Mineralogical and magnetic variations of periglacial loess in SE Tibet reveal mid-Pleistocene expansion of Tibetan glacial activity

Abstract

The formation and evolution of the cryosphere on the Tibetan Plateau is of great significance in understanding the Earth's carbon and climatic system. Periglacial loess deposits in southeastern Tibet offer a means to constrain this history as they contain critical information on glacial grinding and frost shattering processes in high-altitude mountain regions through time, which yield lithogenic fractions of largely loess silts and sands. Based on combined analyses of lithogenic magnetic properties and mineralogical composition, here we find that increasing high mountain production and supply of fresh detrital components since the mid-Pleistocene climate transition (MPT, 1.2–0.7 Ma) led to a substantial increase of the lithogenic susceptibility and decrease in chemical weathering intensity of periglacial loess in southeastern Tibet. The agreement of these findings with similar results from eolian loess on the northern margins of the Tibetan Plateau suggests a plateau-wide glacier expansion during the MPT. Enhanced glacial erosion and freeze-thaw activities occurred in the high-altitude mountain regions of the plateau during the MPT, thereby providing vast amounts of fresh detritus for the formation of loess deposits. High and constant values of lithogenic magnetic parameters in the loess deposits after 0.7–0.6 Ma further indicate that the Middle Pleistocene is a critical period for the establishment of modern-like glacial and periglacial landforms on the Tibetan Plateau. Our results further suggest that enhanced Quaternary glaciation in SE Tibet occurred earlier than in the north, which we attribute to climate cooling combined with regional seasonal snowline lowering. As such, a close relationship exists between global climate changes, development of pan-Tibetan glaciations, and large-scale dust emission.