长江河水^87Sr／^86Sr值的季节性变化及其指示意义：以长江大通站为例

Temporal Variation in Sr and ^(87)Sr/^(86)Sr of Yangtze River:An Example from Datong Hydrological Station

在长江下游大通水文站进行了为期1年的每2周1次的水样采样,分析测试其Sr含量及其同位素组成。结果表明,长江溶解Sr浓度在1.74~2.92μmol/L之间变化,87Sr/86Sr值的范围为0.710125~0.710965,河水Sr含量及同位素组成出现明显的季节性差异,表现为从洪水季节开始87Sr/86Sr值逐步升高,至12月达到最高值后缓慢下降并在下一个洪水季节到来时升高。研究认为,下游河水87Sr/86Sr值的季节性变化主要受流域降雨的时空变化所导致的物源相对贡献比例变化控制。受季风气候影响,当洪水期短期强降雨集中在上游地区时,强烈的风化使得上游贡献增多,致使下游河水Sr同位素组成迅速降低。采样期间(2011年1~5月)中下游的持久干旱致使中下游硅酸盐岩风化对河水Sr贡献减少从而造成河水同位素组成持续降低。同时,基于全年样品数据计算得出长江溶解Sr同位素组成的入海特征值为0.710628,Sr通量为1.9×109mol/a。研究结果表明河水Sr同位素组对流域极端气候的响应较好,可为今后使用历史时期物质Sr同位素研究解释极端气候变化奠定基础。

Temporal variation of dissolved 87^Sr/86^Sr in the Yangtze River is poorly understood compared to other Tibetan rivers. In this study, dissolved Sr and 87^Sr/86^Sr were measured from a temporal series of water samples collected biweekly at Datong Hydrological station over a period of one year. Our results show that Sr concentration in the Yangtze River ranges from 1.74 to 2.92 μmol/L with 87^Sr/86^Sr of 0. 710125 to 0. 710965. The Sr concentration and 87^Sr/86^Sr shows a distinct seasonal variation, with a general increase in 87^Sr/86^Sr ratios from summer to winter and some fluctuations during July and December, then followed by a gradually decrease till the next rainy season. The seasonal variation results from the variation of contributions from different sub-basin due to the spatially and seasonally variable rainfall across the basin. During the flood season, more contribution from upper reach （ low 87^Sr/86^Sr values） due to the strong rainfall decreases the 87^Sr/86^Sr ratio at lower reach. While the severe drought which happened in the middle-lower reaches （high 87^Sr/86^Sr values） from January to May explain the decrease in the later part of the data by the decrease of the contribution from middle-lower reaches. The discharge weighted annual 87^Sr/86^Sr and annual Sr flux of the Yangtze River based on the time series data are 0. 710628 and 1.9×10^9 mol/a, respectively. It was also indicated that dissolved 87^Sr/86^Sr in the Yangtze River is well correlated to the extreme climate events and might contribute to our explanation for reconstructing past climatic changes by using 87^Sr/86^Sr ratios of the sedimentary record in the delta.