Tracing Sr isotopic composition in space and time across the Yangtze River basin

Abstract

Variation of dissolved 87Sr/86Sr in the Yangtze River is poorly documented compared to other Tibetan-sourced rivers. Here, we trace the Sr isotopic composition in space and time across the Yangtze River basin using a systematic sampling strategy. The 87Sr/86Sr values of samples collected at different depths within the water column at three gauge stations located in the upper, middle and lower reaches of the river are very similar, indicating a well-mixed water body at a given location. Data from basin wide-samples shows low 87Sr/86Sr values in the upper reaches while higher 87Sr/86Sr values in the middle-lower reaches which mainly reflect the controls of source rocks. A time series record at the lowermost reaches of the river indicates a temporal variation of 87Sr/86Sr values from 0.7101 to 0.7109, with a discharge weighted value of 0.7106. We suggest that temporal variations of the isotopic composition result from changes in the relative contributions from different terrains associated with spatial variability of rainfall within the basin. A mixing model based on Sr values deduced from basin-wide sampling from different seasons yields similar temporal variations of Sr isotopic composition compared to experimental data. Calculation of Sr fluxes indicates that the upper reaches contribute 3 times as much as the middle and lower reaches to total Sr flux. The annual Sr flux of the Yangtze River to the East China Sea is estimated to be 1.9×109mol·a−1, which is one magnitude higher than that of the Brahmaputra and Ganges.