The global imprint of shale weathering on molybdenum isotope ratios in river waters

Abstract

The molybdenum (Mo) isotope ratios (δ⁹⁸Mo) of marine sediments can preserve information on the redox state of the past ocean. However, a robust interpretation of marine δ⁹⁸Mo records requires an understanding of the main controls on the δ⁹⁸Mo values of riverine inputs. A growing consensus suggests that secondary mineral formation following rock weathering sets the δ⁹⁸Mo values of the dissolved riverine flux. However, variability in the Mo isotope composition of the weathering lithologies, such as sedimentary rocks, might exert an additional control. Here we assemble a dataset for large rivers spanning a wide range of sulfate abundance as a broad tracer of sulfide oxidation, making paired measurements of river water and solid loads.

The riverine dissolved and solid Mo isotopes span a range of +0.3 to +1.9 ‰ and -0.1 to +1.4 ‰, respectively. Our results indicate that both source and process control the isotope composition of dissolved riverine Mo. First, the elemental and isotope partitioning of Mo between river dissolved and solid loads is indicative of the impact of the formation of secondary weathering products. Second, the positive relationship between the dissolved and solid Mo isotope signatures implies the variable weathering of an additional heavy Mo isotope source above and beyond silicate. Consistent with this, silicate weathering alone cannot explain the riverine Mo abundances, calling for an additional Mo-rich source. Comparison between riverine dissolved Mo isotopes and potential sulfide oxidation tracers indicates an important control by sulfide weathering on river dissolved Mo isotope signatures. In this view, the pattern of variation of dissolved Mo isotopes across different weathering regimes may be interpreted in terms of the greater supply-limitation control of sulfide versus silicate weathering at the global scale. Overall, these findings indicate that changes in sulfide oxidation rates on the continents could modify the δ⁹⁸Mo of the global average riverine input to the oceans over geological timescales.