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Microbial processes within the ocean crust are of potential importance in controlling rates of chemical reactions and thereby affecting chemical exchange between the oceans and lithosphere. We here assess the oxidation state of altered ocean crust and estimate the magnitude of microbial biomass production that might be supported by oxidative and nonoxidative alteration. Compilations of Fe₂O₃, FeO, and S concentrations from DSDP/ODP drill core samples representing upper basaltic ocean crust suggest that Fe³⁺/ΣFe increases from 0.15 ± 0.05 to 0.45 ± 0.15 within the first 10–20 Myr of crustal evolution. Within the same time frame 70 ± 25% of primary sulfides in basalt are oxidized. With an annual production of 4.0 ± 1.8 × 10¹⁵ g of upper (500 ± 200 m) crust and average initial concentrations of 8.0 ± 1.3 wt% Fe and 0.125 ± 0.020 wt% S, we estimate annual oxidation rates of 1.7 ± 1.2 × 10¹² mol Fe and 1.1 ± …