global sea level fall, and important changes in deep water circulation, including increased production of Southern Component Water. East Antarctic ice sheet growth and polar cooling also had large effects on global carbon cycling and on the terrestrial biosphere, including aridification of mid-latitude continental regions. Increased stability of the EAIS after 14.8 Ma represents a crucial step in the establishment of late Neogene global climate systems. What controlled these changes in polar climates and the East Antarctic ice sheet? Deep ocean circulation changes probably played a major role in the evolution and variation in polar climates, as they have throughout the Cenozoic. Oxygen and carbon isotopic evidence for warm, saline deep water production in the eastern Tethyan/northern Indian Ocean indicates that meridional heat transport to the Antarctic inhibited Cenozoic polar cooling and EAIS growth during the early middle Miocene from ∼16 to ∼14.8 Ma. Inferred competition between warm low-latitude sources (derived from the eastern Tethyan-northern Indian Ocean) and a cold high-latitude source (Southern Component Water) from ∼16 to 14.8 Ma may have been associated with instability in the Antarctic climate and cryosphere. Reduction of warm, saline deep water flow to the Southern Ocean at ∼14.8 Ma may have decreased meridional heat transport to the Antarctic, cooling the region and leading to increased production of Southern Component Water.These middle Miocene climatic and cryospheric changes in the Antarctic had profound effects on marine and terrestrial climates. As the meridional surface temperature gradient increased, boundaries between climatic zones strengthened, leading to increased aridification of mid-latitude continental regions in Australia, Africa and North and South America, enhancing the development of grasslands and stimulating the evolution of grazing mammals.
5. 1994: Schoell, M., et al. “A molecular organic carbon isotope record of Miocene climate changes.” Science 263.5150 (1994): 1122-1125.  The difference in carbon-13 (¹³C) contents of hopane and sterane biomarkers in the Monterey formation (Naples Beach, California) parallels the Miocene inorganic record of the change in ¹⁸O (δ¹⁸O), reflecting the Miocene evolution from a well-mixed to a highly stratified photic zone (upper 100 meters) in the Pacific. Steranes (δ¹³C = 25.4 ± 0.7 per mil versus the Pee Dee belemnite standard) from shallow photic-zone organisms do not change isotopically throughout the Miocene. In contrast, sulfur-bound C₃₅ hopanes (likely derived from bacterial plankton living at the base of the photic zone) have systematically decreasing ¹³C concentrations in Middle and Late Miocene samples (δ¹³C = –29.5 to –31.5 per mil), consistent with the Middle Miocene formation of a carbon dioxide—rich cold water mass at the base of the photic zone.
6. 1999: Pagani, Mark, Michael A. Arthur, and Katherine H. Freeman. “Miocene evolution of atmospheric carbon dioxide.” Paleoceanography 14.3 (1999): 273-292.  Changes in pCO₂ or ocean circulation are generally invoked to explain warm early Miocene climates and a rapid East Antarctic ice sheet (EAIS) expansion in the middle Miocene. This study reconstructs late Oligocene to late Miocene pCO₂ from ε_p values based on carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifera from Deep Sea Drilling Project sites 588 and 608 and Ocean Drilling Program site 730. Our results indicate that highest pCO₂ occurred during the latest Oligocene (∼350 ppmv) but decreased rapidly at ∼25 Ma. The early and middle Miocene was characterized by low pCO₂ (260–190 ppmv). Lower intervals of pCO₂ correspond to inferred organic carbon burial events and glacial episodes with the lowest concentrations occurring during the middle Miocene. There is no evidence for either high pCO₂ during the late early Miocene climatic optimum or a sharp pCO₂ decrease associated with EAIS growth. Paradoxically, pCO₂ increased following EAIS growth and obtained preindustrial levels by ∼10 Ma. Although we emphasize an oceanographic control on Miocene climate, low pCO₂ could have primed the climate system to respond sensitively to changes in heat and vapor transport.
