Résumé de la publication
Haploops species are amphipods crustaceans who build dense tube mats in muddy environments. Those engineers species play a bio-sedimentological key role in ecosystems processes. Indeed, their constructions modify the local seafloor characteristics (densification and uplift), catching the sediments particles and the currents flows. Their empty tube rooted in the first centimeters of the sea floor sediments allows water circulation and then influences the early digenesis processes. Haploops occurrence in specific environments remains unclear but a link with the biogenic methane observed in the sub-bottom layers has been highlighted. To underline this link, three ecosystems have been investigated in south of Brittany and Loire Atlantic : (1) the Bay of Concarneau, a calm area without any continental supplies (no important rivers) and protected from large swell waves, (2) the offshore of the coast of Le Croisic (Loire Atlantic), a more energetic environment with high sedimentary supplies from the Loire river, poorly protected by the Plateau du Four at the West and (3) the middle of the Loire estuary, a highly disturbed system with important sediment supplies, mainly constituted of clay, from the Loire itself and from the navigation channel dredging. This last area is submitted to tidal and swell influences and is affected by an anthropogenic activity : large commercials ships anchor in this waiting zone of the Port of St Nazaire). In those three areas, the same pattern (coincidence) of Haploops and gas can be observed. The chirp sub-bottom profiles show that under the Haploops fields, gas is stored into the sediment column and in each area, pockmarks occur within these ecosystems. Pockmarks are craterlike figures found in soft, fine grained sediments and formed where seabed’s fluids escape. Those pockmarks repartition perfectly matches with the presence of Haploops in the three studied environments. Our hypothesis is that pockmarks are the link between the sub-bottom gas and the Haploops. As Haploops do not have the necessary bacteria to consume methane, indirect explanation should be proposed.
The pockmarks bring methane to the sediment interface and thus either (i) directly expulse some chemical elements (present in the deeper paleovalleys which are necessary for Haploops development or (ii) the methane is necessary for chemical reactions at seafloor surface, which may be used to locally increase the primary production of phytoplankton. Both of these hypotheses are currently tested and geochemical analyses should help to better understand the early digenesis processes occurring in these particular fields of Haploops.