Relationship between water discharge and sulfate sources of the yangtze river inferred from seasonal variations of sulfur and oxygen isotopic compositions

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Abstract

Resolving sulfate sources in river systems and thus to the global ocean is important for understanding the sulfur biogeochemical cycle. The Yangtze River is one of most important large rivers connecting land and sea, showing a trend of acidification characterized by persistently increasing SO<inf>4</inf><sup>2-</sup> concentration. A weekly sampling campaign from the middle Yangtze River in Wuhan was conducted, over a 1-year period in 2011, to systematically examine the effects of seasonality and associated water discharge on the balance of sulfate sources of the Yangtze River. The SO<inf>4</inf><sup>2-</sup> concentrations varied from 8.6 to 55.4mg/L with a discharge-weighted average of 30.0mg/L, while the SO<inf>4</inf><sup>2-</sup> fluxes varied from 103.2 to 1257.6kg/s with an annual average of 532.5kg/s. The δ<sup>34</sup>S<inf>SO4</inf> values ranged between 6.7‰ and 16.2‰ with a discharge-weighted average of 10.4‰, while the δ<sup>18</sup>O<inf>SO4</inf> values ranged between 1.6‰ and 10.5‰ with a discharge-weighted average of 5.1‰. In comparison to the low water discharge period, the period of high water discharge of the Yangtze River in Wuhan is characterized by 1) a doubling in water discharge with higher water level by 5-10m; 2) a concomitant doubling in SO<inf>4</inf><sup>2-</sup> flux without substantial change in SO<inf>4</inf><sup>2-</sup> concentration; 3) a systematic increase of δ<sup>34</sup>S<inf>SO4</inf> and δ<sup>18</sup>O<inf>SO4</inf> values; 4) a significant increase of sulfate contribution from evaporites dissolution. From the observed seasonal variation of sulfur and oxygen isotopic composition of sulfate, a relationship between the dynamic water discharge and dominant sulfate sources of the Yangtze River was inferred. During the period of low water discharge in January-mid May, the sulfur and oxygen isotopic compositions of sulfate had averaged values of 8.3±0.4‰ for δ<sup>34</sup>S<inf>SO4</inf> and 4.9±0.7‰ for δ<sup>18</sup>O<inf>SO4</inf>, suggesting the riverine SO<inf>4</inf><sup>2-</sup> of the Yangtze Riveris derived from sulfide oxidation, atmospheric deposition and evaporite dissolution; whereas during the period of low water discharge in December, the contribution of anthropogenic sulfate could be important, indicated by relatively higher δ<sup>34</sup>S<inf>SO4</inf> (10.5±1.5‰) and δ<sup>18</sup>O<inf>SO4</inf> (9.6±1.0‰) values. In contrast, during the period of high water discharge in June-September, the dual-isotope compositions were characterized by 14.7±0.7‰ for δ<sup>34</sup>S<inf>SO4</inf> and 8.1±0.7‰ for δ<sup>18</sup>O<inf>SO4</inf>, indicating sulfate from dissolution of evaporites would be a dominant source. The jump event of dual-isotopic composition between low and high water discharge occurred when a half-year drought suddenly turned into the flood season. Due to the water storage dispatch of the Three Gorge Dam, the Hanjiang River could make a greater impact on the variation of water discharge and thus sulfate sources of the Yangtze River during the transition from high to low water discharge in October-November. However, the relationship between water discharge and sulfate sources of the Yangtze River need s to be confirmed by additional years of observation. Further studies on the mechanisms of water discharge controlling sulfate sources of the Yangtze River is urgently required, as well as a mixing model to calculate the contribution of different sulfate sources.

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Li, X., Gan, Y., Zhou, A., & Liu, Y. (2015). Relationship between water discharge and sulfate sources of the yangtze river inferred from seasonal variations of sulfur and oxygen isotopic compositions. Journal of Geochemical Exploration, 153, 30–39. https://doi.org/10.1016/j.gexplo.2015.02.009

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