Ice wedge casts that formed due to thawing of ice wedges are significant signs of past warm periods in Arctic periglacial regions. Such permafrost degradation structures are numerous and very well exposed on both coasts of the Dmitry Laptev Strait, which connects the Laptev Sea with the East Siberian Sea between the southern island of the New Siberian Archipelago (Bolshoy Lyakhovsky Island) and the mainland (Oyogos Yar coast). Cryolithological, sedimentological and palaeo-ecological features were studied in order to obtain detailed records of interglacial permafrost, landscape and climate dynamics in the Northeast Siberian Arctic.The lithostratigraphical structure of Eemian sections on both coasts show similar patterns of two facial different horizons. The lowermost sequences of consists of thawed and subsequently refrozen (so-called taberal) deposits, accumulated in pre-Eemian times and underwent thawing in lake taliks during the Eemian Interglacial when thermokarst processes led to the formation of lakes and thawed deposits below thelakes. The sedimentological and cryolithological features of the pre-Eemian taberal horizons show single whitish laminations which are interpreted as thaw signs, numerous small dark-grey spots representing strongly decomposed organic matter, and a massive cryostructure. Pollen records reveal relatively high amounts of reworked ancient(mineralised) coniferous pollen, whereas ostracod remains are absent. The refreezing of the taberal horizons took place in post-Eemian time and the deposits remained frozen until today. Taberal deposits are covered by the Eemian lacustrine sequence that formed due to warmer conditions during the Interglacial when thermokarst lakes occurred. Under such conditions pre-Eemian ice wedges thawed; at their positions small thermokarst lake basins formed and lacustrine sediments began to accumulate. Distinctive features of the lacustrine sediments are the alternating beds of finely laminated brownish plant detritus and grey sandy silt layers. Synsedimentary slumping structures were additionally observed there.The Eemian pollen records from both coasts show similarities reflecting comparable vegetation. The main difference is the absence of Larix at the northern location. The northern tree line was located along the Dmitry Laptev Strait during the Middle Eemian. High numbers of Glomus spores indicate significant surface disturbance. High number of Artemisia and the presence of herb pollen taxa show that open plant associations werecommon. Numerous spores of dung-inhabiting fungi point to the presence of grazing mammals. Climate conditions in the Eemian were warmer than today in northern Yakutia. According to pollen-based quantitative climate reconstruction the mean air temperatures of the warmest month (MTWA) vary from 7.8 to 9.6 °C for the Eemian thermal optimum (modern MTWA at Bolshoy Lyakhovsky Island: 2.8 °C). Using Eemian plant macrofossil records an open subarctic shrub tundra dominated by Duschekia (Alnus) fruticosa, Betula nana and Ledum palustre and interspersed with lakes and grasslands existed and a MTWA of about 12.5 °C was reconstructed.. Late Eemian pollen records indicate gradual climate deterioration toward to the early Weichselian.The occurrence of numerous well-preserved ostracod remains as well as numerous mollusc shells in the lacustrine horizons points to stable aquatic conditions due to extensive thawing of pre-Eemian ice-rich deposits and the widespread occurrence of thermokarst lakes caused by generally warmer climate conditions. The Eemian ostracodassemblages are dominated by species which tolerate the considerable changes in temperature and salinity regimes that are typical of modern habitats like thermokarst lakes and polygon ponds in the periglacial landscapes of East Siberia.The Eemian lacustrine deposits are discordantly covered by thick Ice Complex deposits of Weichselian age.
Helmholtz Research Programs > PACES I (2009-2013) > TOPIC 3: Lessons from the Past > WP 3.1: Past Polar Climate and inter-hemispheric Coupling