Iron and sulfide oxidation within the basaltic ocean crust: implications for chemolithoautotrophic microbial biomass production

Abstract

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 Fe 2O 3, FeO, and S concentrations from DSDP/ODP drill core samples representing upper basaltic ocean crust suggest that Fe 3+/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 × 10 15 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 × 10 12 mol Fe and 1.1 ± 0.7 × 10 11 mol S. We estimate that 50% of Fe oxidation may be attributed to hydrolysis, producing 4.5 ± 3.0 × 10 11 mol H 2/yr. Thermodynamic and bioenergetic calculations were used to estimate the potential chemolithoautotrophic microbial biomass production within ridge flanks. Combined, aerobic and anaerobic Fe and S oxidation may support production of up to 48 ± 21 × 10 10 g cellular carbon (C). Hydrogen-consuming reactions may support production of a similar or larger microbial biomass if iron reduction, nitrate reduction, or hydrogen oxidation by O 2(aq) are the prevailing metabolic reactions. If autotrophic sulfate reduction or methanogenesis prevail, the potential biomass production is 9 ± 7 × 10 10 g C/yr and 3 ± 2 × 10 10 g C/yr, respectively. Combined primary biomass production of up to ∼1 × 10 12 g C/yr may be similar to that fueled by anaerobic oxidation of organic matter in deep-seated heterotrophic systems. These estimates suggest that water-rock reactions may support significant microbial life within ridge flank hydrothermal systems, These estimates suggest that water-rock reactions may support significant microbial life within ridge flank hydrothermal systems.