RECONSTRUCTING REDOX CONDITIONS USING A MULTIPROXY METAL ISOTOPE APPROACH: A CASE STUDY FROM THE NEOPROTEROZOIC (Invited Presentation)

Reconstructing the paleoredox conditions throughout Earth’s history remains an important avenue to constrain Earth’s biochemical evolution. The causal relationship between oxygenation and biological evolution has still highly debated especially for the evolution of eukaryotic, early animal, proliferation of life, and mass extinction events. While a general broad first-order relationship is starting to emerge, there is a gap in proxy specificity to record non-sulfidic anoxic and low oxygen conditions. There have been intervals with seemingly mismatching redox records/interpretations for various events. A combined multiproxy approach may alleviate some of these issues by providing a more robust interpretation with additional constraints. Additionally, this approach may provide more redox specificity to the position on the redox ladder at a local to global record.

The Neoproterozoic black shale deposits are some of the first identical samples to have measured multiple redox-sensitive elements molybdenum (Mo), thallium (Tl), and vanadium (V) isotopes from multiple localities. Importantly, the local redox conditions are also constrained using Fe speciation. This provides a unique opportunity to compare and contrast these isotope proxies. Each of these isotope proxies has a shared, at least one, sink that is fractionated which is adsorption onto oxides. Thallium isotopes are only fractionated during adsorption to Mn oxides, Mo has differential Fe and Mn oxide fractionation and V remains unknown. Therefore, changes in the global burial of oxides should have a proportionate effect on all three systems although different magnitudes due to the associated isotopic effect. Conversely, each of the proxies has additional sink parameters that could affect the marine signature such as altered oceanic crust, hydrothermal recirculation, and/or variable source fluxes. There are potential local redox effects as well which could alter certain isotopes more than others (e.g. Mo). Lastly, these three isotope systems have variable residence times in the modern ocean which may affect the exact responses. Using the dynamics of these isotope systems will better constrain spatiotemporal oxygenation of the oceans and provide insights into the link between low oxygen redox conditions, and the emergence and evolution of early animals.