Incongruent chemical weathering and adsorption of neodymium drive modulation of neodymium isotope composition of global riverine particulate matter

We report incongruent weathering and differential release of radiogenic neodymium (143Nd) to challenge the widely accepted congruent behaviour of Nd isotopes during weathering of rocks. Investigating two basaltic weathering profiles of the Rajmahal Volcanic Province in India, the incongruent behaviour of minerals and Nd isotopes were established. These results contradict the premise of invariant nature of 143Nd/144Nd (expressed as εNd ) during surface processes, the basis for extensive application of εNd in geological and geochemical studies. We also demonstrate that the εNd of Nd released to the weathering solution is reliably captured by the oxyhydroxide phases of the weathering profiles. Combining these two observations, we hypothesize that riverine particulate εNd is sensitive to adsorption of dissolved Nd and would be regulated by the duration of water-particle interaction during transport. Therefore, we predict that particulate εNd would be different between the high and low discharge periods in a river catchment. Utilising published results of fourteen rivers from across the world, which cover multiple lithologies and climatic regimes, we demonstrate that differential release of Nd isotopes via incongruent weathering and Nd adsorption collectively modulate particulate εNd through the length and duration of river transport. An important implication of this study is that the εNd offset between the leachates and detrital phases of marine sediments can be a robust tracer for investigating continental weathering and transport during glacial-interglacial periods.