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THE ARCTIC OCEAN WARMS FROM BELOW

Posted on: December 8, 2020

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THIS POST IS A PRESENTATION OF THE FINDINGS IN “The Arctic Ocean warms from below
Eddy C. Carmack William J. Williams Sarah L. Zimmermann Fiona A. McLaughlin, Geophysical Research Letters, 2012.

LINK TO FULL TEXT: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL050890

ABSTRACT:  The old (∼450‐year isolation age) and near‐homogenous deep waters of the Canada Basin (CBDW), that are found below ∼2700 m, warmed at a rate of ∼0.0004°C yr−1 between 1993 and 2010. This rate is slightly less than expected from the reported geothermal heat flux (Fg ∼ 50 mW m−2). A deep temperature minimum Tmin layer overlies CBDW within the basin and is also warming at approximately the same rate, suggesting that some geothermal heat escapes vertically through a multi‐stepped, ∼300‐m‐thick deep transitional layer. Double diffusive convection and thermobaric instabilities are identified as possible mechanisms governing this vertical heat transfer. The CBDW found above the lower continental slope of the deep basin maintains higher temperatures than those in the basin interior, consistent with geothermal heat being distributed through a shallower water column, and suggests that heat from the basin interior does not diffuse laterally and escape at the edges.

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The deep waters of the Arctic Ocean form in the Nordic seas and enter the Arctic Ocean through Fram Strait. The Arctic Ocean itself contains two main basins; the Eurasian and Canadian, separated by the Lomonosov Ridge with a sill depth of ∼2000 m (Figure 1a). The Canadian Basin has the largest volume and contains the oldest deep water, with 14C isolation age estimates of ∼450 yr, approximately 200 yr older than those in the Eurasian Basin. In turn, the Canadian Basin is separated by the Alpha‐Mendeleyev ridge complex into the Makarov and Canada Basins. The deep waters of the Canada Basin (CBDW) are near‐homogeneous, varying in potential temperature (θ) by less than 0.001°C between ∼2700 m and the bottom, a feature ascribed to geothermal heating and vertical convection. Reported geothermal heat flux in the Canadian Basin is 40–60 mW/m2, and we use 50 mW/m2 here as a reference flux. Salinity in the Canadian Basin increases with depth to ∼2700 m, but, like θ, is nearly constant below this depth. Björk & Winsor (2006) also observed near‐homogenous bottom waters in the Eurasian Basin, noting a temperature change in the bottom layer which they attributed to geothermal heating. Timmermans etal (2003) said about the Canada Basin bottom waters that temperatures within the CBDW were near‐constant with time, that little heat escaped vertically through the overlying deep transitional layer and that excess heat was lost mainly around the perimeter of the basin. Here, using data from annual surveys of the southern Canada Basin between 2002 and 2010, we re‐examine role of geothermal heating in CBDW, describe temporal and spatial patterns in water mass properties, and propose heat exchange mechanisms that involve diffusive and thermobaric instabilities.

  1. FINDINGS: CBDW appears as a thick, near‐homogenous bottom layer extending from ∼2700 m to the bottom. Above lies an ∼300‐m‐thick deep transitional layer that is characterized by a temperature–salinity step structure, and the top of this layer is marked by a temperature minimum (Tmin) at ∼2400 m, the sill depth at Cooperation Gap on the Alpha‐Mendeleyev Ridge complex. The staircase structure, through which both the mean temperature and mean salinity increase with depth, is observed at all stations across the entire basin and has been a persistent feature for at least two decades. The step structure is typically characterized by three to four mixed layers that are 10–60 m thick and are separated by 2–20 m‐thick interfaces over which changes are δθ ∼ 0.003°C and δS ∼ 0.0007. The θ/S properties of the deep basin show that stratification below the Tmin is marginally stable with respect to density calculated at 3000 m and that those of the Tmin itself closely match those of the Makarov Basin at sill depth, pointing to this as the likely source maintaining the Tmin layer. For any given year the θ of CBDW within the deep basin is laterally near‐uniform, varying by less than 0.0007°C across the full study area, reflecting the isolation of the basin and also the ubiquity of geothermal heating. The θ of the Tmin, however, shows greater spatial variability.
Evolution of the Deep Water in the Canadian Basin in the Arctic Ocean in:  Journal of Physical Oceanography Volume 36 Issue 5 (2006)


