Porcupine Bank Research Articles

Evidence for population structuring of blue whiting (Micromesistius poutassou) in the Northeast Atlantic

AbstractWas, A., Gosling, E., McCrann, K., and Mork, J. 2008. Evidence for population structuring of blue whiting (Micromesistius poutassou) in the Northeast Atlantic. – ICES Journal of Marine Science, 65: 216–225. Many marine fish species are characterized by large population sizes, strong migratory behaviour, high fecundity, and pelagic eggs and larvae that are subject to passive transport by ocean currents, all factors that tend to reduce the rate of development of genetic partitioning among localized populations. The blue whiting (Micromesistius poutassou) is a commercially important gadoid that exhibits all these characteristics, although to date there has been little evidence of genetic heterogeneity except at the latitudinal extremes of its range in the NE Atlantic. Genetic variation was analysed at five microsatellite loci in 16 samples, 14 comprising spawning adults, collected along the continental shelf from 44°N to 60°N, a distance of ∼1900 km. Although pairwise FST values were low (0.0–0.040; mean 0.0097), more than 40% of the estimates were significant, with Celtic Sea and Bay of Biscay samples significantly differentiated from samples from the Porcupine Bank, Hebridean Shelf, Sulisker Bank, and Papa Bank. There was also significant differentiation between samples taken in different years on Rockall Bank. Mantel tests revealed no significant isolation by distance. We used a landscape genetics approach, which combines spatial and genetic information, to detect barriers to gene flow. Four zones of lowered gene flow were identified, generally in concordance with hydrographic patterns, fish spawning behaviour, and the simulated transport of larvae in the NE Atlantic Ocean.

Two new species of Adontorhina Berry, 1947 (Bivalvia: Thyasiridae) from the Porcupine Bank, off the west coast of Ireland

Two new species of Adontorhina Berry, 1947 (Bivalvia: Thyasiridae) from the Porcupine Bank, off the west coast of Ireland

Two new species of Adontorhina Berry, 1947 (Bivalvia: Thyasiridae) from the Porcupine Bank, off the west coast of Ireland

A minute bivalve, Adontorhina keegani, new species (Thyasiridae) is described from the Porcupine Bank, west of Ireland. The new species occurs between 300 and 789 m on the continental slope. The shell is elongate and compressed, with a flattened posterior and complete lack of radial sulcus. The shell has a hinge margin bearing irregular granules and noticeably low umbones, located in the posterior. Hydroids were found growing on the margins of the shell, indicating a shallow burrowing habit. Adontorhina similis, new species, was previously recognized as a European variety of Mendicula pygmaea Verrill & Bush, 1898, but is here shown to be a distinct species. The hinge bears irregular granules, which precludes classification as a Mendicula species. The shell is elongate, moderately inflated with prominent umbones and a pointed posterior margin. The posterior flank of the shell is flattened and does not bear hydroids. In comparison with previously described species of Adontorhina, the new species are more elongate and less inflated.

Juvenile life history of NE Atlantic orange roughy from otolith stable isotopes

Otoliths of pre-recruit orange roughy ( Hoplostethus atlanticus) were sampled from different deep-water habitats (‘flat’ and ‘hill’) and areas (north and south) on the Porcupine Bank. Age-based profiles for stable isotopes of carbon (δ 13C) and oxygen (δ 18O) in these otoliths were developed, which provide a fish life-history record of water depth and metabolic activity. These profiles were consistent among all individuals implying endogenous (ontogenic) influences on the pattern. The data indicate that post-larval orange roughy are mesopelagic active foragers, early juvenile fish move into a low energy deep-demersal phase, and older pre-recruit orange roughy assume the habitat depth and metabolic rate typical of adults. Comparison of otolith stable isotope profiles among areas and habitats on the Porcupine Bank suggest that juvenile orange roughy from South ‘hill’ and ‘flat’ habitats experience differing temperature and metabolic status at certain life stages. This may reflect oceanographic and ecological divergence between the two environments and suggests fine-scale population structure that may reduce resilience to exploitation.

