Reconstructing the natural conditions of the islands of the Lesser Kuril Ridge from the composition of diatom flora of the Late Pleistocene‒Holocene peat deposits

Abstract

Based on studying the diatom flora by use of the radiocarbon dating methods, tephrostratigraph and other available biostratigraphic data, we reconstructed the settings of development of peat deposits in Tanfiliev and Polonskii Islands (Lesser Kuril Ridge) for the end of the Late Pleistocene‒Holocene according to climatic changes, tsunami events and the influence of volcanic ash falls. It is found that the main factor determining the time history of peat bogs is represented by hydroclimatic changes during the Holocene. We identified several (quite well comparable for the two islands) waterlogging and drying stages of peat deposits, and determined the peat accumulation rates for different periods in the Holocene. In the Mid-Holocene (7240‒3660 years ago), the formation of peat deposits was proceeding in conditions of insignificant waterlogging, and the peat accumulation rate was decreasing substantially. An increase in waterlogging started about 3660 years ago. Waterlogging of the peat deposits increased considerably after the fall of volcanic Ta-c and Ma-b ashes. The dry conditions, caused by short-lasting cooling periods, existed about 12, 7 and 1.5 ka. During the final stage of the Late Holocene, peat deposits were developing in water-deficient, strongly acidic conditions. They exhibited the presence of marine kinds of diatoms. In the Early Holocene, diatoms were supplied to the peat bogs as a result of the scour of marine Pleistocene deposits which existed within a land bridge. In the Mid- and Late Holocene, the presence of marine diatoms in peat deposits is associated with tsunami events.