Introduction of isotopically light barium from the Rainbow hydrothermal system into the deep Atlantic Ocean

The marine barium (Ba) cycle is closely connected to the short-timescale carbon cycle, and Ba serves as a valuable paleo proxy for export production, ocean alkalinity, and terrestrial inputs. However, the marine Ba budget is poorly constrained, particularly regarding the fluxes of hydrothermally sourced Ba, which hinders our understanding of the Ba cycle and use of Ba-based proxies. Recent studies have suggested a modern source-sink imbalance of Ba isotopes in the global ocean, with sources being overall isotopically heavier than the sinks, and the hydrothermal Ba inputs were considered isotopically heavy sources. In this study, we present the first investigation of Ba and its isotopes in a non-buoyant hydrothermal plume based on dissolved and particulate samples collected from the Rainbow hydrothermal vent field on the Mid-Atlantic Ridge. Our data reveal strong hydrothermal signals at near-field stations, as evidenced by helium isotopes, accompanied by elevated concentrations of dissolved and particulate Ba. Dissolved Ba isotope compositions (δ¹³⁸Ba) in hydrothermally influenced deep waters (∼0.3 %₀) are lighter than at similar depths of far-field stations (∼0.45 %₀) in the Atlantic Ocean. The concentrations and isotopic compositions of dissolved and labile particulate Ba in the non-buoyant hydrothermal plume can be explained by conservative mixing between a Ba-enriched hydrothermal component and North Atlantic Deep Water. By extrapolating the correlations to the vent fluid endmember, our results suggest that there is negligible removal of Ba, and insignificant modification of Ba isotopic signatures, from the vent fluid endmember to the non-buoyant hydrothermal plume. This indicates that the Rainbow hydrothermal system introduces isotopically light Ba (−0.17 ± 0.05 %₀) to the deep Atlantic Ocean. We estimate that global hydrothermal inputs of Ba are 4.6 ± 2.2 Gmol/yr. These observations highlight the potential of hydrothermal Ba to be an isotopically light source component of the marine Ba isotope budget.