FINESST: Bridging the O2 Gap: Using Rhenium Isotopes to Detect Low O2 in Ancient

FINESST: Bridging the O2 Gap: Using Rhenium Isotopes to Detect Low O2 in Ancient Bridging the O2 Gap: Using Rhenium Isotopes to Detect Low O2 in Ancient Oceans Geochemical data from ancient marine sedimentary rocks indicate that many major steps in the evolution of life are tied to changes in the oxygenation of the oceans and atmosphere (e.g., Lyons et al., 2014; Fig. 1). There are geochemical indicators of ocean-basin-scale oxygenated and anoxic (0 uM O2) conditions, but currently there is not a global geochemical tool to detect when large parts of the oceans are in an intermediate-leaning-toward-anoxic condition termed suboxic (=10 uM O2). Detecting suboxic conditions is particularly important because some aerobic organisms can live in extremely low-O2 waters (down to ~10 nM O2; Stolper et al. 2010), and so it is of great interest to know when large parts of the ocean first crossed from anoxic to suboxic. Rhenium (Re) isotopes (d187Re) measured in marine sedimentary rocks are a novel global paleoredox tool that has the potential to be the first proxy that can detect suboxic conditions. The ability for Re to track suboxic conditions comes from its unique geochemical behavior. Under oxic conditions, Re exists primarily as perrhenate (ReVIIO4-), an ion that is highly unreactive and therefore accumulates in seawater (e.g., Koide et al., 1986; Colodner et al., 1993). In low-O2 conditions, ReVII is reduced to ReIV, leading to efficient removal coupled to organic carbon burial (Colodner et al., 1993,1995; Kendall et al., 2010; Morford et al., 2012). Rhenium has two stable isotopes, 187Re and 185Re. Preliminary data indicate that the light isotope is preferentially reduced, causing a shift in the isotopic composition of the residual Re remaining in seawater (Miller et al., 2015). As a result, d187Re variations in ancient sediments that sample the isotope composition of seawater could provide a sensitive tool to track the global extent of suboxic seafloor through time. I propose to develop the Re isotope proxy to track changes in the extent of suboxic conditions in the ocean. This proxy can be applied to any time in Earth history during which major changes in oceanic oxygenation occurred.