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 (δ¹³⁸Ba_DBa) 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 δ¹³⁸Ba_DBa distributions beyond low salinities, which are well predicted by an ion exchange model. Elevated δ¹³⁸Ba_DBa in the river endmember results from preferential removal of light Ba isotopes by adsorption to fluvial particles. Subsequently, δ¹³⁸Ba_DBa rapidly drops to minimum signatures at increased salinities indicating particle desorption of isotopically light Ba. Nevertheless, the apparently conservative δ¹³⁸Ba_DBa-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.