Extreme precipitation variations in the northeastern Tibetan Plateau during the last millennium
Authors: Wu Duo , Kong Weifeng , Liang Yuan , Guo Shilong , Wang Tao , Huang Jinghua , Pan Lili 
Published: March 2024

With the background of global warming, the hydrological cycle&#160;has&#160;accelerated and the occurrence of extreme precipitation events has also increased, bringing significant impact on agriculture, transportation, and human safety. The investigation of the frequency of historical or ancient extreme precipitation events is helpful for a better management of modern hydrological&#160;disasters&#160;and a reasonable prediction of future precipitation variations. The present&#160;study focuses on Lake Dalzong, an alpine lake located in Xiahe County, Gansu Province, northeastern Tibetan Plateau, and provides a lacustrine record of extreme precipitation variations during the last millennium in the study region.Considering that Lake Dalzong&#160;is a varve&#160;lake, we monthly and seasonally&#160;collected&#160;modern surface sediment&#160;samples,&#160;lake water samples, and precipitation samples from June 2020 to October&#160;2023. By analyzing the hydrogen and oxygen isotopes of&#160;water&#160;samples, we found that the lake is a hydrology open system and the lake water is mainly supplied by precipitation. From the measurements of the surface sediments and typical laminated samples, annual lamination was&#160;successfully identified. It is found that the coarse-grained dark layer was formed under increased precipitation from the summer to the early autumn, as heavy rainfall can&#160;bring exogenous detrital materials into the lake. And the fine-grained light layer was deposited during the freezing period of the lake from mid-November to mid-April of the following year&#160;when lake water was&#160;still&#160;and tiny dead organisms deposited&#160;slowly. Therefore, the varve&#160;layers are ideal archive of extreme precipitation variations in the region.Furthermore,&#160;a continuous sediment core reaching the bedrock,&#160;with a total length of 457.5 cm,&#160;was&#160;obtained&#160;from the center of the lake, and a reliable chronological framework for the past thousand years was established by using 137Cs&#160;and&#160;AMS14C dating, as well as the varve&#160;counting. Based on the analysis of&#160;grain size, XRF&#160;elemental data&#160;with high resolution, and the extreme precipitation variations was&#160;reconstructed. The coarse-grained&#160;dark layer with a thickness of 0.3~1.2 cm was extracted and&#160;defined as the varve event layer.&#160;It matched&#160;well with the peak percentage of coarse grain and the relative content of&#160;Ti&#160;elements, indicating that there&#160;was&#160;a corresponding relationship between the varve event layers&#160;and the extreme precipitation events. The results show that the frequency and intensity of extreme&#160;precipitation events increased during the Little Ice Age but decreased since the Industrial revolution. A further investigation shows&#160;that the extreme precipitation variations in the northeastern Tibetan Plateau&#160;was controlled by the external-driven factors and the ocean-atmosphere interactions in the Earth system.