Isotopic study of the source and cycle of sulfur in the Yamdrok Tso basin, Southern Tibet, China

This study focuses on the inland drainage basin of Yamdrok Tso (Tso means 'lake' in the Tibetan language), which lies at altitudes between 4394 and 4989 m, in southern Tibet. The basin is located in the Tethys Himalayan zone, and consists of seven lakes with different hydrological characteristics. The main purpose of the study was to investigate the source of dissolved SO42− and the sulfur cycle in the groundwater-river-lake system. We measured the hydrogeochemical composition, δ18OH2O, δ34SSO4 and δ18OSO4 of sulfate in river, spring and lake water; the total sulfur content (STotal) and δ34STotal of total sulfur in a lake sediment core; and the δ34Spyrite of pyrite in the bedrock. The results indicate that most of the rivers in the study area are characterized by a high sulfate content with values of up to 270 mg/L. In addition, the δ34Spyrite of pyrite in the bedrock ranges between −4.31‰ and −5.77‰, while the sulfate in river and spring waters has mainly negative values of δ34SSO4 (−7.14‰ to −2.02‰) and δ18OSO4 (−13.2‰ to −4.04‰). In contrast, the waters of closed lakes in the study basin have high values of δ34SSO4 (3.44‰-8.58‰) and δ18OSO4 (7.26‰-10.3‰). The negative δ34SSO4 and δ18OSO4 dual-isotopic composition of sulfate indicates that pyrite weathering controls the dissolved sulfate and other solutes in rivers and spring water, while the anthropogenic pollutant sulfur flux is negligible. Water is the main oxygen source for sulfate derived from the oxidation of pyrite, while Fe3+ is the main direct agent of pyrite S oxidation. The lake water and sediments in the study basin are important sinks/stores of sulfur. The δ34STotal values of total sulfur in a sediment core from a closed and holomictic lake ranges between −8.5‰ and −44.9‰. As evidenced by the strong 34S and 18O enrichment in the residual pool of SO42− in the lake waters, and the marked depletion of 34S in the sediment core, the dissimilatory microbial sulfate reduction (MSR) in the lake sediments generally results in the preferential sequestration of sulfur enriched in 32S. In addition, the magnitude of 34S/32S fractionation between sulfate in the lake water and sedimentary total sulfur can reach up to 48.3‰. This abnormally high fractionation value can be explained mainly by the reoxidation of reduced inorganic sulfur compounds, together with the dispropotionation pathway in the sediments, caused by lake level fluctuations. The changes in STotal and δ34STotal values with depth in the sediment core can also be explained by the same mechanism.