Reconciling the role of clay mineral formation on net alkalinity generation during rock weathering: Insights from reactive transport models of shale, granite, and basalt

Abstract. Clay minerals, while ubiquitous in soil, have wide-ranging and countervailing impacts on global biogeochemical cycles. Their compositional diversity makes predicting their formation conditions and impacts on soil solution chemistry a grand challenge. In this proof of concept study, we unpack the influence of clay mineral formation on alkalinity generation during rock weathering: a key process regulating atmospheric CO₂ levels over geologic time. Because they remove aqueous Si and Al from solution during formation, clay minerals may enhance silicate mineral dissolution and thereby enhance alkalinity generation. However, clays can also offset the alkalinity produced during silicate dissolution by generating acidity when they form. To identify whether, or to what extent, clay minerals enhance alkalinity generation, we employ reactive transport models of idealized granite, basalt, and shale weathering over a range of environmental conditions. Clay mineral formation typically increases net alkalinity generation by enabling greater silicate dissolution. This enhancement is most pronounced at low fluid flow rates and low soil CO₂ concentrations when thermodynamic conditions require clays for continued silicate dissolution and sustained alkalinity generation. However, clay-enhanced alkalinity generation diminishes at low erosion rates when primary mineral supply decreases; in less common instances where chlorite forms, clay formation can lead to a net alkalinity consumption. Overall, clay minerals promote CO₂ drawdown and modify the silicate weathering feedback.