Shallow gas off the Rhône prodelta, Gulf of Lions

Abstract

In areas unaffected by the high flux of organic matter and rapid/thick flood deposition, or in between flood events, the conditions for methanogenesis and gas accumulation have not been met. In these areas, the physical and biological reworking of the surficial sediment may effectively oxidize and mineralize organic matter and limit bacterial methanogenesis in the subsurface. We propose that in the Rhône prodelta flood deposits deliver significant amounts of terrigenous 1 organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake, and leading to the potential for methanogenesis with burial.

Occurrence of pockmarks on the Ortegal Spur continental margin, Northwestern Iberian Peninsula

“…Assuming that the whole time period of the deposits in the study area is Plio-Quaternary, and taking into account the sedimentation model that characterized the Spanish Iberian shelves during this period (Farrán and Maldonado, 1990;Ercilla et al, 1994Ercilla et al, , 2008Lobo et al, 2002, among others), the near-surface sediment layers correspond to low-stands, paleodeltaic distributary systems or shelf-margin deltas that developed during falling and low-stand periods of sea level. The volume of sediment flooring the Ortegal Spur is relatively small levels compared to river fans in which pockmarks related to biogenic fluids have been observed, such as the Po, Rhône, Ebro and Guadalquivir rivers (Baraza and Ercilla, 1996;Maestro et al, 2002;Orange et al, 2005;García-García et al, 2006). We believe that the huge biogenic fluid volume necessary to develop the high density of pockmarks observed in the Ortegal Spur area can not be generated.…”

Control of Quaternary sea‐level changes on gas seeps

“…The ideal environment for the formation of biogenic methane is one containing areas of rapidly accumulating, fine‐grained muddy sediments rich in organic matter [ Rice and Claypool , ; Hovland et al ., ], such as those observed in the study area during sea level falls (Figure b). The sediment accumulated during lowstands is characterized by high organic matter content, with total organic carbon (TOC) levels likely comparable to those presently measured off the Rhône prodelta [ García‐García et al ., ] (TOC between 1 and 2% wt). Such organic matter concentrations are sufficient to allow significant biogenic gas generation [ Jané et al ., ], with the TOC concentrations comparable to those documented in the Congo Basin [ Andresen and Huuse , ] and in the South China Sea [ Sun et al ., ], resulting in the widespread formation of pockmarks [ Hovland and Judd , ].…”

Origin of remineralized organic matter in sediments from the Rhone River prodelta (NW Mediterranean) traced by Δ 14 C and δ 13 C signatures of pore water DIC

“…This would raise the C:N ratio at station A and Z from 7.5 to 9. Oxidation of methane, either aerobic or anaerobic, could potentially occur at depth in these sediments, since methane has been detected at shallow sediment depth (Garcia-Garcia et al, 2006;Rassmann et al, in prep.). As methane is generally depleted in 13 C carbon isotopes, with typical signatures of d 13 C-CH 4 À50‰ (Alperin et al, 1992;Boehme et al, 1996;Whiticar, 1999), its oxidation should produce depleted d 13 C-DIC in pore water.…”