Remote sensing and modelling of bio-physical distribution patterns at Porcupine and Rockall Bank, Northeast Atlantic

Monthly composites of multi-year sea surface temperature (SST) and chlorophyll-a (Chl- a) have been used in combination with ocean model simulations to study bio-physical distribution patterns at Porcupine and Rockall Bank, two large submarine banks in the Northeast Atlantic in close proximity to the European shelf edge. Seven years (January 1998–December 2004) of remotely sensed data have been collated to create monthly climatological fields and to analyse principal spatio-temporal characteristics. At both banks, a region of cooler SST is found over the summit region compared to warmer waters of the surrounding ocean, less apparent in summer when capped by the seasonal thermocline. Enhanced Chl- a levels are found over both banks with a lifetime partly exceeding the bloom period. At Rockall Bank, both SST and Chl- a signals are more pronounced and persistent showing a 30% increase in annual Chl- a levels over the summit area with an even higher ratio in spring and autumn. A combination of physical processes appears to promote the enhanced productivity over both banks through the generation of a quasi-steady dome of cold, dense water during winter convection and upwelling events. This cold dome is associated with the presence of a retentive circulation based on Taylor cap dynamics and tidal rectification processes. The larger and more persistent enhancement of Chl- a levels over Rockall Bank would appear due to its isolated nature as well as its size. In contrast, Porcupine Bank is partly attached to the Irish shelf edge and exposed to the poleward flowing shelf edge current which may strip passive particles from the central bank region. Satellite derived Chl- a spring/summer distributions over the banks have been used to initialise model simulations of passive tracer dispersion. Timescales for the observed lifetime of the remotely sensed Chl- a patches are consistent with model derived retention timescales and simple scaling for the dispersion of passive biological material over the banks. Surface particle residence times over Rockall Bank are estimated to exceed Porcupine Bank values by a factor of two. Finally, the tidal contribution to individual particle motion is found to be large in some Rockall Bank areas, but less important at Porcupine Bank.

Shelf to slope sedimentation processes and the impact of Plio–Pleistocene glaciations in the northeast Atlantic, west of Ireland

The eastern margin of the Rockall Trough, between 54°30′ and 56°30′N, straddles the southern limit of the glaciated North Atlantic margin. A complex system of canyons, channels and slope failure features on the steep margins of the trough is resolved on the TRIM (TOBI Rockall Irish Margins Project) deep-towed sidescan survey. The canyons consist of a complex headwall gullied region, produced by multiple slope failure events, and interconnect to form a cauliform pattern in plan view. Individual gullies are U-shaped in cross-section in the upper parts of the margin and are connected downslope to single, deeply-incised, V-shaped channels. These channels feed extensive aprons of sediment on the lower slope supplied by failure of the headwall regions and sediment transport along the shelf into the canyon headwalls. The change in the canyon morphology systems reflects a more abundant supply of glaciomarine sedimentary debris from Pleistocene ice sheets, whose southerly limit lay along this part of the eastern Rockall margin. A good correlation between acoustic sidescan backscatter and more deeply penetrating 3.5 kHz profiler data suggests that sediment transport pathways were stable through time. Slope failure escarpments occur in association with the canyon systems along the upper part of the slope. Canyons are absent in the north of the area where the slope gradient is lowest. This is interpreted as the result of progradation of sheet-like flows of glaciomarine debris across the southern part of the Donegal Fan. A poor correlation between acoustic backscatter and sub-bottom profile data indicates that sediment transport pathways varied through time in the region. Evidence for strong northeast-flowing bottom currents is present along the upper slope and shelf. These currents transport and sort sediments along the entire eastern margin of the Rockall Trough from the south of the Porcupine Bank to the Hebrides Terrace Seamount.

