Alkalinity and elemental cycles in present and past ocean: Insight from geochemical modeling and alkali and alkaline earth metal isotopes

This chapter provides an overview of near-surface geochemical processes operating on Earth, with special emphasis placed on (i) marine weathering such as alteration and dissolution of silicates, carbonates and terrigenous riverine particles in the ocean, complemented by (ii) reverse weathering reactions leading to marine authigenic clay formation, and the impact of these phenomena on ocean alkalinity budget and the chemical and isotope composition of seawater. Model simulations of the above processes provide estimates of the global marine fluxes of major cations (Na+, K+, Mg2+, Ca2+) and alkalinity in the ocean induced by silicate weathering and dissolution of terrigenous material in seawater. Additional constraints on silicate vs. carbonate weathering, oceanic/coastal CaCO3 cycling, and paleo-seawater reconstructions are provided via the stable and radiogenic isotope systems of alkali and alkaline earth metals (Li, K, Mg, Ca, and Sr isotopes) that are discussed within the context of marine and reverse weathering in the present and past ocean.