Veerle A.I. Huvenne

Large deep-water coral banks in the Porcupine Basin, southwest of Ireland.

The Porcupine Basin, southwest of Ireland, was one of the earliest sites from where the deep-water corals Lophelia sp. and Madrepora sp. were recovered. These deep-water corals have since been found all along the Atlantic margins of Europe, in water depths ranging from 50 to more than 2000 m. Recent geophysical studies have demonstrated the mound-building potential of deep-water corals. Available data indicate that three major provinces of coral bank occurrences can be identified in the Porcupine Basin: (1) high-relief surface mounds which have a dimension of 1 by 5 km and a height up to 200 m (‘Hovland’ mounds), flanked to the north by (2) a swarm of buried mounds, somewhat smaller (up to 90 m), and with more irregular shapes than those recognised in area 1 (‘Magellan’ mounds), and (3) outcropping or buried, conical mounds (single or in elongated clusters, up to 150 m high) occurring on the southeastern slope of the basin (‘Belgica’ mounds). As far as can be inferred from shallow cores, the surface lithology predominantly consists of an upper layer rich in foraminiferal sand and terrigenous silty clay with intercalations of biogenic rubble. The banks host a remarkable number of colonies of living and dead Lophelia pertusa and Madrepora oculata. The living and dead assemblages are underlain by a significant layer of coral debris in a muddy matrix. Deep-water coral debris together with a living association of the same species covers the surface of the ‘Belgica’ and ‘Hovland’ mounds, which may suggest that these corals have played a significant role in the development of the mound structures. The capacity for mound formation by scleractinian corals in the aphotic zone has been known for some time. Examples are found at different locations along the shelves and the continental margins of the North Atlantic. The role of the corals in these deep-water build-ups is still a point of debate. Though the genesis and initial control of mound settings in this basin might be related to hydrocarbon seeps, it appears that the major development of the Porcupine coral banks in recent geological times has most likely been controlled by oceanic circulation and dynamics in water masses and nutrient supply.

The Discovery of New Deep-Sea Hydrothermal Vent Communities in the Southern Ocean and Implications for Biogeography

A survey of Antarctic waters along the East Scotia Ridge in the Southern Ocean reveals a new vent biogeographic province among previously uncharacterized deep-sea hydrothermal vent communities.

Marine litter distribution and density in European seas, from the shelves to deep basins

Anthropogenic litter is present in all marine habitats, from beaches to the most remote points in the oceans. On the seafloor, marine litter, particularly plastic, can accumulate in high densities with deleterious consequences for its inhabitants. Yet, because of the high cost involved with sampling the seafloor, no large-scale assessment of distribution patterns was available to date. Here, we present data on litter distribution and density collected during 588 video and trawl surveys across 32 sites in European waters. We found litter to be present in the deepest areas and at locations as remote from land as the Charlie-Gibbs Fracture Zone across the Mid-Atlantic Ridge. The highest litter density occurs in submarine canyons, whilst the lowest density can be found on continental shelves and on ocean ridges. Plastic was the most prevalent litter item found on the seafloor. Litter from fishing activities (derelict fishing lines and nets) was particularly common on seamounts, banks, mounds and ocean ridges. Our results highlight the extent of the problem and the need for action to prevent increasing accumulation of litter in marine environments.

Seismic evidence of current-controlled sedimentation in the Belgica mound province, upper Porcupine slope, southwest of Ireland

