COSMOGENIC NUCLIDE CONSTRAINTS ON THE TIMING OF LITTLE ICE AGE GLACIAL ADVANCES IN THE EASTERN TIAN SHAN, CHINA

Glaciers provide critical freshwater supply for arid and semi-arid region in Central Asia; thus, it is of critical importance to understanding the dynamics of glaciers in this region, especially during the past several hundreds of years. Scholars have identified a set of fresh and bouldery moraines, a few hundred meters downstream from present glaciers, as the landforms formed during the Little Ice Age (LIA) in Central Asian highlands. However, few studies have been conducted to constrain the formation ages of these moraines. We constrained the timing and extent of LIA glacial advances based on 55 cosmogenic ¹⁰Be exposure ages obtained from eight glacial valleys in the eastern Tian Shan, China. Our results revealed three major glacial advances around 790±300 yr, 430±100 yr, and 270±50 yr in the last millennium. The maximum glacial advance occurred around 430±100 yr in most regions, whereas an early maximum advance was recorded around 790±300 yr in the easternmost Tian Shan. It was likely that the late glacial advance around 430±100 yr was more extensive in most regions and the deposit of this event presumably reworked or covered deposits from the early event. We also observed considerable nuclide inheritance on ¹⁰Be surface exposure dating of the LIA moraines, especially in front of small and thin glaciers. In particular, ¹⁰Be exposure ages on the LIA moraines in front of small and thin glaciers are widely scattered and much older than the LIA period. Historical topographic maps indicate that these small and thin glaciers were more extensive in the early 1960s, even some of our ¹⁰Be sample sites were located close to or within the ice coverage at that time. Glacial deposits transported by these glaciers may be originated mainly from reworked deposits formed prior to the LIA glacial advances, leading to varied nuclide inheritance to produce apparently old and widely scattered exposure ages.