Sponge diversity and community composition in Irish bathyal coral reefs

Sponge diversity and community composition in bathyal cold water coral reefs (CWRs) were examined at 500-900 m depth on the southeastern slopes of Rockall Bank and the northwestern slope of Porcupine Bank, to the west of Ireland in 2004 and 2005 with boxcores. A total of 104 boxcore samples, supplemented with 10 trawl/dredge attempts, were analyzed for the presence and abundance of sponges, using microscopical examination of (sub)samples of collected coral branches, and semi-quantitative macroscopic examination. Approximate minimum size of identified and counted sponge individuals was 1 mm. Literature data were added to the Porcupine Bank results to compensate for a less intensive sampling program in that location. Species richness and abundance were determined at local (sample diversity, pooled-sample diversity, local reef diversity), between-reef (diversity of two reef areas at 15 km distance), and regional scales (diversity of three reef areas over a distance of 200 km). Abiotic and biotic parameters including depth, the presence and cover of live coral, dead coral and sand, local reef, and orientation towards the nearest reef mound summit, were included in a constrained ordination technique (RDA); a Monte Carlo forward selection procedure was used to obtain significant predictors of variation in composition. The results of this analysis were compared with unconstrained ordination (PCA) and cluster analysis. The presence of live coral, depth and the local reefs C1 and C3 proved to be significant predictors of variation in sponge composition. The PCA and cluster analysis confirmed these results. Sample species richness was consistently heterogeneous from zero species and individuals up to 57 species and 90 individuals per (boxcore) sample. Species richness of local reefs determined from pooled samples showed the three localities studied to have similar species richness, namely 105-122 species in each location. Species richness was highest in samples with relatively low live coral cover. As in the RDA, live coral presence and depth appeared to be responsible for most of the variation observed in the cluster results. Cluster analysis of Bray-Curtis dissimilarity values of the pooled samples of all three reef localities using presence/absence data of all available samples indicated that distance appeared to structure the composition of the sponge assemblages of the three reef mound areas, but much less so within and among local reefs. Bathyal reefs of the regions to the west of Ireland were found to have a combined sponge species richness of 191 species, exceeding the richness of individual reef mound areas by c. 38-45%. Sponge presence in CWRs is clearly structured and controlled by biotic and abiotic factors. In particular, live coral presence appears a significant predictor of CWR sponge composition and diversity.

Fecundity and Maturity of Orange Roughy (Hoplostethus atlanticus Collett 1889) on the Porcupine Bank, Northeast Atlantic

The first, comprehensive analysis of the fecundity and reproductive maturity of orange roughy, Hoplostethus atlanticus, from a specific area in the Northeast Atlantic is reported. Specimens were collected from aboard vessels, targeting this species on the Porcupine Bank (ICES subarea VII), in waters of between 1400 m and 1,650 m depth. Between September and December 2002, a non-random, stratified, sampling protocol was implemented by on board fisheries biologists to collect mature female fish between 300 mm and 540 mm SL. Ovaries from 65 individuals, representing the majority of 10 mm SL size classes, formed the analytical sample. A novel, digital method of oocyte counting was developed, allowing digital images of oocytes to be annotated, counted and stored. Total fecundity ranged between 20,352 and 244,578 oocytes per female and mean total fecundity was estimated to be 97,368 oocytes per female (SD = 48,322). Relative fecundity was estimated to be 33,376 oocytes per kg (SD = 11,407). Fecundity was shown not to decrease with age. Macroscopic analyses showed that 50% of females were not mature until they reached 27.5 years and 37 cm SL. Comparison with stocks from the southern hemisphere indicate that orange roughy from the Northeast Atlantic mature at a larger size and generally have a higher mean fecundity than those found in the southern hemisphere. This may reflect differences in growth rates influenced by environmental variables and fishing pressure.

Benthic dynamics at the carbonate mound regions of the Porcupine Sea Bight continental margin

A brief review is given of some dynamical processes that influence the benthic dynamics within the carbonate mound provinces located at the Porcupine Bank/Sea Bight margin, NE Atlantic. The depth range of the mounds in this region (600–1,000 m) marks the upper boundary of the Mediterranean outflow water above which Eastern North Atlantic Water dominates. Both water masses are carried northwards by the eastern boundary slope current. In the benthic boundary layer both the action of internal waves, and other tidal period baroclinic waves, may enhance the bottom currents and add to both the residual and maximum flow strength. Both residual and maximum bottom currents vary at different mound locations, with stronger currents found at Belgica (SE Porcupine Sea Bight) mound and Pelagia (NW Porcupine Bank) mound regions, whilst weakest currents are found at the Hovland and Magellan Mounds at the northern Sea Bight margin. The differences may be attributed to the presence of internal waves (Pelagia) or bottom intensified diurnal waves (Belgica). These different dynamical regimes are likely to have implications for the distribution patterns of live coral at the different locations.

