On Genesis of the Lower Kolyma Edoma Based on New AMS 14C Dates from Duvanny Yar

In recent years, careful attention has been paid to the study of ice complex (loess-like silts enclosing thick ice wedges) as one of the most significant sources of organic matter incoming to the Arctic watershed rivers. In this connection in 2008-09, in the scopes of the POLARIS project, we conducted studies aimed to refine structure and genesis of this complex, widespread on the Kolyma lowland. New radiocarbon data were obtained on the upper edoma part from the stratotype exposure Duvanny Yar, situated on the Kolyma River right bank, 40 km downstream of the Omolon River mouth.

The studied section (N68 37.8' E159 08.6') is located on the eastern end of a big thermokarst circus which had formed at the margin part of the gently sloping accumulative plain cut by the Kolyma River. Here, from the edge of slope (32-35 m a.s.l.) downward to the depth of 70 cm, a soil layer of brownish silt with relatively low ice content is distributed. Downward, in the interval 70-75 cm, a transfer layer of ice-rich dark-grey silt pinching out in this section is observed. The transfer layer is underlain at the depth of 75 cm by ice complex, composed of ice wedges and enclosing dark grey silts.

Samples of organic matter (thread-like roots in situ) were collected from frozen silts above an ice wedge head and from enclosing silts. AMS 14C analysis was performed in the National Ocean Science AMS laboratory. The resulting dates were the following: 260+/-30 (interval 70 cm from the top); 29500+/-200 and 26600+/-170 (90-95 cm); 32600+/-230 (130-135 cm); 32600+/-220 (165-170 cm); 33400+/-240 (195-200 cm); 32900+/-310 (230-235 cm); 27900+/-270 (255-260 cm).

The obtained data indicate that the accumulation of the ice complex upper part took place during MIS3 (Karginsky period). The age inversions observed in the section in such a close range (2 m) attest to redeposition of the material downslope, supporting a deluvial-solifluction hypothesis of the edoma formation (Konischev, 1983). The lack of dates referred to MIS2 (Sartan period) assumes erosion of edoma which began in the end of MIS3. It could be supposed that the recognized erosion is explained by an uplift of the territory. Thus, an input of organic matter to rivers began, presumably, as early as the end of Late Pleistocene due to erosion and increased in Holocene in the result of thermokarst processes induced by global warming.