One aspect of this which is not GAU (geology as usual), raised by both S&S and separately by Wadhams, concerns anthropogenic warming of Arctic Ocean water at the depth of the continental shelf, ie at the interface with submerged permafrost. This could provide an accelerant to loss of impermeable gas cap, not so much by plain thermal diffusion as by cryo-geological mechanisms described by S&S.
Recall here that about a third of this ocean, especially on the Siberian side, is very shallow, well within range of wind, wave and tidal turbulent mixing. Early and persistent seasonal loss of ice cover attributable to anthropogenic Arctic amplification allows enhanced solar heat adsorption and provides much longer fetches for wind to mix up stratification.
Wadhams sees a significant difference between submerged permafrost meeting sea water near the latter's freezing point of -1.8ºC vs contacting sea water above its own melting temperature which is more like 0ºC (since permafrost ice is freshwater ice formed on land). Consequently, geology-as-usual may be going off the trolley tracks. In this view, the late timing within the Holocene cycle is being seriously supplemented by man-made effects.
IP Semiletov is a co-author on 3 papers at AGU17. I did not see abstracts for N Shakhova; her four 2017 papers are listed below.
http://envisionation.co.uk/index.php/nick-breeze/203-subsea-permafrost-on-east-siberian-arctic-shelf-now-in-accelerated-declinePP51B-1069: Deglacial remobilization of permafrost carbon to sediments along the East Siberian Arctic Seas
J Martens et al
Current climate change is expected to thaw large quantities of permafrost carbon (PF-C) and expose it to degradation which emits greenhouse gases (i.e. CO2 and CH4). Warming causes a gradual deepening of the seasonally thawed active layer surface of permafrost soils, but also the abrupt collapse of deeper Ice Complex Deposits (ICD), especially along Siberian coastlines. It was recently hypothesized that past warming already induced large-scale permafrost degradation after the last glacial, which ultimately amplified climate forcing. We here assess the mobilization of PF-C to East Siberian Arctic Sea sediments during these warming periods.
We perform source apportionment using bulk carbon isotopes together with terrestrial biomarkers (CuO-derived lignin phenols) as indicators for PF-C transfer. We apply these techniques to sediment cores from the Chukchi Sea and the southern Lomonosov Ridge.
We found that PF-C fluxes during the Bølling-Allerød warming (14.7 to 12.7 cal ka BP), the Younger Dryas cooling (12.7 to 11.7 cal ka BP) and the early Holocene warming (until 11 cal ka BP) were overall higher than mid and late Holocene fluxes. In the Chukchi Sea, PF-C burial was 2x higher during the deglaciation (7.2 g m-2 a-1) than in the mid and late Holocene (3.6 g m-2 a-1), and ICD were the dominant source of PF-C (79.1%). Smaller fractions originated from the active layer (9.1%) and marine sources (11.7%).
We conclude that thermo-erosion of ICD released large amounts of PF-C to the Chukchi Sea, likely driven by climate warming and the deglacial sea level rise. This contrasts to earlier analyses of Laptev Sea sediments where active layer material from river transport dominated the carbon flux.
Preliminary data on lignin phenol concentrations of Lomonosov Ridge sediments suggest that the postglacial remobilization of PF-C was one order of magnitude higher (10x) than during both the preceding glacial and the subsequent Holocene. We will apply source apportionments between coastal erosion of ICD and river export of active layer material for the outer East Siberian Arctic Seas.
Our findings demonstrate remobilization of PF-C during past warming events and suggest that current climate change might cause a similar cascade of permafrost destabilization and, thus, accelerate climate warming.
PP54A-03: Late Holocene sea ice conditions in Herald Canyon, Chukchi Sea
C Pearce et al
Sea ice in the Arctic Ocean has been in steady decline in recent decades and, based on satellite data, the retreat is most pronounced in the Chukchi and Beaufort seas. Historical observations suggest that the recent changes were unprecedented during the last 150 years, but for a longer time perspective, we rely on the geological record. For this study, we analyzed sediment samples from two piston cores from Herald Canyon in the Chukchi Sea, collected during the 2014 SWERUS-C3 Arctic Ocean Expedition.
