2000 years of frequent turbidite activity in the Capbreton Canyon (Bay of Biscay)

• First bathymetric acquisition of the entire Capbreton Canyon
• 2000 years of annual turbidite activity within the canyon
• Continuum from turbidite activity to nepheloid layers on successively higher terraces
• Disconnection from the Adour River changed sediment petrology, but not volume

Capbreton Canyon extends from 200 m water depth near the Aquitanian coast to 3000 m water depth before abruptly turning northward and merging with the Cap-Ferret systems. The Capbreton Canyon has been disconnected from the Adour River input since 1310 AD. Despite this situation, modern sedimentary activity is evident in the first kilometres from the head. Present sedimentation in the Capbreton Canyon was studied by combined analysis of acoustic data and cores. Cores from the topographic highs document moderate hemipelagic sedimentation and record the global climatic signal of the Bay of Biscay. Within the canyon, at ± 80 km from the head, on terraces bordering the thalweg at 1600 m water depth, cores attest to recent turbidite activity. Massive medium sand is restricted to the thalweg floor, where sediment transfer dominates. Fine-grained turbidites are preferentially accumulated on the shallower terraces near the thalweg (the accumulation rate is 3 and 1 cm/year at 75 m and 125 m above the thalweg, respectively). On a higher terrace (225 m), turbidite sequences are absent or too thin to be preserved from bioturbation. The sedimentation rate is lower (approximately 0.17 cm/year) but still 10 times higher than the hemipelagic sedimentation outside the Capbreton Canyon. We demonstrate that frequent turbidity currents (1/year) are currently responsible for very high accumulation of dominantly fine-grained sediments on the terraces that are the nearest from the thalweg. We assume an impact of nepheloid layers on the sedimentation of the highest terraces. Looking for any impact on sediment supply of the disconnection of the Capbreton Canyon head from the Adour River mouth, it appears that the impact on the sedimentation affected the nature of the particles, which are less carbonate-rich, rather than the quantity of sediments. This result suggests that the longshore drift at the canyon head and hydrodynamic events are major factors in the sedimentation of Capbreton Canyon.