The Porcupine Seabight is an embayment that takes a particular position in the NE Atlantic slope. Sonographs, a few current measurements and hydrodynamic modelling suggest the presence of a strong northward-flowing bottom current, locally enhanced by internal tides, affecting the eastern slope of the Seabight. At this location a province of coral banks is described, expressed as mounds lined up in along-slope-trending ridges. In this paper, very high-resolution single-channel seismic profiles are used to evaluate to what extent the bottom currents influenced the deposition of the sediments surrounding the mounds throughout the Late Cenozoic. Three seismostratigraphic units (P1, P2 and P3) can be identified in the Belgica mound area, separated by two margin-wide discontinuities (RD2 and RD1). Within Unit P1 (probably Early to Middle Miocene) upslope-migrating sediment waves are observed, suggesting strong bottom currents were already active in the Miocene. After an early Middle Miocene erosion event, represented by reflector RD2, an acoustically transparent layer (Unit P2) of as yet unknown lithology was deposited in the studied area. A second margin-wide erosional event, marked by the Late Pliocene RD1 reflector, removed a large part of Unit P2 and has cut deeply into Unit P1. Subsequently, the Belgica mounds were constructed spectacularly fast on topographic irregularities on the RD1 paleobathymetry. The onlap within the Quaternary Unit P3, which surrounds these mounds, suggests that the mounds were already present before the deposition of P3 and were big enough to affect the intensity of the currents around them. Furthermore, the channels and the mounds are, together with the complex oceanographic regime, the key morphological elements responsible for the shaping of a contourite system in the Belgica mound area during the Quaternary. One drift body is formed by an inferred south–north-directed current, with a drift levee and associated channel located on its western side. Between this channel and the mounds, large-scale sediment waves suggest an intensified bottom current running along the foot of the steep flanks of the mounds. The Belgica mounds are embedded in another drift body. Here, an interaction of bottom and turbidity currents is suggested, creating short turbidite channels at the southern and northern flanks of the mounds. Locally, small confined drifts can be observed where Unit P3 is deposited in a narrow passage made by the paleobathymetry of RD1 and the mounds.

Textural analyses of sidescan sonar imagery from two mound provinces in the Porcupine Seabight

Large mound structures have been discovered in the Porcupine Seabight (Northeast Atlantic) at a depth of 500–1200 m, associated with the growth of cold-water deep-sea coral species such as Lophelia pertusa (L.) or Madrepora oculata (L.). During the Training Through Research cruise in 1997, high-resolution OREtech sidescan sonar data were acquired over two provinces of these structures. This article focuses on the presentation and quantitative interpretation of representative sections of these sidescan data from areas around the Belgica and Hovland mounds. Several image analysis tools were used, but texture analysis, based on grey level co-occurrence matrices, gave the best results. Entropy and homogeneity indices were calculated, and the resulting images made it possible to discriminate between different seabed features on a quantitative basis. Mounds, moats and background sediments could be delineated accurately, and the image textures could be linked to the actual seafloor appearance through core descriptions and deep-towed video data. A major difference was found in the acoustic returns from the two provinces studied: the Belgica province shows much rougher textures. This is due to an actual difference in seabed roughness, caused by a difference in bottom currents and sediment dynamics in the two areas. The combined effect of the northward-directed North Atlantic slope current and superimposed internal waves and tides appears to be much stronger in the Belgica province. The reported difference in current strength might well influence the growth of the deep-water coral species in both areas.

A picture on the wall: innovative mapping reveals cold-water coral refuge in submarine canyon.

Cold-water corals are azooxanthellate species found throughout the ocean at water depths down to 5000 m. They occur in patches, reefs or large mound structures up to 380 m high, and as ecosystem engineers create important habitats for a diverse fauna. However, the majority of these habitats are now within reach of deep-sea bottom trawling. Many have been severely damaged or are under threat, despite recent protection initiatives. Here we present a cold-water coral habitat type that so far has been overlooked – quite literally – and that has received minimal impact from human activities. Vertical and overhanging cliffs in deep-sea canyons, revealed using an innovative approach to marine habitat mapping, are shown to provide the perfect substratum for extensive cold-water coral-based communities. Typical canyon-related processes, including locally enhanced internal tides and focussed downslope organic carbon transport, provide favourable environmental conditions (current regime, food input) to sustain the communities, even outside the optimal depth and density envelopes reported elsewhere in the NE Atlantic. Our findings show that deep-sea canyons can form natural refuges for faunal communities sensitive to anthropogenic disturbance, and have the potential to fulfil the crucial role of larval sources for the recolonisation of damaged sites elsewhere on the margin.

A 3D seismic study of the morphology and spatial distribution of buried coral banks in the Porcupine Basin, SW of Ireland

