A local clay-varve chronology of Onega Ice Lake, NW Russia

Abstract

Onega Ice Lake developed in front of the receding Late Weichselian ice margin in Russian Karelia. Glacial varves that formed in Onega Ice Lake have been earlier studied by means of varve counts, palaeomagnetism and 14C AMS dates from small lakes north of modern Lake Onega and from Lake Onega proper. Synchronous changes in magnetic parameters and similar stratigraphy of these varve records together with the existence of basin-wide marker interval of pink-coloured varves have been used for core-to-core correlation and palaeogeographic interpretations. Unfortunately, there are missing varve-to-varve correlations between these cores. We present a 1155 yr long local varve chronology based on 3 parallel overlapping cores from two small lakes in the Zaonezhsky Peninsula at the northern coast of Lake Onega. Varve counts and matching of varve series were made from digital images. Following the textural, structural, colour and thickness changes of varves within Onega Ice Lake varve series, six characteristic varve units are identified. These clay units reflect changing sedimentary conditions at the time of accumulation that in general can be interpreted as a progressively increasing distance to the retreating ice margin. Variations in varve thickness superimposed on that overall trend, namely two episodes of abrupt and consistent decrease in varve thickness, do not correspond to known lake-level changes or shifts in outflow directions. AMS chronology for deglaciation of Lake Onega basin gives some evidences for correlation of above intervals of decreased varve thickness with the GI-1c2 and GI-1B cold events in NGRIP δ 18O event stratigraphy but further age estimations are needed to confirm presented correlations. Interval of reddish-brown varves – earlier described also as the ‘pink horizon’ – was observed in all studied sequences, and according to varve correlation, at the same stratigraphical level, which proves its basin-wide synchronous appearance. Ca 100 km south of our study area, Saarnisto and Saarinen (2001) reported ca 200 more varves compared to our results. Visual correlation of varve thickness graphs places these extra varves to the proximal part of the series and indicate a rapid ice recession from the main lake basin to the northern Lake Onega area with the ice recession rate ca 500 m per year.