Holocene glacier fluctuations and climate changes in the southeastern part of the Russian Altai (South Siberia) based on a radiocarbon chronology

摘要

This study investigates glacier dynamic and climatic variations in the southeastern part of the Russian Altai (SE Altai) during the last 7000 years. Recent glacier retreats and ice melting in moraines has led to exhumation of organic material allowing the possibility of radiocarbon dating. We report here 57 new radiocarbon dates from wood remains buried by moraines and from proglacial forefields, from peat layers and lacustrine sediments that cover moraines, from dead trees at the upper tree limit, and from rock glaciers on trough slopes from six glacial valleys in the North Chuya Range, SE Altai. Such a numerous dataset for the vast but unified in neotectonic and climatic conditions area is presented for the first time the history of research in the Altai.Together with 62 previously published radiocarbon ages, mainly of fossil soils and peat layers in the foot of the ranges in SE Altai, they form the basis for understanding the relative magnitudes and timing of the most important glacial and climatic events of SE Altai. New data refute the traditional concept of the Russian Altai Holocene glaciations as a consecutive retreat of the late Würm glaciers and argue their complete degradation at the head of trough valleys at least 7000 cal. years BP. Moraine complexes of three Holocene glacial stages are morphologically expressed in trough valleys of the North Chuya range. They correlate with three identified periods of glacial advances: from 4900 to 4200 cal. years BP (Akkem stage), from 2300 to 1700 cal. years BP (Historical stage) and in the 13th-19th centuries (Little Ice Age (LIA) or Aktru stage). The coincident extremes of lowering temperature and increasing precipitation during the Akkem stage led to abrupt glacier advances and forming of the most remote moraine complexes downstream in the valleys. Following glacier advances had distinctly smaller magnitudes. In addition to the radiocarbon data, the time limits of the Historical stage were defined more precisely using dendrochronological and archaeological data from Scythian burials of Pazyryk culture in SE Altai. Repeated forest regrowth in the presently glaciatiated area indicates significant retreat or even complete glacier degradation during interstage warming. The decreases of glacier length in the following stages argues for intensification of aridity in the SE Altai during the second half of the Holocene. The thermal minimum in the middle of 19th century, the greatest in the last millennium, did not positively influence the mass balance of glaciers, which also supports this conclusion.