Origin,Growth and demise of the cold-water coral mound Challenger (IODP Site 1317)
The Integrated Ocean Drilling Program (IODP) Expedition 307 was proposed to obtain evidence for understanding the origin and evolution of a 155 m high deepwater carbonate mound in the Porcupine Seabight. The major aim of this study is the reconstruction of environmental parameters using well-developed paleoceanographic proxies derived from calcareous tests and skeletons of benthic organisms based on sediment cores from this expedition. In particular, this study uses different archives such as scleractinian cold-water corals and calcitic foraminifers. The second chapter (published in Marine Geology 2011) reports on a high-resolution record of the mound base. Stable oxygen and carbon isotopes measured in several benthic and planktonic foraminifers as well as sortable silt analyses document the start-up phase of coral growth. Mound initiation and further development coincide with the intensification of Mediterranean Outflow Water (MOW) characterized by oceanographic conditions favourable for rapid cold-water coral growth. Furthermore excursions in foraminiferal δ13C values and increased flow conditions indicate erosional intervals, which overprinted probably diagenetically the original geochemical signals. The third chapter (to be submitted to Geology), also based on sediments from the mound base, shows that these ecosystems only thrive under specific oceanographic conditions. Based on core material, not only from Challenger Mound (IODP Expedition 307) but also from the Propeller Mound, we reconstructed paleo-seawater densities from oxygen isotope ratios in benthic foraminifera. Results clearly indicate results demonstrate that cold-water coral mound development occurred when a density window of sigma-theta (σΘ) = 27.35–27.55 kg m-3 was present in the ambient bottom water. Therefore we conclude that seawater density is reflecting one of the major controlling factors favoring mound growth and highlights the sensitivity of these ecosystems to environmental changes. The fourth chapter (submitted to Earth Planetary Science Letters) demonstrates the use of paleotemperature proxies in the scleractinian reef building cold-water coral Lophelia pertusa. Temperature calibrations are based on L. pertusa samples from temperature range of 5.9°- 13.65°C originating from the European continental margin and the Mediterranean Sea. ix Results could not confirm earlier findings of Rüggeberg et al. (2008) that δ88/86Sr in Lophelia skeleton is positively correlated with temperature and may serve as a potential paleotemperature proxy. Results rather show that δ88/86Sr is inversely correlated with temperature in samples from the North Atlantic. However, this temperature effect appears to be superimposed by changes in the ocean carbonate system. Furthermore, this sample set of L. pertusa clearly shows the temperature dependency of elemental ratios such as Mg/Li and Sr/Ca. The Mg/Li ratio may serve as a new paleotemperature proxy in scleractinian cold- water corals, whereas the Sr/Ca ratio needs more detailed research. The fifth chapter (to be submitted) focuses on the long-term controlling mechanisms of cold- water coral mound growth in the Porcupine Seabight. Here, different paleo-proxies such as Mg/Ca, δ13C and δ18O in foraminifera and Mg/Li, Ba/Ca and U/Ca in cold-water coral L. pertusa were used to reconstruct paleoenvironmental parameters. Based on existing and additional age determinations (87Sr/86Sr, Th/U) previous findings were supported. However, our data point to an earlier mound initiation at ~3 Ma coincidently with the intensification of the Mediterranean Ouflow Water (MOW). Foraminiferal temperature records reveal that early mound development occurred in glacial and interglacial conditions, whereas the recent mound decline was caused by high amplitude excursions of the last interglacial/glacial cycles. In particular, coral Mg/LiLophelia temperatures indicate that coral growth occurred within a temperature range of 8 to 10°C, comparable to the recent measured settings in the Porcupine Seabight. Hence, results imply that the variations in intermediate water masses (Mediterranean Outflow Water, Eastern North Atlantic Water,) are the main trigger for mound growth and decline in the Porcupine Seabight. Moreover prior to the mid-Pleistocene rapid Challenger Mound growth benefited from a stable boundary layer between the MOW and the Eastern North Atlantic Water (ENAW) at which organic matter and nutrients settled on.