Cold-water Coral Carbonate Mounds Research Articles

What could form a 35-km lineament of carbonate mounds on the ocean floor?

A multibeam survey carried out in November 2019 reveals a lineament of seafloor protrusions on the Santos Basin (SW Atlantic) continental slope. Shallow seismic data and seafloor composition suggest the lineament comprises a 35 km long arrangement of Cold-Water Coral (CWC) carbonate mounds, here named the Tupana Carbonate Ridge (TCR), defined in this contribution for the first time. Insights into genetic controlling factors of the TCR led to the analysis of single-channel seismic profiles that cross the lineament. Interpretation of deep seismic reflection data suggests a stratigraphic control provided by the lateral arrangement of coastal deposits formed during the protracted progradation that characterized a significant portion of Santos Basin's depositional history. Coastal sandstones encased in low-angle shelf-margin progradational wedges constitute a potential hydrocarbon reservoir supplied by the flow of fluids toward the surface through vertical structures. The ultimate source of fluids seems to be provided by halokinetic movements on a region depleted of salts known as the Albian Gap domain. This work questions the role of seafloor hydrodynamics in shaping the TCR morphology by eroding the carbonate mounds and forming elongated contourite systems.

Biomass Mapping for an Improved Understanding of the Contribution of Cold-Water Coral Carbonate Mounds to C and N Cycling

This study used a novel approach combining biological, environmental, and ecosystem function data of the Logachev cold-water coral carbonate mound province to predictively map coral framework (bio)mass. A more accurate representation and quantification of cold-water coral reef ecosystem functions such as Carbon and Nitrogen stock and turnover were given by accounting for the spatial heterogeneity. Our results indicate that 45% is covered by dead and only 3% by live coral framework. The remaining 51%, is covered by fine sediments. It is estimated that 75,034–93,534 tons (T) of live coral framework is present in the area, of which ∼10% (7,747–9,316 T) consists of Cinorg and ∼1% (411–1,061 T) of Corg. A much larger amount of 3,485,828–4,357,435 T (60:1 dead:live ratio) dead coral framework contained ∼11% (418,299–522,892 T) Cinorg and <1% (0–16 T) Corg. The nutrient turnover by dead coral framework is the largest, contributing 45–51% (2,596–3,626 T) C year–1 and 30–62% (290–1,989 T) N year–1 to the total turnover in the area. Live coral framework turns over 1,656–2,828 T C year–1 and 53–286 T N year–1. Sediments contribute between 1,216–1,512 T C year–1 and 629–919 T N year–1 to the area’s benthic organic matter mineralization. However, this amount is likely higher as sediments baffled by coral framework might play a much more critical role in reefs CN cycling than previously assumed. Our calculations showed that the area overturns 1–3.4 times the C compared to a soft-sediment area at a similar depth. With only 5–9% of the primary productivity reaching the corals via natural deposition, this study indicated that the supply of food largely depends on local hydrodynamical food supply mechanisms and the reefs ability to retain and recycle nutrients. Climate-induced changes in primary production, local hydrodynamical food supply and the dissolution of particle-baffling coral framework could have severe implications for the survival and functioning of cold-water coral reefs.

The Diversity and Ecological Role of Non-scleractinian Corals (Antipatharia and Alcyonacea) on Scleractinian Cold-Water Coral Mounds

Cold-water coral carbonate mounds, created by framework-building scleractinian corals, are also important habitats for non-scleractinian corals, whose ecology and role are understudied in deep-sea environments. This paper describes the diversity, ecology and role of non-scleractinian corals on scleractinian cold-water coral carbonate mounds in the Logachev Mound Province, Rockall Bank, NE Atlantic. In total ten non-scleractinian species were identified, which were mapped out along eight ROV video transects. Eight species were identified as black corals (three belonging to the family Schizopathidae, one each to the Leiopathidae, Cladopathidae, and Antipathidae and two to an unknown family) and two as gorgonians (Isididae and Plexauridae). The most abundant species were Leiopathes sp. and Parantipathes sp. 2. Areas with a high diversity of non-scleractinian corals are interpreted to offer sufficient food, weak inter-species competition and the presence of heterogeneous and hard settlement substrates. A difference in the density and occurrence of small vs. large colonies of Leiopathes sp. was also observed, which is likely related to a difference in the stability of the substrate they choose for settlement. Non-scleractinian corals, especially black corals, are an important habitat for crabs, crinoids, and shrimps in the Logachev Mound Province. The carrier crab Paromola sp. was observed carrying the plexaurid Paramuricea sp. and a black coral species belonging to the genus Parantipathes, a behavior believed to provide the crab with camouflage or potentially a defense mechanism against predators. More information on the ecophysiology of non-scleractinian corals and fine-scale local organic matter supply are needed to understand what drives differences in their spatial distribution and community structure.

