GRAIN SIZE DISTRIBUTION OF BOTTOM SEDIMENTS IN SMALL LAKES OF THE SHOKSHA RIDGE, KARELIA, AS A PROXY FOR PALEO-HYDRODYNAMIC SEDIMENTATION ENVIRONMENTS

Abstract

The paper examines the grain size distribution of bottom sediments of lakes in the Shoksha Ridge area, Karelia, to elucidate howdeglaciation and formation of the Onego Ice Lake basin proceeded. A sequence of bottom sediments of Lake Anashkino and two sediment sequences of Lake Rzhanoe were studied, and lithostratigraphy of the sediment cores was described. The grain size compositions of the 9.32-10.32m sequence from Lake Anashkino and the 2.5-6.52 m sequence from Lake Rzhanoye were analyzed. The study revealed features of the bottomsediment fractions distribution along the depth of occurrence. Paleo-hydrodynamic sedimentation environments were identified using grain sizeanalysis. Lake Anashkino bottom sediments of a 9.96–10.31 m horizon, represented by silt and fine-grained sand, were deposited under conditions of highly dynamic shallowing (decrease in the water level of Onego Ice Lake). The upper part of sediments (9.32–9.92 m), represented by clayey silt mixed with fine-grained sand, accumulated under weak flow conditions. Paleo-hydrodynamic sedimentationenvironments in Lake Rzhanoye were characterized by alternation of highflow conditions (5.36-6.23 m, 6.31-6.32 m, 6.47-6.48 m) and weak flowconditions (2.50-4.88 m, 6.28-6.29 m, 6.30-6.31 m, 6.37-6.45 m, 6.51-6.52 m). This is possibly due to the dynamics of the decrease in the waterlevel of Onego Ice Lake and the formation of small isolated lakes.In the process of the formation of bottom sediments of small lakes of the Shoksha Ridge during the period of intensive changes in thelandscapes, outlines and levels of Onego Ice Lake at the transition from the late Pleistocene to Holocene, against a shift from nival to humid climate,we note a change in sedimentation conditionsin the bottom sediments of the two studied lakes – Anashkino and Rzhanoe – from highly dynamic(flowing conditions, decrease in water level of Onego Ice Lake) to weakly flowing, in the conditions of isolation of the lakes and formation ofmodern small lakes.