(a) Schematic of Canada Basin Bottom Water (CBDW) structure and processes; MB is the Makarov Basin, A/M is the Alpha Mendeleyev Ridge, CB is the Canada Basin, Slope is the continental slope in the south and east. E and D are entrainment and detrainment associated with a hypothetical descending plume of cold salty water from the shelf. (b–d) Changes in potential temperature (red), salinity (blue) and density in the central Canada Basin at JOIS station CB‐15 at 77N, 140W (see Figure 1b, green dot) in 2003 (thin lines) and 2010 (thick lines). The profile from 2000 m to the bottom is shown in Figure 2b, the deep transitional layer (DTL) from 2450 to 2750 m is shown in Figure 2c and potential temperature vs. salinity is shown in Figure 2d with contours of potential density referenced to 3000 db.

Temporal Trends and Spatial Variation
[6] The time sequence of potential temperature (θ) profiles in the deep Canada Basin from 2002 to 2010 reveals the steady increase in temperature of the CBDW (Figure 3). As shown below, this rate of warming is consistent with, but slightly less than, the geothermal heat flux. A least‐squares fit to all available CBDW temperature data from 1993–2010 for stations deeper than 3000 m gives a rate of warming of ∼0.0004°C/yr; a similar rate of warming is observed in the Tmin, although there is a much larger spatial variation (Figure 4a). A least‐squares fit to the associated salinity data from 1993 to 2010 shows no discernable trend for either the CDBW or the Tmin.

LINK TO FULL TEXT: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012GL050890

Geothermal heat flux from (a) Pollack et al. (1993) and (b) Davies (2013).  | Download Scientific Diagram

RELATED POST ON RELEVANT GEOLOGY OF THE ARCTIC

LINK: https://tambonthongchai.com/2021/02/27/geological-features-of-the-arctic/

CONCLUSIONS: THE ARCTIC IS GEOLOGICALLY ACTIVE WITH SIGNIFICANT SOURCES OF GEOTHERMAL HEAT FLOW FROM THE MIDDLE OF THE PLANET (MANTLE AND CORE). THEREFORE, OCEAN TEMPERATURE DYNAMICS AND ICE MELT EVENTS HERE CANNOT BE UNDERSTOOD PURELY IN ATMOSPHERIC TERMS. YET, WHAT WE SEE IN CLIMATE SCIENCE IS AN EXCLUSIVE RELIANCE OF ATMOSPHERIC PHENOMENA TO EXPLAIN ALL OCEAN WARMING AND ICE MELT PHENOMENA IN THE ARCTIC. THE BIBLIOGRAPHY BELOW SHEDS FURTHER LIGHT ON THIS ISSUE.

A relevant bibliography is provided below where empirical evidence of these location specific geothermal activity is presented. Principal investigators in this area are Yasmina Martos, a NASA scientist, and Ralf Greve, Glaciologist, Hokkaido University.

Bio - Yasmina M Martos Martin
DRYASMINA MARTOS: NASA SCIENTIST
New estimates for the rise in sea levels due to ice sheet mass loss under  future climate change | Hokkaido University
RALF GREVE (RIGHT) HOKKAIDO UNIVERSITY GLACIOLOGIST (not sure who is buddy is)

