Abstract:

The spatial characteristics of grain size distribution, total organic matter (TOC) and stable carbon isotopic composition (δ¹³C_TOC) in surface sediments of a desiccating alkaline lake in the southern Otindag sandy land were studied to explore their environmental implications. We found that clay and silt fractions were dominated in water surface and desiccated lakebed while TOC concentrations were generally less than 1% in most surficial samples. In the halophytic meadow to the east, the sand fraction moderately increased while TOC increased up 10.75% on average. From west to east, δ¹³C_TOC became more negative from the lake (-24.88‰) to desicacated lakebed (-25.17‰) and halophytic meadow (-27.93‰) areas. End-member mixing analysis (EMMA) showed that the increases in the saltation and creeping end members were correlated to lake regression and vegetation degradation in the catchment. For Qrdin Nuur, the relatively low TOC and depleted δ¹³C_TOC might indicate the absence of hydrophytes and major contributions of terrestrial C₃ plants to sedimentary organic matter. Furthermore, when the seasonal lake dries up, the bared lakebed, composed mainly of clay and silt fractions, would experience strong wind erosion, leading to increased sand fraction and reduced salinity therefore the invasion of halophytes. Then TOC increased significantly following the appearance of halophytes, and the subsequent succession of vegetation and the increasing C₃ biomass would likely lead to a decline in δ¹³C_TOC. Thus, by studying the modern sedimentary processes and mechanism of Qrdin Nuur, we have deepened our understanding of the sedimentation and ecological evolution in desiccating alkaline lakes in the Otindag sandy land, thereby providing an important basis for paleoenvironmental reconstruction.

Key words: lacustrine surface sediments, organic carbon isotope, end-member modeling analysis, spatial distribution, lake regression, Otindag sandy land