Sulfur cycling in the Yellow River and the sulfate flux to the ocean

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© 2019 Elsevier B.V. River sulfate flux is a major input into the ocean reservoir and plays a critical role in the global sulfur cycle. Constrained riverine sulfate isotope compositions are useful in gaining a better understanding of the secular changes in the sulfur isotopic value (δ34SSO4) of the ocean. The sulfate flux from rivers flowing through arid areas is often ignored due to their low water discharge, but these rivers generally contain large amounts of sulfate duo to water condensation, gypsum dissolution and anthropogenic effluents. Hence, their effects on the global sulfur cycle are still unclear. The Yellow River (YR) flows across the eastern Qinghai-Tibet Plateau (QTP) and arid Chinese Loess Plateau (CLP) with a length of 5464 km. As one typical large river flowing across arid climate areas coupled with extensive human activities in the world, the YR is an ideal site to illustrate this problem. The riverine sulfate sources in the different reaches of the YR were determined by δ34SSO4 and δ18OSO4 values, and sulfur cycling concept model in the Yellow River Basin (YRB) was established to explain the natural and anthropogenic effects on sulfate flux coupled with δ34SSO4 values from the YR to the ocean. Our data showed that gypsum-derived sulfate controls the δ34SSO4 of the YR and its contribution increased from 30% to 90% downstream with mean values of 71%, 84%, and 87% for the upstream, midstream, and downstream segments of the river, respectively. The annual sulfate flux to the ocean ranges from 0.026 Tmol/yr to 0.043 Tmol/yr with an average of 0.036 Tmol/yr in this century, and the average flux-weighted δ34SSO4 value was +8.5‰. Compared with the natural global riverine sulfate flux to the ocean, the YR contributes nearly 1.1% of the sulfate flux. This combined with its positive δ34SSO4 value makes this arid area an important contributor to the sulfate flux and the global sulfur cycle.

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Chemical Geology

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