Farkas, Juraj, Wallmann, Klaus , Mosley, Luke, Staudigel, Philip, Zheng, Xin-Yuan, Leyden, Emily, Shao, Yuexiao, Fryda, Jiri, Holmden, Chris and Eisenhauer, Anton (2024) Alkalinity and elemental cycles in present and past ocean: Insight from geochemical modeling and alkali and alkaline earth metal isotopes. In: Reference Module in Earth Systems and Environmental Sciences. . Elsevier, Rotterdam, pp. 1-55. DOI 10.1016/B978-0-323-99762-1.00037-1.

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Abstract

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.

Key points

• Impact of weathering processes on marine elemental cycles and the ocean alkalinity budget.
• Alteration and dissolution of silicate minerals and riverine particles in the ocean quantified via thermodynamic equilibrium (PHREEQC) calculations, in seawater and top sediment settings.
• Estimates of global ocean fluxes of dissolved cations (Na+ , K+ , Mg 2+ , Ca2+ ) and alkalinity induced by alteration and dissolution of terrigenous material in seawater and marine sediments.
• Principles and mechanisms of isotope variability in nature (mass-dependent and radiogenic isotope effects) observed for
alkali and alkaline earth metals.
• Silicate vs. carbonate weathering and coastal carbon/carbonate cycling constrained via stable and radiogenic Ca and Sr, and Li isotopes.
• Oceanic processes, marine carbonate chemistry (alkalinization vs. acidification), and paleo-seawater reconstructions constrained via d44 Ca, d88
Sr, d26 Mg proxies and numerical (MATLAB) modeling.
• Emerging metal isotope proxies (d41
K) for silicate and reverse weathering in the ocean.

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