Semi-Automatic Versus Manual Mapping of Cold-Water Coral Carbonate Mounds Located Offshore Norway

Cold-water coral reefs are hotspots of biological diversity and play an important role as carbonate factories in the global carbon cycle. Reef-building corals can be found in cold oceanic waters around the world. Detailed knowledge on the spatial location and distribution of coral reefs is of importance for spatial management, conservation and science. Carbonate mounds (reefs) are readily identifiable in high-resolution multibeam echosounder data but systematic mapping programs have relied mostly on visual interpretation and manual digitizing so far. Developing more automated methods will help to reduce the time spent on this laborious task and will additionally lead to more objective and reproducible results. In this paper, we present an attempt at testing whether rule-based classification can replace manual mapping when mapping cold-water coral carbonate mounds. To that end, we have estimated and compared the accuracies of manual mapping, pixel-based terrain analysis and object-based image analysis. To verify the mapping results, we created a reference dataset of presence/absence points agreed upon by three mapping experts. There were no statistically significant differences in the overall accuracies of the maps produced by the three approaches. We conclude that semi-automated rule-based methods might be a viable option for mapping carbonate mounds with high spatial detail over large areas.

Mapping of Cold-Water Coral Carbonate Mounds Based on Geomorphometric Features: An Object-Based Approach

Cold-water coral reefs are rich, yet fragile ecosystems found in colder oceanic waters. Knowledge of their spatial distribution on continental shelves, slopes, seamounts and ridge systems is vital for marine spatial planning and conservation. Cold-water corals frequently form conspicuous carbonate mounds of varying sizes, which are identifiable from multibeam echosounder bathymetry and derived geomorphometric attributes. However, the often-large number of mounds makes manual interpretation and mapping a tedious process. We present a methodology that combines image segmentation and random forest spatial prediction with the aim to derive maps of carbonate mounds and an associated measure of confidence. We demonstrate our method based on multibeam echosounder data from Iverryggen on the mid-Norwegian shelf. We identified the image-object mean planar curvature as the most important predictor. The presence and absence of carbonate mounds is mapped with high accuracy. Spatially-explicit confidence in the predictions is derived from the predicted probability and whether the predictions are within or outside the modelled range of values and is generally high. We plan to apply the showcased method to other areas of the Norwegian continental shelf and slope where multibeam echosounder data have been collected with the aim to provide crucial information for marine spatial planning.

Carbonate mound development in contrasting settings on the Irish margin

Cold-water coral carbonate mounds, formed by framework building cold-water corals, are found in several mound provinces on the Irish margin. Differences in cold-water coral mound development rates and sediment composition between mounds at the southwest Rockall Trough margin and the Galway Mound in the Porcupine Seabight are investigated. Variations in sediment composition in the two mound provinces are related to the local environmental conditions and sediment sources. Mound accumulation rates are possibly higher at the Galway Mound probably due to a higher influx of hemipelagic fine grained non-carbonate sediments. In both cold-water coral mound areas, mound growth has been continuous for the last ca 11,000 years, before this period several hiatuses and unconformities exist in the mound record. The most recent unconformity can be correlated across multiple mounds and mound provinces at the Irish margin on the basis of apparent age. On the southwest Rockall Trough margin these hiatuses/unconformities are associated with post-depositional aragonite dissolution in, and lithification of, certain intervals, while at Galway Mound no lithification occurs. This study revealed that the influx and types of material transported to cold-water coral mounds may have a direct impact on the carbonate mound accumulation rate and on post-depositional processes. Significantly, the Logachev Mounds on the SW Rockall Trough margin accumulate slower but, because they contain lithified layers, are less susceptible to erosion. This net effect may account for their larger size compared to the Belgica Mounds.

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 2.1Ma. 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 1Ma. During the late Pleistocene (over the last ∼0.5Myr) 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.

