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Geophysical exploration of an active pockmark field in the Bay of Concarneau,southern Brittany,and implications for resident suspension feeders |
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| Authors: | Agnès Baltzer Axel Ehrhold Carinne Rigolet Aurélie Souron Céline Cordier Hélène Clouet Stanislas F Dubois |
| Institution: | 1. UMR 6554, Geolittomer, IGARUN, Université de Nantes, Campus du Tertre, BP 81227, 44312, Nantes cedex, France 2. Ifremer, Centre de Brest, Unité de Recherche des Géosciences Marines, BP 70, 29280, Plouzané, France 3. Ifremer, Centre de Brest, Unité Dynamiques de l’Environnement C?tier, BP 70, 29280, Plouzané, France 4. UMR CNRS 6143, 24 rue des Tilleuls, Université de Caen (Campus 1), 14000, Caen cedex, France |
| Abstract: | About a decade ago, a large field of pockmarks (individual features up to 30 m in diameter and <2 m deep) was discovered in water depths of 15–40 m in the Bay of Concarneau in southern Brittany along the French Atlantic coast, covering an overall area of 36 km2 and characterised by unusually high pockmark densities in places reaching 2,500 per square kilometre. As revealed by geophysical swath and subbottom profile data ground-truthed by sediment cores collected during two campaigns in 2005 and 2009, the confines of the pockmark field show a spectacular spatial association with those of a vast expanse of tube mats formed by a benthic community of the suspension-feeding amphipod Haploops nirae. The present study complements those findings with subbottom chirp profiles, seabed sonar imagery and ultrasonic backscatter data from the water column acquired in April 2011. Results show that pockmark distribution is influenced by the thickness of Holocene deposits covering an Oligocene palaeo-valley system. Two groups of pockmarks were identified: (1) a group of large (>10 m diameter), more widely scattered pockmarks deeply rooted (up to 8 ms two-way travel time, TWTT) in the Holocene palaeo-valley infills, and (2) a group of smaller, more densely spaced pockmarks shallowly rooted (up to 2 ms TWTT) in interfluve deposits. Pockmark pore water analyses revealed high methane concentrations peaking at ca. 400 μl/l at 22 and 30 cm core depth in silty sediments immediately above Haploops-bearing layers. Water column data indicate acoustic plumes above pockmarks, implying ongoing pockmark activity. Pockmark gas and/or fluid expulsion resulting in increased turbidity (resuspension of, amongst others, freshly settled phytoplankton) could at least partly account for the strong spatial association with the phytoplankton-feeding H. nirae in the Bay of Concarneau, exacerbating impacts of anthropogenically induced eutrophication and growing offshore trawling activities. Tidally driven hydraulic pumping in gas-charged pockmarks represents a good candidate as large-scale short-term triggering mechanism of pockmark activation, in addition to episodic regional seismic activity. |
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