Weathering intensity and lithium isotopes: A reactive transport perspective

Abstract

Lithium isotopes have emerged as a powerful tool to probe the response of global weathering to changes in climate. Due to the preferential incorporation of ⁶Li into clay minerals during chemical weathering, the isotope ratio δ⁷Li may be used to interrogate the balance of primary mineral dissolution and clay precipitation. This balance has been linked to relative rates of chemical and physical denudation, such that dissolved δ⁷Li (δ⁷Li_diss) is highest at moderate weathering intensities when chemical and physical denudation are comparable. However, we argue that current theory linking δ⁷Li to weathering regimes through fluid travel times are unable to explain observations of low δ⁷Li and high Li concentrations in rapidly eroding settings. In this study, we re-examine the relationships between δ⁷Li, Li concentration, and weathering regime by incorporating Li isotopes into simulations of weathering profiles using a reactive transport model (CrunchFlow) that includes advective fluxes of regolith to simulate variable erosion rates in response to uplift. In these simulations, fractionation is implemented through a kinetic fractionation factor during clay precipitation, which allows the δ⁷Li of dissolved and suspended loads in the model to vary as a function of Li/Al ratios in primary and secondary minerals. When the model is run over a range of infiltration and erosion rates, simulations reproduce observed global patterns of δ⁷Li_diss and suspended load δ⁷Li as a function of weathering intensity, controlled primarily by water travel times and mineral residence times in weathered bedrock. We find that reduced water travel times at low weathering intensity, however, are inconsistent with observations of high Li concentrations. As an alternative, we demonstrate how the rapid weathering of soluble, Li-rich minerals such as chlorite under low weathering intensities may resolve this apparent discrepancy between data and theory. We also suggest that observed patterns are consistent with geothermal Li sources under low weathering intensities. This work offers a foundation guiding future studies in testing potential mechanisms underlying global riverine δ⁷Li_diss.