Cold-water coral carbonate mounds as unique palaeo-archives: the Plio-Pleistocene Challenger Mound record (NE Atlantic)

Through the interplay of a stabilising cold-water coral framework and a dynamic sedimentary environment, cold-water coral carbonate mounds create distinctive centres of bio-geological accumulation in often complex (continental margin) settings. The IODP Expedition 307 drilling of the Challenger Mound (eastern Porcupine Seabight; NE Atlantic) not only retrieved the first complete developmental history of a coral carbonate mound, it also exposed a unique, Early-Pleistocene sedimentary sequence of exceptional resolution along the mid-latitudinal NE Atlantic margin.In this study, a comprehensive assessment of the Challenger Mound as an archive of Quaternary palaeo-environmental change and long-term coral carbonate mound development is presented. New and existing environmental proxy records, including clay mineralogy, planktonic foraminifer and calcareous nannofossil biostratigraphy and assemblage counts, planktonic foraminifer oxygen isotopes and siliciclastic particle-size, are thereby discussed within a refined chronostratigraphic and climatic context.Overall, the development of the Challenger Mound shows a strong affinity to the Plio-Pleistocene evolution of the Northern Hemisphere climate system, albeit not being completely in phase with it. The two major oceanographic and climatic transitions of the Plio-Pleistocene – the Late Pliocene/Early Pleistocene intensification of continental ice-sheet development and the mid-Pleistocene transition to the more extremely variable and more extensively glaciated late Quaternary – mark two major thresholds in Challenger Mound development: its Late Pliocene (>2.74 Ma) origin and its Middle–Late Pleistocene to recent decline. Distinct surface-water perturbations (i.e. water-mass/polar front migrations, productivity changes, melt-water pulses) are identified throughout the sequence, which can be linked to the intensity and extent of ice development on the nearby British–Irish Isles since the earliest Pleistocene. Glaciation-induced shifts in surface-water primary productivity are thereby proposed to fundamentally control cold-water coral growth, which in turn influences on-mound sediment accumulation and, hence, coral carbonate mound development throughout the Pleistocene. As local factors, such as proximal ice-sheet dynamics and on-mound changes in cold-water coral density, significantly affected the development of the Challenger Mound, they can potentially explain the nature of its palaeo-record and its offsets with the periodicities of global climate variability. On the other hand, owing to this unique setting, a regionally exceptional, high-resolution palaeo-record of Early Pleistocene (ca 2.6 to 2.1 Ma) environmental change (including early British–Irish ice-sheet development), broadly in phase with the 41 ka-paced global climate system, is preserved in the lower Challenger Mound. All in all, the Challenger Mound record highlights the wider relevance of coral carbonate mound archives and their potential to capture unique records from dynamic (continental margin) environments.

Foraminiferal biodiversity associated with cold-water coral carbonate mounds and open slope of SE Rockall Bank (Irish continental margin—NE Atlantic)

Cold-water coral (CWC) ecosystems are hotspots of macro- and microfaunal biodiversity and provide refuge for a wide variety of deep-sea species. We investigated how the abundance and biodiversity of ‘live’ (Rose Bengal stained) foraminifera varies with, and is related to, the occurrence of CWC on the Rockall Bank (NE Atlantic). Qualitative and quantitative analyses were performed on 21 replicate samples from 8 deep-sea stations, including 4 stations on CWC-covered carbonate mounds at depths of 567–657 m, and 4 stations on the adjacent slope at depths of 469–1958 m where CWC were absent. This sampling strategy enabled us to demonstrate that sediments surrounding the living CWC were characterised by higher foraminiferal abundance and biodiversity than open-slope sediments from the same area. A total of 163 foraminiferal species was identified. The dominant species in CWC sediments were: Spirillina vivipara, Allogromiid sp. 1, Globocassidulina subglobosa, Adercotryma wrighti, Eponides pusillus, Ehrenbergina carinata, Planulina ariminensis, Trochammina inflata and Paratrochammina challengeri. Foraminifera were nearly absent in adjacent open slope areas subject to strong tidal currents and characterised by coarse grained deposits. We suggest that CWC create a heterogeneous three-dimensional substrate offering microhabitats to a diverse benthic foraminiferal community.

