Ice Complex Islands and Bars With Frozen Sea Floor on the Eastern Siberia Shelf.

Between the eighteenth and twentieth centuries, on the Eastern Siberia Shelf there were many islands build of ice-rich syncryogenic Ice Complex (IC). The Ice Complex was accumulated on the surface of emergent arctic shelf and coastal lowlands during the Late Pleistocene. Ground ice content in IC riches 90-95 percent by volume. Remains of animal bones, vegetation and other dispersed organic matter were preserved inside IC. The recent destruction of IC leads to thawing of this organic matter and involving it in carbon cycle. The first wide-range destruction of IC by thermokarst lakes began around 12,8 kyr B.P. Active formation of thaw lakes took place before the shelf was submerged under seawater. Sea transgression transformed thaw lakes into gulfs that were named "thermokarst lagoons". This process resulted in a dramatic winding of shoreline, an increase of shore thermo-erosion, and a higher rate of Holocene transgression expansion. During the last approximately 9 kyr, southward shoreline displacement reached from 300 km on the Laptev Sea and up to 800 km on the Eastern Siberia Sea shelf. At the same time, the outflow of suspended sediments on the outer shelf and on the continental slope decreased. Thaw lakes and "thermokarst lagoons" that trapped sediments caused a decrease in the outflow of sediments. Lake taliks submerged under seawater became predominantly closed subsea taliks. Capes and peninsulas, that divided "thermokarst lagoons", were gradually transformed into IC-islands. IC- islands endured very active shore thermal erosion, resulting in the constant decrease of the areas of relic IC-islands. The majority of relic islands disappeared and became sandy bars as a result of "sea floor thermal erosion". Only a few IC-islands can still be found on the shallow arctic shelves of the seas mentioned above. The bottoms of those bars are frozen, and covered only by a thin layer of sand. A Schematic map of existing IC- islands and those that have disappeared during the last three centuries has been compiled using published and archive data. Seawater depth over the former IC-islands is constantly increasing due to the sea floor thermal erosion. This process does not have a very good scientific explanation. Different investigators took bathymetry measurements at the sandy bars. By compiling these measurements, the approximate rate of seawater depth increase has been calculated. This parameter reflects the "sea floor thermal erosion" rate under modern hydrological conditions. At the places of the former Vasil'evsky and Semenovsky IC-islands this rates were equal approximately 0,12- 0,2 m/year. We were able to calculate the approximate time needed for the total destruction of IC-islands on the Eastern Siberia shelf by using seawater depth data from well studied bars and by taking into account rates of sea floor erosion.