The 2.6 Ma depositional sequence from the Challenger cold-water coral carbonate mound (IODP Exp. 307): Sediment contributors and hydrodynamic palaeo-environments

During IODP Expedition 307, the first complete sequence through a cold-water coral carbonate mound (a bio-geological seafloor feature created through successive stages of cold-water coral mediated sediment accumulation) was successfully acquired. The full recovery of the Challenger Mound, one of the large (ca. 155 m high) coral carbonate mounds along the NE Atlantic continental margin (Belgica mound province, eastern Porcupine Seabight) facilitates for the first time the study of the entire development of a coral carbonate mound and, hence, allows the identification of the environmental conditions driving and maintaining the entire build-up of these remarkable seafloor habitats.In this study, the different sediment contributors to the Challenger Mound are identified and assessed throughout its entire 2.6 Ma long development. High-resolution siliciclastic particle-size end-member modelling and its ground-truthing (XRD, quartz-sand surface microtextures) indicate the influence of an all dominant contour-current system, operating in variable energetic modes, for most of the sediment accumulation history of the Challenger Mound. Only local, short-distance current-controlled sediment re-dispersal is evidenced, while iceberg rafting is identified as an important depositional mechanism throughout the whole mound development period. Furthermore, evidence for icebergs repeatedly reaching the eastern Porcupine Seabight continental margin, even in the early stages of Northern Hemisphere glacial expansion, is preserved in the mound sequence.Supporting the existing coral-stratigraphy, the Challenger Mound depositional sequence reveals a two-phase development, separated by a significant hiatus. The lower mound-phase (M1; 2.6–1.7 Ma) indicates a semi-continuous, steadily changing, fast accumulating current-controlled depositional environment. The condensed upper mound-phase (M2; 998–1.5 ka) bears witness of a distinct shift to a more glacially-influenced, low accumulation environment, most likely resulting from a reduced capacity to deposit and/or preserve sediments. Cold-water coral density at the site of deposition is assumed to play an essential role in sediment deposition and preservation on Challenger Mound. It enabled the preservation of a unique, higher resolution record in the lower M1 sequence in the otherwise erosive/non-depositional Early-Pleistocene environment along the NW European continental margin. The potential of coral carbonate mounds as intermediate water depth, continental margin, (Plio-)Pleistocene palaeo-archives is thereby showcased