Mid- to Late Cenozoic canyon development on the eastern margin of the Rockall Trough, offshore Ireland

The sediment-undersupplied eastern margin of the Rockall Trough, west of Ireland, is incised by numerous canyons and slope failure features. Swath multibeam bathymetry has been integrated with 2D seismic profiles to constrain the Neogene evolution of the slope and its canyons. The morphology varies along the margin, with canyon heads located at mid-slope depths in the south but extending onto the shelf in the north. West of Porcupine Bank, slope gullies connect with a distributative channel system on the trough floor, while north of Porcupine Bank the basin floor is flat and featureless. Draped fault-blocks and deep structures exerted an important influence on slope gradients, canyon extent and geometry. A ‘bottom driven’ upslope–retrogressive slope failure mechanism is inferred for canyon formation. They were initiated by failure localisation following widespread slope rotation and instability linked to differential subsidence that produced a latest Eocene–early Oligocene (C30) regional unconformity. In the NE Rockall, where the greatest density of canyons occurs, a large mass failure wedge directly overlies the C30 surface and the seabed canyons have incised the upper part of the wedge. Axial profile data indicate that canyons in the NE Rockall Trough formed in Mid-Cenozoic times but were locally reutilised as sediment conduits during Plio-Pleistocene slope progradation.

Organic matter quality and supply to deep-water coral/mound systems of the NW European Continental Margin

Comparison of five deep-water coral (DWC)/mound ecosystems along the European Continental Margin shows that suspended particulate organic matter (sPOM), a potential food source, is lipid rich and of high quality. However, there are differences between the sites. The Darwin and Pelagia Mounds (N. Rockall Trough and N. Porcupine Bank, respectively) have higher proportions of labile particulate lipids (including high proportions of polyunsaturated fatty acids) in the benthic boundary layer than Logachev, Hovland and Belgica Mounds (Rockall Bank, S. Porcupine Bank and Porcupine Seabight, respectively). The high quality sPOM could be transported downslope from the euphotic zone. There is some evidence for inter-annual variability at some sites (e.g. Hovland and Logachev Mounds) as large differences in suspended lipid and particulate organic carbon concentrations were observed over the sampling period. Elevated total organic carbon contents of sediments at mound sites, relative to control sites in some cases (particularly Darwin Mounds), probably reflect local hydrodynamic control and the trapping of sPOM by the DWC. Fresh POM can be relatively rapidly transferred to significant depth (up to 8 cm) through bioturbation that is evident at all sites. There is no clear evidence of present day hydrocarbon seepage at any of the sites.

Seasonal distribution of orange roughy ( Hoplostethus atlanticus) on the Porcupine Bank west of Ireland

The Porcupine Bank west of Ireland comprises broad, gently sloping areas interspersed with complexes of hills and canyons. An Irish deepwater fleet has exploited orange roughy ( Hoplostethus atlanticus) here since 2000. Seasonal aggregations are targeted on hills and a year-round bycatch is taken in a flat ground mixed species fishery, with roughy being caught at 675–1450 m depth. Orange roughy ( N = 3575) were sampled at sea in both of these fisheries from August 2003 to April 2004 and during February 2005. Standard length (SL) and stage of sexual maturity were recorded, with adult fish varying between 29 and 60 cm SL. Spawning of orange roughy extended from January to April and peaked in late February to early March in both 2004 and 2005. Mean SL/haul for orange roughy differed significantly between the spawning season (January–April) and the rest of the year on each of ‘flat’ and ‘hill’ habitats. Mean SL/haul also differed between ‘flat’ and ‘hill’ during the spawning season but not during the rest of the year (Mann–Whitney tests, P < 0.05). Proportions of orange roughy >30 cm SL/haul that would not spawn in the current year did not differ between ‘hill’ and ‘flat’ during the spawning season (Mann–Whitney test, P = 0.07) but spawning ogives for the two habitats were significantly different ( F = 18.81, d.f. = 2.78, P < 0.01) and provided a lesser L 50 for roughy on the ‘hills’ (34.10 cm SL) than on the ‘flat’ (37.00 cm SL). Hill aggregations appeared to comprise larger fish with a higher probability of spawning in the current year and hence seasonal migration of fish from flat ground to spawning aggregations on the hills is suggested.

