The Porcupine Sediment Drift complex

About 10 years ago, the Porcupine Seabight, off Ireland, started receiving more attention of the deep-sea scientific community due the discovery of the deep-water coral banks (Henriet et al., 1998; De Mol et al., 2002). Nowadays it is widely accepted that the main drivers of these special ecosystems are the hydrodynamic regime, nutrient (vs. sediment) supply and the (pa1aeo)topography of the seafloor (which are both undoubtedly linked to palaeoceanography and climate). Especially in the Belgica mound province, located on the eastern slope of the Porcupine Seabight, the activity and intensity of bottom currents throughout the Neogene has been decisive in the "coral bank life cycle" and the construction of several sediment and/or contourite drift bodies (Van Rooij et al., 2007a). Here, we would like to present the main scientific results of 10 years of research on the Porcupine Sediment Drift complex (PSD). The main dataset exits out of 1500 km of high-resolution single channel sparker seismic profiles (RCMG), together with multibeam bathymetry (Beyer et al., 2000) and TOBI Sidescan sonar imagery (NIOZ-UCC-RCMG). Groundtruthing of recent deposits happened by means of piston coring or Calypso coring (Van Rooij et al., 2007b). A longer time record was drilled during IODP Expedition 307 in May 2005 (IODP 307 Expedition Scientists, 2005).Although the main PSD has a Quaternary age, underlying deposits infer the presence of intense bottom currents already since Oligocene times. At least 2 buried elongate mounded drifts can be distinguished, sometimes accompanied with a sediment wave field. However, most of these features have been eroded by an erosion event that has been dated on IODP Site U1318 from 8.96 Ma to 1.24 Ma (Kano et al., submitted). This period of 7.5 Ma erosion has greatly affected this part of the margin. It has created a terraced palaeotopography, suggesting a step-wise erosional history, culminating in the formation of one big north-south channel. According to more preliminary dating from the IODP 307 expedition, the coral banks already started to grow from 2.3 Ma ago. However, the onset of sediment drift deposition is generally estimated at the start of the Pleistocene. The PSD is built up in 3 parts. Basinward (west) of the north-south channel, a long elongate mounded drift is built. Upslope (east) from this channel towards the first coral banks, a complex of plastered drifts is inferred. The most intriguing feature however, is the presence of small mounded contourite drifts in between the coral banks. The latter also plays an important role in the termination of coral bank growth, as demonstrated within the southern tail of the PSD. A RN Marion Dufresne Calypso core on the crest of such a small contourite drift has inferred a more sluggish bottom current during glacial times with muddy contourites. During interglacial times, bottom currents are much more intense and can even created relatively large sandy contourite deposits