The Herald Canyon is a local depression across the Chukchi Shelf, and acts as one of the main pathways for Pacific Water to the Arctic Ocean after entering through the narrow and shallow Bering Strait. The study site lies at the modern-day seasonal sea ice minimum edge, and is thus an ideal location for the reconstruction of past sea ice variability.
Both sediment cores contain late Holocene deposits characterized by high sediment accumulation rates (100-300 cm/kyr). Core 2-PC1 from the shallow canyon flank (57 m water depth) is 8 meter long and extends back to 4200 cal yrs BP, while the upper 3 meters of Core 4-PC1 from the central canyon (120 mwd) cover the last ~3000 years. The chronologies of the cores are based on radiocarbon dates and the 3.6 ka Aniakchak CFE II tephra, which is used as an absolute age marker to calculate the marine radiocarbon reservoir age.
Analysis of biomarkers for sea ice and surface water productivity indicate stable sea ice conditions throughout the entire late Holocene, ending with an abrupt increase of phytoplankton sterols in the very top of both sediment sequences. The shift is accompanied by a sudden increase in coarse sediments (> 125 µm) and a minor change in δ
13C
org.
We interpret this transition in the top sediments as a community turnover in primary producers from sea ice to open water biota. Most importantly, our results indicate that the ongoing rapid ice retreat in the Chukchi Sea of recent decades was
unprecedented during the last 4000 years.
PP54A-02: The Deglacial to Holocene Paleoceanography of Bering Strait: Results From the SWERUS-C3 Program (Invited)
M Jakobsson
The multi-disciplinary SWERUS-C3 Program was carried out on a two-leg 90-day long expedition in 2014 with Swedish icebreaker Oden. One component of the expedition consisted of geophysical mapping and coring of Herald Canyon, located on the Chukchi Sea shelf north of the Bering Strait in the western Arctic Ocean.
Herald Canyon is strategically placed to capture the history of the Pacific-Arctic Ocean connection and related changes in Arctic Ocean paleoceanography.
We provide a new age constraint of 11 cal ka BP on sediments from the uppermost slope for the initial flooding of the Bering Land Bridge and
reestablishment of the Pacific-Arctic Ocean connection following the last glaciation. This age corresponds to meltwater pulse 1b (MWP1b) known as a post-Younger Dryas warming in many sea level and paleoclimate records.
High late Holocene sedimentation rates in Herald Canyon permitted paleo-ceanographic reconstructions of ocean circulation and sea ice cover at centennial scales throughout the late Holocene. Evidence suggests varying influence from inflowing Pacific water into the western Arctic Ocean including some evidence for quasi-cyclic variability in several paleoceanographic parameters, such as micro-paleontological assemblages, isotope geochemistry and sediment physical properties.
U13B-13: Implementation of an acoustic-based methane flux estimation methodology in the Eastern Siberian Arctic Sea
EF Weidner et al
Quantifying methane flux originating from marine seep systems in climatically sensitive regions is of critically importance for current and future climate studies. Yet, the methane contribution from these systems has been difficult to estimate given the broad spatial scale of the ocean and the heterogeneity of seep activity.
One such region is the Eastern Siberian Arctic Sea (ESAS), where bubble release into the shallow water column (<40 meters average depth) facilitates transport of methane to the atmosphere without oxidation. Quantifying the current seep methane flux from the ESAS is necessary to understand not only the total ocean methane budget, but also to provide baseline estimates against which future climate-induced changes can be measured.
At the 2016 AGU fall meeting, we presented a new acoustic-based flux methodology using a calibrated broadband split-beam echosounder. The broad (14-24 kHz) bandwidth provides a vertical resolution of 10 cm, making possible the identification of single bubbles. After calibration using 64 mm copper sphere of known backscatter, the acoustic backscatter of individual bubbles is measured and compared to analytical models to estimate bubble radius. Additionally, bubbles are precisely located and traced upwards through the water column to estimate rise velocity. The combination of radius and rise velocity allows for gas flux estimation.
