Alluvial plains formation in response to 100-ka glacial–interglacial cycles since the Middle Pleistocene in the southern Tian Shan, NW China

Abstract

Abstract The tenet of the Milankovitch theory is that Northern Hemisphere solar insolation, controlled by Earth s orbital is responsible for cyclic climate variations. Eccentricity in this context is responsible for cyclic variability of 100\u202fka and may drive periodicity in climate change during the middle and late Pleistocene. Although abundant evidence exists for 100\u202fka cyclic climatic fluctuations driving glacial-interglacial cycles, our knowledge about the record of cyclic climatic changes preserved in terrestrial landscapes remains limited, especially over multi-millennial (several glacial-interglacial) timescales. The intermontane Yanqi Basin in the southeastern Tian Shan, Central Asia, preserves at least seven generations of alluvial surfaces spanning multiple glacial cycles and yields insights into the record of climatic fluctuations that lead to aggradation and incision of river systems. We complement existing chronologic data with two new cosmogenic 10Be depth profiles and four surface exposure ages of alluvial surface abandonment in the south Tian Shan piedmont within the Yanqi Basin. New surface-exposure dating reveals the successive abandonment of these surfaces with a 100\u202fka cyclicity between 5.1\u202f±\u202f0.9 and ~550\u202fka and indicates that there were multiple aggradation-incision cycles. The superimposed 100\u202fka\u202fcycles of aggradation-incision are best explained by eccentricity-driven climate change. Within these cycles, sediment is produced during glaciations by glacial and periglacial erosion. However, due to the dry and cold conditions of the glacial periods in central Asia, there is little discharge available to transport and evacuate the sediment from the valleys to the piedmont. Later, during the glacial to interglacial transition, higher precipitation and high discharge due to glacial melt water resulted in higher transport capacity of the river beyond a threshold to evacuate sediment. Subsequent rapid incision occurs after the upstream sediment reservoir is depleted and its timing corresponds to the glacial to interglacial transition period in our study area. Compared with the aggradation and incision of the northern piedmont of the Tian Shan governed by the Westerlies, the sedimentary systems of the South Tian Shan piedmont within the Yanqi Basin are mainly controlled by eccentricity-driven glacial-interglacial cycles.