Holocene climate change in the Alta Mountains and its implication for human activities

Genetic and archaeological evidence shows that an increased human population and long-distance socioeconomic exchanges in the Altai-Sayan region of Central Asia occurred in the early Bronze Age, some five thousand years ago. However, the local/regional environmental and climatic drivers behind the enhanced cultural exchanges between east and west are poorly understood. We obtained a multiproxy-based Holocene climate history from the sediments of Kanas Lake and neighboring Tiewaike Lake, in the southern Altai Mountains, with the objective of determining the role of climate change in human migration and cultural exchanges. Records of the silicon isotope composition of diatom silica (δ۳۰Sidiatom) and the biogenic silica (BSi) content indicate an exceptionally warm climate during ~۶.۵–۳.۶ kyr BP. During ۴.۷–۴.۳ kyr BP, especially, a peak in δ۳۰Sidiatom reflects enhanced lake thermal stratification and periodic nutrient limitation as indicated by concomitant decreasing BSi content. Supported by the widely-recorded humid climate of the middle to late Holocene in arid Central Asia, our geochemical results indicate a significantly warm and wet climate in the Altai Mountain region during ۶.۵–۳.۶ kyr, which promotes an increased prehistoric human population of pastoralist early Bronze Age Afanasievo Culture (~۵.۱–۴.۵ kyr) that migrated from the Yamnaya Culture (~۵.۵–۴.۵ kyr). Intriguingly, this unusually warm climatic interval (۴.۸-۳.۶ kyr) is simultaneous with the appearance of the thermophilic algal species Pediastrum simplex in Bosten Lake in southern Xinjiang. The climate cooled during ۴.۲-۴.۱ kyr and ۳.۶-۳.۵ kyr, as indicated by the decrease or absence of Pediastrum simplex. These cold events may have triggered the southward human migration out of the Altai region, resulting in the widespread distribution of archaeological sites at lower latitudes, in the Tienshan Mountains, and in the desert-oasis areas of the Tarim Basin in southern Xinjiang.