Reassessing the dissolved molybdenum isotopic composition of ocean inputs: The effect of chemical weathering and groundwater

The abundance and isotopic composition of molybdenum (Mo) in marine sediments has been used to reconstruct the evolution of the ocean's oxidation-reduction (redox) potential. However, the utility of Mo as a redox tracer relies on quantifying the flux and isotopic composition of Mo delivered to the ocean, and understanding how these parameters may change as a function of the extent of chemical weathering in the terrestrial environment. Here we present Mo data from groundwater and river samples across the Hawaiian Islands where the duration of chemical weathering is well defined, and a complementary data set from rivers draining a variety of sedimentary, metamorphic, and igneous lithologies. Groundwater flow dominates Mo transport at the earliest stage of weathering, and there is little Mo isotopic fractionation from bedrock. As the duration of chemical weathering increases, rivers draining shallower flow paths become the main vector for water transport to the oceans. Molybdenum concentrations in these rivers are two orders of magnitude lower than in groundwater and show greater isotopic fractionation relative to bedrock. Data from rivers draining diverse sedimentary, metamorphic, and igneous lithologies show highly variable Mo isotopic signatures but consistently small Mo fluxes. We find that both groundwater and rivers with high Mo fluxes are isotopically similar to the average continental crust, suggesting that the range of Mo isotopic compositions delivered to the oceans is likely to be limited to a narrow range close to average crustal values.