Here, we follow up with the completed implementation of this methodology applied to the Herald Canyon region of the western ESAS. From the
68 recognized seeps, bubble radii and rise velocity were computed for more than 550 individual bubbles. The range of bubble radii, 1-6 mm, is comparable to those published by other investigators, while the radius dependent rise velocities are consistent with published models. Methane flux for the Herald Canyon region was estimated by extrapolation from individual seep flux values.
1. Sonar gas flux estimation by bubble insonification: application to methane bubble flux from seep areas in the outer Laptev Sea
I Leifer, D Chernykh, N Shakhova…
https://www.the-cryosphere.net/11/1333/2017/tc-11-1333-2017.pdf open access
Sonar surveys provide an effective mechanism for mapping seabed methane flux
emissions, with Arctic submerged permafrost seepage having great potential to significantly
affect climate. We created in situ engineered bubble plumes from 40 m depth with fluxes ... seepage-mapped spatial patterns suggested subsurface geologic control attributing methane fluxes to the current state of subsea permafrost.
On a century timescale, methane (CH4) is the next most important anthropogenic greenhouse gas after CO2. However, on a decadal timescale comparable to its atmospheric lifetime, CH4 is more important to the atmospheric radiative balance than CO2 (Forster 2007; Fig. 2.21
https://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf)
ESAS seepage is on a dramatically larger scale with∼ 30 000 plumes manually identified in just two transects. Seepage densities up to ∼ 3000 seep bubble plumes per km2 were found transecting
a single hotspot. Based on the hotspot size (18 400 km2), an order of magnitude estimate suggests 60 million seep plumes for the hotspot alone.
2. The origin of methane in the East Siberian Arctic Shelf unraveled with triple isotope analysis
CJ Sapart, N Shakhova, I Semiletov, J Jansen…
https://www.biogeosciences.net/14/2283/2017/bg-14-2283-2017.pdf open access
CH4 concentration and triple isotope composition were analyzed on gas extracted from sediment and water sampled at numerous locations on the shallow ESAS from 2007 to 2013. We find high concentrations (up to 500 µM) of CH4 in the pore water of the partially thawed subsea permafrost of this region.
For all sediment cores, both hydrogen and carbon isotope data reveal the predominant occurrence of CH4 that is
not of thermogenic origin as it has long been thought, but resultant from
microbial CH4 formation. At some locations, meltwater from buried meteoric ice and/or old organic matter preserved in the subsea permafrost were used as substrates.
Radiocarbon data demonstrate that the
CH4 present in the ESAS sediment is of Pleistocene age or older... Our sediment data suggest that at locations where bubble plumes have been observed, CH4 can
escape anaerobic oxidation in the surface sediment.
3. Current rates and mechanisms of subsea permafrost degradation in the East Siberian Arctic Shelf
N Shakhova, I Semiletov, O Gustafsson…
https://www.nature.com/articles/ncomms15872 open access
Here we present results of the first comprehensive scientific re-drilling to show that subsea permafrost in the near-shore zone of the ESAS has a downward movement of the ice-bonded permafrost table of ∼14 cm year−1 over the past 31–32 years. Our data reveal polygonal thermokarst patterns on the seafloor and gas-migration associated with submerged taliks, ice scouring and pockmarks.
4. Discovery and characterization of submarine groundwater discharge in the Siberian Arctic seas: a case study in the Buor-Khaya Gulf, Laptev Sea
AN Charkin, MR van der Loeff, NE Shakhova 2017
https://www.the-cryosphere.net/11/2305/2017/tc-11-2305-2017.pdf open access
It has been suggested that increasing terrestrial water discharge to the Arctic Ocean
may partly occur as submarine groundwater discharge (SGD), yet there are no direct
observations of this phenomenon in the Arctic shelf seas... Another possible mechanism for preventing taliks from freezing and/or preventing talik formation could be groundwater flow through coastal sediments, especially in the areas underlain by faults