7. 1999: Pagani, Mark, Katherine H. Freeman, and Michael A. Arthur. “Late Miocene atmospheric CO2 concentrations and the expansion of C4 grasses.” Science 285.5429 (1999): 876-879.  The global expansion of C₄ grasslands in the late Miocene has been attributed to a large-scale decrease in atmospheric carbon dioxide (CO₂) concentrations. This triggering mechanism is controversial, in part because of a lack of direct evidence for change in the partial pressure of CO₂(pCO₂) and because other factors are also important determinants in controlling plant-type distributions. Alkenone-based pCO₂ estimates for the late Miocene indicate that pCO₂ increased from 14 to 9 million years ago and stabilized at preindustrial values by 9 million years ago. The estimates presented here provide no evidence for major changes in pCO₂ during the late Miocene. Thus, C₄ plant expansion was likely driven by additional factors, possibly a tectonically related episode of enhanced low-latitude aridity or changes in seasonal precipitation patterns on a global scale (or both).
8. 2001: Turco, E., et al. “Punctuated evolution of global climate cooling during the Late Middle to Early Late Miocene: High‐resolution planktonic foraminiferal and oxygen isotope records from the Mediterranean.” Paleoceanography 16.4 (2001): 405-423.  High‐resolution planktonic foraminiferal and oxygen isotope records are presented from a Mediterranean deep marine succession, dated astronomically between 12.12 and 9.78 Ma. Planktonic and benthic oxygen isotope records are punctuated by two episodes of δ¹⁸O increase, which have astronomical ages of 11.4 and 10.4 Ma and correspond to the Mi5 and Mi6 events of Miller et al. [1991a]. These ice growth events coincide with low‐amplitude variations in the 1.2 Myr obliquity cycle and are accompanied by significant faunal changes in the Mediterranean, such as the arrival of neogloboquadrinids, the increase in abundance of the G. apertura‐G. obliquus group, and the areal differentiation between N. atlantica and N. acostaensis. Short‐term variations in the planktonic foraminiferal and oxygen isotope records correspond to dominantly precession‐controlled sedimentary cycles. Features of the sapropel/grey marl layers indicate that the short‐term astronomically controlled circum‐Mediterranean climate changes remained basically the same over the last 12 Myr.
9. 2004: Shevenell, Amelia E., James P. Kennett, and David W. Lea. “Middle Miocene southern ocean cooling and Antarctic cryosphere expansion.” Science 305.5691 (2004): 1766-1770.  Magnesium/calcium data from Southern Ocean planktonic foraminifera demonstrate that high-latitude (∼55°S) southwest Pacific sea surface temperatures (SSTs) cooled 6° to 7°C during the middle Miocene climate transition (14.2 to 13.8 million years ago). Stepwise surface cooling is paced by eccentricity forcing and precedes Antarctic cryosphere expansion by ∼60 thousand years, suggesting the involvement of additional feedbacks during this interval of inferred low-atmospheric partial pressure of CO₂ (pCO₂). Comparing SSTs and global carbon cycling proxies challenges the notion that episodic pCO₂ drawdown drove this major Cenozoic climate transition. SST, salinity, and ice-volume trends suggest instead that orbitally paced ocean circulation changes altered meridional heat/vapor transport, triggering ice growth and global cooling.
10. 2005: Westerhold, T., Torsten Bickert, and Ursula Röhl. “Middle to late Miocene oxygen isotope stratigraphy of ODP site 1085 (SE Atlantic): new constrains on Miocene climate variability and sea-level fluctuations.” Palaeogeography, Palaeoclimatology, Palaeoecology 217.3 (2005): 205-222. The middle Miocene δ¹⁸O increase represents a fundamental change in earth’s climate system due to a major expansion and permanent establishment of the East Antarctic Ice Sheet accompanied by some effect of deepwater cooling. The long-term cooling trend in the middle to late Miocene was superimposed by several punctuated periods of glaciations (Mi-Events) characterized by oxygen isotopic shifts that have been related to the waxing and waning of the Antarctic ice-sheet and bottom water cooling. Here, we present a high-resolution benthic stable oxygen isotope record from ODP Site 1085 located at the southwestern African continental margin that provides a detailed chronology for the middle to late Miocene (13.9–7.3 Ma) climate transition in the eastern South Atlantic. A composite Fe intensity record obtained by XRF core scanning ODP Sites 1085 and 1087 was used to construct an astronomically calibrated chronology based on orbital tuning. The oxygen isotope data exhibit four distinct δ¹⁸O excursions, which have astronomical ages of 13.8, 13.2, 11.7, and 10.4 Ma and correspond to the Mi3, Mi4, Mi5, and Mi6 events. A global climate record was extracted from the oxygen isotopic composition. Both long- and short-term variabilities in the climate record are discussed in terms of sea-level and deep-water temperature changes. The oxygen isotope data support a causal link between sequence boundaries traced from the shelf and glacioeustatic changes due to ice-sheet growth. Spectral analysis of the benthic δ¹⁸O record shows strong power in the 400-kyr and 100-kyr bands documenting a paleoceanographic response to eccentricity-modulated variations in precession. A spectral peak around 180-kyr might be related to the asymmetry of the obliquity cycle indicating that the response of the dominantly unipolar Antarctic ice-sheet to obliquity-induced variations probably controlled the middle to late Miocene climate system. Maxima in the δ¹⁸O record, interpreted as glacial periods, correspond to minima in 100-kyr eccentricity cycle and minima in the 174-kyr obliquity modulation. Strong middle to late Miocene glacial events are associated with 400-kyr eccentricity minima and obliquity modulation minima. Thus, fluctuations in the amplitude of obliquity and eccentricity seem to be the driving force for the middle to late Miocene climate variability.