A record of fluctuating bottom currents on the slopes west of the Porcupine Bank, offshore Ireland — implications for Late Quaternary climate forcing

Both local and regional controls on slope sedimentation west of Porcupine Bank are assessed using an array of 25 gravity cores, integrated with shallow seismic, TOBI side-scan and high-resolution bathymetry data. The cores were retrieved from an area of smooth, distally steepened slope (between 52° and 53°N) in water depths of 950 to 2750 m. The slope here is unmodified by gravity failures and is swept by bottom currents that flow from S to N along the margin. The cores reveal a coherent shallow stratigraphy that can be traced along and between transects at upper-, mid- and lower-slope levels. AMS 14C dating, oxygen-isotopes and carbonate profiles suggest the cored record could extend as far back as 500 ka in the longest cores, with most cores providing details of the slope response to the last interglacial, last glacial and Holocene forcing. The facies indicate deposition was dominated by a combination of bottom currents, ice-rafting and hemipelagic settling, with carbonate-prone deposits during interglacials, and siliciclastic deposits during glacials. Inferred contourites imply that strong currents operated during interglacials, with weaker current reworking during glacial conditions. A pair of erosion surfaces record significant mid- and upper-slope scouring during Marine Isotope Stage (MIS 3) and in the Early Holocene. The lateral facies distribution implies stronger currents at shallower levels on the slope, although there is evidence that the core of the current migrated up and down the slope, and that sand might locally have spilt down-slope. The bathymetry influenced both the wider geometry of the condensed contourite sheet and the local thickness and facies variation across the slope. A significant result of the study is the identification of a pair of thin sand–mud contourite couplets that record enhanced bottom-current reworking corresponding to periods of interstadial warming during MIS 3. The couplets can be correlated to the terrestrial records onshore Ireland and imply that the NE Atlantic margin oceanographic and onshore climate records are strongly coupled at interstadial level.

Genetic differentiation of blue whiting (Micromesistius poutassou Risso) populations at the extremes of the species range and at the Hebrides–Porcupine Bank spawning grounds

AbstractThe blue whiting, Micromesistius poutassou (Teleostei, Gadidae) is found between latitudes 26° and 82°N along the continental margin of the Northeast Atlantic, with smaller populations in the Northwest Atlantic and the Mediterranean. There is an annual spawning aggregation on the Porcupine Bank and Hebridean Shelf (west of Ireland and Scotland, respectively), where most of the blue whiting population of the Northeast Atlantic spawns. Analysis of samples from the Barents Sea, the Northeast Atlantic, and the Mediterranean (n = 850, 11 samples) using one minisatellite and five microsatellite loci revealed significant geographic heterogeneity and isolated populations at the extremes of the species range in the Barents Sea and the Mediterranean. Furthermore, there was evidence of genetic heterogeneity among samples taken during the spawning season on the Porcupine Bank and Hebridean Shelf, with highly significant differentiation between the samples taken in the Hebrides in 1992 and 1998.

