Current rates and mechanisms of subsea permafrost degradation in the East Siberian Arctic Shelf
- Nature Communications 8, Article number: 15872 (2017)
- doi:10.1038/ncomms15872
- Download Citation
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.
(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
4, 5, 6, 7, 8, 9 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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