The Moira Mounds, small cold-water coral mounds in the Porcupine Seabight, NE Atlantic: Part B—Evaluating the impact of sediment dynamics through high-resolution ROV-borne bathymetric mapping

Large cold-water coral carbonate mounds are well-known from several locations along the north-east Atlantic continental margin. With these mound structures, which can measure up to 350m in height, often smaller mounds are found in the same hydrodynamic settings. This paper illustrates the importance of local sediment dynamics on the morphology and development of small-sized mounds, the Moira Mounds, in the Belgica Mound Province (Porcupine Seabight, SW of Ireland) through ROV-borne microbathymetry and video data. The data set illustrates the sedimentary setting of small mounds in the Porcupine Seabight, gives new insights in the sedimentary processes acting around the small mounds and sheds more light on the interaction between hydrodynamics, sedimentation and erosion, cold-water coral mound initiation and mound growth. The high-resolution bathymetry over the Moira Mounds reveals the presence of several well-delineated zones, each characterized by typical sedimentary structures bearing witness of active sediment transport. The most important ones are: (1) regular E–W directed sediment waves, (2) irregular overgrown sediment waves, and (3) furrows and ridges associated with straight-crested sediment waves. The Moira Mounds appear as small-mounded features, subcircular in shape, measuring 30–50m across and up to ~5m tall. The evolution of the Moira Mounds can be explained by the interaction between cold-water coral growth and sediment baffling through a positive feedback mechanism. However, it is suggested that the small mound build-ups represent nowadays rather a way of mound formation under stressed conditions due to high sediment fluxes, than an initial phase of extensive mound growth.

Microbial community composition and biogeochemical processes in cold-water coral carbonate mounds in the Gulf of Cadiz, on the Moroccan margin

This study reports on the microbial community composition of Bacteria and Archaea in a cluster of carbonate mounds located in the Gulf of Cadiz on the Moroccan margin at water depths between 500 and 600m amidst a field of mud volcanoes. Although nearly no live corals were observed on the mound surfaces, gravity cores examined from both Alpha and Beta Mound (MD 169 MiCROSYSTEMS cruise, cores MD08-3218 and MD08-3214, respectively) display a high number of cold-water coral fragments (scleractinians) throughout the cores. Molecular techniques, using taxonomic (16S rRNA gene PCR-DGGE) and functional (mcrA and dsrAB) gene markers, were applied to assess prokaryotic diversity. This was compared with the distribution of total cell counts and a novel approach of ATP-activity measurements to identify microbial activity hot spots during core sub-sampling. Despite broad similarities in the prokaryotic community at the two sites, some differences were observed at specific depth intervals, in correlation with differences in ATP activity. Alpha and Beta Mound showed evidence for the presence of a shallow sulfate–methane transition zone (SMTZ) with increased sulfate reduction rates indicating the presence of microbially mediated anaerobic oxidation of methane (AOM). The key marker gene for methanotrophy and methanogenesis (mcrA) was only found at Alpha Mound within the SMTZ where AOM is occurring. In contrast, sulfate-reducing prokaryotes (dsrAB) were identified in cores from both mounds but at different depths, correlating with increased ATP activity and SR rates. This study demonstrates that the cold-water coral carbonate mounds of the Pen Duick Escarpment host a diverse and metabolically active prokaryotic population. Although it remains unclear to what extent prokaryotes are important for the initiation of a cold-water coral carbonate mound, our results provide some evidence that microbes may play a role in the cementation of the mound sediment through microbially induced carbonate precipitation, and thereby help preserve the mound structure.

Paleoenvironmental reconstruction of Challenger Mound initiation in the Porcupine Seabight, NE Atlantic