Wavelet and multitaper coherence methods for assessing the elastic thickness of the Irish Atlantic margin

SUMMARY There have been some inconsistencies in estimates of the effective elastic thickness of the continental lithosphere Te based upon admittance or coherence relationships between gravity and topography. This paper compares multitaper and wavelet methods to analyse the coherence between Bouguer gravity and bathymetric data over the Irish Atlantic margin. The analyses show that similar lateral Te variations can be recovered from the data, but demonstrate that the size of the data window can give rise to a significant downward bias in Te estimates. A seismically constrained 3-D gravity inversion over the Rockall basin shows the presence of surface and subsurface loads whose ratio is loosely correlated with load ratio variations generated from the wavelet coherence method. The Te and load ratio, f variations can be plausibly related to major geological structures on the margin. If the load ratio variations can be interpreted geologically, it implies that spectral based methods to estimate effective elastic thickness must incorporate subsurface loads within the underlying theoretical model. On the Irish Atlantic margin, Te is generally low (6‐18 km) and is associated with a NE‐SW Caledonian trend. The weakest lithosphere is in the southern Rockall basin, Porcupine bank and Porcupine basin and the strongest lithosphere is along the Rockall‐Hatton region. The low Te values are consistent with results from other passive margins. The reasons for such low Te values on the Irish Atlantic margin remain unclear, but may be the consequence of Te being frozen into the lithosphere when loads were emplaced during continental breakup and temperature gradients were high. The process of sedimentation and the presence of fluids may be contributory factors. There is an indication of a geological and rheological divide between the Rockall‐Hatton region and the Rockall basin, possibly associated with the Caledonian orogenic front.

Simulation of mackerel (Scomber scombrus) recruitment with an individual‐based model and comparison with field data

AbstractAn individual‐based model (IBM) for the simulation of year‐to‐year survival during the early life‐history stages of the north‐east Atlantic stock of mackerel (Scomber scombrus) was developed within the EU funded Shelf‐Edge Advection, Mortality and Recruitment (SEAMAR) programme. The IBM included transport, growth and survival and was used to track the passive movement of mackerel eggs, larvae and post‐larvae and determine their distribution and abundance after approximately 2 months of drift. One of the main outputs from the IBM, namely distributions and numbers of surviving post‐larvae, are compared with field data as recruit (age‐0/age‐1 juveniles) distribution and abundance for the years 1998, 1999 and 2000. The juvenile distributions show more inter‐annual and spatial variability than the modelled distributions of survivors; this may be due to the restriction of using the same initial egg distribution for all 3 yr of simulation. The IBM simulations indicate two main recruitment areas for the north‐east Atlantic stock of mackerel, these being Porcupine Bank and the south‐eastern Bay of Biscay. These areas correspond to areas of high juvenile catches, although the juveniles generally have a more widespread distribution than the model simulations. The best agreement between modelled data and field data for distribution (juveniles and model survivors) is for the year 1998. The juvenile catches in different representative nursery areas are totalled to give a field abundance index (FAI). This index is compared with a model survivor index (MSI) which is calculated from the total of survivors for the whole spawning season. The MSI compares favourably with the FAI for 1998 and 1999 but not for 2000; in this year, juvenile catches dropped sharply compared with the previous years but there was no equivalent drop in modelled survivors.

An individual‐based model of the early life history of mackerel (Scomber scombrus) in the eastern North Atlantic, simulating transport, growth and mortality

AbstractThe main purpose of this paper is to provide the core description of the modelling exercise within the Shelf Edge Advection Mortality And Recruitment (SEAMAR) programme. An individual‐based model (IBM) was developed for the prediction of year‐to‐year survival of the early life‐history stages of mackerel (Scomber scombrus) in the eastern North Atlantic. The IBM is one of two components of the model system. The first component is a circulation model to provide physical input data for the IBM. The circulation model is a geographical variant of the HAMburg Shelf Ocean Model (HAMSOM). The second component is the IBM, which is an i‐space configuration model in which large numbers of individuals are followed as discrete entities to simulate the transport, growth and mortality of mackerel eggs, larvae and post‐larvae. Larval and post‐larval growth is modelled as a function of length, temperature and food distribution; mortality is modelled as a function of length and absolute growth rate. Each particle is considered as a super‐individual representing 106 eggs at the outset of the simulation, and then declining according to the mortality function. Simulations were carried out for the years 1998–2000. Results showed concentrations of particles at Porcupine Bank and the adjacent Irish shelf, along the Celtic Sea shelf‐edge, and in the southern Bay of Biscay. High survival was observed only at Porcupine and the adjacent shelf areas, and, more patchily, around the coastal margin of Biscay. The low survival along the shelf‐edge of the Celtic Sea was due to the consistently low estimates of food availability in that area.

