Grain-size distribution patterns of the surface sediments and their indications for water depth at Wanlongwoma Lake, source area of the Yellow River

Grain size of lake sediments is an important environmental proxy indicator, and its climatic/environmental significances often show specific regional characteristics due to the variable tectonic settings and watershed hydrological processes of various lakes. In this study, we selected Wanlongwoma Lake as the research object. Wanlongwoma Lake(34°37'13.17″~34°35'55.02″N, 97°18'37.33″~97°30'21.57″E; 4300~4400m a. s. l.), is located in the source area of the Yellow River in northeastern Tibetan Plateau. The lake is a small open shallow lake, with a maximum water depth of about 1.5m. The Wanlongwoma Lake has the characteristics of alpine climate with a long dry and windy cold season lasting from October to next June, with warm and rainy season from May to September. The annual average temperature is about -2.9℃and the annual average rainfall is about 350mm during 1991~2020. Most of the rainfall in this region is mainly concentrated from May to September.By analyzing the grain-size composition and distribution of 55 surface lake sediment samples from Wanlongwoma Lake, the dominant factors controlling the grain-size distribution were revealed. The results show that the mean grain size of the surface lake sediments of Wanlongwoma Lake is from 20.89 to 152.29μm, the grain size separation coefficient is between 1.87 and 2.94. It is revealed that the main component of sediments is silt, with an average content of 51.76%, while the average content of silt and sand are 21.03%and 27.22%, respectively. The material sources supply system and the hydrodynamic conditions of runoff into the lake are the key factors controlling the spatial distribution of grain size of surface sediment in Wanlongwoma Lake. In the southeast of the lake, the grain size is generally coarse under the inference of the rivers entering the lake. The southeastern part of the lake is close to bare land, where the sand content is the highest under the influence of high-speed winter wind together with the rivers. There is a significant negative correlation between the sedimentary grain size and water depth, that is, good sorting and fine grain size correlates to deep water depth, while poor sorting and coarser grain size correlates to shallow water depth. The study of grain size of surface lake sediments of Wanlongwoma Lake is of great significance to understand the material input and hydrodynamic conditions of the lake and can be applied to study the paleoclimate/paleoenvironment on the Tibetan Plateau.