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Controls on methane concentration and stable isotope ( δ 2 H‐CH 4 and δ 13 C‐CH 4 ) distributions in the water columns of the Black Sea and Cariaco Basin
Author(s) -
Kessler J. D.,
Reeburgh W. S.,
Tyler S. C.
Publication year - 2006
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2005gb002571
Subject(s) - stable isotope ratio , geology , isotopes of carbon , isotope , isotope fractionation , methane , seafloor spreading , anoxic waters , isotopic signature , kinetic isotope effect , oceanography , deuterium , chemistry , physics , organic chemistry , quantum mechanics
Methane (CH 4 ) concentration and stable isotope ( δ 2 H‐CH 4 and δ 13 C‐CH 4 ) depth distributions show large differences in the water columns of the Earth's largest CH 4 ‐containing anoxic basins, the Black Sea and Cariaco Basin. In the deep basins, the between‐basin stable isotope differences are large, 83‰ for δ 2 H‐CH 4 and 9‰ for δ 13 C‐CH 4 , and the distributions are mirror images of one another. The major sink in both basins, anaerobic oxidation of CH 4 , results in such extensive isotope fractionation that little direct information can be obtained regarding sources. Recent measurements of natural 14 C‐CH 4 show that the CH 4 geochemistry in both basins is dominated (∼64 to 98%) by inputs of fossil (radiocarbon‐free) CH 4 from seafloor seeps. We derive open‐system kinetic isotope effect equations and use a one‐dimensional (vertical) stable isotope box model that, along with isotope budgets developed using radiocarbon, permits a quantitative treatment of the stable isotope differences. We show that two main factors control the CH 4 concentration and stable isotope differences: (1) the depth distributions of the input of CH 4 from seafloor seeps and (2) anaerobic oxidation of CH 4 under open‐system steady state conditions in the Black Sea and open‐system non‐steady‐state conditions in the Cariaco Basin.