The understanding of the paleoenvironment during initiation and early development of deep cold-water coral carbonate mounds in the NE Atlantic is currently a focus of international research. The Integrated Ocean Drilling Program (IODP) Expedition 307 drilled the 155m high Challenger Mound in the Porcupine Seabight (SW off Ireland) in order to investigate for the first time sediments from the base of a giant carbonate mound. In this study we focus in high resolution on 12m of sediments from Site 1317 encompassing the mound base. The mound initiation and start-up phase coincide with the intensification of the Northern Hemisphere Glaciation (INHG) at around 2.7Ma. Further carbonate mound development seems to be strongly dependent on rapid changes in paleoceanographic and climatic conditions at the Pliocene–Pleistocene boundary, especially characterized and caused by the interaction of intermediate water masses, the Mediterranean Outflow Water (MOW), the Eastern North Atlantic Water (ENAW) and the influence of Southern Component Water (SCW).This study is based on well-established proxies such as δ18O and δ13C of planktonic (Globigerina bulloides) and benthic foraminifera (Fontbotia wuellerstorfi, Discanomalina coronata, Lobatula lobatula, Lobatula antarctica, and Planulina ariminensis) as well as grain size parameters to identify the paleoenvironmental and paleoecological setting favourable for the initial coral colonization on the mound. Stable oxygen and carbon isotope records of benthic foraminiferal species indicate that L. lobatula provides a reliable isotopic signature for paleoenvironmental reconstructions. In particular, δ18O values of L. lobatula indicate that initial mound growth started in a glacial mode with moderate excursions in δ18O values. Carbon isotope values of D. coronata are significantly offset compared to other epibenthic species. This offset may be related to vital effects. Bottom water temperatures, calculated using standard equations based on δ18O of foraminiferal tests, range between 7 and 11°C, consistent with the known temperature range conducive for cold-water coral growth and development.Bottom currents transporting intermediate water masses of southern origin (Mediterranean and Bay of Biscay) enhanced at 2.6Ma supporting first coral settlements with the INHG. The benthic δ13C and the sortable silt records indicate that the early Pleistocene hydrodynamic regime was characterized by weaker current intensities associated with vertical movements of MOW or its replacement by SCW at intermediate depth. After these sluggish phases enhanced MOW flow dominated again and led to stronger current intensities and most probably sediment erosion on Challenger Mound. Erosion in combination with early diagenetic (oxidation) processes overprinted the sediment layers as indicated by dissolved coral skeletons, the increase in Ca-content and sediment density, minimum δ13Cplanktonic values, as well as the occurrence of gypsum and pyrite, implying a careful evaluation of original and overprinted geochemical signals. We conclude that the Challenger Mound development was already influenced by short-term variability of water masses from southern origin and possible erosional events comparable to the late Pleistocene setting.

The importance of the permanent thermocline to the cold water coral carbonate mound distribution in the NE Atlantic

A prominent feature of the NW European continental slope is the presence of numerous cold water coral carbonate mounds that are clustered in a number of provinces. These provinces occupy a relatively narrow depth range along the continental slope: 95% of all coral carbonate mounds identified on the Irish seabed have their mound bases between 500 and 1000 m water depths, with a peak in distribution at ∼ 650 m water depth. The distribution in mound base depths is skewed with a tail extending from the maximum at 650 m to deeper depths. This distribution brackets the depth of the permanent thermocline in the NE Atlantic (600–1000 m) formed below the base of the winter mixed layer. It is shown that the permanent thermocline is associated with the strongest residual near seabed current flow, with typical residual current speeds up to 2–3 times larger at the thermocline depth compared to other depths. The strong vertical density gradient associated with the permanent thermocline, together with the steep continental slope at those depths, also enhances the energy of certain periodic motions such as internal waves and baroclinic tidal currents. These dynamic conditions favour mound growth through the promotion of significant along-slope sediment transport and also provide large across-slope sediment movement and organic matter fluxes. The stability of the thermocline structure is likely the key in providing favourable conditions over long time scales that allow mound growth through sediment baffling processes.

Microbial lipid markers within and adjacent to Challenger Mound in the Belgica carbonate mound province, Porcupine Basin, offshore Ireland (IODP Expedition 307)

During the Integrated Ocean Drilling Program (IODP) Expedition 307 for the first time a cold-water coral carbonate mound was drilled down through its base into the underlying sediments. In the current study, sample material from within and below Challenger Mound, located in the Belgica carbonate mound province in the Porcupine Basin offshore Ireland, was investigated for its distribution of microbial communities and gas composition using biogeochemical and geochemical approaches to elucidate the question on the initiation of carbonate mounds. Past and living microbial populations are lower in the mound section compared to the underlying sediments or sediments of an upslope reference site. A reason for this might be a reduced substrate feedstock, reflected by low total organic carbon (TOC) contents, in the once coral dominated mound sequence. In contrast, in the reference site a lithostratigraphic sequence with comparatively high TOC contents shows higher abundances of both past and present microbial communities, indicating favourable living conditions from time of sedimentation until today. Composition and isotopic values of gases below the mound base seem to point to a mixed gas of biogenic and thermogenic origin with a higher proportion of biogenic gas. Oil-derived hydrocarbons were not detected at the mound site. This suggests that at least in the investigated part of the mound base the upward flow of fossil hydrocarbons, being one hypothesis for the initiation of the formation of carbonate mounds, seems to be only of minor significance.

