The Mo, Tl, and U isotope paleoredox proxies: comparisons, contrasts, and future directions

Abstract

Sedimentary molybdenum (Mo), thallium (Tl), and uranium (U) isotope ratios can potentially track global-scale changes to past ocean oxygenation levels. In this review we summarize and describe the unique strengths and weaknesses of these “paleoredox proxies”. We emphasize two critically important proxy requirements: (1) a unique redox sensitivity and (2) a reliable archive for tracking this sensitivity. The Mo isotope proxy does a poor job satisfying both requirements. Thallium isotopes are sensitive to seafloor Mn oxide burial. Multiple archives exist for tracking this sensitivity, the most reliable being mudrocks formed in open ocean settings under reducing conditions. Uranium isotopes are sensitive to U(VI) reduction. The conditions of reduction have important impacts on this sensitivity and need to be taken into consideration. Carbonates can be a viable archive for U isotopes, so long as offsets observed in modern environments are taken into account. Future studies should consider leveraging underappreciated controls on each isotope system such as water column depth and organic carbon burial. These relationships could help integrate metal isotope data into new geochemical models. Some amount of focus should also be placed on how metal isotope cycles might have operated differently in past oceans.