Paleoenvironmental reconstruction of Challenger Mound initiation in the Porcupine Seabight, NE Atlantic

The understanding of the paleoenvironment during initiation and early development of deep cold-water coral carbonate mounds in the NE Atlantic is currently a focus of international research. The Integrated Ocean Drilling Program (IODP) Expedition 307 drilled the 155 m high Challenger Mound in the Porcupine Seabight (SW off Ireland) in order to investigate for the first time sediments from the base of a giant carbonate mound. In this study we focus in high resolution on 12 m of sediments from Site 1317 encompassing the mound base. The mound initiation and start-up phase coincide with the intensification of the Northern Hemisphere Glaciation (INHG) at around 2.7 Ma. Further carbonate mound development seems to be strongly dependent on rapid changes in paleoceanographic and climatic conditions at the Pliocene-Pleistocene boundary, especially characterized and caused by the interaction of intermediate water masses, the Mediterranean Outflow Water (MOW), the Eastern North Atlantic Water (ENAW) and the influence of Southern Component Water (SCW). This study is based on well-established proxies such as d18O and d13C of planktonic (Globigerina bulloides) and benthic foraminifera (Fontbotia wuellerstorfi, Discanomalina coronata, Lobatula lobatula, Lobatula antarctica, and Planulina ariminensis) as well as grain size parameters to identify the paleoenvironmental and paleoecological setting favourable for the initial coral colonization on the mound. Stable oxygen and carbon isotope records of benthic foraminiferal species indicate that L. lobatula provides a reliable isotopic signature for paleoenvironmental reconstructions. In particular, d18O values of L. lobatula indicate that initial mound growth started in a glacial mode with moderate excursions in d18O values. Carbon isotope values of D. coronata are significantly offset compared to other epibenthic species. This offset may be related to vital effects. Bottom water temperatures, calculated using standard equations based on d18O of foraminiferal tests, range between 7 and 11°C. consistent with the known temperature range conducive for cold-water coral growth and development. Bottom currents transporting intermediate water masses of southern origin (Mediterranean and Bay of Biscay) enhanced at 2.6 Ma supporting first coral settlements with the INHG. The benthic d13C and the sortable silt records indicate that the early Pleistocene hydrodynamic regime was characterized by weaker current intensities associated with vertical movements of MOW or its replacement by SCW at intermediate depth. After these sluggish phases enhanced MOW flow dominated again and led to stronger current intensities and most probably sediment erosion on Challenger Mound. Erosion in combination with early diagenetic (oxidation) processes overprinted the sediment layers as indicated by dissolved coral skeletons, the increase in Ca-content and sediment density, minimum d13Cplanktonic values, as well as the occurrence of gypsum and pyrite, implying a careful evaluation of original and overprinted geochemical signals. We conclude that the Challenger Mound development was already influenced by short-term variability of water masses from southern origin and possible erosional events comparable to the late Pleistocene setting