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

Abstract

The Tibetan Plateau and the Himalayas influence monsoonal circulation patterns and represent the water towers of humanity. Modern coupling of atmospheric circulation and the hydrological cycle over and on the plateau can be observed and delineated, and the factors that control lake levels can be identified. Recent monitoring of lakes has shown that many have grown for decades due to higher meltwater inflow or stronger rainfall from different sources, depending on the location. The long-term dynamics of lakes, however, can be best described by using high-resolution climate archives. We focus here on the often controversial discussion of Holocene lake development and use the Bangong Co drainage basin on the western Tibetan Plateau as a case study. We aim to identify the factors that influence lake level, such as monsoonal or convective precipitation and meltwater. To accomplish this, shells of the aquatic gastropod genus Radix were collected from an early Middle Holocene sediment sequence in the Nama Chu subcatchment of the eastern Bangong Co, and sclerochronlogical isotope patterns for five shells were obtained at a weekly to submonthly resolution. Our data suggest that at ca. 7.5 ka, monsoonal rainfall was higher than it is today. However, summer precipitation was not continuous but occurred as extended moisture pulses. This result implies that the northern boundary of the SW Asian monsoon was similar to that today. We identify convective rainfall events significantly stronger than today and relate these events to higher soil moisture and larger lake surface areas under higher insolation conditions. The regional amount of meltwater corresponds with the westerly derived winter snowfall. 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 for studying other lake systems on the Tibetan Plateau from which fossil Radix shells can be obtained.