Open AccessA survey of methane isotope abundance ( 14 C, 13 C, 2 H) from five nearshore marine basins that reveals unusual radiocarbon levels in subsurface waters
Author(s) -
Kessler J. D.,
Reeburgh W. S.,
Valentine D. L.,
Kinnaman F. S.,
Peltzer E. T.,
Brewer P. G.,
Southon J.,
Tyler S. C.
Publication year - 2008
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2008jc004822
Subject(s) - geology , petroleum seep , clathrate hydrate , water column , methane , sediment , oceanography , radiocarbon dating , isotopes of carbon , bay , environmental chemistry , total organic carbon , hydrate , geomorphology , chemistry , paleontology , organic chemistry
Methane (CH 4 ) in the subsurface ocean is often supersaturated compared to equilibrium with the modern atmosphere. In order to investigate sources of CH 4 to the subsurface ocean, isotope surveys ( 14 C‐CH 4, δ 13 C‐CH 4 , δ 2 H‐CH 4 ) were conducted at five locations: Skan Bay (SB), Santa Barbara Basin (SBB), Santa Monica Basin (SMB), Cariaco Basin (CB), and the Guaymas Basin (GB). Depth distributions of CH 4 concentration and isotopic abundance were determined for both the sediment and water column at the SB, SBB, SMB, and CB sites; CH 4 emitted from seeps on the continental shelf adjacent to the SBB as well as seeps and decomposing clathrate hydrates in the GB was also collected, purified, and analyzed. Methane isotope distributions in the sediments were consistent with known methanogenic and methanotrophic activity; seep‐ and clathrate‐hydrate‐derived CH 4 was found to be depleted in radiocarbon. However, surprising results were obtained in the water column at all sites investigated. In SB the radiocarbon content of the subsurface CH 4 concentration maximum was on average 41% less than its suspected sediment CH 4 source, suggesting CH 4 seepage in the bay. In the SBB, SMB, and CB, the 14 C‐CH 4 contents in the subsurface ocean were 1.2 to 3.6 times greater than modern carbon quantities suggesting a source of 14 C from atmospheric nuclear weapons testing, nuclear power plant effluents, or cosmogenic isotope production.