Cold-water carbonate mounds as palaeo-archives : the Plio-Pleistocene sediment record from the Challenger Mound, northeast Atlantic

During IODP Expedition 307, the first (and so far only) complete sequence through a cold-water coral carbonate mound was successfully drilled. After decades of research on contemporary to sub-recent coral carbonate mound environments, the complete recovery of the Challenger Mound record, sampling one of the large mounds along the Irish sector of the NE Atlantic continental margin (eastern Porcupine Seabight; Belgica mound province), allowed for the first time the investigation of long-term mound development, from mound initiation to decline. Here, we present an overview of the palaeo-environmental (i.e. hydrodynamic, oceanographic and climatic) signal captured in the entire Challenger Mound sequence (Hole U1317E). A high-resolution multi-proxy characterisation of the ca. 155m long mound matrix sediment record was conducted, encompassing a wide array of sedimentological, mineralogical, geochemical and stratigraphic techniques. These included, amongst others, siliciclastic particle-size analysis, X-ray diffraction phase quantification, isotopic fingerprinting of target elements for provenance purposes, calcareous nannofossil and planktonic foraminifer biostratigraphy and assemblage counts. In this way we aimed to (1) identify the controls on Challenger Mound development throughout the different phases of its entire Plio-Pleistocene to recent build-up, and (2) assess the unique character of coral carbonate mounds as recorders of Quaternary palaeo-environmental change at intermediate water depth in the NE Atlantic. Overall, Challenger Mound development shows a strong affinity to the general climate variability of the Northern Hemisphere, although not being completely in phase with it. The major oceanographic and climatic rearrangements of the Plio-Pleistocene, such as those associated with the Late Pliocene/Early Pleistocene intensification of continental ice-sheet development (ca. 2.75 – 2.55 Ma)1 or the orbital frequency changes during the mid-Pleistocene climate transition (ca. 1.2 – 0.6 Ma)2, seem responsible for the two significant thresholds in Challenger Mound development: its Late Pliocene origin and its Middle-Late Pleistocene to recent decline. However, local influences such as proximal (British-Irish) ice-sheet dynamics and on-mound changes in cold-water coral density seem to have a stronger control on Challenger Mound development and may have induced the offset between global climate and Challenger Mound proxy record variability. On the other hand, owing to this, a unique, high-resolution palaeo-record of regional Early-Pleistocene environmental change (including early British-Irish ice-sheet development3) is preserved in the lower Challenger Mound, covering a period that is not recorded in the general sedimentary sequence of the area. The Challenger Mound record, albeit with restricted Late Quaternary preservation, highlights the potential of coral carbonate mounds as excellent palaeo-recorders, providing us with unique records from ‘complex’ continental margin environments