PROBING CONTINENTAL WEATHERING AT GLOBAL SCALE USING LI, MG AND SI ISOTOPES IN FLUVIALSEDIMENTS

This study reports Li and Mg elemental and isotope compositions of the clay- and silt-size fraction of sediments from major rivers worldwide, which, together with previously published data for Si isotopes, are used in order to re-evaluate their potential as silicate weathering proxies at basin and global scale.

Amongst the three isotopic systems, Li isotopes display the largest variations and differences between clays and silts, while no significant size-dependent differences are observed with Mg isotopes. World river clays display significant and correlated enrichments in Li and Mg compared to silts, indicating higher abundances than the upper continental crust (UCC), and consistent with preferential incorporation of Li and Mg into neoformed secondary phases.

In agreement with recent works, we show that a provenance proxy is required for deconvolving the weathering signal from measured clay d⁷Li values, using either Nd isotopes or average estimates for bedrock d⁷Li compositions. A methodology is proposed to quantify the dissolution / neoformation rate ratio (i.e. the weathering incongruency degree).

In contrast with the distribution of Si isotope ratios (d³⁰Si) in fluvial clays, which display correlations with environmental parameters, the weathering signal extracted from Li isotopes is found to be independent from both clay mineralogy and climate. This finding contrasts with previous inferences of a link between the evolution of clay-bound d⁷Li and climate change during the Late Quaternary.

Using our dataset, we provide the first global estimates for average Si, Mg and Li isotope compositions and concentrations in the clay- and silt-size fractions of fluvial sediments entering the ocean. When compared to the UCC, our estimates indicate systematic imbalance away from steady-state caused by an excess of elemental dissolved fluxes. Whether this excess is caused by enhanced mineral dissolution rates following the last deglaciation period or by potential anthropogenic contamination remains to be elucidated.