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Tuesday, June 22, 2010

D. Nicolsky & N. Shakhova, ERL, 5 (2010), Modeling sub-sea permafrost in the East Siberian Arctic Shelf: the Dmitry Laptev Strait

Environ. Res. Lett., 5 (January-March 2010) 015006; doi: 10.1088/1748-9326/5/1/015006

Modeling sub-sea permafrost in the East Siberian Arctic Shelf: the Dmitry Laptev Strait
D. Nicolsky1 and N. Shakhova2
1 Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99709, USA
2 International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK 99709, USA

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Received 8 November 2009; accepted 17 February 2010; published 25 March 2010.


The present state of sub-sea permafrost modeling does not agree with certain observational data on the permafrost state within the East Siberian Arctic Shelf. This suggests a need to consider other mechanisms of permafrost destabilization after the recent ocean transgression. We propose development of open taliks wherever thaw lakes and river paleo-valleys were submerged shelf-wide as a possible mechanism for the degradation of sub-sea permafrost. To test the hypothesis we performed numerical modeling of permafrost dynamics in the Dmitry Laptev Strait area. We achieved sufficient agreement with the observed distribution of thawed and frozen layers to suggest that the proposed mechanism of permafrost destabilization is plausible.
1. Introduction
Arctic warming can re-introduce carbon, accumulated in terrestrial and sub-sea permafrost for millennia, back into the present day atmosphere and biosphere biogeochemical cycle (ACIA 2004), and consequently might lead to acceleration of certain feedback processes affecting global climate dynamics. The current state of sub-sea permafrost beneath the East Siberian Arctic Shelf (ESAS) is a potential key to understanding whether and how methane, preserved in seabed reservoirs, escapes to the atmosphere. Figure 1 shows the extent of the sub-sea permafrost around the Dmitry Laptev Strait (DLS) and under the Laptev and East Siberian Seas.

Figure 1

Figure 1. Lateral extent of permafrost in the Laptev and East Siberian Seas, the Dmitry Laptev Strait, and a transect along which several boreholes into sub-sea permafrost were drilled. 

Unlike the terrestrial permafrost in the Arctic, which experienced a change in its thermal regime caused by a 6–7 °C mean annual air temperature increase since the last glacial maximum (Frenzel et al. 1992), sub-sea permafrost has been subjected to additional drastic transformations, e.g. inundation by the ocean, which resulted in warming the permafrost environment by as much as 17 °C (Soloviev et al. 1987, Kim et al. 1999, Romanovskii & Hubberten 2001, Romanovskii et al. 2005). The present understanding of the current thermal state and stability of submarine permafrost in the ESAS, is primarily based on modeling results (Soloviev et al. 1987, Kim et al. 1999, Delisle 2000, Romanovskii & Hubberten 2001, Romanovskii et al. 2005). Two basic mechanisms are proposed to explain permafrost dynamics after the inundation: the so-called upward degradation under geothermal heat flux in the areas underlain by fault zones (Romanovskii & Hubberten 2001), and the so-called downward degradation under the warming effect of large river bodies (Delisle 2000). Modeling results based on either mechanism suggest the existence of open taliks—a body of unfrozen ground connecting sub- and supra-permafrost waters—within limited areas of fault zones and those influenced by large rivers. According to model results in Romanovskii et al. (2005), the area of seabed endowed with open taliks is less than 5% of the Laptev and East Siberian Seas, and no open taliks can develop within the DLS area due to its distance from large river bodies.

One possible mechanism which allows for formation of open taliks is thawing of the permafrost beneath thaw lakes, submerged several thousand years ago during the ocean transgression, after taliks had already begun to form. Such thaw lakes were abundantly spread on the Laptev Sea coastal plain and their interactions with the ocean are described in Romanovskii et al. (2000). These authors limit their thermokarst theory to near-shore places, located between the present day shoreline and the isobath – 20 m. Noting that the maximum depth in the DLS area does not exceed 15 m, we model permafrost dynamics underneath the submerged thaw lakes, located in the near-shore areas of the ESAS such as in the DLS, by further developing the ideas of Romanovskii et al. (2000). Outside of the ESAS, in the North American Arctic, research on the sub-sea permafrost was conducted, e.g., by Mackay (1972), Osterkamp and Harrison (1985). Several researchers, such as Nixon (1986), Taylor et al. (1996), attempted modeling of the sub-sea permafrost dynamics on the North American Arctic shelf. One of the key distinctions of their modeling efforts from those of Romanovskii et al. (2000), Delisle (2000) is that the former included the influence of salt contamination on the permafrost temperature dynamics.

In this work, we try to combine the ideas of Taylor et al. (1996), Romanovskii et al. (2000) and show that degradation of the salt-contaminated sub-sea permafrost can lead to formation of open taliks in the DLS area. These taliks can serve as pathways for methane in the marine permafrost, and hence provide a justification for widespread methane observations in the DLS area (Shakhova et al. 2005, Shakhova & Semiletov 2007).

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