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ArticleCompositional and isotopic characteristics of hydrocarbons generated by a hydrothermal experiment simulating seafloor sediment alteration stepwise heating from 275 to 361 degrees C at 30 MPa(The Geochemical Society of Japan, 2019-08-08) Kawagucci, Shinsuke ; Seewald, Jeffrey S.We conducted a laboratory hydrothermal experiment that simulated generation of low molecular-weight hydrocarbons during seafloor sediment alteration at 275–361°C and 30 MPa. The abundance and carbon and hydrogen stable isotope composition of low molecular weight thermogenic hydrocarbons in the fluids were determined. In general, the abundance of C1−C4 alkanes increased with time. The abundance of CH4 relative to C2−C4 alkanes as reflected by C1/C2+ ratios showed progressive increases from 1.2 to 4.3 with continued sediment heating. Alkenes were enriched in early phase and decreased with time. Carbon isotope ratios (δ13C) of thermogenic CH4 ranged between –42.0~−24.2‰. Carbon isotope ratios of C2H6 and C3H8 were similar to each other throughout the experiment (δ13C = –28.0~−20.3‰). In general, the carbon isotope ratios of C1−C4 alkanes were more close to those of substrate organic matter in larger carbon numbers and at later periods of the experiment. Hydrogen isotope ratios (δD) of CH4 varied from –325~−262‰, more negative than those expected at the isotope equilibrium between CH4 and H2O. Compared with results from the experiment, natural hydrothermal fluids show higher C1/C2+ ratio, more diverse δ13CCH4 values among the fields, higher δ13CC2 values, and higher δDCH4 values. The differences likely result from lower maturity of the experimental fluid and biogenic methane contribution to the natural fluids.
ArticleCool, alkaline serpentinite formation fluid regime with scarce microbial habitability and possible abiotic synthesis beneath the South Chamorro Seamount(Springer, 2018-11-14) Kawagucci, Shinsuke ; Miyazaki, Junichi ; Morono, Yuki ; Seewald, Jeffrey S. ; Wheat, C. Geoffrey ; Takai, KenSouth Chamorro Seamount (SCS) is a blueschist-bearing serpentinite mud volcano in the Mariana forearc. Previous scientific drilling conducted at SCS revealed highly alkaline, sulfate-rich formation fluids resulting from slab-derived fluid upwelling combined with serpentinization both beneath and within the seamount. In the present study, a time-series of ROV dives spanning 1000 days was conducted to collect discharging alkaline fluids from the cased Ocean Drilling Program (ODP) Hole 1200C (hereafter the CORK fluid). The CORK fluids were analyzed for chemical compositions (including dissolved gas) and microbial community composition/function. Compared to the ODP porewater, the CORK fluids were generally identical in concentration of major ions, with the exception of significant sulfate depletion and enrichment in sulfide, alkalinity, and methane. Microbiological analyses of the CORK fluids revealed little biomass and functional activity, despite habitable temperature conditions. The post-drilling sulfate depletion is likely attributable to sulfate reduction coupled with oxidation of methane (and hydrogen), probably triggered by the drilling and casing operations. Multiple lines of evidence suggest that abiotic organic synthesis associated with serpentinization is the most plausible source of the abundant methane in the CORK fluid. The SCS formation fluid regime presented here may represent the first example on Earth where abiotic syntheses are conspicuous with little biotic processes, despite a condition with sufficient bioavailable energy potentials and temperatures within the habitable range.