11. 2006: Jiménez-Moreno, Gonzalo. “Progressive substitution of a subtropical forest for a temperate one during the middle Miocene climate cooling in Central Europe according to palynological data from cores Tengelic-2 and Hidas-53 (Pannonian Basin, Hungary).” Review of Palaeobotany and Palynology 142.1-2 (2006): 1-14. The palynological analysis in the Karpatian–Sarmatian (late Early-Middle Miocene) interval of the cores Tengelic-2 and Hidas-53 (Hungary) reveals the existence of a forest organized in altitudinal belts, developed in a subtropical–warm temperate humid climate, reflecting the so-called Miocene climatic optimum. Pollen changes from the late early Miocene to the late middle Miocene have been observed and are related to climatic changes. The vegetation during the Burdigalian and the Langhian was dominated by thermophilous elements such as evergreen trees and Engelhardia, typical of a present day rain and evergreen forest at low altitudes (i.e. SE China). During the Serravallian several thermophilous elements strongly decreased, and some of them disappeared from the central European area. Thus, the rain and evergreen–deciduous mixed forest suffered a great transformation due to the loss and decrease in the abundance of several evergreen plants. This kind of vegetation was progressively substituted by deciduous and mesothermic plants such as deciduous Quercus, and Fagus, Alnus, Acer, Carpinus, Ulmus, Zelkova, etc. At the same time, the presence of altitude coniferous trees increased. This climatic cooling is correlated with global and regional climatic changes.
12. 2007: Holbourn, Ann, et al. “Orbitally-paced climate evolution during the middle Miocene “Monterey” carbon-isotope excursion.” Earth and Planetary Science Letters 261.3 (2007): 534-550.  One of the most enigmatic features of Cenozoic long-term climate evolution is the long-lasting positive carbon-isotope excursion or “Monterey Excursion”, which started during a period of global warmth after 16.9 Ma and ended at ∼ 13.5 Ma, approximately 400 kyr after major expansion of the Antarctic ice-sheet. We present high-resolution (1–9 kyr) astronomically-tuned climate proxy records in two complete sedimentary successions from the northwestern and southeastern Pacific (ODP Sites 1146 and 1237), which shed new light on the middle Miocene carbon-isotope excursion and associated climatic transition over the interval 17.1–12.7 Ma. We recognize three distinct climate phases with different imprints of orbital variations into the climatic signals (1146 and 1237 δ¹⁸O, δ¹³C; 1237 XRF Fe, fraction > 63 μm): (1) climate optimum prior to 14.7 Ma characterized by minimum ice volume and prominent 100 and 400 kyr variability, (2) long-term cooling from 14.7 to 13.9 Ma, principally driven by obliquity and culminating with rapid cryosphere expansion and global cooling at the onset of the last and most pronounced δ¹³C increase, (3) “Icehouse” mode after 13.9 Ma with distinct 100 kyr variability and improved ventilation of the deep Pacific. The “Monterey” carbon-isotope excursion (16.9–13.5 Ma) consists overall of nine 400 kyr cycles, which show high coherence with the long eccentricity period. Superposed on these low-frequency oscillations are high-frequency variations (100 kyr), which closely track the amplitude modulation of the short eccentricity period. In contrast to δ¹³C, the δ¹⁸O signal additionally shows significant power in the 41 kyr band, and the 1.2 Myr amplitude modulation of the obliquity cycle is clearly imprinted in the 1146 δ¹⁸O signal. Our results suggest that eccentricity was a prime pacemaker of middle Miocene climate evolution through the modulation of long-term carbon budgets and that obliquity-paced changes in high-latitude seasonality favored the transition into the “Icehouse” climate.
