Glacial-interglacial contourite dynamics on the Irish margin over the last 340,000 years: a new biostratigraphy and reinterpretation for IODP Site U1318 on the Porcupine Seabight

IODP Expedition 307 cored an 95m thick sediment contourite drift at site U1318B, on the eastern fringe of the Porcupine Seabight (northeast Atlantic). Detailed sedimentological and microfossil analyses reveals a 340,000 year record of ice sheet advance and retreat and the resultant changes in contourite and glacio-marine deposition. New microfossil data suggests a reappraisal of the strigraphy for this borehole. A basal late Miocene carbonate unit is overlain unconformably by a series of sediment drift deposits with alternation between active interglacial bottom-currents and lower energy glacial conditions near the ice sheet margin. Microfossil data suggest that this region experienced mixed polar water conditions (with abundant Neogloboquadryina pachyderma sinistral) for most of the last 400,000 years with occasional periods of subarctic conditions and inflows of warmer Mediterranean Outflow Water (Globorotalia inflata, Neogloboquadryina incompta and other warmer water species). The region was north of the polar front during Marine Isotope Stages 8, 6 and possible during MIS 4. IRD detritus (250μm) are most common when the region lay south of the polar front. Mean sortable silt data suggest generally high current velocities during warmer phases with the lowest current velocities when the region was north of the polar front.