Biogeochemistry of dissolved methane and hydrogen in basement fluids of the sediment-buried Juan de Fuca Ridge flank at Boreholes (CORKs) 1301A, 1362A and 1362B: methane isotopic compositions

The ocean crust is the largest aquifer system on Earth. Within the sediment-buried 3.5 Myr basaltic crust of the eastern Juan de Fuca Ridge (JFR) flank, the circulating basement fluids have moderate temperature (~65°C) and potentially harbor a substantial subseafloor biosphere. With dissolved oxygen and nitrate exhausted, sulfate may serve as the major electron acceptor in this environment. This study aims to evaluate the availability and the biogeochemistry of two important electron donors, methane and hydrogen, for the subseafloor biosphere. Basement fluids were collected via stainless steel and ethylene-tetrafluoroethylene fluoropolymer (ETFE) fluid delivery lines associated with Integrated Ocean Drilling Program (IODP) Circulation Obviation Retrofit Kits (CORKs) that extend from basement depths to outlet ports at the seafloor. Three CORKs were visited; 1301A, 1362A and 1362B lie within 200 to 500 m of each other, and their fluid intakes lie at ~30, ~60, and ~50 m below the sediment-basement interface (mbs), respectively. In addition, CORK 1362A contains a second intake at a deep (~200 mbs) horizon. The basement fluids from the three CORKs contained significantly higher concentrations of methane (1.5-13μM) and hydrogen (0.05-1.1 μM) than in bottom seawater (0.002 and 0.0004, respectively), indicating that prevalence and availability of both methane and hydrogen as electron donors for the subseafloor biosphere. Thermodynamic calculations show that sulfate reduction coupled with either methane or hydrogen oxidation is energy yielding in the oceanic basement. The δ13C values of methane ranged from -43×1‰ to -58×0.3‰; the δ2H values of methane in CORKs 1301A, 1362A and 1362B fluids were 57×5‰, -262×2‰, -209×2‰, respectively. The isotopic compositions suggest that methane in the basement fluid is of biogenic origin. Interestingly, the δ2H value of methane in the CORK 1301A fluids is far more positive than that in other marine environments investigated so far (Martens et al., 1999; Kessler et al., 2006; Kessler et al., 2008). The positive δ2H value of methane is best explained by partial microbial oxidation of biogenic methane, which has an initial isotopic composition similar to that from CORK 1362A and 1362B borehole fluid. High-throughput sequencing of the small subunit ribosomal RNA gene indicates the presence of methanogenic Euryarchaeota (e.g. Methanobacteria) in each of the borehole fluid samples described here. On average, fluid samples from boreholes 1362A and 1362B possessed a relatively higher abundance of known methanogens compared to borehole 1301A, consistent with higher methane concentration in 1362A and 1362B relative to 1301A fluids. Methane-oxidizing bacterial lineages from the phyla Proteobacteria and Verrucomicrobia were also detected; however, these groups were less abundant relative to the putative methane-producing groups. In conclusion, our study shows that methane and hydrogen are available electron donors and that methane is produced and potentially consumed by microorganisms in the oceanic basement. The data presented will guide incubation experiments using basement fluid in order to better understand the methane production/utilization processes within the oceanic basement.