Live foraminiferal faunas from a 2800 m deep lower canyon station from the Bay of Biscay: Faunal response to focusing of refractory organic matter

A 2800 m deep station was sampled on three occasions, in January 1999, June 1999 and April 2000, in the lower part of Cap-Ferret Canyon (Bay of Biscay). This area is characterised by a rapid accumulation of fine-grained sediments and by important inputs of reworked organic matter in an intermediate state of decay. Diagenetic reactions within the sediment follow the well-established depth sequence resulting from the oxidation of organic deposits by different electron acceptors. At our station, live benthic foraminiferal faunas differ strongly from faunas previously collected at nearby open slope sites at a comparable water depth. Spectacularly high densities of deep infaunal species are observed in the deeper parts of the sediment for all three sampling periods. In our opinion, these high deep infaunal densities are a direct response to the massive flux of partially degraded organic matter, which is slowly introduced into the deeper parts of the sediment, where it induces a rather stable succession of redox gradients. Melonis barleeanus lives in the dysoxic part of the sediment whereas Globobulimina affinis appears preferentially close to the zero oxygen boundary. Both taxa occupy niches where the highest content of Mn (III, IV)-oxides and -oxihydroxides and Fe (III)-oxides are recorded. The fact that most of the geochemical reactions within the sediment are directly or indirectly catalysed by heterotrophic and chemolithoautotrophic bacterial consortia could suggest that deep infaunal foraminifera may be highly specialised protozoans able to feed on, or live in symbiosis with these prokaryotic communities.