Friday, 23 June 2017

This is the paper from Shakhova, Semiletov et. al.

Current rates and mechanisms of subsea permafrost degradation in the East Siberian Arctic Shelf

Nature,9 May, 2017

Abstract

The rates of subsea permafrost degradation and occurrence of gas-migration pathways are key factors controlling the East Siberian Arctic Shelf (ESAS) methane (CH4) emissions, yet these factors still require assessment. It is thought that after inundation, permafrost-degradation rates would decrease over time and submerged thaw-lake taliks would freeze; therefore, no CH4 release would occur for millennia. 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. Knowing the rate and mechanisms of subsea permafrost degradation is a prerequisite to meaningful predictions of near-future CH4 release in the Arctic.
Introduction

Arctic coastal zone permafrost (ground that remains ≤0 °C for ≥2 year) developed when the Northern Hemisphere cooled ∼2.5 Myr ago1. Most subsea permafrost formed on the continental shelves when the shelves were exposed during periods of low sea level associated with times of major glacial activity2. As the glaciers eventually melted, the sea level rose, which submerged this permafrost3. Inundation can markedly change permafrost properties because the permafrost is warmed by as much as 17 °C by the overlying seawater4. The following factors were suggested to determine the evolution of subsea permafrost after inundation: duration of submergence compared with the duration of previous exposure above the sea surface; thermal state and thickness of permafrost before inundation; coastal morphology and hydro- and lithodynamics; shoreline configuration and retreat rate; pre-existing thermokarst (that is, the process by which characteristic landforms result from the thawing of ice-rich permafrost or the melting of massive ice) accompanied by formation of thaw lakes; bottom water temperature and salinity; and sediment composition, including ice content5,6,7,8.
Warming of the East Siberian Arctic Shelf (ESAS) began ∼12–13 thousand years (kyr) ago when the entire shelf area was exposed above sea level, forming a major fraction of the coastal plain7. By the time of inundation, numerous thaw lakes underlain by taliks had developed in that area due to thermokarst9. A talik is a layer or body of unfrozen ground in a permafrost area in which the temperature is above 0 °C due to the local thermal regime of the ground10. The fate of these thermokarst-induced features in the near-shore zone, only recently inundated, has long been debated8,9,11,12. The widely accepted hypothesis is that the <0 °C bottom seawater temperature would halt thermokarst formation and cause taliks to freeze by creating a negative temperature profile in the sediments7,8,9. However, no observational evidence to confirm this hypothesis has existed to date.
On the contrary, some authors suggested that seawater could be transported into the sediments at sufficient rates to lower the freezing point of the sediment pore water, even in ice-bonded permafrost13,14. In addition, via convective fingering, seawater transport rates could be orders of magnitude greater than heat conduction from the surface15, perhaps preventing freezing of the submerged thaw-lake taliks and, thus, causing advanced top–down permafrost degradation16. In addition, the ESAS near-shore zone is largely affected by riverine runoff, which causes the mean annual bottom seawater temperature to be >0 °C (ref. 17). Heat flux from large rivers can cause deep talik formation beneath riverbeds; it has been suggested that such taliks might exist below the paleo rivers18. A significant area of the ESAS is affected by paleo-river valleys19. A substantial part of submerged ESAS permafrost consists of ice complexes (ICs), which are Late Pleistocene ice-rich syncryogenic deposits with massive ice wedges20,21. Before inundation, ICs are subjected to two destructive processes: thermo-denudation (upslope permafrost retreat under the influence of insolation and heat flux on the slope) and thermo-abrasion (mechanically and thermally caused retreat of exposed permafrost due to seawater and wind erosion)22,23. Some authors believe that after ICs are submerged, they are subjected to thermo-abrasion and chemical- and current-induced seafloor erosion, to list a few such destructive processes24,25.
Permafrost-degradation rates could be evaluated by assessing changes in the ice-bonded permafrost table (IBPT) position. The position of the IBPT in the ESAS has been investigated using seismic techniques26,27. However, there are problems with high attenuation of the reflected seismic signal where sediments contain gas28 and/or reflect variability in permafrost properties29,30. Methods based on electrical properties of frozen/unfrozen ground were shown to be applicable in shallow coastal waters31,32. Poor knowledge of the physical and chemical processes occurring within subsea permafrost, combined with a lack of observational data for model calibration, restricts further progress in modelling the current state of subsea permafrost and associated methane (CH4) releases in the ESAS16,33. It is, therefore, necessary to conduct comprehensive geocryological investigations.
This study aimed to document subsea permafrost-degradation rates after submergence by directly studying frozen ground samples recovered from drilled boreholes, and interpreting geophysical data collected during repeated observations in the study area. On the basis of results of first comprehensive scientific re-drilling investigation of subsea permafrost in the ESAS, here we present observation-based demonstration of thawing of subsea permafrost resolved over decadal scale. Interpretation of geophysical data calibrated by drilling allows resolving on inter-annual scale upward migration of shallow gas. We demonstrate that thermokarst occurs after inundation, submerged thaw lakes not always freeze and could serve as gas-migration paths, and ice scouring serves as important mechanism of permafrost disturbance associated with gas releases. Knowing the rate and mechanisms of subsea permafrost degradation is a prerequisite to meaningful predictions of near-future CH4 release on the Arctic shelf.
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Results

Study area

In 2008–2014, we conducted four marine expeditions and four drilling campaigns in the study area between 70–74° N and 129–131° E with focus on the near-shore Laptev Sea southeast of the Lena Delta, the Buor-Khaya Bay (BKB, between 70–74° N and 129–131° E), and the Dmitry Laptev Strait (DLS, between 72.5–73.5° N and 138–143° E, Fig. 1). In drilling campaigns, we investigated the thermal regime, geomorphology, lithology and geocryology of sediment cores extracted from drilled boreholes and sediments sampled along the drilling transect (Fig. 2). We also performed few geoelectrical surveys, results of which were validated by direct measurement of electrical resistivity of recovered sediments. In marine expeditions, we collected conductivity-temperature-depth (CTD) data, performed high-resolution sub-bottom profiling, sonar-derived imagery and visual observations (using an autonomous underwater vehicle) of geomorphological features of the seafloor (subsea thermokarst, ice scouring and pockmarks) associated with gas releases.
Figure 1: Study area bathymetry and position of the investigated sites.
Figure 1
(a) Red and blue rectangles mark study areas, where drilling was conducted in 2011–2014, position of the sites investigated in marine expeditions, data from which are presented inFigs 456789 are shown as black triangles (2D sites) and red lines (transects); two black crosses in the blue rectangle show position of the drilling transect conducted in 2012–2014 (shown enlarged in b,c); (b) position of the boreholes drilled in March 2011–2013; (c) enlarged position of the drilling transect performed the northern part of MI in 2012–2014.

Read the paper HERE


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