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

Julien Middleton, Ph.D., 2022
Tristan Horner, Advisor

Carbon export represents a climatically important sink of atmospheric carbon dioxide. As organic carbon is not well preserved in the sediment record, other proxies, including the chemistry of barium (Ba), allow investigation of carbon export through geologic time. This thesis seeks to understand controls on the formation, cycling, and isotopic signature of the main particulate phase of marine barium, the mineral barite (BaSO4). To that end, I characterize the depth, spatial region, and general controls on particulate Ba formation through shipboard experimentation and find that formation occurs in the surface ocean of the high latitudes in association with large particles and microbial activity. Next, I characterize the effect of ion exchange on BaSO4 a laboratory setting. Ion exchange occurs rapidly and imparts a characteristic offset between the Ba isotope composition of dissolved Ba and BaSO4, 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. Environmental findings are consistent with laboratory data, suggesting that this mode of isotopic fractionation impacts Ba isotopes in the environment and must be accounted for when applying Ba based climate proxies.