Geothermal effect on riverine molybdenum isotopes: Implications for its riverine input into the ocean

Molybdenum (Mo) isotopes preserved in marine sediments serve as a premier proxy for reconstructing Earth’s oxygenation history. However, their reliability depends on accurately deciphering the mechanisms driving spatiotemporal variations in Mo isotopes in river water—the ocean’s dominant Mo source. Chemical weathering of continental rocks is widely recognized as the primary control on δ98Mo values in the dissolved riverine flux. Nevertheless, continental geothermal inputs, which discharge into major river networks along tectonically active zones worldwide, may exert an additional, yet underexplored control on riverine Mo isotope signatures. This study presents the first systematic time-series investigation of geothermal impacts on riverine δ98Mo, utilizing the tectonically active Liuxihe River Basin (South China) as a natural laboratory. Our findings reveal that geothermal inputs along fault zones dramatically alter riverine Mo isotopes, depressing δ98Mo values from a weathering baseline of 0.99‰ to as low as −0.99‰, a 2‰ shift exceeding all previously documented riverine variations. Mixing models indicate that geothermal sources can contribute up to 56% of dissolved Mo in affected river segments. Given the ubiquity of continental geothermal systems along major plate boundaries and intracontinental fault zones, this previously unrecognized control may fundamentally transform our understanding of the global riverine Mo flux. These results carry significant implications for refining global Mo budgets to incorporate isotopically light continental geothermal inputs and for reinterpreting paleo-redox records by considering variable riverine end members rather than fixed crustal values.