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

Abstract

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.

Strengthened Indian summer monsoon brought more rainfall to the western Tibetan Plateau during the early Holocene

“…This resulted in increased rainfall over northern India, but a decline in water vapor input from the Bay of Bengal, leading to a greater increase in precipitation for the western TP rather than the eastern TP. A recent study proposed that during the early middle Holocene (7.5 ka ago) monsoonal moisture did not penetrate onto the plateau significantly more than at present, but that rainfall events were more frequent and more intense, with the average annual precipitation in western TP (Bangong Lake) likely several times higher than modern [13]. This study is consistent with shoreline-based precipitation variability estimations and our KCM model simulation results.…”

Mid-Holocene palaeohydrochemistry and palaeohydrology of Yamdrok Yumtso, southern Tibetan Plateau, reconstructed from δ18O and δ13C of fossil shells of the gastropod Radix auricularia

“…In the PQ section, as illustrated in Figure 3d, it is clear that most of the δ 13 C shell values of fossil Radix auricularia within Units 4-2 are lower than 0‰ and more positive than the value of Radix auricularia in modern Yamdrok Yumtso (−1.9‰; Supplemental Table S1, available online; Figure 3d). Decomposition of organic matter, the mineralogical substrate, exchange with atmospheric CO 2 and photosynthesis of aquatic plants all affect the δ 13 C composition of the dissolved inorganic carbon within lakes which provides the basic material for the shells of freshwater gastropods (Taft et al, 2020). The δ 13 C shell of Radix sp.…”

“…In addition, the δ 13 C and δ 18 O values of the shells of Radix sp. have previously been used to infer palaeohydrological and palaeoclimatic changes in the Tibetan Plateau and elsewhere (Chen et al, 2016a(Chen et al, , 2017a(Chen et al, , 2017bHailemichael et al, 2002;Hu et al, 2017;Mischke et al, 2002;Taft et al, 2012Taft et al, , 2013Taft et al, , 2014Taft et al, , 2020Weynell et al, 2016). The geochemical information recorded in a single shell of Radix sp.…”