Gas Seepage along the Edge of the Aquitaine Shelf (France): Origin and Local Fluxes

Abstract

During the scientific expedition GAZCOGNE2 at the Bay of Biscay nine gas seeps were sampled for the first time and their flux was measured using an in situ pressure-preservation sampler (PEGAZ, © IFREMER). Overall, three sites were investigated to determine the nature and the origin of the gases bubbling at the seafloor and forming acoustic plumes into the water column, as this was the question raised from the first geologic study of the area. This has guided our study and accordingly corresponds to the main purpose of the present article. Thus, the molecular and isotopic ( D and 13 C) analyses revealed that the gas seeps were primarily composed of methane. Both methane and ethane are of microbial origin, and the former has been generated by microbial reduction of carbon dioxide. Heavier hydrocarbons accounted for less than 0.06% mol of the total amount. Despite the microbial origin of methane, the samples exhibit subtle differences with respect to the 13 C CH 4 values, which varied between −72.7 and −66.1‰. It has been suggested that such a discrepancy was predominantly governed by the occurrence of anaerobic methane oxidation. The PEGAZ sampler also enabled us to estimate the local gas fluxes from the sampled streams. The resulting values are extremely heterogeneous between seeps, ranging from 35 to 368 mLn⋅min −1 . Assuming a steady discharge, the mean calculated methane emission for the nine seeps is of 38 kmol⋅yr −1 . Considering the extent of the seep area, this very local estimate suggests that the Aquitaine Shelf is a very appropriate place to study methane discharge and its fate on continental shelves.

Multiple gas reservoirs are responsible for the gas emissions along the Marmara fault network

“…The seeps were often composed of several gas streams close to each other, and the more vigorous stream was sampled by deploying our gas-bubble sampler PEGAZ (for PrElèvement de GAZ). This sampler was designed to collect gas bubbles and preserve the sample at the in situ pressure until its recovery (Lanteri and Bignon, 2007;Ruffine et al, 2017). Aliquots of gas were then subsampled at low pressure (2-4 bars) in 12-mL pre-evacuated vials from Labco® by connecting the PEGAZ sampler to a custom-made gas transfer system (Charlou et al, 2004).…”

“…Intensive gas emissions are common and widespread processes in oceanic and continental marine basins. Amongst the areas on Earth where they have been densely observed, one can cite offshore Siberia (Shakhova et al, 2014), the Norwegian continental margin including the well-studied Hakon Mosby Mud Volcano (Gentz et al, 2014;Sauer et al, 2015;Westbrook et al, 2009), the North Sea (Borges et al, 2016;McGinnis et al, 2011;von Deimling et al, 2011), the Black Sea (Klaucke et al, 2006;Roemer et al, 2012a), the Sea of Marmara (Dupré et al, 2012;Dupré et al, 2010a), the Aquitaine Shelf (Dupré et al, 2014;Ruffine et al, 2017), the Central Nile Deep-Sea Fan (Dupré et al, 2010b;Roemer et al, 2014a), the US Atlantic Margin (Skarke et al, 2014;Weinstein et al, 2016), the Gulf of Mexico (Bernard et al, 1976;Hu et al, 2012), the Santa Barbara Basin (Clark et al, 2010), the Hydrate Ridge (Haeckel et al, 2004;Milkov et al, 2005;Philip et al, 2016), the Makran continental margin (Roemer et al, 2012b), the South China Sea (Di et al, 2014;Huang et al, 2009), the Japan Sea (Aoyama et al, 2007), as well as offshore New Zealand (Greinert et al, 2010) and the Southern Ocean (Roemer et al, 2014b). Such phenomena occur either as dissolved or free gases, and they lead to the formation of specific sites called cold seeps (Hovland and Judd, 1988;Suess, 2014;Talukder, 2012).…”

Molecular and Isotopic Composition of Hydrate-Bound, Dissolved and Free Gases in the Amazon Deep-Sea Fan and Slope Sediments, Brazil

“…Low εc values may be partly explained by the fact that methane is transported by both diffusion as a dissolved phase and advection as gas bubbles, in which the latter is probably an important process owing to the presence of large quantities of gas venting on the water column [21]. The acoustic chimneys in the Amazon fan and adjacent slope area are usually associated with the presence of numerous fractures [53][54][55], which may act as potential fluid conduits for the upward migration of microbial gas or thermogenic gases [56]. Methane transport in the gas phase forms massive hydrates at the chimney sites in this basin [21].…”

“…This process preferentially takes up methane with the lighter carbon atom [66], therefore producing a 12 C-enriched CO 2 , which leads to a decrease of εc, as shown in Figure 3. Ruffine et al [56] determined the nature and origin of the gases bubbling at the Bay of Biscay. Their data were also plotted outside the appointed fields due to the very light carbon isotopic composition of CO 2 .…”

“…Their data were also plotted outside the appointed fields due to the very light carbon isotopic composition of CO 2 . They attributed the very light carbon isotopic composition to anaerobic oxidation of methane, coupled with carbonate precipitation within the sedimentary column [56]. Recently, archaeal 16S rRNA gene sequences were recovered that were characteristic of anaerobic methane oxidizers from biodegraded oil reservoirs.…”