Using the model for sediment thermophysics, we have obtained estimates of the Arctic shelf permafrost state under changes in the seafloor temperature based on simulations with climate models from the 6th phase of the Coupled Model Intercomparison Project (CMIP6) through the end of the 23rd century. We used the estimated data of three models from the CMIP6 driven by the scenario with high anthropogenic greenhouse gas emissions SSP5-8.5. Within the historical period only minor differences between the scenarios are found, but in the 22nd-23rd centuries, permafrost thawing is 5-10 times faster and the differences between the scenarios with various datasets of seafloor temperatures grow. The average rates of subsea permafrost degradation were 1-2 cm per year for the period of 1950-2015, 5 cm per year for 2015-2100, and 10 cm per year for the period of 2100-2300.
Large reserves of carbon are preserved under conditions of subsea permafrost in the bottom sediments of the Arctic shelf. The existence of permafrost has created the necessary conditions for the thermodynamic stability of methane hydrates. Using a mathematical model that describes the thermal state of the sediment, we analyzed the dynamics of the permafrost and methane hydrates stability zone of the Arctic shelf bottom sediments for 100 thousand years in the future. Climate changes are considered under an idealized scenario of CO2 emissions into the atmosphere and changes in the parameters of the earth's orbit. The simulations for the next 100 kyr found that at the middle and shallow parts of the shelf the subsea permafrost survives, at least, for 9 kyr after the emission onset or even for several tens of kiloyears. Model estimates of methane emission from the Arctic shelf sediments to the water amounts up to 10 g/m2 per year.
Based on the regional ocean-ice model SibCIOM, calculations have been made for modeling the Arctic Ocean water masses conditions and dissolved methane transfer for the period from 1970 to 2019. We have performed the model analysis of methane emissions from the Arctic seas caused by gas release at the "ocean-bottom" interface. It is shown that the East Siberian shelf seas provide significant contributions to the total methane emission in the region. The spatial variability of the methane emissions into the atmosphere is primarily due to the region's peculiarities and ice conditions circulation. The estimated methane flux from the Arctic shelf seas has amounted to 2 Tg per year.
Mathematical modeling is used to study the role of natural events in the evolution and current state of the permafrost zone and the gas hydrates stability zone in the northern part of the East Siberian shelf. One of the main events of the last 200 thousand years was the cover glaciation of the end of the Middle Pleistocene, which left sheet ice in the sediments of the Anzhu Islands (New Siberia and Faddeevsky), 20-30 m thick and 2-3 km long. A paleogeographic scenario of the shelf development in the last 200 thousand years and a geological structure model, which characterizes the composition and thermophysical properties of rocks, have been constructed. A model of thermophysical processes in the shelf sediments was used for a geological section with a thickness of 1500 m to calculate the temperature of the rocks. The simulation results showed the following. The permafrost thickness in the glacial region is 30-160 m less, and the base of the gas hydrate stability zone is located 140-600 m closer to the seabed surface than in non-glacial conditions.
The changes in the characteristics of submarine permafrost within the ocean shelf have been analyzed using a mathematical model describing the thermal state of the soil and supplemented with the salt diffusion equation. The model is complemented with reconstructions of climate changes at the Arctic shelf over the past 400 thousand years with two combinations of air temperature and sea level reconstructions. The estimates of the subsea permafrost sensitivity to the uncertainty of paleoclimatic reconstructions of air temperature and ocean level have been obtained. The time scales of the Arctic shelf submarine permafrost response to climate change in the glacial cycles have been estimated. The temporal scale of the propagation of the thermal signal in the permafrost layer of the shelf sediment amounted to 4-12 thousand years.
The dissociation of the pore metastable relict gas hydrates in permafrost as a consequence of the observed climate changes is one the possible reasons for the formation of gas craters in the northern part of West Siberia. Using the heat transfer model in the glacier-soil system, the temperature field in the permafrost under the ice sheet was calculated. The methane hydrate stability zone in the permafrost soils with typical and increased geothermal flows was determined. The results have showen that about 60 thousand years ago under the ice sheets on the Yamal Peninsula in the Pleistocene, the methane hydrates could form near the surface. Formed near the surface, methane hydrates could later transform into the relict hydrates.
