Earth and Planetary Science Letters 119 (1993) 85-101. doi:10.1016/0012-821X(93)90008-WReceived by publisher: 1992-08-18Harvest Date: 2016-01-04 12:19:37DOI: 10.1016/0012-821X(93)90008-WPage Range: 85-10

No abstract

Sediment-covered basalt on the flanks of mid-ocean ridges constitutes most of Earth's oceanic crust, but the composition and metabolic function of its microbial ecosystem are largely unknown. By drilling into 3.5-million-year-old subseafloor basalt, we demonstrated the presence of methane-and sulfurcycling microbes on the eastern flank of the Juan de Fuca Ridge. Depth horizons with functional genes indicative of methane-cycling and sulfate-reducing microorganisms are enriched in solid-phase sulfur and total organic carbon, host delta C-13-and delta S-34-isotopic values with a biological imprint, and show clear signs of microbial activity when incubated in the laboratory. Downcore changes in carbon and sulfur cycling show discrete geochemical intervals with chemoautotrophic delta C-13 signatures locally attenuated by heterotrophic metabolism. Main text Subseafloor basaltic crust represents the largest habitable zone by volume on Earth (1). Chemical reactions of basalt with seawater flowing through fractures release energy that may support chemosynthetic communities. Microbes exploiting these reactions are known from basalt exposed at the seafloor, where the oxidation of reduced sulfur (S) and iron (Fe) from basalt with dissolved oxygen and nitrate from seawater supports high microbial biomass and diversity (2, 3). Multiple lines of indirect evidence that include textural alterations (4), depletions in δ 34 S-pyrite (FeS 2) (5) and δ 13 C-dissolved inorganic carbon (DIC) (6), and DNA sequences from

In this study, we report on coupled Fe-and S-isotope systematics of hydrothermal fluids and sulfide deposits from the East Pacific Rise at 9-10°N to better constrain processes affecting Feisotope fractionation in hydrothermal environments. We aim to address three fundamental questions: (1) is there significant Fe isotope fractionation during sulfide precipitation? (2) Is there significant variability of Fe-isotope composition of the hydrothermal fluids reflecting sulfide precipitation in subsurface environments? (3) Are there any systematics between Fe-and Sisotopes in sulfide minerals? The results show that chalcopyrite, precipitating in the interior wall of a hydrothermal chimney displays a limited range of δ 56 Fe values and δ 34 S values, between-0.11 to-0.33‰ and 2.2 to 2.6‰ respectively. The δ 56 Fe values are, on average, slightly higher by 0.14‰ relative to coeval vent fluid composition while δ 34 S values suggest significant S-isotope fractionation (-0.6±0.2‰) during chalcopyrite precipitation. In contrast, systematically lower δ 56 Fe and δ 34 S values relative to hydrothermal fluids, by up to 0.91‰ and 2.0‰ respectively, are observed in pyrite and marcasite precipitating in the interior of active chimneys. These results suggest isotope disequilibrium in both Fe-and S-isotopes due to S-isotopic exchange between hydrothermal H 2 S and seawater SO 4 2followed by rapid formation of pyrite from FeS precursors, thus preserving the effects of a strong kinetic Fe-isotope fractionation during FeS precipitation. In contrast, δ 56 Fe and δ 34 S values of pyrite from inactive massive sulfides, which show evidence of extensive late-stage reworking, are essentially similar to the hydrothermal fluids. Multiple stages of remineralization of ancient chimney deposits at the seafloor appear to produce minimal Feisotope fractionation. Similar affects are indicated during subsurface sulfide precipitation as demonstrated by the lack of systematic differences between δ 56 Fe values in both hightemperature, Fe-rich black smokers and lower temperature, Fe-depleted vents.