Effect of calcite precipitation on stable strontium isotopic compositions: Insights from riverine and pool waters in a karst cave

Radiogenic and stable Sr isotope ratios (87Sr/86Sr, δ88/86Sr) serve as potential tracers for evaluation of weathering sources and associated physicochemical processes in terrestrial environments. Despite carbonate weathering sourcing two-thirds of the total continental Sr flux to the ocean, however, few attempts have been aimed at studying stable Sr isotope fractionation in carbonate-dominated aquifers. Here we present a monthly-resolved monitoring dataset of major ion concentrations and Sr isotope ratios for karst riverine and pool waters at the Luofang Cave in the Central China to understand the potential controls responsible for water δ88/86Sr compositions under various climatic conditions. With the exception of some pool waters in the dry period, significantly heavier δ88/86Sr were detected in the riverine and pool waters (0.20‰ to 0.42‰) compared to bedrock δ88/86Sr (0.24‰ to 0.28‰), which can't be interpreted solely by lithology. A sequential leaching experiment of the local soils suggests limited stable Sr isotope fractionation during incongruent silicate mineral dissolution. Local plant utilization and biological activity in the soil overlying the cave are possible processes for causing the observed heavier δ88/86Sr in cave water. However, air partial pressure of CO2 in cave, monitoring water pH, and Sr isotopic mass balance calculations suggest that plant utilization is unlikely to be the primary control on the heavier water δ88/86Sr. On the other hand, significant stable Sr isotope fractionation was found between the cave dripping waters and calcite precipitates, and the Δ88/86Srcar-aq was estimated to be − 0.15‰ ± 0.07‰ (2SD). Precipitation of secondary calcites might be a potential process responsible for the heavier δ88/86Sr in the waters. The negative correlation between δ88/86Sr and Sr/Na ratios in the cave waters is consistent with a scenario of preferential incorporation of the light Sr isotope by solid phase. Furthermore, by combining the calcite saturation states and water pH, our results suggest that calcite precipitation is the most likely process controlling δ88/86Sr compositions in cave water. We demonstrate that secondary calcite precipitation in carbonate-dominated catchments could be important in controlling the riverine water δ88/86Sr compositions.