Intra-seasonal hydrological processes on the western Tibetan Plateau: Monsoonal and convective rainfall events ∼7.5 ka ago

Abstract. Billions of people depend on the precipitation of the Asian monsoons. The Tibetan Plateau and the Himalayas on the one hand strongly influence the monsoonal circulation pattern and on the other hand represent water towers of humanity. Understanding the dynamics of the Asian monsoons is one of the prime targets in climate research. Modern coupling of atmospheric circulation and hydrological cycle over and on the plateau can be observed and outlined, and lake level controlling factors be identified. Recent monitoring of lakes showed that many of them have grown at least for decades, the causes being higher meltwater inflow or stronger rainfall of different sources, depending on the particular location of a drainage basin. The long-term dynamics, however, can be described best with the aid of high-resolution climate archives. We focus here on the often controversial discussion of Holocene lake development and selected the Bangong Co drainage basin on the western Tibetan Plateau as a case site. The aim of our study is, to identify the factors influencing lake level such as monsoonal or convective precipitation and meltwater. For doing so, shells of the aquatic gastropod genus Radix were collected from an early Middle Holocene sediment sequence in the Nama Chu sub-catchment of the eastern Bangong Co and sclerochronlogical isotope patterns of five shells obtained in weekly to sub-monthly resolution. Our data suggests that during ca. 7.5 ka ago, monsoonal rainfall was higher than today. However, summer precipitation was not continuous but affected the area as extended moisture pulses. This implicates that the northern boundary of the SW Asian monsoon was similar to modern times. We could identify convective rainfall events significantly stronger than today. We relate this to higher soil moisture and larger lake surface areas under higher insolation. The regional meltwater amount corresponds with westerly-derived winter snowfall. The snowfall amount was probably similar to modern times. Exceptionally heavy δ¹³C values archived in the shells were likely, at least partly, triggered by biogenic methane production. We suggest that our approach is suitable to study other lake systems on the Tibetan Plateau from which fossil Radix shells can be obtained. It may thus help to infer palaeo-weather patterns across the plateau.