Effect of damming on riverine strontium geochemical behavior: Evidence from 87Sr/86Sr analysis

Abstract

River damming alters biogeochemical cycles in river continuum. Yet, its effect on the biogeochemical behavior of riverine strontium (Sr) is still unclear. Here, we measured riverine 87Sr/86Sr in both dissolved and particulate phases and relevant parameters in karst cascade reservoirs of Southwest China, and simulated 87Sr/86Sr fractionation during biological processes through experimental incubation of model phytoplankton. The results showed that the dissolved 87Sr/86Sr was rather homogeneous across the water body and nearly identical to inorganic particulate-bound 87Sr/86Sr, and reservoir Sr mainly sourced from carbonate weathering. However, the dissolved Sr concentrations were stratified and increased with depths of the reservoir water columns. This stratification was likely caused by phytoplankton and the precipitation and dissolution of calcite, with the stratified strength controlled by reservoir hydraulic loads. A long-term loads along cascade reservoirs thus could result in a significant increase in dissolved Sr concentrations rather than 87Sr/86Sr. The culture experiment indicated that the dissolved Sr was massively captured by the phytoplankton during which insignificant 87Sr/86Sr fractionation occurred. Thus, the 87Sr/86Sr of reservoir phytoplankton would conserve the dissolved 87Sr/86Sr. The distinctly lower 87Sr/86Sr of phytoplankton than terrestrial organic particulates highlights its potential to distinguish autochthonous and allochthonous sources of reservoir particulate matter. This study demonstrated that damming largely alters the elemental and isotopic distribution of riverine Sr and would deepen the understanding of Sr biogeochemistry in dammed rivers.