Sequence stratigraphic models predict increased sediment delivery to deep-water areas during sea-level lowstand. The Armorican margin (Bay of Biscay, western European margin) is an interesting area to test this hypothesis because the margin has a wide continental shelf, still partly flooded during sea-level lowstand, and the Armorican turbidite system has experienced fluctuating sediment fluxes since the last glacial period. The stratigraphic response of the Armorican turbidite system to sea-level oscillations and climate changes was assessed for the last 35,000 years through the study of the Guilcher, Crozon and Audierne levees. Millenial-timescale resolution chronostratigraphy allowed us to reconstruct the sediment accumulation and turbidite frequency, thickness and grain-size over this period of time.
We found that the Armorican turbidite system was sediment-starved during highstand conditions (ca. 8–0 ka) and that glacial conditions favoured sediment delivery to the deep Bay of Biscay. However, contrary to what would be expected from sequence stratigraphic models, the turbidite flux did not reach a maximum during the LGM lowstand (ca. 26–20 ka) but at the onset of Termination I (between ca. 20 and 17 ka). This makes the Armorican turbidite system a transgression-dominated one. This sediment pulse can be interpreted as a huge increase in the meltwater discharge of the Fleuve Manche palaeoriver in response to the decay of the British and Fennoscandian ice-sheets. At that time, despite the rising sea-level, a large deltaic system had to have developed on the outer shelf, leading to the delivery of the Fleuve Manche sediment load into the canyon heads. On the other hand, our dataset suggest that the delivery of sediment into canyons was mainly forced by the winnowing and reworking of the sediment stored on the wide, drowned shelf during the last glacial period (between ca. 35 and 20 ka, and between 17 and 8 ka). These findings illustrate the competing influences of accommodation and sediment supply on the Armorican margin over the last 35,000 years, with a shelf acting as a buffer for the sediment supply signal for most of the period, except during the last deglaciation. At that time, the western European sediment-routing system was reactive, the climatic signal rapidly propagated from the southern limb of the European ice-sheet to the Armorican turbidite system. Finally, our study demonstrates that precise reconstruction of turbidite flux in deep-water areas, added to knowledge about the morphology of the margin and the palaeoenvironmental changes (fluvial system, shoreline position, etc.), are crucial for determining the response of turbidite systems to external forcing.
Highlights
? Sediment accumulation, vertical grain-size and turbidite thickness trends from levees of the Armorican turbidite system (western European margin) are examined ? Competing influence of accommodation and sediment supply on the Armorican margin over the last 35,000 years ? The wide continental shelf act as a buffer to sediment supply signal for most of the period, except during the last deglaciation ? The growth of the Armorican turbidite system during the last deglaciation is controlled by the Fleuve Manche palaeoriver and the decaying European ice-sheet