Anaerobic oxidation of methane in a cold-water coral carbonate mound from the Gulf of Cadiz

The Gulf of Cadiz is an area of mud volcanism and gas venting through the seafloor. In addition, several cold-water coral carbonate mounds have been discovered at the Pen Duick escarpment amidst the El Arraiche mud volcano field on the Moroccan margin. One of these mounds -named Alpha mound- has been studied to examine the impact of the presence of methane on pore-water geochemistry, potential sulphate reduction (SR) rate and dissolved inorganic carbon (DIC) budget of the mound in comparison with off-mound and off-escarpment locations. Pore-water profiles of sulphate, sulphide, methane, and DIC from the on-mound location showed the presence of a sulphate to methane transition zone at 350 cm below the sea floor. This was well correlated with an increase in SR activity. 13C-depleted DIC at the transition zone (−21.9‰ vs. Vienna Pee Dee Belemnite) indicated that microbial methane oxidation significantly contribute to the DIC budget of the mound. The Alpha mound thus represents a new carbonate mound type where the presence and anaerobic oxidation of methane has an important imprint on both geochemistry and DIC isotopic signature and budget of this carbonate mound.

The 2.6 Ma depositional sequence from the Challenger cold-water coral carbonate mound (IODP Exp. 307): Sediment contributors and hydrodynamic palaeo-environments

During IODP Expedition 307, the first complete sequence through a cold-water coral carbonate mound (a bio-geological seafloor feature created through successive stages of cold-water coral mediated sediment accumulation) was successfully acquired. The full recovery of the Challenger Mound, one of the large (ca. 155 m high) coral carbonate mounds along the NE Atlantic continental margin (Belgica mound province, eastern Porcupine Seabight) facilitates for the first time the study of the entire development of a coral carbonate mound and, hence, allows the identification of the environmental conditions driving and maintaining the entire build-up of these remarkable seafloor habitats. In this study, the different sediment contributors to the Challenger Mound are identified and assessed throughout its entire 2.6 Ma long development. High-resolution siliciclastic particle-size end-member modelling and its ground-truthing (XRD, quartz-sand surface microtextures) indicate the influence of an all dominant contour-current system, operating in variable energetic modes, for most of the sediment accumulation history of the Challenger Mound. Only local, short-distance current-controlled sediment re-dispersal is evidenced, while iceberg rafting is identified as an important depositional mechanism throughout the whole mound development period. Furthermore, evidence for icebergs repeatedly reaching the eastern Porcupine Seabight continental margin, even in the early stages of Northern Hemisphere glacial expansion, is preserved in the mound sequence. Supporting the existing coral-stratigraphy, the Challenger Mound depositional sequence reveals a two-phase development, separated by a significant hiatus. The lower mound-phase (M1; 2.6–1.7 Ma) indicates a semi-continuous, steadily changing, fast accumulating current-controlled depositional environment. The condensed upper mound-phase (M2; 998–1.5 ka) bears witness of a distinct shift to a more glacially-influenced, low accumulation environment, most likely resulting from a reduced capacity to deposit and/or preserve sediments. Cold-water coral density at the site of deposition is assumed to play an essential role in sediment deposition and preservation on Challenger Mound. It enabled the preservation of a unique, higher resolution record in the lower M1 sequence in the otherwise erosive/non-depositional Early-Pleistocene environment along the NW European continental margin. The potential of coral carbonate mounds as intermediate water depth, continental margin, (Plio-)Pleistocene palaeo-archives is thereby showcased.

Sediment dynamics and palaeo-environmental context at key stages in the Challenger cold-water coral mound formation: Clues from sediment deposits at the mound base

