Presenter: Rachel Phillips (University of Texas at El Paso)

Description:
Thallium (Tl) isotope ratios are promising tracers of marine oxygenation as seawater Tl isotopic composition is primarily controlled by manganese (Mn) oxide burial, which in turn requires free oxygen to form in the water column. However, previous studies have hypothesized that only hexagonal birnessite, which dominates hydrogenetic ferromanganese deposits, has the capacity to fractionate Tl isotopes, whereas other mineralogical forms of Mn oxide, such as todorokite and triclinic birnessite, do not. These two minerals are common in hydrothermal and diagenetic marine Mn oxide deposits. Constraining the underlying mechanisms and magnitudes of Tl isotopic fractionation is, therefore, critical to place robust constraints on the impact of different Mn oxide minerals on the marine Tl isotope composition. Here, we experimentally investigate Tl sorption behavior on triclinic birnessite and todorokite in artificial seawater at a range of Tl concentrations and time intervals. Both triclinic birnessite and todorokite exhibit relatively weak Tl sorption with minimal isotopic fractionation, consistent with previous hypotheses. X-ray absorption near-edge structure (XANES) results suggest no change in Mn oxidation state in either mineral upon Tl sorption. Despite the lack of Tl isotope fractionation and Mn oxidation state change, X-ray diffraction (XRD) results suggest that Tl sorption affects the crystal structure of both minerals, especially the increased interlayer distance of triclinic birnessite due to Tl sorption. Overall, our results indicate a strong mineralogical control on Tl sorption behavior. Particularly, hydrothermal and diagenetic Mn oxides have negligible effects on Tl, emphasizing the role of hydrogenetic ferromanganese deposits in controlling the global marine Tl budget.