Permafrost sequences exposed at the coast of the southernmost New Siberian Island are studied multidisciplinary by a Russian-German team using cryolithology, sedimentology, geochronology, geochemistry of ground ice, and bioindicators. The oldest horizon contains remains of a periglacial reworked Palaeogene weathering crust as proved by the occurrence of weathering products like kaolinite and montmorillonite. Separate epigenetic ice wedges and the absence of bioindicators also characterize this horizon. Saalian climate fluctuations are documented in two sedimentological units formed c. 200-120 ky ago. The lower unit (c. 200-170 ky) is very ice-rich and contains large ice wedges. Cryolithologically it is similar to the Late Pleistocene deposits of the so-called Ice Complex. The lower part of this unit contains pollen assemblages of sparse grass-sedge vegetation and reflects stadial environment. The upper part of this ice-rich unit is characterized by pollen spectra of dense grass-dominated tundra reflecting interstadial conditions. This Saalian Ice Complex deposits were eroded and covered by a younger Saalian unit (c. 170-120 ky). Well-sorted fine-grained sand contains less ground ice and pollen spectra of sparse grass-sedge dominated vegetation assigned to a Late Saalian Stadial. The accumulation of these loess-like floodplain and lake deposits, and the formation of ice wedge polygon systems took place under extremely cold and dry conditions. The following unit, assigned to the Eemian Interglacial, contains large ice wedge casts with many paleoecological evidences of interglacial environment. Humid and warm conditions resulted in thawing of ice wedge systems and the formation of ice wedge casts and thermokarst lakes. Subsequently, the permafrost sequences were locally eroded down to the old Ice Complex deposits. Lacustrine and loess-like floodplain deposits with ice wedge polygon systems were accumulated again during the Early Weichselian stadial (c. 100-50 ky) under extremely cold and dry conditions. They consist of fine-grained, well-sorted sands with rare grass and sedge pollen. These deposits turn gradually into an about 20 m thick ice-rich Late Weichselian Ice Complex horizon, dated 50-28 ky BP and containing the pollen spectra of typical mammoth tundra-steppe associations. The Ice Complex contains big ice wedge polygon systems. It represents a swampy, poorly drained habitat, which existed under extreme continental climate. Whereas peaty deposits of the Middle Weichselian Interstadial (c. 40-30 ky BP) occur regularly, Late Weichselian Stadial sequences were not found. The Ice Complex deposits appear to be eroded during that time and covered by Late Glacial/Holocene deposits afterwards. Active thermokarst processes during the Late Pleistocene/Holocene transition (c. 12-10 ky BP) resulted in the formation of large thermokarst depressions. New ice wedge polygon systems were formed during the Late Holocene climate deterioration. The Late Pleistocene/Holocene transition, including the Allerød warming and Younger Dryas cooling events, is preserved within lacustrine thermokarst deposits in a thermokarst depression flanking the Late Weichselian Ice Complex sequences.
Helmholtz Research Programs > MARCOPOLI (2004-2008) > POL6-Earth climate variability since the Pliocene