Stable Barium Isotope Dynamics During Estuarine Mixing

Stable barium isotopes are a potential proxy for riverine inputs into the ocean that reflect monsoon variability and climate change. However, dissolved Ba isotope (δ138BaDBa) geochemistry in river estuaries, a dynamic land to ocean transition zone, has rarely been systematically examined to date. Here, we show that significant Ba isotope fractionation occurs at near‐zero salinities in the Yangtze and Pearl River Estuary, whereas conservative mixing dominates δ138BaDBa distributions beyond low salinities, which are well predicted by an ion exchange model. Elevated δ138BaDBa in the river endmember results from preferential removal of light Ba isotopes by adsorption to fluvial particles. Subsequently, δ138BaDBa rapidly drops to minimum signatures at increased salinities indicating particle desorption of isotopically light Ba. Nevertheless, the apparently conservative δ138BaDBa‐salinity relationship beyond the low‐salinity minimum in both estuaries provides a modern calibration for using Ba isotopes as a proxy for paleosalinity and river water inputs into the ocean. Plain Language Summary Barium is a nutrient‐type trace element in the ocean and has been widely used for tracing circulation, biological productivity, and land‐ocean interactions. Riverine input is the largest source of oceanic Ba but the Ba properties of this input can be altered during transport along river estuaries. The recent development of stable Ba isotope measurements provides a novel tool to investigate the geochemical behavior of Ba during estuarine mixing between river water and seawater. We thus systematically investigated the dissolved Ba isotope distributions along the entire salinity gradient in two Asian large river estuaries. We found that significant Ba isotope fractionation occurs at near‐zero salinities in the Yangtze and Pearl River Estuary, which mainly results from particle adsorption‐desorption processes. This fractionation substantially alters the original Ba properties of the river water endmember and significantly affects the global ocean Ba mass balance. Beyond low salinities of both estuaries, however, Ba isotopes and salinity show a conservative relationship predominantly controlled by water mass mixing. This is an important calibration for using stable Ba isotopes as a proxy for past sea surface salinity under the influence of large river plumes, which reflects the variability of precipitation and the impact of climate change. Key Points We present dissolved Ba isotop