Cosmogenic Be-10 Constraints of the “Little Ice Age” Glacial Advances in the Eastern Tian Shan, China

Abstract:

The Little Ice Age (LIA) glacial advances, represented as a set of “fresh and bouldery” moraines a few hundred meters downstream from modern glaciers, have been widely recognized in Central Asian highlands. However, few studies have been conducted to constrain the formation ages of these moraines. Glaciers provide critical freshwater supply in Central Asia, so understanding the dynamics of glaciers, especially in the past a few hundreds of years, is of great importance. In a NSF funded project reported here, we aim to constrain the formation ages of these putative LIA moraines in the eastern Tian Shan, China, using cosmogenic ¹⁰Be surface exposure dating. Our initial results showed ¹⁰Be exposure ages ranging from 0.17±0.10 ka to 0.28±0.10 ka, from 0.38±0.03 ka to 0.86±0.08 ka, and from 0.36±0.04 ka to 0.41±0.10 ka for these moraines from the Bayinbuluke Valley in the Nalati Range, the Turgan Valley in the Karlik Range, and the Daxi Valley in the Tianger Range, respectively. We are currently processing 30 samples for the “LIA” moraines in six valleys located in the Heigou Valley of the Bogeda Range, the Haxilegen Pass in the Borohoro Range, and the Urumqi River headwaters in the Tianger Range. Except for one older age of 5.1±0.6 ka, which might be an outlier, our recently measured four ages range from 0.19±0.04 ka to 0.61±0.10 ka. These ages suggest that these fresh moraines were formed during the LIA, but glaciers might advance to their maximum extents slightly differently in different sites. This work demonstrates the importance of absolute dating in reconstructing past glacial fluctuations, and provides quantitative evidence in assessing the impact of glacier change on the ecosystem and society in recent centuries.