Water column barium sulfate dissolution and shielding by organic matter aggregates: Implications for the pelagic barite proxy

Pelagic barite (BaSO4) and related proxies are useful tools for reconstructing the marine carbon cycle. The factors controlling pelagic barite dissolution in the ocean water column are poorly understood, which adds uncertainty to Ba-based reconstructions. Here, we conducted static laboratory incubations to test the sensitivity of barium sulfate dissolution rate to a range of commonly occurring seawater pH, salinity, and temperature conditions. We observed relatively rapid dissolution rates ranging from 1.7 ± 0.4 to 3.4 ± 0.8 pg BaSO4 day−1 for these experiments, and we did not observe statistically significant differences in the rate of dissolution with varying pH, salinity, or temperature. The slowest dissolution rate observed in these experiments suggests that an average barium sulfate crystal would survive in the ocean water column just 6.2 ± 0.3 days. We estimate that an average isolated pelagic barite crystal would take 67 years to sink down through the water column, so our experiments imply that solitary pelagic barite crystals do not survive this transit. We conducted an additional experiment on a roller table to assess the impact of organic matter aggregates on barium sulfate dissolution. Free barium sulfate crystals incubated on the roller table dissolved even more rapidly than crystals in the static experiment (19 ± 7 pg BaSO4 day−1), but barium sulfate crystals incubated with organic matter aggregates showed little sign of dissolution over time. Our findings suggest that organic matter aggregates play a vital role in shielding pelagic barite from dissolution in the water column. This implies that pelagic barite in ocean sediments records the arrival of organic detritus to the seafloor, not just barite crystal formation in sinking organic matter in the upper water column. Additional work is needed to determine which aspects of the marine carbon cycle the pelagic barite proxy captures.