An industrial 3D seismic data volume, supplemented by high-resolution 2D seismics, was used to study part of a province of buried mound structures in the Porcupine Basin, southwest of Ireland. These ‘Magellan’ mounds and their associated moat structures, interpreted as scour marks, were mapped semi-automatically from time-structure and isopach maps. Image analysis techniques such as a tophat transformation (mathematical morphology) were applied for feature extraction. Size measures of both mounds and moats were derived from the resulting maps and summarised by means of some descriptive statistics. Spatial variability in mound occurrence and characteristics was investigated. Comparison with other mound structures in the area allowed the Magellan mounds to be identified as ‘coral banks’, associated with the growth of cold-water deep-sea coral species such as Lophelia pertusa (L.) and Madrepora oculata (L.). Mound growth clearly started in a single ‘event’, confined in time and space. Bottom currents and oceanographic characteristics of the surrounding water masses influenced this sudden process and the further mound development. However, the analysis of the 3D seismic data set did not allow us to identify unambiguously the actual cause for the sudden mound start-up. The mounds appear to have formed a dense cluster of structures of moderate size, which are significantly elongated in a N–S direction. They are associated with even more elongated moats, implying a periodically reversing N–S-directed current influence. A spatial density of one mound per km2 was measured, which remains more or less constant over the area investigated. Mound width and cross-sectional area and moat shape gradually change across the mound province, due to spatially changing environmental conditions at the initial growth stages of the mounds and during their further development (interplay between current regime and sedimentation).

The seabed appearance of different coral bank provinces in the Porcupine Seabight, NE Atlantic: results from sidescan sonar and ROV seabed mapping

Carbonate mounds, identified as deep-water coral banks, have been reported recently from three provinces in the Porcupine Seabight, SW of Ireland. As yet, the mechanisms behind their formation and development are only partly understood. This contribution discusses their seabed appearance and present-day sedimentary environment, based on a large-scale TOBI sidescan sonar mapping carried out in 2002, and on detailed ROV video records from specific sites within the three mound provinces, collected in 2001. The study of the present-day characteristics and variability of these mounds can help to understand their development history in the past.

Europe’s Grand Canyon: Nazaré submarine canyon

The Nazare submarine canyon extends similar to 210 km westward from the coast of Portugal, down to a water depth of > 4300 m. The considerable habitat heterogeneity found throughout the canyon is affected by strong currents and high turbidity, especially in the upper parts of the canyon. The canyon morphology comprises steep slopes, scarps, terraces, and overhangs, and a deeply incised thalweg is found in the lower part of the canyon. The seabed within the canyon is composed of varying proportions of rock and sediments that range from sand to fine mud. This great variation in physical environment is reflected by the varied fauna inhabiting the canyon. Diversity tends to decrease with depth, but there is also continual replacement of species with increasing water depth. Certain groups, such as the gorgonians and sea lilies, tend to be found on rocky surfaces, while large protozoans dominate the sediments at 3400-m depth. In addition to describing the fauna of Nazare Canyon, we discuss experiments undertaken as part of the HERMES project to elucidate the ecosystem function processes operating in the deeper parts of the canyon.

New insights into the morphology, fill, and remarkable longevity (>0.2 m.y.) of modern deep-water erosional scours along the northeast Atlantic margin

A series of large-scale erosional scours are described from four modern deep-water canyon and/or channel systems along the northeast Atlantic continental margin. Regional-scale geophysical data indicate that most scours occur in zones of rapid flow expansion, such as canyon and/or channel termini and margins. High-resolution images of the scours cover ∼25 km² at 2 × 2 m pixel size, and were obtained at depths of 4200–4900 m using Autosub6000, an autonomous underwater vehicle equipped with an EM2000 multibeam bathymetry system. Sedimentological and microfossil-based chronological data of scour fills and interscour areas were obtained via accurately located piston cores that targeted specific sites within imaged areas. These core data reveal a number of key findings. (1) Deep-water scours can be very long lived (>0.2 m.y. ) and may undergo discrete phases of isolation, amalgamation, and infilling. (2) Deep-water scours can develop via a composite of cutting and filling events with periodicities of between tens of thousands and hundreds of thousands of years. (3) Immediately adjacent scours may have strikingly different sedimentological histories and do not necessarily evolve contemporaneously. (4) Scour infills are typically out of phase with sedimentation in intrascour areas, having thin sands internally and thick sands externally, or thick muds internally and thin muds externally. (5) Erosional hiatuses within scour fills may represent hundreds of thousands of years of time, and yet leave little visible record. Four distinct morphologies of scour are identified that range from 40 to 3170 m wide and 8 to 48 m deep: spoon shaped, heel shaped, crescent shaped, and oval shaped. Isolated scours are shown to coalesce laterally into broad regions of amalgamated scour that may be several kilometers across. The combined morphosedimentological data set is used to examine some of the putative formative mechanisms for scour genesis.