Abiotic methane seepage in the Ronda peridotite massif,southern Spain |
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| Institution: | 1. Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Italy, and Faculty of Environmental Science and Engineering, Babes Bolyai University, Cluj-Napoca, Romania;2. Centre of Hydrogeology, University of Malaga, Spain;3. School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada;4. CERENA/CEPGIST, Instituto Superior Técnico, Universidade de Lisboa, Portugal;5. Centro de Ciências e Tecnologias Nucleares, Instituto Superior Técnico, Universidade de Lisboa, Portugal;6. Department of Life Sciences and Center of Neuroscience and Cell Biology, University of Coimbra, Portugal;7. Water Research Institute, University of Granada, Spain;1. University of California Los Angeles, United States;2. University of Toronto, Canada;3. Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Italy;4. Faculty of Environmental Science and Engineering, Babes-Bolyai University, Cluj-Napoca, Romania;5. Carnegie Institution of Washington, United States;6. University of Oxford, United Kingdom;7. Princeton University, United States;8. Istanbul University, Republic of Turkey;9. Stockholm University, Sweden;10. Universidade de Lisboa, Portugal;11. University of Southern California, United States;12. Simons Foundation, United States;13. Rice University, United States;14. Shell International Exploration and Production Inc., United States;1. Department of Geological Sciences, UCB 399, University of Colorado, Boulder, CO 80309, USA;2. Department of Ocean and Earth Science, University of Southampton, SO14 3ZH, UK;3. Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA;4. Earth Sciences Division, MS 70A-4418, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA;5. Department of Ecology and Evolutionary Biology, CIRES 215, University of Colorado, Boulder, CO 80309,;6. Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA;7. Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843, USA;1. Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 1, via di Vigna Murata 605, 00143, Rome, Italy;2. Department of Physics and Earth Sciences University of Ferrara, Via Saragat 1, 44122, Ferrara, Italy;3. Consiglio Nazionale delle Ricerche, Istituto di Geologia Ambientale e Geoingegneria, Rome, Italy;4. Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, via di Vigna Murata 605, 00143, Rome, Italy;5. Faculty of Environmental Science and Engineering, Babes-Bolyai University, Cluj-Napoca, Romania;6. Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Via Uguccione della Faggiola 32, 56126, Pisa, Italy;7. HYDRA Marine Sciences GmbH, Burgweg 4, 76547, Sinzheim, Germany;8. HYDRA Field Station, Loc. Fetovaia, 57034, Campo nell’Elba, Italy;9. Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, via Ugo La Malfa 153, 90146, Palermo, Italy;1. Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy;2. Dipartimento di Scienze della Terra e del Mare (DiSTeM), University of Palermo, Italy;3. Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, Greece |
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| Abstract: | Abiotic methane in serpentinized peridotites (MSP) has implications for energy resource exploration, planetary geology, subsurface microbiology and astrobiology. Once considered a rare occurrence on Earth, reports of MSP are increasing for numerous localities worldwide in low temperature, land-based springs and seeps. We report the discovery of six methane-rich water springs and two ponds with active gas bubbling in the Ronda peridotite massif, in southern Spain. Water is hyperalkaline with typical hydrochemical features of active serpentinization (pH: 10.7 to 11.7, T: 17.1 to 21.5 °C, Ca–OH facies). Dissolved CH4 concentrations range from 0.1 to 3.2 mg/L. The methane stable C and H isotope ratios in the natural spring and bubbling sites (δ13CCH4: ?12.3 to ?37‰ VPDB; δ2HCH4: ?280 to ?333‰ VSMOW) indicate a predominant abiotic origin. In contrast, springs with manmade water systems, i.e., pipes or fountains, appear to have mixed biotic-abiotic origin (δ13CCH4: ?44 to ?69‰; δ2HCH4: ?180 to ?319‰). Radiocarbon (14C) analyses show that methane C in a natural spring is older than ca. 50,000 y BP, whereas dissolved inorganic carbon (DIC) analysed in all springs has an apparent 14C age ranging from modern to 2334 y BP. Therefore most, if not all, of the CH4 is allochthonous, i.e., not generated from the carbon in the hyperalkaline water. Methane is also released as bubbles in natural ponds and as diffuse seepages (～101–102 mg CH4 m?2d?1) from the ground up to several tens of metres from the seeps and springs, albeit with no overt visual evidence. These data suggest that the gas follows independent migration pathways, potentially along faults or fracture systems, physically isolated from the hyperalkaline springs. Methane does not seem to be genetically related to the hyperalkaline water, which may only act as a carrier of the gas. Gas-bearing springs, vents and invisible microseepage in land-based peridotites are more common than previously thought. In addition to other geological sources, MSP is potentially a natural source of methane for the troposphere and requires more worldwide flux measurements. |
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