13. 2009: You, Y., et al. “Simulation of the middle Miocene climate optimum.” Geophysical Research Letters 36.4 (2009).  Proxy data constraining land and ocean surface paleo‐temperatures indicate that the Middle Miocene Climate Optimum (MMCO), a global warming event at ∼15 Ma, had a global annual mean surface temperature of 18.4°C, about 3°C higher than present and equivalent to the warming predicted for the next century. We apply the latest National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM3.1 and Land Model CLM3.0 coupled to a slab ocean to examine sensitivity of MMCO climate to varying ocean heat fluxes derived from paleo sea surface temperatures (SSTs) and atmospheric carbon dioxide concentrations, using detailed reconstructions of Middle Miocene boundary conditions including paleogeography, elevation, vegetation and surface temperatures. Our model suggests that to maintain MMCO warmth consistent with proxy data, the required atmospheric CO₂ concentration is about 460–580 ppmv, narrowed from the most recent estimate of 300–600 ppmv.  [FULL TEXT]
14. 2013: Badger, Marcus PS, et al. “CO2 drawdown following the middle Miocene expansion of the Antarctic Ice Sheet.” Paleoceanography 28.1 (2013): 42-53.  The development of a permanent, stable ice sheet in East Antarctica happened during the middle Miocene, about 14 million years (Myr) ago. The middle Miocene therefore represents one of the distinct phases of rapid change in the transition from the “greenhouse” of the early Eocene to the “icehouse” of the present day. Carbonate carbon isotope records of the period immediately following the main stage of ice sheet development reveal a major perturbation in the carbon system, represented by the positive δ¹³C excursion known as carbon maximum 6 (“CM6”), which has traditionally been interpreted as reflecting increased burial of organic matter and atmospheric pCO₂drawdown. More recently, it has been suggested that the δ¹³C excursion records a negative feedback resulting from the reduction of silicate weathering and an increase in atmospheric pCO₂. Here we present high‐resolution multi‐proxy (alkenone carbon and foraminiferal boron isotope) records of atmospheric carbon dioxide and sea surface temperature across CM6. Similar to previously published records spanning this interval, our records document a world of generally low (~300 ppm) atmospheric pCO₂ at a time generally accepted to be much warmer than today. Crucially, they also reveal a pCO₂decrease with associated cooling, which demonstrates that the carbon burial hypothesis for CM6 is feasible and could have acted as a positive feedback on global cooling. [FULL TEXT]
15. 2014: Greenop, Rosanna, et al. “Middle Miocene climate instability associated with high‐amplitude CO2 variability.” Paleoceanography and Paleoclimatology 29.9 (2014): 845-853.  The amplitude of climatic change, as recorded in the benthic oxygen isotope record, has varied throughout geological time. During the late Pleistocene, changes in the atmospheric concentration of carbon dioxide (CO₂) are an important control on this amplitude of variability. The contribution of CO₂ to climate variability during the pre‐Quaternary however is unknown. Here we present a new boron isotope‐based CO₂record for the transition into the middle Miocene Climatic Optimum (MCO) between 15.5 and 17 Myr that shows pronounced variability between 300 ppm and 500 ppm on a roughly 100 kyr time scale during the MCO. The CO₂ changes reconstructed for the Miocene are ~2 times larger in absolute terms (300 to 500 ppm compared to 180 to 280 ppm) than those associated with the late Pleistocene and ~15% larger in terms of climate forcing. In contrast, however, variability in the contemporaneous benthic oxygen isotope record (at ~1‰) is approximately two thirds the amplitude of that seen during the late Pleistocene. These observations indicate a lower overall sensitivity to CO₂ forcing for Miocene (Antarctic only) ice sheets than their late Pleistocene (Antarctic plus lower latitude northern hemisphere) counterparts. When our Miocene CO₂ record is compared to the estimated changes in contemporaneous δ¹⁸O_sw (ice volume), they point to the existence of two reservoirs of ice on Antarctica. One of these reservoirs appears stable, while a second reservoir shows a level of dynamism that contradicts the results of coupled climate‐ice sheet model experiments given the CO₂ concentrations that we reconstruct. [FULL TEXT]

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