Barium isotope signatures of barite–fluid ion exchange in Equatorial Pacific sediments

Abstract

The isotope composition of barium (Ba) in the mineral barite (BaSO4) is emerging as a powerful tracer of the pelagic Ba, carbon, and sulfur cycles. However, it is critical to identify and constrain processes that may alter the primary isotope composition of Ba, δ138Ba, in BaSO4, particularly during early diagenesis. To this end, we analyzed the Ba isotope composition of porewaters and co-located BaSO4 in sediments from the Equatorial Pacific and performed a series of laboratory experiments with these same BaSO4 to assess rates of Ba isotope alteration. We find that sedimentary BaSO4 exhibit Ba isotope compositions ≈+0.1‰ and are offset by ≈−0.16‰ relative to ambient porewaters. Experiments using isotope-labeled seawater show extensive and rapid transfer of Ba ions between the solid and fluid phase through coupled BaSO4 dissolution-precipitation, even at chemical equilibrium. Using published values for Ba isotope fractionation during BaSO4 precipitation and dissolution, we calculate that co-located BaSO4 and porewaters should exhibit Ba isotope offset of −0.17‰ at isotopic equilibrium, similar to the offsets observed in Equatorial Pacific sediments. Altogether, the field data, laboratory experiments, and calculations indicate that ion exchange occurs in Equatorial Pacific sediments and that this process also drives the observed Ba isotope offsets between pelagic BaSO4 and porewaters. This finding implies that ion exchange may alter the isotope composition of Ba in sedimentary BaSO4, though the degree of alteration will depend on the proportion of Ba held in the solid phase. More broadly, this study provides an example of how ion-exchange-mediated processes are widespread in marine geochemistry and that these processes likely affect other metals and minerals beyond Ba and BaSO4.