Silicate Weathering and Diagenetic Reaction Balances in Deltaic Muds

Reactions between terrigenous sediments, marine-biogenic substances and seawater modulate multiple biogeochemical cycles, but the dynamics and factors governing these reactions are poorly constrained. Deltaic mobile muds are a major sedimentary facies along river-dominated ocean margins through which most terrigenous sediment transits and mixes with marine-biogenic matter, representing efficient and globally significant batch reactors. Here, we present a process-based model that combines equilibrium aqueous chemistry with kinetic concepts from sediment biogeochemistry and mineral sciences to explore the solution-mediated interplay of organic and inorganic matter alteration in episodically reworked deltaic muds. The model reproduces observed diagenetic conditions and product suites over the seasonal timescales relevant to deltaic systems and indicates a systematic and dynamic coupling between the sedimentary cycles of H⁺, C, P, Fe, S, Si, Mg, K, and Ca. We used the model in combination with published field observations and concepts of authigenic mineral occurrences to develop a generalized explanatory framework for silicate weathering fluxes and diagenetic reaction balances in marine sediments. Diagenetic silicate weathering is represented by a continuum of reaction balances with acid (reverse) and alkaline (forward) endmembers that is moderated by sediment sources, which determine the sediment’s weathering potential, and depositional environments, which govern the expression of this potential. Reverse weathering dominates in seasonally reworked, low-latitude deltaic muds, where green clays form rapidly from lateritic river sediments and biogenic silica under suboxic conditions. High mineral precipitation rates and protracted sediment remobilization drive large solute fluxes from/to these sediments. Net forward silicate weathering becomes more likely under steady, sustained anoxic conditions, particularly in volcanically-influenced settings and at minimal pre-weathering of sediment sources. These results further our understanding of the role silicate weathering and marine sediments play in global biogeochemistry and Earth system evolution, and can aid targeted ‘enhanced weathering’ strategies to environmental governance.