Extreme precipitation variations in the northeastern Tibetan Plateau during the last millennium

With the background of global warming, the hydrological cycle has 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 disasters and a reasonable prediction of future precipitation variations. The present 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 is a varve lake, we monthly and seasonally collected modern surface sediment samples, lake water samples, and precipitation samples from June 2020 to October 2023. By analyzing the hydrogen and oxygen isotopes of water 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 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 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 when lake water was still and tiny dead organisms deposited slowly. Therefore, the varve layers are ideal archive of extreme precipitation variations in the region.

Furthermore, a continuous sediment core reaching the bedrock, with a total length of 457.5 cm, was obtained from the center of the lake, and a reliable chronological framework for the past thousand years was established by using ¹³⁷Cs and AMS¹⁴C dating, as well as the varve counting. Based on the analysis of grain size, XRF elemental data with high resolution, and the extreme precipitation variations was reconstructed. The coarse-grained dark layer with a thickness of 0.3~1.2 cm was extracted and defined as the varve event layer. It matched well with the peak percentage of coarse grain and the relative content of Ti elements, indicating that there was a corresponding relationship between the varve event layers and the extreme precipitation events. The results show that the frequency and intensity of extreme precipitation events increased during the Little Ice Age but decreased since the Industrial revolution. A further investigation shows that the extreme precipitation variations in the northeastern Tibetan Plateau was controlled by the external-driven factors and the ocean-atmosphere interactions in the Earth system.