A new species of amphipod crustacean, Pleusymtes comitari sp. nov., associated with gorgonians on deep-water coral reefs off Ireland

A new species of pleustid amphipod of the genus Pleusymtes is described from the west coast of Ireland. It was found to be common in association with an Acanthogorgia sp. gorgonian at 725 and 900 m depth on coral habitats in the continental shelf-break areas of the Porcupine Seabight and Porcupine Bank. It is assumed to be a commensal due to its modified pereopod dactyls.

Structure and development of giant carbonate mounds at the SW and SE Rockall Trough margins, NE Atlantic Ocean

High-resolution seismic reflection profiling carried out in 1997–1999 showed that giant carbonate mounds occur between 500 and 1200 m water depth along both the SE and SW margins of Rockall Trough. The mounds rise 5–300 m above the surrounding seafloor and have diameters at their bases of up to 5 km. Buried mounds, at relatively shallow depths below the seafloor, are also found. Both individual and complex clusters of mounds can be recognized. Smaller and individual, sometimes buried mounds are found at the upper slope. On the SW Rockall Trough margin, higher, steeper and individual mounds are found deeper downslope (900–1100 m). At the middle slope the mounds merge into a complex structure and form complex clusters with a very irregular upper surface and an apparent lack of internal reflectors (600–1000 m depth). Initial results of high-resolution 3D-seismic profiling at 30 m line spacing in August 2000 in two boxes of 750 m×14.5 km covering a small, central part of the SW Rockall Trough mound area indicate two stages of mound development here, the latest probably of Pliocene–Holocene age. This age, based on the presence of a regionally recognizable unconformity below the mounds, is also assumed for the mounds of the SE Rockall Trough (Porcupine Bank) margin.

Giant carbonate mud mounds in the southern Rockall Trough

Seismic profiles, sidescan sonar data, bottom photography and sampling proved for the first time that there are giant carbonate mounds (bioherms) in the Rockall Trough. The Pelagia Mounds, on the northern Porcupine Bank, are relatively isolated whereas most of the Logachev Mounds, on the southeast Rockall Bank, form a field of closely spaced, contiguous mounds. They are all found on the upper slope at depths between about 500 and 1200 m and as a result of this study about 500 have been mapped. They have a variety of shapes and the larger ones are very steep-sided, up to 350 m high and 2 km wide at the base. Sediment samples show that the mounds consist of pale coloured muds, mainly aragonite, usually with live and/or dead cold-water corals at the seafloor and with buried dead corals. Coral thickets appear to be detected as a brush-like signature on high resolution profiles and as a speckled pattern on high resolution sidescan sonar records. Shelly sands are found on the seabed between the mounds. Currents in the mound areas are strong enough to transport sands along slope as medium-sized sand waves and to prevent pelagic deposition, thus providing surfaces for initial coral settlement by the winnowing of fines to leave glacial dropstones and exposed rocks. These observations show that the poleward-directed upper slope current of the eastern North Atlantic extends further south than hitherto known and that there is a southwesterly directed current on Rockall Bank. There are waves on the Logachev Mounds, with a wavelength of about 20–30 m, that are thought to be moulded in carbonate muds by across-slope directed internal tidal currents and/or cascading currents. Mounds develop above erosional surfaces seen on seismic data. The huge amount of carbonate mud seems to be produced by the rapid growth and breakdown of cold-water corals living in a very favourable environment. Intermediate nepheloid layers formed upstream by the strong currents may provide the food source for the deep water suspension feeders. Conditions for giant mound growth require an ideal balance between current speed and sedimentation rate, among other factors. No evidence for the suggested link to methane seepage has been found so far.