The Re/Mo redox proxy reconsidered

Abstract

Compared to continental crust, marine sediments accumulating beneath O2 depleted waters are enriched in Re and Mo, with Re/Mo ratios that vary by more than two orders of magnitude, a remarkably large range. Deposition of both elements begins simultaneously, and both come to finite aqueous concentration asymptotes together, neither being drawn down to analytical detection limits. To explain these aqueous-phase observations and to reconcile them with laboratory evidence, a previous coprecipitation hypothesis is further developed here. The proposed coprecipitate is a nanoscale sulfide, Fe(Mo,Re)S4 that transforms irreversibly by internal electron transfer to a product represented as Fe(Mo,Re)(S2)S2. Irreversibility preserves information about redox conditions at the time of sedimentation unless sedimentary rocks later become subjected to oxidation or thermal maturation. Compelling support for this mechanism comes from the continuously euxinic Black Sea. A Rayleigh fractionation process characterized by a partition coefficient of 0.38 explains Re/Mo ratios in both its water column and its sediments. On the other hand, in sediments where overlying waters range from oxic to intermittently euxinic, and where Re and Mo deposition occurs only beneath the sediment–water interface, the observed >100-fold range of Re/Mo ratios is too large to be explained solely by Rayleigh fractionation. Instead, it is attributed to modification of pore waters by decomposition of unstable precursory phases. Low solid phase Re/Mo ratios are produced by pore waters modified by dissolution of Mo-rich, Mn,Fe oxyhydroxides; high ratios reflect dissolution of the so-far uncharacterized Re-rich particles that are known to create dissolved Re anomalies in shallow pore waters. Rhenium often follows iodine in biology, suggesting that the Re-rich particles could be biogenic and might participate in the marine iodine cycle. A significant correlation between particulate Re/Mo ratios and biogenic CaCO3 in the Cariaco Basin water column suggests that some of these Re-rich particles could be unstable biocalcification precursors like vaterite and amorphous CaCO3. Compared to δ98Mo, another redox proxy, Re/Mo ratios provide more nuanced redox information, especially under conditions that fall between oxic and sulfidic extremes. Thus Re/Mo ratios are potentially advantageous redox proxies that so far have been underutilized.