NSFGEO-NERC: Constraining the oxic marine sink of novel metal isotope proxies to underpin paleoceanographic reconstructions

The evolution of life is intimately linked to ocean chemistry. Throughout time, organisms have adapted to - and driven changes in - their marine chemical environments. One of the most profound examples from life's history was the proliferation of oxygenic photosynthesis. Higher oxygen levels were toxic to many existing organisms and changed how micronutrient metals cycle in the ocean, thus having direct and indirect effects on the evolutionary pathway of life. Reconstructing the timing and sequence of these changes in ocean biogeochemistry is vital to understanding the evolution of life and its dependence and effects on global climate over Earth history.
Because changes in ocean and atmosphere oxygenation control much of the ocean's biogeochemical evolution, it is imperative to establish proxies that can monitor these changes across wide ranges of redox (e.g. from highly reducing to very oxygenated). To understand redox patterns and associated effects on nutrient cycling and life evolution in the past, a burgeoning field is investigating novel metal isotope systems that are sensitive to - and can be used as proxies for - these changes. Of particular promise are the Mo, Tl, U and Zn isotope proxies, which each respond uniquely to changes in redox conditions. Records from ancient sediment and rock archives reveal significant and systematic variations in these isotopic systems that have been used to estimate changes in the extent of anoxic conditions in the global oceans.
Accurate interpretations of the proxy record mandate a detailed understanding of the isotopic compositions of every input and output in the seawater budget. Although it is often assumed that changes in the anoxic sink are the primary means to change the marine isotopic budgets of Mo, Tl, U and Zn, a significant portion of these elements are removed into the vast expanses of deep oxic seafloor. The deep-sea oxic sink of Mo, Tl, U and Zn in the ocean is mainly comprised of ferro-manganese (Fe-Mn) oxides in the form of coatings on abyssal pelagic red clays as well as a minor amount of pure Fe-Mn crusts and nodules. Large isotopic fractionations of these elements have been documented for crusts and nodules and most paleo-seawater studies assume that these fractionation factors are the same for the oxides in pelagic sediment. However, fractionation factors for abyssal pelagic sediment have never explicitly and systematically been analyzed. Preliminary data generated for this proposal suggests that fractionation factors are distinct between pelagic sediment and Fe-Mn crusts and nodules, suggesting isotope proxy data from ancient sediment archives may need to be re-interpreted accordingly.
Here, we propose to constrain the oxic sink of key paleoceanographic proxies by performing the first global-scale investigation of Mo, Tl, U and Zn isotopes in deep-sea oxic marine sediment. The isotope data will be compared with published seawater isotope compositions to determine robust isotope fractionation factors for each element during incorporation into diverse deep-sea oxic sediments. We will combine the results with estimates of global burial fluxes for these elements into the oxic sink, based on the decay of Th isotopes. Ultimately, the constraints provided by this project will become a benchmark for future research of novel metal isotopes and significantly improve quantitative estimates of past variations in ocean oxygenation and biogeochemistry