Submarine weathering reactions associated with cation-rich (‘forward’) and cation-poor (‘reverse’) materials are significant, yet poorly understood parts of marine biogeochemical cycles, especially in coastal zones, where abundant lithogenic material is delivered by rivers and mixes with marine biogenic particles. Because of their different reaction stoichiometry, forward and reverse weathering processes have opposing effects on inorganic carbon, alkalinity and major element cycling in the ocean. The dominance of either forward or reverse weathering processes has been demonstrated to vary at different coastal marine sites, fueling speculation about controls of rates and stoichiometry of these diagenetic weathering reactions. We constructed a simple kinetic box model of coupled organic and inorganic particle diagenesis in river-influenced coastal systems to disentangle effects of (1) pH-Eh regimes, driven by prevalent microbial pathways of organic matter decay, and of (2) mineralogical variability of sediment substrates, i.e., the mixture of different terrestrial and marine inputs. In theory, highly pre-weathered and biogenic silica-rich substrates altering in suboxic settings dominated by microbial iron reduction favor solute and alkalinity consumption by reverse weathering. In contrast, alteration of cation-rich, volcanogenic and basaltic material in CO2-enriched oxic or methanogenic zones may rather drive a forward weathering process and alkalinity generation. Because the spatial distribution of both factors, sediment input and local redox processes, likely has changed during Earth’s history, substantial variability of differential element fluxes in submarine weathering processes would be expected, confounding our current understanding of weathering feedbacks in the Earth system.