The estimates of the uncertainty for the model simulated subsea permafrost characteristics relative to the uncertainty of paleoclimatic reconstructions of ocean level are obtained. This is done by using the model for thermophysical processes in the subsea sediments. This model is driven by four time series of temperature at the sediment top, TB, which is constructed for the last 400 kyr by using different combinations of the same reconstruction of the past surface air temperature but different sea level reconstructions. At each time instant t and each variable Y, the uncertainty metric is defined as a ratio ▵Y (t) / Ym(t), where ▵Y (t) is spread of the values of Y for different TB time series, and Ym(t) is the mean of Y over different realizations corresponding to different TB. The root-mean-square calculated value of thus defined metric for different time intervals is ≤ 50% for permafrost base depth with the exception of isolated time intervals and / or the deepest part of the shelf. This uncertainty is not symmetric with respect to the sign of the sea level uncertainty. In turn, uncertainty for the hydrate stability zone thickness is small for shallow shelf but becomes pronounced for intermediate and deep shelves. The most uncertainty is due to uncertainty of dates for oceanic regressions and transgressions.
The paper presents the results of mathematical modeling of the subsea permafrost dynamics and distribution on the East Arctic shelf. In this study the geographic distribution of the geothermal flow values was taken into account. Estimates of the permafrost thickness on the Laptev and the East Siberian seas shelf have been obtained. The depth of occurrence of the lower boundary of the frozen layer on the shelf was about 50-700 m. The position of the submarine permafrost upper boundary in the bottom sediments of the seas of the East Arctic substantially depends on the depth of the sea and the content of salts. It was found that the upper boundary of frozen rocks is located at a depth of 15-30 m below the seabed, depending on the shelf region. The increased heat flux leads to a significant decrease in the thickness of the submarine permafrost on the outer shelf and in the rift areas.
The objective of the present study is to analyze the interactions between a methane hydrates stability zone and the ocean temperature variations and to define the hydrate sensitivity to the contemporary warming in the Arctic Ocean. To obtain the spatial–temporary variability of the ocean bottom temperature we employ the ICMMG regional Arctic-North Atlantic ocean model that has been developed in the Institute of Computational Mathematics and Mathematical Geophysics. With the ice-ocean model the Arctic bottom water temperatures were analyzed. The resulting warming ocean bottom water is spatially inhomogeneous, with a strong impact by the Atlantic inflow on shallow regions of 200-500 m depth. Results of the mathematical modeling of the dynamics of methane hydrate stability zone in the Arctic Ocean sediment are reported. We find that the reduction of the methane hydrate stability zone occurs in the Arctic Ocean between 250 and 400 m water depths within the upper 100 m of sediment in the area influenced by the Atlantic inflow. We have identified the areas of the Arctic Ocean where an increase in methane release is probable to occur at the present time.
A one-dimensional model for the heat transfer with a phase change is proposed to simulate the dynamics of the subsea permafrost on the Laptev Sea shelf. By now we investigate the evolution of the subsea permafrost since the last glacial maximum, taking into account a possible development of the thermokarst lakes. This paper also discusses the permafrost-related gas hydrate stability zone. The permafrost within most of the Laptev Sea shelf (≤ 50 m water depth) is estimated as 440 - 560 m given a heat flow of 60 mW/m2, and 178 - 265 m given a heat flow of 100 mW/m2 . The thermokarst lakes play an important role in the heat exchange between the atmosphere and sediments. The subsea permafrost evolution also depends on thermokarst lakes existence. Scenarios show that the permafrost thawing from top to bottom leads to the formation of the closed talik. The effect of a heat flux for fault zones results in higher sediment temperatures and in a more rapid destruction of the permafrost and hydrate zone. The proposed model predicts the development of open taliks at the shelf sites with water depths of 10 – 20 m in active tectonic faults.
The recent and the future warming in the Arctic may have a potential to cause rapid changes in the Earth’s system. The global warming could lead to destabilization of the subsea permafrost and cause a release of methane into the water column. The state of permafrost in the Arctic is the key to understanding whether the methane, stored in the permafrostrelated gas hydrate, can escape to the atmosphere. Results of the mathematical modeling of the dynamics of submarine permafrost and methane hydrate stability zone in the sediments of the East Siberian Arctic shelf are reported. The thickness of permafrost on the shelf is 170 - 320 m for the geothermal heat flux 60 mW/m2 according to the results of experiments. The permafrost modeling indicates that after the seafloor warming from 1948 to 2012 the permafrost deepening down to 1-25 m. A significant degradation of the subsea permafrost down to 10 - 70 m is expected in the next 50 - 100 years.
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