THE RELEVANT BIBLIOGRAPHY: PART-1: THE ARCTIC

  1. Fahnestock, Mark, et al. “High geothermal heat flow, basal melt, and the origin of rapid ice flow in central Greenland.” Science 294.5550 (2001): 2338-2342. Age-depth relations from internal layering reveal a large region of rapid basal melting in Greenland. Melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet. Locally, high melt rates indicate geothermal fluxes 15 to 30 times continental background. The southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin.
  2. Rezvanbehbahani, Soroush, et al. “Predicting the geothermal heat flux in Greenland: A machine learning approach.” Geophysical Research Letters 44.24 (2017): 12-271. Geothermal heat flux (GHF) is a crucial boundary condition for making accurate predictions of ice sheet mass loss, yet it is poorly known in Greenland due to inaccessibility of the bedrock. Here we use a machine learning algorithm on a large collection of relevant geologic features and global GHF measurements and produce a GHF map of Greenland that we argue is within ∼15% accuracy. The main features of our predicted GHF map include a large region with high GHF in central‐north Greenland surrounding the NorthGRIP ice core site, and hot spots in the Jakobshavn Isbræ catchment, upstream of Petermann Gletscher, and near the terminus of Nioghalvfjerdsfjorden glacier. Our model also captures the trajectory of Greenland movement over the Icelandic plume by predicting a stripe of elevated GHF in central‐east Greenland. Finally, we show that our model can produce substantially more accurate predictions if additional measurements of GHF in Greenland are provided. FULL TEXT: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL075661
  3. van der Veen, Cornelis J., et al. “Subglacial topography and geothermal heat flux: Potential interactions with drainage of the Greenland ice sheet.” Geophysical research letters 34.12 (2007). Many of the outlet glaciers in Greenland overlie deep and narrow trenches cut into the bedrock. It is well known that pronounced topography intensifies the geothermal heat flux in deep valleys and attenuates this flux on mountains. Here we investigate the magnitude of this effect for two subglacial trenches in Greenland. Heat flux variations are estimated for idealized geometries using solutions for plane slopes derived by Lachenbruch (1968). It is found that for channels such as the one under Jakobshavn Isbræ, topographic effects may increase the local geothermal heat flux by as much as 100%.
  4. Greve, Ralf. “Relation of measured basal temperatures and the spatial distribution of the geothermal heat flux for the Greenland ice sheet.” Annals of Glaciology 42 (2005): 424-432The thermomechanical, three-dimensional ice-sheet model SICOPOLIS is applied to the Greenland ice sheet. Simulations over two glacial–interglacial cycles are carried out, driven by a climatic forcing interpolated between present conditions and Last Glacial Maximum anomalies. Based on the global heat-flow representation by Pollack and others (1993), we attempt to constrain the spatial pattern of the geothermal heat flux by comparing simulation results to direct measurements of basal temperatures at the GRIP, NorthGRIP, Camp Century and Dye 3 ice-core locations. The heat-flux map shows an increasing trend from west to east, a high-heat-flux anomaly around NorthGRIP with values up to 135 mWm–2 and a low-heat-flux anomaly around Dye 3 with values down to 20 mW m–2. Validation is provided by the generally good fit between observed and measured ice thicknesses. Residual discrepancies are most likely due to deficiencies of the input precipitation rate and further variability of the geothermal heat flux not captured here.
  5. Smith‐Johnsen, Silje, et al. “Sensitivity of the Northeast Greenland Ice Stream to geothermal heat.” Journal of Geophysical Research: Earth Surface 125.1 (2020): e2019JF005252. Recent observations of ice flow surface velocities have helped improve our understanding of basal processes on Greenland and Antarctica, though these processes still constitute some of the largest uncertainties driving ice flow change today. The Northeast Greenland Ice Stream is driven largely by basal sliding, believed to be related to subglacial hydrology and the availability of heat. Characterization of the uncertainties associated with Northeast Greenland Ice Stream is crucial for constraining Greenland’s potential contribution to sea level rise in the upcoming centuries. Here, we expand upon past work using the Ice Sheet System Model to quantify the uncertainties in models of the ice flow in the Northeast Greenland Ice Stream by perturbing the geothermal heat flux. Utilizing a subglacial hydrology model simulating sliding beneath the Greenland Ice Sheet, we investigate the sensitivity of the Northeast Greenland Ice Stream ice flow to various estimates of geothermal heat flux, and implications of basal heat flux uncertainties on modeling the hydrological processes beneath Greenland’s major ice stream. We find that the uncertainty due to sliding at the bed is 10 times greater than the uncertainty associated with internal ice viscosity. Geothermal heat flux dictates the size of the area of the subglacial drainage system and its efficiency. The uncertainty of ice discharge from the Northeast Greenland Ice Stream to the ocean due to uncertainties in the geothermal heat flux is estimated at 2.10 Gt/yr. This highlights the urgency in obtaining better constraints on the highly uncertain subglacial hydrology parameters.