IODP Expedition 307, targeting the 160 m high Challenger Mound and its surroundings in the Porcupine Seabight, NE Atlantic, was the first occasion of scientific drilling of a cold-water coral carbonate mound. Such mound structures are found at several locations along the continental margin but are especially numerous off Ireland. All rooted on a common unconformity (RD1) and embedded in drift sediments, the mounds in the Porcupine Seabight remain enigmatic structures, and their initial trigger and formation mechanisms are still not entirely clear. This paper discusses the sedimentary environment during the initial stages of Challenger Mound, and at the start-up of the embedding sediment drift. The results are interpreted within the regional palaeo-environmental context. Based on detailed grain-size analyses and planktonic foraminifera assemblage counts, a 14-m interval overlying the regional base-of-mound unconformity RD1 is characterised at IODP Sites U1317 (on mound), U1316 (off mound), and U1318 (background site). Several sedimentary facies are identified and interpreted in relation to regional current dynamics. Using the foraminifera counts, existing age models for the initial stages of on-mound and off-mound sedimentation are refined. Sedimentation within the initial mound was characterised by a two-mode system, with the observed cyclicities related to glacial/interglacial stages. However, the contrast in environmental conditions between the stages was less extreme than observed in the most recent glacial/interglacial cycles, allowing continuous cold-water coral growth. This sustained presence of coral framework was the key factor for fast mound build-up, baffling sediments at periods of slack currents, and protecting them from renewed erosion during high-current events. The off-mound and background sedimentation consisted mainly of a succession of contourite beds, ranging from sandy contourites in the initial stages to muddy contourites higher up in the sequence, representing the true onset of drift sedimentation. The latter illustrate the increasing importance of glacial conditions after the Mid-Pleistocene Revolution. The overall findings are summarised in a descriptive conceptual model.

Franken Mound: facies and biocoenoses on a newly-discovered “carbonate mound” on the western Rockall Bank, NE Atlantic

Cold-water coral carbonate mounds are widespread along the Irish continental margin. Whereas the Porcupine Seabight and the Rockall Trough are relatively well studied with regard to mound topography, coral coverage, and benthic life diversity, the situation on the western Rockall Bank is rather unknown. Detailed facies and biocoenoses mapping based on video footage analyses was conducted on the newly-discovered Franken Mound. Facies were identified ranging between cliff-like to planar hardgrounds and soft sediments that are partly rippled. A variety of biocoenoses are associated with these facies comprising discrete live coral colonies, dense live and dead coral framework coverage, abundant to scattered coral debris, and a soft sediment faunal community, whereas the latter is three times less speciose as biocoenoses containing live framework-building corals. The facies and biocoenosis classes are supplemented by exposed dropstones, lost fishery nets, and rubbish. The distribution of the classes clearly indicates a close relationship with local current effects and current intensification. Due to the dominance of dead coral framework and the partially exposed internal sediment sequences on the mound flanks, it is assumed that Franken Mound is approaching the “mound retirement” mound growth state.

Hydrodynamics and cold-water coral facies distribution related to recent sedimentary processes at Galway Mound west of Ireland

Cold-water coral carbonate mound development is the result of complex and interactive hydrographical, biological and geological processes that can result in morphostructures several hundred meters high. The case study presented here investigates one of these large mounds – Galway Mound – in the eastern Porcupine Seabight to build an understanding of mound forming processes and the driving factors. For the first time, bottom current data have been recorded at six locations over a mound thus allowing an interpretation of the local flow field to be made. In addition to the overall flow pattern in the Porcupine Seabight, the recorded data display distinct diurnal tides. Comparison of the local flow field, coral facies distributions, current induced seabed features and grain size distributions over the Galway Mound highlights a correlation between the abundance of living corals with areas of enhanced bottom currents. However, the interplay of contour currents, tidal currents and the local topography further influences the coral facies and results in a distinct asymmetry in the coral facies distribution at Galway Mound. By baffling sediment, the corals also affect sedimentation on the mound.

Morphology and environment of cold-water coral carbonate mounds on the NW European margin

Cold-water coral carbonate mounds, owing their presence mainly to the framework building coral Lophelia pertusa and the activity of associated organisms, are common along the European margin with their spatial distribution allowing them to be divided into a number of mound provinces. Variation in mound attributes are explored via a series of case studies on mound provinces that have been the most intensely investigated: Belgica, Hovland, Pelagia, Logachev and Norwegian Mounds. Morphological variation between mound provinces is discussed under the premise that mound morphology is an expression of the environmental conditions under which mounds are initiated and grow. Cold-water coral carbonate mounds can be divided into those exhibiting “inherited” morphologies (where mound morphology reflects the morphology of the colonised features) and “developed” morphology (where the mounds assume their own gross morphology mainly reflecting dominant hydrodynamic controls). Finer-scale, surface morphological features mainly reflecting biological growth forms are also discussed.