Max Planck Institute for Marine Microbiology, Federal Republic of Germany

Cold-water corals occur widely on the deep sea bed, and may cover as large an area as the better-known shallow warm-water corals. They can form giant mound structures such as those found in the Belgica Mound Province in the Porcupine Seabight, 100 km west of Ireland. The Belgica province is a spectacular landscape of 66 partly buried subconical mounds in water depths of 600-900 m, reaching up to 160 m high with slopes as steep as 45 degrees . The surface of the mounds is known through swath bathymetry, remotely operated vehicle deployments, and shallow coring, but the internal composition and history of the mounds could only be speculated on until May 2005, when IODP Expedition 307 recovered the first complete section through to the base of Challenger Mound. Expedition scientists found that cold-water corals started to grow at this location about 2.6 Ma--the same time as the establishment of Northern Hemisphere glaciation and modern NE Atlantic water stratification. Subsequent mound growth was closely linked to the oceanographic regime. In addition to drilling Challenger Mound (Site U1317), Sites U1316 and U1318 recovered sequences downslope and upslope to establish the surrounding geological context. A total of 1400 m of sediment was recovered from these 3 sites during 12 days of drilling. We were able to demonstrate that such mounds are built and structurally supported by cold-water corals, rather than by microbially precipitated carbonate fueled by hydrocarbon seeps, as had previously been proposed. The branching coral skeletons buffered fine-grained sediments and constructed the remarkable 155 m high mound and stabilized the steep slopes. Cold-water corals are suspension feeders sensitive to changes in temperature and nutrient supply, yet they were able to maintain a deep-water mound community for a 1.5 Myr period under glacial/interglacial climatic change, waxing and waning with the prevailing oceanographic regime. Strontium isotope dating of corals and molluscs provided the age history of the mound, and the oxygen isotope record of benthic foraminifera has further refined it. The role of prokaryotes in diagenesis of layers in the mound is being investigated. Drilling on Expedition 307 provided insight into the much more ancient carbonate mud mounds in the geological record, which have a size and shape similar to the modern mounds of the Porcupine Seabight, but differ in their composition and early lithification. The abundance of modern mounds suggests that the fossil mounds may have formed in deeper water than is commonly supposed. There is also diversity among modern carbonate mounds. For example, at the northern end of the Porcupine Seabight there are thousands of smaller-scale coral mounds, the young mounds offshore of Morocco are found alongside mud volcanoes and methane seeps, and the algal mounds in the Florida Strait are different again. There remains much to be discovered by deep drilling of these fascinating geo-biological structures.