  6. Martos, Yasmina M., et al. “Geothermal heat flux reveals the Iceland hotspot track underneath Greenland.” Geophysical research letters 45.16 (2018): 8214-8222. Curie depths beneath Greenland are revealed by spectral analysis of data from the World Digital Magnetic Anomaly Map 2. A thermal model of the lithosphere then provides a corresponding geothermal heat flux map. This new map exhibits significantly higher frequency but lower amplitude variation than earlier heat flux maps and provides an important boundary condition for numerical ice‐sheet models and interpretation of borehole temperature profiles. In addition, it reveals new geologically significant features. Notably, we identify a prominent quasi‐linear elevated geothermal heat flux anomaly running northwest–southeast across Greenland. We interpret this feature to be the relic of the passage of the Iceland hotspot from 80 to 50 Ma. The expected partial melting of the lithosphere and magmatic underplating or intrusion into the lower crust is compatible with models of observed satellite gravity data and recent seismic observations. Our geological interpretation has implications for the geodynamic evolution of Greenland https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL078289
  7. Artemieva, Irina M. “Lithosphere thermal thickness and geothermal heat flux in Greenland from a new thermal isostasy method.” Earth-Science Reviews 188 (2019): 469-481. Lithosphere thermal structure in Greenland is poorly known and models based on seismic and magnetic data are inconsistent, while growing awareness in the fate of the ice sheet in Greenland requires reliable constraints on geothermal heat flux (GHF) from the Earth’s interior in the region where conventional heat flux measurements are nearly absent. The lithosphere structure of Greenland remains controversial, while its geological evolution is constrained by direct observations in the narrow ice-free zone along the coasts. The effect of the Iceland hotspot on the lithosphere structure is also debated. Here I describe a new thermal isostasy method which I use to calculate upper mantle temperature anomalies, lithosphere thickness, and GHF in Greenland from seismic data on the Moho depth, topography and ice thickness. To verify the model results, the predicted GHF values are compared to available measurements and show a good agreement. Thick (200–270 km) cratonic lithosphere of SW Greenland with GHF of ca. 40 mW/m2 thins to 180–190 km towards central Greenland without a clear boundary between the Archean and Proterozoic blocks, and the deepest lithosphere keel is observed beneath the largest kimberlite province in West Greenland. The NW-SE belt with an anomalously thin (100–120 km) lithosphere and GHF of 60–70 mW/m2 crosses north-central Greenland from coast to coast and it may mark the Iceland hotspot track. In East Greenland this anomalous belt merges with a strong GHF anomaly of >100 mW/m2 in the Fjordland region. The anomaly is associated with a strong lithosphere thinning, possibly to the Moho, that requires advective heat transfer such as above active magma chambers, which would accelerate ice basal melting. The anomaly may extend 500 km inland with possibly a significant contribution of ice melt to the ice-drainage system of Greenland.
  8. Greve, Ralf, and Kolumban Hutter. “Polythermal three-dimensional modelling of the Greenland ice sheet with varied geothermal heat flux.” Annals of Glaciology 21 (1995): 8-12. Computations over 50 000 years into steady state with Greve’s polythermal ice-sheet model and its numerical code are performed for the Greenland ice sheet with today’s climatological input (surface temperature and accumulation function) and three values of the geothermal heat flux: (42, 54.6, 29.4) mW m−2. It is shown that through the thermomechanical coupling the geometry as well as the thermal regime, in particular that close to the bed, respond surprisingly strongly to the basal thermal heat input. The most sensitive variable is the basal temperature field, but the maximum height of the summit also varies by more than ±100m. Furthermore, some intercomparison of the model outputs with the real ice sheet is carried out, showing that the model provides reasonable results for the ice-sheet geometry as well as for the englacial temperatures.
Active Volcano Found Under Antarctic Ice: Eruption Could Raise Sea Levels

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