Present deep-submarine canyons activity in the Bay of Biscay (NE Atlantic)

Present sedimentation in three canyons of the Bay of Biscay (Audierne, Blackmud and Capbreton) is studied by the combined analysis of cores and current meter data collected over a 7 month period. At the current meter mooring locations, interface cores were collected to characterize the recent sedimentation processes. In the two canyons located in the Northern part of the Bay of Biscay (Audierne and Blackmud), there is no evidence of recent sedimentary deposits. Canyons are by-passing or erosive areas. In the southern part of the Bay of Biscay (Capbreton), recent turbidite are deposited. In the three canyons, current meters recorded energetic currents with velocities showing alternating upslope and downslope motions, and a period corresponding to the semi-diurnal component M2. These currents are supposed to be related to deep internal tides. The high speed of the current (1 m/s) in Audierne and Blackmud is consistent with the lack of preservation of recent sediments on the canyon floors. In Capbreton Canyon, the magnitude of currents is less and recent turbidites are preserved. In addition to periodical current motion, small magnitude gravity event corresponding to a low-concentration turbulent surge or a high-concentration nepheloid layer initiated during a storm was recorded during the mooring period. These results suggest that deep-sea canyons in the Bay of Biscay have behaviour at present varying between by passing or erosion areas and sediment trapping. These examples suggest that sediments are moved up and down by low-energy, tide-initiated hydrodynamic events during most of the time. During higher magnitude, short-duration gravity events, the sediments are transferred down canyon towards the deep sea.