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Woods Hole Oceanographic Institution

Tristan J. Horner

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Publications
»Ba-isotopic fractionation in seawater mediated by barite cycling and oceanic circulation
»Persistence of deeply sourced iron in the Pacific Ocean
»Cd-isotopic evidence for increasing primary productivity during the Late Permian anoxic event
»Constraints on the vital effect in coccolithophore and dinoflagellate calcite by oxygen isotopic modification of seawater
»Cadmium isotope variations in the Southern Ocean
»Nonspecific uptake and homeostasis drive the oceanic cadmium cycle
»A common reference material for cadmium isotope studies - NIST SRM 3108 Cd
»Isotopic fractionation of cadmium into calcite
»Natural and Anthropogenic Cd Isotope Variations
»Ferromanganese crusts as archives of deep water Cd isotope compositions


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Horner, T.J., C.W. Kinsley, and S.G. Nielsen, Barium-isotopic fractionation in seawater mediated by barite cycling and oceanic circulation, Earth Planet. Sci. Lett., in press

The marine biogeochemical cycle of Ba is thought to be controlled by particulate BaSO4 (barite) precipitation associated with the microbial oxidation of organic carbon and its subsequent dissolution in the BaSO4-undersaturated water column. Despite many of these processes being largely unique to Ba cycling, concentrations of Ba and Si in seawater exhibit a strong linear correlation. The reasons for this correlation are ambiguous, as are the depth ranges corresponding to the most active BaSO4 cycling and the intermediate sources of Ba to particulate BaSO4. Stable isotopic analyses of dissolved Ba in seawater should help address these issues, as Ba-isotopic compositions are predicted to be sensitive to the physical and biogeochemical process that cycle Ba. We report a new methodology for the determination of dissolved Ba-isotopic compositions in seawater and results from a 4,500 m depth profile in the South Atlantic at 39.99 °S, 0.92 °E that exhibit oceanographically-consistent variation with depth. These data reveal that water masses obtain their [Ba]- and Ba-isotopic signatures when at or near the surface, which relates to the cycling of marine BaSO4. The shallow origin of these signatures requires that the substantial Ba-isotopic variation in the bathypelagic region was inherited from when those deep waters were last ventilated. Indeed, the depth profile is well explained by the conservative mixing of water masses with distinct [Ba] and Ba-isotopic compositions. This leads us to conclude that large scale oceanic circulation is important for sustaining the similar oceanographic distributions of Ba and Si in the South Atlantic, and possibly elsewhere. These data demonstrate that the processes of organic carbon oxidation, BaSO4 cycling, and Ba-isotopic fractionation in seawater are closely coupled, such that Ba-isotopic analyses harbor great potential as a tracer of the carbon cycle in the modern and paleo-oceans.

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