Noble gas anomalies related to high-intensity methane gas seeps in the Black Sea |
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| Authors: | CP Holzner DF McGinnis CJ Schubert R Kipfer DM Imboden |
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| Institution: | 1. Lamont-Doherty Earth Observatory, Columbia University, 61 Rt. 9W, Palisades, NY 10964, USA;2. FR Geochemie, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin Germany;3. Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel;1. Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA;2. Department of Physics and Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA;3. Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA;4. Department of Earth and Environmental Sciences and Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027, USA;5. Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, IL 60607, USA;6. Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA;7. Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA;8. Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland;9. Scripps Institution of Oceanography, University of California, San Diego, CA 92093, USA;10. Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, USA;11. GEOMAR Helmholtz Center for Ocean Research Kiel, Marine Biogeochemistry, Kiel, Germany;12. Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA |
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| Abstract: | Dissolved noble gases and tritium were analyzed at a series of high-intensity methane gas seeps in the Black Sea to study the transport and gas exchange induced by bubble-streams in the water column. These processes affect marine methane emissions to the atmosphere and are therefore relevant to climate warming. The seep areas investigated are located in the Dnepr paleo-delta, west of Crimea, and in the Sorokin Trough mud volcano area, south-east of Crimea. Noble gas concentration profiles at active seep sites revealed prominent anomalies compared to reference profiles that are unaffected by outgassing. Supersaturations of the light noble gases helium and neon observed relatively close to the sea floor are interpreted as effects of gas exchange between the water and the rising bubbles. Depletions of the heavy noble gases argon, krypton and xenon that were detected above an active, bubble-releasing mud volcano appear to be related to the injection of fluids depleted in noble gases that undergo vertical transport in the water column due to small density differences. In both cases, the noble gas anomalies clearly document seep-specific processes which are difficult to detect by other methods. Helium is generally enriched in the deep water of the Black Sea due to terrigenic input. Although exceptionally high helium concentrations observed in one seep area indicate a locally elevated helium flux, most of the seeps studied seem to be negligible sources of terrigenic helium. Noble gas analyses of sediment pore waters from the vicinity of a mud volcano showed large vertical gradients in helium concentrations. The helium isotope signature of the pore waters points to a crustal origin for helium, whereas the deep water of the Black Sea also contains a small mantle-type component. |
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