The Role of Manganese Oxide Mineralogy in Thallium Isotopic Fractionation Upon Sorption

Thallium (Tl) isotopes are used to reconstruct changes in global ocean oxygenation by tracking changes in past sedimentary manganese (Mn) oxide burial. This utility is born out of the large positive Tl isotope fractionation associated with sorption onto Mn oxides. While it is understood that interactions between Tl and Mn oxides are mineral-specific, the associated effects on Tl isotope fractionation are largely unexplored. Here, we experimentally investigate Tl isotopic fractionation during sorption to the two Mn oxides triclinic birnessite and todorokite in artificial seawater at a range of Tl concentrations and reaction durations. Both triclinic birnessite and todorokite exhibit relatively weak Tl sorption with minimal isotopic fractionation, with fractionation factors of equilibrated samples ranging from 0.99999 to 1.00010 (average -1.000040 & PLUSMN; 0.00003, 1sd) and 1.00002 to 1.00005 (average -1.000032 & PLUSMN; 0.00001, 1sd), respectively. X-ray absorption spectroscopy analyses imply no change in Mn oxidation state in either mineral upon Tl sorption. Despite the lack of Tl isotopic fractionation and Mn oxidation state change, X-ray diffraction and Raman spectroscopy spectra of triclinic birnessite suggest that Tl sorption caused a partial transformation of the layered structure to a tunnel-structured Mn oxide similar to todorokite. Overall, our results indicate that Tl sorption onto Mn oxides and associated isotopic fractionation is strongly controlled by mineralogy. Hydrogenetic Mn oxide-rich sediments dominated by hexagonal birnessite likely play a much more important role in seawater Tl isotope mass-balance than do sediments dominated by hydrothermal and diagenetic Mn oxides that are comprised largely of triclinic birnessites and todorokite.