Himalayan metamorphic CO2 fluxes: Quantitative constraints from hydrothermal springs |
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| Authors: | John A Becker Mike J Bickle Albert Galy Tim JB Holland |
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| Institution: | 1. Department of Geological Sciences, California State University, Long Beach, CA 90840, USA;2. Department of Geology and Geophysics, School of Ocean and Earth Sciences and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA;3. Water Resources Research Center, University of Hawaii at Manoa, Honolulu, HI 96822, USA;4. State of Hawaii, Department of Health, Honolulu, HI 96822, USA;1. Department of Geophysics, Stanford University, CA 94305-2215, USA;2. Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory, CA 94720, USA;3. National Geophysical Research Institute, Hyderabad 500007, India;4. Department of Marine Geosciences, University of Haifa, Haifa 31905, Israel;5. Department of Geosciences, San Francisco State University, CA 94132, USA;1. Fluids and Volatiles Laboratory, Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0244, USA;2. Now at Planetary Geosciences Institute, Department of Earth Planetary Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA;3. Now at Centre de Recherches Pétrographiques et Géochimiques, CNRS-UL, 15 rue Notre-Dame des Pauvres, BP20, 54501 Vandoeuvre-lès-Nancy Cedex, France;4. Nordic Volcanological Centre, Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, IS-101 Reykjavik, Iceland;1. Department of Geology, FCFM, Universidad de Chile, 8370450 Santiago, Chile;2. Andean Geothermal Center of Excellence (CEGA), FCFM, Universidad de Chile, 8370450 Santiago, Chile;3. AORI, Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba 277-8564, Japan;4. Department of Structural and Geotechnical Engineering, Pontificia Universidad Católica de Chile, 7820436 Santiago, Chile |
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| Abstract: | Hot springs in the Marsyandi Valley, Nepal, vent CO2 sourced from metamorphic fluids that mix with shallow groundwaters before degassing near the Earth's surface. The δ13C of spring waters ranges up to + 13‰, while that of the coexisting free gas phase is close to ? 4‰. Empirical and thermodynamic modelling of this isotopic fractionation suggests > 97 ± 1% CO2 degassing. The calculated minimum total CO2 degassing in the Marsyandi catchment is 5.4 × 109 mol/yr from a Cl-based estimate of the spring water discharge to the Marsyandi River and the fraction of CO2 degassed. Extrapolated to the whole of the Himalayas, this implies a probable minimum metamorphic CO2 flux of 0.9 × 1012 mol/yr, or ～ 13% of solid Earth CO2 degassing. The calculated flux is a factor of three greater than the estimated CO2 drawdown by silicate weathering in the Himalayas. Himalayan metamorphic degassing contributes a significant fraction of the global solid Earth CO2 flux and implies that metamorphism may cause changes in long-term climate that oppose those resulting from the orogenic forcing of chemical weatherability. |
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