Authigenic carbon entombed in methane-soaked sediments from the northeastern transform margin of the Guaymas Basin, Gulf of California

Extensive ROV-based sampling and exploration of the seafloor was conducted along an eroded transform-parallel fault scarp on the northeastern side of the Guaymas Basin in the Gulf of California to observe the nature of fluids venting from the seafloor, measure the record left by methane-venting on the carbonates from this area, and determine the association with gas hydrate. One gas vent vigorous enough to generate a water-column gas plume traceable for over 800 m above the seafloor was found to emanate from a similar to 10-cm-wide orifice on the eroded scarp face. Sediment temperature measurements and topography on a sub-bottom reflector recorded in a transform-parallel seismic reflection profile identified a subsurface thermal anomaly beneath the gas vent. Active chemosynthetic biological communities (CBCs) and extensive authigenic carbonates that coalesce into distinct chemoherm structures were encountered elsewhere along the eroded transform-parallel scarp. The carbon isotopic composition of methane bubbles flowing vigorously from the gas vent (-53.6+/-0.8 parts per thousand PDB) is comparable to methane found in sediment cores taken within the CBCs distributed along the scarp (-51.9+/-8.1 parts per thousand PDB). However, the delta C-13 value of the CO2 in the vent gas (+12.4+/- 1.1 parts per thousand PDB) is very distinct from those for dissolved inorganic carbon (DIC) (-35.8 parts per thousand to -2.9 parts per thousand PDB) found elsewhere along the scarp, including underneath CBCs. The delta C-13 values of the carbonate-rich sediments and rocks exposed on the seafloor today also span an unusually large range (-40.9 parts per thousand to +12.9 parts per thousand PDB) and suggest two distinct populations of authigenic carbonate materials were sampled. Unconsolidated sediments and some carbonate rocks, which have lithologic evidence for near-seafloor formation, have negative delta C-13 values, while carbonate rocks that clearly formed in the subsurface have positive delta C-13 values (up to +23.0 parts per thousand) close to that measured for CO2 in the vent gas. There appears to be two carbon sources for the authigenic carbonates: (1) deeply-sourced, isotopically heavy CO2 (similar to+12 parts per thousand); and (2) isotopically light DIC derived from local anaerobic oxidation of methane at the sulfate-methane interface in the shallow subsurface. Addition of isotopically light methane-derived carbon at the seafloor may completely mask the isotopically heavy CO, signature (+12.4 parts per thousand) in the underlying sediments. Thus, the authigenic carbonates may have formed from the same methane- and carbon dioxide-bearing fluid, but under different migration and alteration conditions, depending oil how it migrated through the sediment column. (C) 2007 Elsevier Ltd. All rights reserved.