Geochemistry and geobiology of a present-day serpentinization site in California: The Cedars
Ultra-basic (pH 11–12) reducing (−656 to −585 mV) groundwater springs discharging from serpentinized peridotite of The Cedars, CA, were investigated for their geochemistry and geobiology. The spring waters investigated were of meteoric origin; however, geochemical modeling suggests that there were two sources of groundwater, a shallow source with sufficient contact with The Cedars' peridotite body to be altered geochemically by serpentinization, and a deeper groundwater source that not only flows through the peridotite body but was also in contact with the marine sediments of the Franciscan Subduction Complex (FSC) below the peridotite body. We propose that the groundwater discharging from lower elevations (GPS1 and CS1) reflect the geochemistry of the deeper groundwater in contact with FSC, while groundwaters discharging from springs at higher elevations (NS1 and BSC) were a mixture of the shallow peridotite-only groundwater and the deeper groundwater that has been in contact with the FSC. Cell densities of suspended microbes within these waters were extremely low. In the NS1 and BSC spring fluids, cell densities ranged from 10^2 to 10^3 cells/ml, while suspended cells at GPS were lower than 10 cells/mL. However, glass slides incubated in the BSC and GPS1 springs for 2–3 weeks were colonized by cells with densities ranging from 10^6 to 10^7 cells/cm^2 attached to their surfaces. All of the springs were very low (⩽1 μM) in several essential elements and electron acceptors (e.g. nitrate/ammonium, sulfate, and phosphate) required for (microbial) growth, which is not uncommon at sites of continental serpentinization. Gases rich in N_2, H_2, and CH_4 were exsolving from the springs. The stable carbon isotope value (δ^(13)C_(CH4) = −68 ± 0.6‰) and the CH_4/C_(2+) (>10^3) of methane and other gaseous hydrocarbons exsolving from NS1 were typical of microbially sourced methane, whereas the isotope values and the CH_4/C_(2+) of BSC and CS1 springs were more enriched in ^(13)C and had CH_4/C_(2+) < 10^3, suggesting a mixture of microbial and non-microbial methane. The concentrations of aromatic compounds, and ethane, propane, iso- and n-butane were well described by simple physical mixing between the aromatic- and alkane-poor, shallow groundwater and the relatively aromatic, and alkane-rich groundwater that flows through both the peridotite and the FSC suggesting that these aromatic and alkane compounds originated in the deeper FSC groundwater and are not produced in the shallow peridotite-only groundwater. The aromatic compounds most probably originated from the diagenesis/degradation of organic matter in the marine sediments below the peridotite body, while the gaseous alkanes may have multiple sources including thermal degradation of the organic matter in the marine sediments below the peridotite body and possibly by abiogenic reactions occurring within the peridotite body. This geochemical study demonstrates the complexity of The Cedars, and the possible sources of hydrocarbons at continental sites of serpentinization.
Additional Information© 2013 Elsevier Ltd. Received 30 July 2012; accepted in revised form 14 January 2013; available online 10 February 2013. Associate editor: Tom McCollom. The authors would like to thank David McCrory and Roger Raiche and The Cedars Friends for all of their inspirational support and great help over the years and for allowing us access to The Cedars; Bob Coleman, Barbara Sherwood Lollar, Norm Sleep, and Mark Wilson for their critical comments; Shunichi Ishii for his contributions to Fig. 7; and Greg Wanger (JCVI) and Jennifer Eigenbrode for their help in the field; and Andrea Cheung, Ana Abraztsova, Ryan Thacher, and Heidi Kavanagh for lending their laboratory expertise. The authors would also like to thank the 3 anonymous Reviewers and the Editor, Tom McCollom, for their detailed reviews and critical comments. This research was funded by the Carnegie Institution of Washington Postdoctoral Fellowship program, the Lewis and Clark Fund for Exploration and Field Research in Astrobiology, and the United States National Science Foundation Grant No 1024872. M.L. Fogel was supported by the NASA Astrobiology Institute through Cooperative Agreement NNA04CC09A.
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