MOLYBDENUM ISOTOPE RECORDS AS A POTENTIAL NEW PROXY FOR PALEOCEANOGRAPHY

Abstract

New high-precision isotope ratios of dissolved Mo in seawater from different ocean basins and depths show a homogeneous isotope composition (‘mean ocean water 98Mo/95Mo’ (MOMO)), as expected from its long ocean residence time (800 kyr). This composition appears to have been constant for the past 60 Myr at a 1–3 Myr time resolution as indicated from thick sections of Fe–Mn crusts from the Atlantic and Pacific. These records yield a constant offset from MOMO (average of −3.1 and −2.9‰). They are similar to our new data on recent oxic Mo sinks: pelagic sediments and six Fe–Mn crust surface layers range from −2.7 to −2.9‰ and −2.7 to −3.1‰, respectively. Recent suboxic Mo sinks from open ocean basins display heavier and more variable isotope ratios (−0.7 to −1.6‰ relative to MOMO). Crustal Mo sources were characterized by measuring two granites (and a mild acid leach of one granite), seven volcanic rocks and two clastic sediments. All show a narrow range of compositions (−2.0 to −2.3‰). These data indicate that isotope fractionation by chemical weathering and magmatic processes is insignificant on a global scale. They therefore represent good estimates of the composition of dissolved Mo input to the oceans and that of the average continental crust. Thus, the Mo input into the oceans appears to be distributed into lighter oxic sinks and heavier reducing sinks. This is consistent with steady-state conditions in the modern ocean. The constant isotope offset between oxic sediments and seawater suggests that the relative amounts of oxic and reducing Mo removal fluxes have not varied by more than 10% over the last 60 Myr. An equilibrium fractionation process is proposed assuming that Mo isotope fractionation occurs between (dominant) MoO42− and (minor) Mo(OH)6 species in solution, of which the latter is preferentially scavenged.