ASSESSING POTENTIAL DIAGENETIC SIGNATURES ON VANADIUM AND THALLIUM ISOTOPES (Invited Presentation)

Vanadium (V) and thallium (Tl) isotopes are new metal isotope proxies that have the potential to track local to global redox conditions with different redox sensitivities. While elemental concentrations or enrichments are effective proxies to record significant environmental perturbations, they can, however, be affected by early diagenesis. Isotopic signatures of V and Tl may have the potential to record initial redox conditions and could be less susceptible to later overprinting. Our current understanding of V and Tl suggests that the modern marine isotopic signature is dominantly controlled by the fractionation and global (ferro-manganese – Fe-Mn) oxide burial flux. The burial of Fe-Mn oxides is directly tied to the local redox conditions in the water column and diagenetic sedimentary environments. Additionally, the potential for a Fe-Mn shuttle to overprint the overlying conditions has been suggested for other isotope systems in the modern and ancient ocean. A compressed redox ladder in which oxide reduction occurs near sulfide production has the potential to record the isotopically fractionated elements, thus providing a false positive signature for oxic conditions.

Recently published results suggest that V isotopes are fractionated during adsorption onto Fe-Mn oxides, while Tl isotopes are only fractionated by low-temperature Mn oxides. Core top studies of V isotope signatures from modern environments directly correlates with bottom water oxygen conditions with a progressive depletion under more oxic conditions – providing the ability to fingerprint low but non-zero oxygen. Thallium isotopes record oxic seawater conditions for regions that are permanently anoxic as Mn oxides are dissolved prior to the Tl sequestration into the sedimentary pyrite. This suggests that early diagenetic pyrite faithfully captures the overlying oxic seawater value. Neither V nor Tl record an isotopic signal for an Fe-Mn shuttle in the Black Sea, which suggests these systems are not as affected by this mechanism as Fe isotopes, but further research is required to fully assess this idea. To date, early diagenetic pyrite under low oxygen conditions does not record oxide isotope signatures but further research is required to fully assess this possibility. Lastly, Miocene Tl isotopes associated with phosphogenesis may be slightly fractionated from seawater but remain relatively close to the expected values even through multiple phases of reworking and diagenetic remobilization.