New Isotopic Constraints on the Sources of Methane at Sites of Active Continental Serpentinization

Friday, 19 December 2014
David T Wang1,2, Danielle Gruen1,2, Penny L Morrill3, Amanda Rietze4, Kenneth H Nealson5, Michael D. Kubo6, Dawn Cardace7, Matthew O Schrenk8,9, Tori M Hoehler6, Thomas M McCollom10, Giuseppe Etiope11, Hakan Hosgormez12, Martin Schoell13 and Shuhei Ono1, (1)MIT, Cambridge, MA, United States, (2)WHOI, Woods Hole, MA, United States, (3)Memorial University of Newfoundland, St John's, NL, Canada, (4)Memorial University of Newfoundland, St John's, Canada, (5)University of Southern California, Los Angeles, CA, United States, (6)NASA Ames Research Center, Moffett Field, CA, United States, (7)University of Rhode Island, Kingston, RI, United States, (8)East Carolina University, Greenville, NC, United States, (9)Michigan State University, East Lansing, MI, United States, (10)Univ Colorado, Boulder, CO, United States, (11)Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Roma, Italy, (12)Istanbul University, Istanbul, Turkey, (13)GasConsult International Inc., Berkeley, CA, United States
At continental sites of serpentinization, high concentrations of reduced gases (e.g., H2, CH4) are frequently found in association with highly-alkaline groundwater. Identification of the process(es) responsible for the generation of methane—as well as the source(s) of C & H—in these environments has been challenging. The difficulty is due to both the wide range of processes (microbial, thermal, abiotic) that could be involved, and the limited number of parameters that are accessible to currently-available analytical technologies (e.g., δ13C, δD).

The recent development of a new technique based on tunable infrared laser spectroscopy [1] has enabled the fully-resolved quantification of four isotopologues of methane: 12CH4, 13CH4, 12CH3D, and 13CH3D, a doubly-substituted (“clumped”) isotopologue. We used this technique to measure 13CH3D in gases sampled from continental sites of serpentinization, in order to provide independent constraints on C–H bond-forming processes involved in the generation of the methane found in these systems. Our study sites are hosted in ultramafic units that are presently undergoing serpentinization. These include The Cedars peridotite body (Calif., USA) [2], the Coast Range Ophiolite Microbial Observatory (Calif., USA) [3], and the Chimaera seep (Tekirova Ophiolite, Turkey) [4].

Preliminary measurements indicate that Δ13CH3D (the deviation of the abundance of 13CH3D from the stochastic distribution) in methane sampled from these sites spans nearly the entire range of thermodynamically-predicted values, from >+5‰ (13CH3D-based apparent equilibrium temperature < 45 °C) to ~0‰ (Tapparent → ∞). The new 13CH3D data is complemented by conventional geochemical analyses (e.g., dissolved ions/organics, δ13C, δD) on samples collected during the same field campaigns. Our study demonstrates that the measurement of 13CH3D provides a new dimension of isotopic constraints for unraveling the complex processes controlling the distribution of methane, and the flow of energy and carbon, in areas of active continental serpentinization.

[1] Ono et al. (2014) Anal. Chem. 86, 6487.

[2] Morrill et al. (2013) Geochim. Cosmochim. Acta 109, 222.

[3] Cardace et al. (2013) Sci. Dril. 16, 45.

[4] Etiope et al. (2011) Earth Planet. Sci. Lett. 310, 96.