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

Origin, growth and demise of 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

Paleoseawater density reconstruction and its implication for cold-water coral carbonate mounds in the northeast Atlantic through time

Carbonate buildups and mounds are impressive biogenic structures throughout Earth history. In the recent NE Atlantic, cold-water coral (CWC) reefs form giant carbonate mounds of up to 300 m of elevation. The expansion of these coral carbonate mounds is paced by climatic changes during the past 2.7 Myr. Environmental control on their development is directly linked to controls on its main constructors, the reef-building CWCs. Seawater density has been identified as one of the main controlling parameter of CWC growth in the NE Atlantic. One possibility is the formation of a pycnocline above the carbonate mounds, which is increasing the hydrodynamic regime, supporting elevated food supply, and possibly facilitating the distribution of coral larvae. The potential to reconstruct past seawater densities from stable oxygen isotopes of benthic foraminifera has been further developed: a regional equation gives reliable results for three different settings, peak interglacials (e.g., Holocene), peak glacials (e.g., Last Glacial Maximum), and intermediate setting (between the two extremes). Seawater densities are reconstructed for two different NE Atlantic CWC carbonate mounds in the Porcupine Seabight indicating that the development of carbonate mounds is predominantly found at a seawater density range between 27.3 and 27.7 kg m⁻³ (σΘ notation). Comparable to recent conditions, we interpret the reconstructed density range as a pycnocline serving as boundary layer, on which currents develop, carrying nutrition and possibly coral larvae. The close correlation of CWC reef growth with reconstructed seawater densities through the Pleistocene highlights the importance of pycnoclines and intermediate water mass dynamics

The influence of seawater pH on U / Ca ratios in the scleractinian cold-water coral Lophelia pertusa

The increasing pCO2 in seawater is a serious threat for marine calcifiers and alters the biogeochemistry of the ocean. Therefore, the reconstruction of past-seawater properties and their impact on marine ecosystems is an important way to investigate the underlying mechanisms and to better constrain the effects of possible changes in the future ocean. Cold-water coral (CWC) ecosystems are biodiversity hotspots. Living close to aragonite-undersaturation, these corals serve as living laboratories as well as archives to reconstruct the boundary conditions of their calcification under the carbonate system of the ocean. We investigated the reef-building CWC Lophelia pertusa as a recorder of intermediate ocean seawater pH. This species-specific field calibration is based on a unique sample set of live in-situ collected L. pertusa and corresponding seawater samples. These data demonstrate that uranium speciation and skeletal incorporation for azooxanthellate scleractinian CWCs is pH dependent. However, this also indicates that internal pH up-regulation of the coral does not play a role in uranium incorporation into the majority of the skeleton of L. pertusa. This study suggests L. pertusa provides a new archive for the reconstruction of intermediate water mass pH and hence may help to constrain tipping points for ecosystem dynamics and evolutionary characteristics in a changing ocean

The influence of seawater pH on U / Ca ratios in the scleractinian cold-water coral Lophelia pertusa

The increasing pCO2 in seawater is a serious threat for marine calcifiers and alters the biogeochemistry of the ocean. Therefore, the reconstruction of past-seawater properties and their impact on marine ecosystems is an important way to investigate the underlying mechanisms and to better constrain the effects of possible changes in the future ocean. Cold-water coral (CWC) ecosystems are biodiversity hotspots. Living close to aragonite-undersaturation, these corals serve as living laboratories as well as archives to reconstruct the boundary conditions of their calcification under the carbonate system of the ocean. We investigated the reef-building CWC Lophelia pertusa as a recorder of intermediate ocean seawater pH. This species-specific field calibration is based on a unique sample set of live in-situ collected L. pertusa and corresponding seawater samples. These data demonstrate that uranium speciation and skeletal incorporation for azooxanthellate scleractinian CWCs is pH dependent. However, this also indicates that internal pH up-regulation of the coral does not play a role in uranium incorporation into the majority of the skeleton of L. pertusa. This study suggests L. pertusa provides a new archive for the reconstruction of intermediate water mass pH and hence may help to constrain tipping points for ecosystem dynamics and evolutionary characteristics in a changing ocean

Monsoonal forcing of cold-water coral growth off southeastern Brazil during the past 160 kyr

Cold-water corals (CWCs) constitute important deep-water ecosystems that are under increasing environmental pressure due to ocean acidification and global warming. The sensitivity of these deep-water ecosystems to environmental change is demonstrated by abundant paleorecords drilled through CWC mounds that reveal characteristic alterations between rapid formation and dormant or erosive phases. Previous studies have identified several central parameters for driving or inhibiting CWC growth such as food supply, oxygenation, and the carbon saturation state of bottom water, yet there are still large uncertainties about the relative importance of the different environmental parameters. To advance this debate we have performed a multiproxy study on a sediment core retrieved from the 25 m high Bowie Mound, located at 866 m water depth on the continental slope off southeastern Brazil, a structure built up mainly by the CWC Solenosmilia variabilis. Our results indicate a multifactorial control on CWC growth at Bowie Mound during the past ∼ 160 kyr, which reveals distinct formation pulses during northern high-latitude glacial cold events (Heinrich stadials, HSs) largely associated with anomalously strong monsoonal rainfall over the continent. The ensuing enhanced runoff elevated the terrigenous nutrient and organic-matter supply to the continental margin and likely boosted marine productivity. The dispersal of food particles towards the CWC colonies during HSs was facilitated by the highly dynamic hydraulic conditions along the continental slope that prevailed throughout glacial periods. These conditions caused the emplacement of a pronounced nepheloid layer above Bowie Mound, thereby aiding the concentration and along-slope dispersal of organic matter. Our study thus emphasizes the impact of continental climate variability on a highly vulnerable deep-marine ecosystem

Environmental boundary conditions of cold-water coral mound growth over the last 3 million years in the Porcupine Seabight, Northeast Atlantic

IODP Expedition 307 made it for the first time possible to investigate the entire body of a cold-water coral carbonate mound. Here we provide new insights into the long-term history of Challenger Mound on the European continental margin off Ireland. This study is based on age determinations (230Th/U, 87Sr/86Sr) and geochemical signals (Mg/Li and Ba/Ca) measured in the scleractinian cold-water coral Lophelia pertusa from IODP Site 1317 in the Porcupine Seabight. The paleoceanographic reconstructions reveal that coral growth in the Porcupine Seabight was restricted to specific oceanographic conditions such as enhanced export of primary production and Bottom-Water Temperatures (BWT) between ∼8 and 10 °C, related to the water mass stratification of the Mediterranean Outflow Water (MOW) and Eastern North Atlantic Water (ENAW). The geochemical signals from the coral skeletons can be explained by the close interaction between cold-water coral growth, sea-surface productivity and the surrounding water masses – the boundary layer between MOW and ENAW. Enhanced sea-surface productivity and the build-up of a stable water mass stratification between ENAW and MOW caused enhanced nutrient supply at intermediate water depths and facilitated a steady mound growth between ∼3.0 and 2.1 Ma. With the decrease in sea-surface productivity and related reduced export productivity the food supply was insufficient for rapid coral mound growth between ∼1.7 and 1 Ma. During the late Pleistocene (over the last ∼0.5 Myr) mound growth was restricted to interglacial periods. During glacials the water mass boundary between ENAW/MOW probably was below the mound summit and hence food supply was not sufficient for corals to grow