Barium isotope cycling in the marine environment: Pathways of fractionation and implications for paleoceanographic applications

Abstract

Removal of particulate organic carbon (POC) from sunlit surface waters into the deep ocean represents a climatically important sink of atmospheric carbon dioxide (CO₂), linking the biogeochemical cycling of POC to CO₂-driven climate change. As POC is not well preserved in the sediment record, other proxies, including the chemistry of barium (Ba) in the ocean and through the sedimentary record, offer an avenue to investigate oceanic carbon export through Earth’s history. This thesis seeks to constrain the controls on the formation, cycling, and isotopic signature of the main particulate phase of marine barium, the mineral barite (BaSO₄) through its inception in the water column, during deposition, and ultimately into the rock record. To that end, I characterize the depth, spatial region, and general controls on particulate Ba formation in the South Pacific Ocean through shipboard experimentation and find that particulate Ba forms mainly in the surface of the Polar Frontal Zone in the presence of large particles and microbial activity. Next, I characterize the effect of ion exchange on BaSO₄, a process previously unstudied under marine conditions, in a laboratory setting. Ion exchange occurs rapidly between dissolved Ba and BaSO₄ and imparts a characteristic net offset between the Ba isotope composition of the dissolved and solid phase, which arises through a combination of Ba isotope fractionation during both precipitation and dissolution. Finally, I investigate the role of ion exchange in marine settings using co-located pore fluids and sedimented BaSO₄. Modeling of natural samples produce results that are consistent with the laboratory study, suggesting that this mode of isotopic fractionation impacts Ba isotopes in the environment and must be accounted for when applying Ba based climate proxies.