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Geofluids
Volume 2017 (2017), Article ID 4240818, 13 pages
https://doi.org/10.1155/2017/4240818
Research Article

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

1IFREMER, Unité des Géosciences Marines, 29280 Plouzané, France
2IFREMER, Unité des Recherches et Développements Technologiques, 29280 Plouzané, France
3IFREMER, Unité des Ecosystèmes Profonds, 29280 Plouzané, France
4IFPEN, 1-4 avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex, France
5TOTAL, avenue Larribau, 64000 Pau, France

Correspondence should be addressed to Livio Ruffine; rf.remerfi@eniffur.oivil

Received 26 January 2017; Accepted 8 March 2017; Published 25 July 2017

Academic Editor: Timothy S. Collett

Copyright © 2017 Livio Ruffine et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 (δD and δ13C) 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 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.