HR: 1340h
AN: C13A-0260    [Abstracts]
TI: Numeric Apporach to the Thermal Evolution of Submarine Permafrost on the Laptev-Sea Shelf / Siberia
AU: * Junker, R
EM: ralf.junker@uni-bremen.de
AF: Universitaet Bremen, Klagenfurter Strasse, Bremen, 28359 Germany
AU: Kaul, N
EM: nkaul@uni-bremen.de
AF: Universitaet Bremen, Klagenfurter Strasse, Bremen, 28359 Germany
AB: Almost 24 percent of the northern hemisphere are under permafrost influence. A special variety is the submarine or sub-sea permafrost proposed on the Laptev-Sea shelf in Siberia. However, temperatures far below the freezing point can establish in soils only under subaeric conditions without ice-cover. Submarine permafrost is considered to be a relic terrestrial permafrost formed on the shallow Laptev-Sea shelf during times of sealevel lowstand of the last galcioeustatic cycle. Vast areas of the shelf down to the present 120 meters isobath were exposed to acrtic mean annual temperatures of less than -15øC. At the end of the last glacial maximum, about 13.000 ky B.P., transgression of arctic sea-water started to drown the permafrost. Nowadays, the epicontinental Laptev-Sea provides water depth less than 40 meters throughout most of the shelf area. Rates of coastal retreat of approx. 5 m/y make the transition zone of land and sea a rapidly changing system that holds significant keys to the understanding of temporal and spatial development of the submarine permafrost. Due to the contact to relatively warm arctic saline water above (about -1.5 øC in average), the submarine permafrost assimilates heat energy from the seawater. This energy flux leads to a warming of the submarine permafrost close to the thaw temperature of fresh water in the pore space. The present state of permafrost degeneration is largely unknown. In order to learn more about the process of degeneration and to determine locations of special interest for fourthcoming expeditions, a numeric model based on finite-element method was developed. Assuming, that the process of heat diffusion strongly outweights convective mechanisms, a diffusion equation was used to compute the thermal development of the permafrost beginning from 50 ky B.P up to now. The model takes account of regional climate change, transgression of sea water/coastal retreat, basal heat flux, thermal rock-paramters for water saturated sediments and latent heat of fusion. Applying all this parameters to the numeric solution of the equation of heat diffusion, leads to spatial present-day temperature distribution across Laptev Sea. Results of numerical modelling show, that the submarine permafrost responds very sensitive to temperature changes applied to the upper boundary condition - in this case seawater (-1.5 øC). Already 1000 years after transgression, the model is close to a thermal equillibrium with sea water. All areas of Laptev-Sea that have been covered with sea water earlier than app. 3 ky B.P. show largely normal temperature gradients over depth. Much lower temperatures, probably retated with ice bonded permafrost, have been preserved only in less than 30 km distance from the coast. According to the model, the rate of transgression / coastal retreat strongly controls the distribution and conservation of submarine permafrost. This raises the question, whether the ice in the pore-space of submarine permafrost has been conserved further off the coast where temperature is close to its thawing temperature. At this crucial point probably other side effects and minor related paramter like depression of freezing point due to grain size, infiltration with salt water and variations in basal heat flux, become more important and could make a difference.
DE: 9320 Asia
DE: 9604 Cenozoic
DE: 3230 Numerical solutions
DE: 1823 Frozen ground
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting