Introduction

Cold-water corals and slope instabilities are features that have been reported from numerous places along the Atlantic European margin (Mienert and Weaver 2003 and references therein; Freiwald et al. 2004) (Fig. 1). Both can be considered as local sedimentary processes abruptly altering the seabed and in doing so, they may influence each other. In this paper, we discuss the potential relationships between cold-water coral bank development and submarine landsliding from a number of case studies. Both cold-water coral bank development and submarine landsliding can change the local seabed morphology, near-bottom currents and sediment transport, and that can lead to seafloor burial or to the local exposure of subsurface geology. The present knowledge on the spatial distribution of landslides and cold-water coral banks along the European margin demonstrates that only at a few locations a direct spatial relation is observed between the two. However, the development of coral banks can also be influenced by landslides that occur at a distance, for example through temporary enhancements of the sediment load in the water column.

Fig. 1
figure 1

Cold-water coral occurrences (red dots) and main submarine landslides (brown) along the NE Atlantic margin and adjacent Mediterranean Sea. Black boxes show the location of the case studies. Bathymetry from GEBCO. Cold-water coral occurrences from UNEP (Freiwald et al. 2004) and the HERMES database. Submarine landslide occurrences after Weaver et al. (2000)

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This paper focuses on four representative areas along the NE Atlantic margin, where there is a joint occurrence of cold-water coral banks and landslides:

  • The Magellan Mound Province in the Porcupine Seabight contains mainly fossil coral banks overlying a slab slide that may have indirectly influenced the coral bank development due to enhanced fluid flow.

  • The Sula Ridge Reef Complex, which shows indications of indirect impact on the present reef by sediment turbulence resulting from the Storegga slide events.

  • The coral accumulations near the scarp of the Storegga slide complex, which may be related to the escape of fluids and may profit from the slide-generated rough seafloor.

  • The Mauritania coral bank range, which seems to be located near or on top of a landslide scarp whose retrogression seems to be stopped or delayed by coral bank development.

Cold-water corals

Cold-water corals are azooxanthellate coral species that can live at great depth and in colder waters than their tropical counterparts. They are abundant in the world’s oceans, but only in a few places they build large coral banks (Freiwald et al. 2004). Basically, the environmental requirements for framework-building cold-water corals to settle and grow can be reduced to the following two:

A suitable substratum Cold-water corals are found on hard substrates like dropstones, consolidated sediments, outcropping rocks, carbonate crusts (Gulf of Mexico, Schroeder et al. 2005, pebbles Wilson 1979), worm tubes, shells, coral debris and on offshore constructions (Mortensen 2000; Roberts 2000). A hard substratum is believed to be a primary requirement for settling of framework-building cold-water corals as it provides stable anchorage in a dynamic environment.

Food availability Until recently, little was known about the feeding habits of azooxanthellae. Several authors (e.g. Roberts et al. 2006) have demonstrated that Lophelia pertusa, the dominant framework builder, is preferentially a carnivore. The polyps are able to capture and ingest living zooplankton such as copepods, chaetognaths and crustacea drifting over the corals. The size of food particles that L. pertusa is able to capture is probably related to current velocity. Large food items are probably captured when current velocities are high, while fine-grained organic particles become food sources in addition to zooplankton when the current speed is low. Corals are frequently reported from sites with locally accelerated currents, such as continental slope areas where internal (tidal) waves enhance food supply to the seabed (De Mol et al. 2002; Fosså et al. 2005; Frederiksen et al. 1992; Kenyon et al. 2003; Lindberg and Mienert 2005). Recent research has shown that cold-water corals are largely fed by primary productivity in surface waters and subsequent food transport to the seafloor (White 2007). The food availability is also a function of water mass stratification or the level of the pycnocline. The density stratification extends the residence time of potential food in the water column, either fresh or degraded organic material, and hence makes it available for longer. While in general, cold-water corals occur in a broad variety of environmental conditions, build-ups only appear at specific locations with particularly favourable conditions. Regionally, cold-water corals are often found in well-defined depth ranges parallel to the shelf break or to the rim of offshore banks and seamounts, under a relatively narrow range of temperature, salinity and current regimes (Roberts et al., 2006).

Cold-water coral banks

Cold-water coral banks can be defined as seafloor elevations consisting of a coral framework and mud. Coral banks represent the most complex development stage of cold-water coral accumulations and show great similarities with shallow-water coral reefs. Principally, a coral bank is characterised by three distinct units: (1) a cap of living coral colonies, which rests on (2) an open spaced but dead coral framework and debris zone, and (3) a zone of coral framework and debris that is clogged solidly with sediment. Recolonisation by corals may take place on each of these three delineated zones, although the greatest density of living coral colonies is generally observed on the top and upper flanks of the bank (Mortensen et al. 1995). Coral bank morphology varies from small coral topped mounds (ca. 75 m in diameter and 5 m high) (e.g. Darwin mounds in the Rockall Trough, Masson et al. 2003) over extensive coral framework constructions such as the Sula Ridge Reef Complex (up to 30 m high, 100 m wide and 14 km in length; Freiwald et al. 2002) to huge coral banks clustered along the Rockall Trough (e.g. Mienis et al. 2006; van Weering et al. 2003), offshore Mauritania (Colman et al. 2005) and in the Porcupine Seabight (De Mol et al. 2002), with heights of up to 380 m and several kilometres in length.

The localised occurrence of large coral banks stirred an intense debate on their genesis and development. Their development has been attributed to specific oceanographic conditions (e.g. Colman et al. 2005; De Mol et al. 2005; Mienis et al. 2007), to local hydrocarbon seepage (e.g. Henriet et al. 1998; Hovland et al. 1994), and to seepage-related processes controlling the nutrient supply to the ecosystem (Henriet et al. 2002; Hovland 2008). In their original model, Hovland et al. (1994) presented a concept in which the first condition for the formation of coral banks is the generation of hydrocarbons at depth. A second step would be that some of the hydrocarbons find their way to the surface in a focused manner, through faults and fissures. The seabed there would be locally eroded, pockmarks would be created by the seepage and the local seawater would be provided with nourishment on which bacteria and microorganisms would depend (“cloud” in water column). As time passes by, organisms and their skeletal remains would accumulate, whereas authigenic carbonates would precipitate locally, cementing the sediments and skeletal debris and forming a settling ground for the cold-water corals. However, so far no clear evidence has been found for the necessity of (hydrocarbon) seepage for cold-water coral growth or coral bank development. The recent IODP Exp. 307 demonstrated that at least some of these mounds are entirely built up of coral framework embedded in a (fine-grained) sediment matrix (Expedition Scientists 2005). In order to develop a huge coral build-up, equilibrium has to be found between coral growth and sedimentary processes, as sediment particle size, suspended load and deposition rates are known physical determinants of coral performance (Veron 1995). Coral bank growth rates must exceed sediment accumulation rates; otherwise, young corals would be overwhelmed with sediment and get buried. This environment can be found in areas with low sedimentation rates, such as pockmarks, steep slopes and areas with strong bottom currents. This suggests that in areas with significant near-bottom sediment transport, at least during the initial growing phase, coral banks must develop quickly in the vertical direction, while the sediment may already be filling the lower parts of the framework. As the framework becomes higher, it will gradually be more influenced by the suspended load in the water column and progressively less influenced by the sedimentary bed load. In this way, the active biological growth on the upper flanks of a coral bank mainly results in the vertical development of the bank, while its horizontal development is limited due to the sediment stress near the bottom. Fine suspended particles may be processed by filter feeders to extract nutritive organic particles. However, as pointed above, very high turbidity is lethal to the corals. Therefore, the tallest coral banks often develop in areas where local sedimentation rates are lowest (e.g. De Mol et al. 2005, 2007; Huvenne et al. 2007). Coral bank optimum development occurs inside a threshold range of turbidity, sufficient to fill the cavities to strengthen the framework and provide enough nutrients, but in the meantime low enough to provide the corals a substratum to settle on and prevent burial. This subtle equilibrium amongst sedimentation and coral growth is suggested to be the key factor behind cold-water coral bank formation (De Mol et al. 2002, 2005; Huvenne et al. 2003) and is influenced by the sediment dynamic processes, varying in time and space, acting along the continental margins where the coral banks are found. However, the processes behind the development of cold-water coral banks are not fully understood and remain a main multidisciplinary research topic. Similar to tropical reefs, high-energy short-lived physical events like benthic storms or landsliding may dramatically impact the survival chances of a given coral bank, yet others may enter in a standby situation for a short time and resume development.

Submarine landslides

Submarine landsliding, globally one of the most significant processes for downslope sediment transport, can move catastrophically enormous amounts of sediment in geologically instantaneous events. Submarine landslides range from rock falls and rotational/translational slides to debris flows and mud flows. They can occur as a combination of several retrograding slope failures, which create large, up to several kilometres long headwall scarps with clear terrace-like negative offsets on the seafloor (Canals et al. 2004; Locat and Lee 2002). In addition to their major impact on the seafloor morphology, submarine landslides are known as one of the main triggers of turbidity currents (Masson 1996) and, therefore, may strongly influence the ocean physical environment and the survival chances of cold-water coral banks.

Submarine slope failures occur when the external forces (i.e. the combination of gravity, seismicity and seepage) determining the shear stress on sediment packages exceeds the internal shear strength of the sediments (Lee et al. 1999). Shear strength is inversely related to pore fluid pressure (Hampton et al. 1996), which might build up through rapid sedimentation and inefficient dewatering, or through fluid flow from deeper levels within the sediment package. Hence, the main cause for submarine landsliding is often the occurrence of one or more “weak layers”, which become unstable due to a change in external conditions (Canals et al. 2004). Triggers for submarine landslide initiation include oversteepening, seismic activity, storm-wave loading, rapid accumulation and underconsolidation, gas charging, gas hydrate dissociation, sea level and tidal effects, fluid seepage, glacial loading and volcanic island growth (Locat and Lee 2002).

The majority of submarine landslides in the North Atlantic are generated on slopes with pre-sliding angles of less than 2°, in a water depth window of 1,000–1,300 m (Huhnerbach and Masson 2004). These depth ranges indicate that, on average, most coral banks are presently located at depths shallower than submarine landslides, with the exception of the Storegga slide (140–500 m) and the Mauritanian Slide Complex (500 m). Pockmarks are known to occur in association with slides and slumps (Bünz et al. 2003; Hovland et al. 2002; Lastras et al. 2004). Their presence suggests that discontinuities and/or unconformities within sediment packages are more effective fluid conduits than stratigraphically homogeneous sections that are dominated by intergranular fluid flow (Abrams 1992; Brown 2000) and are thus, more likely the conduits responsible for pockmark development (Orange et al. 1999). Fluid escape features have been observed near landslide scarps that are attributed to the release of fluids due to the drop of lithological overload after a landslide (Lastras et al. 2004; Bouriak et al. 2000; Bunz et al. 2005). In addition, landslide deposits are mostly chaotic in texture with blocky patterns, which create fractures and fissures that can lead to preferential migration paths if fluids occur in the underlying strata.

Submarine landslides versus cold-water coral bank development

Conceptual approach

The effect of submarine landslides on cold-water coral bank development can be twofold. A subdivision should be made between processes associated with the landsliding event itself, which generally have a negative effect (i.e. coral bank destruction or burial), and the resulting features and post-sliding processes that may promote the development of coral banks.

Erosion along the pathway of a slope failure clearly is the most lethal effect on cold-water coral bank development, but also the associated turbidity and burial may play a role. Coral recruits survive short-term exposure to low levels of nutrients and low levels of fine sedimentation (Fabricius et al. 2003), but larger amounts may cause permanent damage. However, although the deposition of a sediment cloud may kill new recruits or even mature corals, it does not necessarily imply the end of the coral bank. Often it only means a short interruption of coral bank development, followed by recolonisation. Only when the sediment input stands high for a longer period, or the biological community cannot re-establish itself, the coral bank development stops and the banks will eventually become buried.

In terms of beneficial effects, cold-water coral banks have mostly been observed on elevated seafloor features, characterised by low sedimentation rates and increased local turbulence (e.g. channel flanks, scarps, ridges, iceberg plough marks and tills) (De Mol et al. 2005; Freiwald et al. 1999, 2004). Elevated seafloor features are common on slide scars and of atop debris flow and landslide deposits. Landsliding can also lead to the exposure of (over)consolidated sediment forming relatively strengthened and steep seafloors.

The effect of cold-water corals on the landsliding potential of a continental margin is less easy to predict. One could argue that the coral framework and large build-ups may strengthen certain sections of seabed, especially where slopes are steeper and therefore may be more prone to downslope transport (e.g. retrogressive sliding). However, the large build-ups may also create an additional loading on the seabed, and may—directly or indirectly—influence the dewatering of the sediments, which in turn will affect the sliding potential of the strata.

The next paragraphs discuss four case studies, which illustrate in more detail the variable nature of the relations in between cold-water coral banks and submarine landslides.

Submarine landslides as precursors of coral bank development? Magellan Mound Province, Porcupine Seabight, Ireland

Observations

The Magellan Mound Province is one of the four known cold-water coral bank provinces in the Porcupine Seabight, west of Ireland (Fig. 1) (Huvenne et al. 2003, 2007). The origin, growth and burial of the mounds in this province have been studied in detail using a large and varied data set comprising 2D and 3D seismic, side-scan sonar imagery and ROV video data.

More than 1,000 densely spaced (about 1.2 mound/km2) and mainly buried mounds have been identified in the Magellan area (Huvenne et al. 2007). They were shaped by N/S oscillating currents creating elongated moats around the mound structures and causing a significant N/S elongation in the mound morphology. The Magellan mounds are on average about 62-m high and 340 × 415-m wide and are buried in a Pleistocene drift sequence. Seismic evidence shows that all mounds are rooted on one single reflector (Fig. 2) as observed in the other mound provinces of the Porcupine Seabight (De Mol et al. 2002; Van Rooij et al. 2003). This indicates that the mounds were initiated at a confined moment in geological time, in a spatially widespread, but sharply delineated event, after a period of drift sedimentation under a contour-parallel current regime.

Fig. 2
figure 2

Representative seismic reflection profile across the Magellan Mound Province illustrating the mounds embedded in drift sediments and rooted on the mound base (MB) yellow reflector. The chaotic appearance of the slide deposit is seen clearly at the lower part of the seismic profile. The sharp downdip termination (slide toe) and small scarp in the top slide unconformity (TS headwall scarp) are indicated, as are some other key reflections (SF seafloor, MS ‘mound shaped’ reflector, BS base of slide reflector) (Huvenne et al. 2003). 3D seismic data kindly provided by Statoil Exploration (Ireland) Ltd. and partners

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At shallow depth below the mounds, but still separated from them by a unit of drift sediments, an intensely faulted interval with chaotic reflector configuration has been identified and interpreted as a buried slope failure (Figs. 2, 3) (Bailey et al. 2003; Huvenne et al. 2002). The headwall maximum height is about 25 m, while the maximum thickness of the failed slab itself is 120 m at its distal end, ca. 30 km downslope of the headwall. The most striking characteristic, however, is that the main body of the slide contains a set of relatively undisturbed polygonal blocks of 100–500 m in diameter (Fig. 3). Their sharp edges contrast with the surrounding chaotic matrix, and display evidence of both extensional and reverse offsets, with indication of sediment mobilisation in the form of small ‘diapiric’ ridges at the base of the slope failure (Bailey et al. 2003; Huvenne et al. 2002).

Fig. 3
figure 3

a Gradient map of the TS unconformity (see Fig. 2) within the 3D seismic volume of the Magellan Mound Province, showing the slide toe and headwall scarp (stronger gradients are darker). Mound positions are shown. Mounds mapped from 2D seismic data appear as triangles. Red diamonds indicate the locations where the slide toe could be mapped from the 2D seismic data. The thick black line is a tentative indication of the extent of the slide toe. b Amplitude map of the TS unconformity (see Fig. 2), shifted downwards by 43 ms (indicated in black on the profile in Fig. 2), illustrating the blocky pattern of the slide interval. See location in box (modified from Huvenne et al. 2002, 2007)

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The observed features are similar to those in polygonal fault systems (Cartwright and Lonergan 1996), and therefore, by analogy, illustrate the effect of the build-up and release of overpressure in a possibly shale-rich layer that also may have induced the slab sliding. It appears as if the slide ‘froze’ in an early stage of development. It is likely that the break-up of the slab allowed enough of the overpressure to be released, increasing the basal friction and the loss of momentum sufficiently to halt the mass movement.

Features indicative of seepage have been found in the study area; pockmarks can be observed at several horizons in the seismic data. However, they generally occur with a fairly low spatial density, although one large cluster of pockmarks is located in a horizon above the headwall scarp (Huvenne et al. 2003) (Fig. 4). These pockmarks clearly indicate an overpressure release confined in geological time and space, which could have been caused by fluids concentrated upslope along the slide package.

Fig. 4
figure 4

Shaded relief map of the horizon PM (see inset), illuminated from the NE. A large cluster of pockmarks is found in the NW part of the data (see inset), above the location of the slide headwall. Other pockmark occurrences can be seen towards the SE, but they are much less numerous. The mounds appear as irregular features in the SE part of the data (modified after Huvenne et al. 2003)

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Discussion

At first sight, 2D seismic reflection profiles suggest that the Magellan mound distribution has a spatial relationship with the underlying slab slide. A model proposed by Henriet et al. (1998, 2001) suggested that episodic occurrence of seepage could be at the origin of both processes. The theory is based on refuelling and releasing of underlying gas preferentially along vertical faults. The release of gas may have caused the build-up of the weak layer and the trigger for the slide, while at a later stage focussing of gas seepage along the block edges in the slide would have influenced the initiation of the coral banks. Furthermore, the theory suggested that the release of gas could have facilitated the formation of autigenic carbonate crusts and a nutrient-rich environment of cold seeps, hence fulfilling the basic requirements for coral growth, such as a hard substratum, low sedimentation (by gas seepage) and the flourishing of a microbial community at the base of the food chain to which the corals belong.

However, the above observations do not support a relation between the underlying polygonal slab slide and the Magellan mound occurrence. The apparent spatial relationship between coral banks and slide only occurs locally; Magellan mounds are also found in the north-eastern part of the province, which does not overlay the slide (Fig. 4), while towards the south there is a large part of the slide that is not overlain by mounds. Furthermore, till present, no large fluid escape features in the underlying strata and above the slab slide have been observed in the seismic data and no seep proxies have been detected in short sediment cores from the area. On the other hand, the influence on the mounds of the seepage expressed by the field of pockmarks does not seem to be major. The pockmarks are located quite far away from the coral banks, in the upslope direction. They formed after the deposition of the MS seismic horizon, which marks the onset of burial of most of the Magellan mounds (Huvenne et al. 2007). Hence, if this pockmark field indicates a major seepage and fluid venting event, it did not have an obvious effect on the development of the Magellan coral banks. Moreover, no new mounds were formed in the pockmark area, thus indicating that the seepage or, more generally, the sedimentary environment in that area was not particularly favourable to coral settlement.

Overall, there is no one to one spatial relation between the slide, the pockmarks and the coral banks and it seems unlikely, in this case, that there is a direct link between submarine landsliding and the coral mounds.

Cold-water coral reefs damaged by fast sedimentation resulting from landsliding events: the Sula Ridge Reef Complex

Observations

Sula Ridge is a north-eastward-plunging spur made of westwards dipping Cretaceous and Palaeocene sandstones from 240 to 340 m water depth on the mid-Norwegian shelf (Bugge et al. 1987) (Figs. 1, 5). Underneath the Sula Ridge crest, a Palaeocene claystone with some terrestrial and tuffaceous influence is observed on seismic profiles. This stratigraphic layer is more resistant to erosion than the adjacent units, which created an escarpment as result of differential ice-erosion during glacial periods. Towards the east, less competent Jurassic to Cretaceous units constitute the Suladjupet depression (Freiwald et al. 2002). Quaternary sediments of varying thickness are lying unconformably on the Mesozoic and Tertiary units. Acoustic evidence indicates the presence of free gas, in agreement with the occurrence of pockmarks in the area and with locally elevated hydrocarbon contents in the surface sediments (Hovland et al. 1998).

Fig. 5
figure 5

Overview map of the cold-water corals (Fosså et al. 2002) and submarine landslides (Weaver et al. 2000) occurrence along the Norwegian margin

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The coral reef complex is concentrated along the northwesterly-facing crest of the Sula Ridge (Figs. 5, 6). Side-scan sonar and echo-sounding mapping of the Sula Ridge showed that the main coral reef complex is more than 13-km-long and can be up to 500-m wide (Freiwald et al. 2002). The average reef height is 15 m but individual reefs are as high as 35 m. The most active and lively coral growth is located near the crest of the ridge. Towards the southwest and the northeast, single discrete patch reefs have been observed (Mortensen et al. 1995). West of the main coral reef, a group of 3–8 m high single coral patches is found, consisting of broken L. pertusa fragments, with their framework filled with clayey sands containing abraded Late Pleistocene benthic foraminifera (Freiwald et al. 1999). This deposit has been identified along the entire region and also occurs in the terraces that fringe the main coral reef. The oldest available dating from a Lophelia fragment from the Sula Ridge Reef Complex is 8,150 calendar years BP (Hovland et al. 1998; Hovland and Thomsen 1997).

Fig. 6
figure 6

Interpreted seismic record across the Sula Ridge Reef Complex showing dipping beds of Palaeocene and Cretaceous age. Alive reefal build-ups are represented as dark cone-shaped reliefs on the upper part of the ridge. A detailed bathymetric map illustrates two distinct reef areas: Area A has a dense coral colonisation over, 14 km-long following the main topographic structure. Area B is characterised by scattered low-relief mounds on the gently sloping western flank mainly clogged with sediments (modified after Freiwald et al. 2002). The conceptual sketch illustrates the ‘hydraulic theory’ for deep-water coral reef development, which is based on the seepage of fluids after removing a sediment load by a submarine landslide. The seepage might promote a higher concentration of bacteria, which promotes on their turn a higher nutrient level for higher species in the local food web. The increase in nutrients can be locally redistributed by turbulence and oceanographic currents and promote coral bank development near the headwalls of landslides (Hovland and Mortensen 1999; Hovland 2008)

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Discussion

Two genetic models have been proposed for the Sula Ridge Reef Complex. The first model relates the coral growth near the crest of Sula Ridge to the outcropping Palaeocene layers with their enhanced acoustic amplitudes and potential indications for hydrocarbon leakage. It is suggested that hydrocarbon seepage stimulates bacterial activity in the nearby water column. The enriched microbiological activity serves as nourishment for zooplankton, which in turn feeds the sessile suspension-feeder communities, in this case the cold-water corals (Hovland and Thomsen 1997).

Alternatively, Freiwald et al. (2002) explain the reef development by bentho-pelagic coupling as a result of the particular temperature and salinity gradients, and seasonal food pulses, related to the North Atlantic Current. The resulting environmental conditions open the ecological window for Lophelia, whose settlement is furthermore favoured by the glacially shaped seabed morphology.

According to Freiwald et al. (1999), the development of the Sula Reef was steered by rapid coral growth and sediment infill of the coral framework from external (pelagic sedimentation and episodic terrigenous inputs) and internal (carbonate mud from bioerosion) sources. Near-bottom currents and seabed morphology govern the sedimentation on the Norwegian shelf. Therefore, it should be expected that coral patches located at the base are more affected by sediment draping than those on the upper flanks of the Sula Ridge Reef Complex. The clayey sands with Pleistocene signature that fill the broken coral framework in the fringing terraces are not in stratigraphical order, and may therefore represent reworked and transported materials (Freiwald et al. 1999) They appear to have stopped the coral development and to have sealed the early stage of reef growth (dated to be around 8 Ky) on the Sula Ridge, and may represent an event that suddenly introduced a cloud of mostly fine-grained sediment in suspension leading to high instantaneous sedimentation.

The most likely candidate for a large instantaneous source of reworked sediments in the mid-Holocene is one of the several large submarine slides that have been reported along the Norwegian margin. The best candidates are the multi-staged Storegga Slide, about 100 km southwest of the Sula Ridge, and the Trænadjupet Slide. The last phase in the Storegga Slide complex occurred between 7.3 and 8.1 ka BP (Haflidason et al. 2004, 2005), while the Trænadjupet Slide has been dated at 4.0 ka BP (Evans et al. 2005; Laberg et al. 2000, 2002). The dating results of the corals indicate that the initial coral reef complex was already in place when the last Storegga sliding phase occurred.

Submarine landsliding events resuspend fine-grained sediments leading to higher turbidity in the water column that may spread over large areas (Talling et al. 2007; Frenz et al. 2008). Fast fine sediment accumulation has clearly affected the polyps and many coral patches on the western flank of the Sula Ridge, facing the Storegga Slide, which seem to have been buried and not recolonised (Freiwald et al., 2002). The sediment infill of the base of the reefs strengthened the build-ups by filling up the cavities. The reefs located higher upslope also became affected by the fine sediment, which mostly interrupted the coral growth. There is no evidence that the uppermost parts of the reefs suffered a lot from the temporary increase in turbidity (Freiwald et al. 1999).

After this fast sedimentation event, coral debris mounds were left on the lower part of the Sula Ridge, while further coral growth took place on the upper flanks, leading the reef to develop to its present size. Cold-water corals are common along the Norwegian margin, and recolonisation of the higher located debris mounds, which provided a hard substratum in an area of low sedimentation with suitable oceanographic conditions, might have been a natural phenomena.

In addition to the impact of the suspended sediment, Freiwald et al. (1999) also suggested a physical destruction of the coral reefs due to the sliding event in the Storegga area. The only physical destructive phenomenon at 75–100 km distance from the submarine landslide could have been a tsunami wave. It is proven that a tsunami wave was generated after the last sliding event around 7,250 ± 250 14C BP (Haflidason et al. 2005). The largest Storegga tsunami wave had a run-up of up to 20 m and an estimated 40 m amplitude in the Sula area (Bondevik et al. 2005). A tsunami wave transmits energy over the entire water column, although the relatively low level of energy on the bottom possibly is too low to cause direct significant damage to coral build-ups. The estimated bottom current velocity on the shelf during the Storegga tsunami may have been in the order of 1–2 m/s (Bondevik et al. 2005), which is in the order of peak current velocities observed in other coral regions (White 2003). Hence, the coral damage and the formation of debris mounds might in this case rather have resulted from the natural collapse of unstable dead coral framework, due to natural bioerosion or weakening of the framework after the polyps died through smothering.

Coral reefs at landslide escarpments: Storegga Slide

Besides its destructive effects on parts of the Sula Ridge Reef Complex, the Storegga Slide may also have had some positive effects on cold-water coral growth. The Norwegian name ‘Storegga’ actually refers to the abrupt termination of the continental shelf of mid-Norway, and represents the top (shoulder) of the continental slope. The Storegga Slide is one of the largest submarine landslides in the world (Canals et al. 2004; Haflidason et al. 2005), which caused the steepest continental slope off mid-Norway (Fig. 1).

Observations

Lophelia reefs have been reported by fishermen and are known from visual inspection just above and in the slide escarpment area (Hovland and Risk 2003; Parsons et al. 2005) (Fig. 5). Along the northward continuation of the main slide escarpment at about 500-m water depth, several partially buried L. pertusa patches have been observed (Parsons et al. 2005). Evidence for seepage in this area has been given by Bouriak et al. (2000) and Bunz et al. (2005), and buried coral patches are found close to an area of cracks and pockmarks (Parsons et al. 2005). However, those patches are not in direct conjunction with the main reefs. Still, the headwall and upper reaches of the Storegga Slide form a dewatering seepage-prone area due to the numerous sedimentary bedding planes that have been cut off by the landslide erosion, and to the exposure of overpressured sediments (Hovland and Risk 2003).

The Storegga Slide escarpment is also characterised by boulders and coarse rock fragments transported during the glaciations and redistributed during landsliding (Parsons et al. 2005). Although coral patches are common along the Norwegian shelf-slope area, Fosså et al. (2002) illustrate that the highest density of corals is found along relatively steep seabed morphology, associated with iceberg plough marks or glacial deposits and especially along the slide escarpment near the Storegga-Sormannsneset area (Fig. 5).

Discussion

The higher occurrences of corals in the Storegga area is suggested to be related to oceanographic conditions (Fosså et al. 2002) and to the seabed morphology, which generates local turbulence and favours a low sedimentation regime also caused by strong currents. Thiem et al. (2006) used a numerical model to analyse the L. pertusa distribution and food supply mechanisms along the Norwegian continental shelf, and concluded that food supply is a key factor for reef development. Mortensen (2001) demonstrates that L. pertusa can live on a variety of fresh and dead sources of organic matter. In areas with high turbulence, the corals have a higher particle encounter rate and will be more concentrated (Thiem et al. 2006).

Hovland and Mortensen (1999) proposed a ‘hydraulic theory’ to explain the presence of cold-water coral reefs on the edge of major slide scarps as a result of dewatering of the downslope eroded and exposed strata (Fig. 6). As mentioned above, it is assumed by several authors that seepage might indirectly enhance nutrient availability for the corals, although so far no direct connection between seepage and coral growth has been demonstrated. Some indirect connections are suggested by a microbiological study of sponge tissue recovered from a Norwegian cold-water coral reef environment at the ‘Kristin’ hydrocarbon field (Jensen et al. 2008). A small number of genes indicating the presence of sulphide-nitrite and iodide-oxidising bacteria have been identified, but additional research is necessary to clearly confirm the presence and understand the role of these organisms in the reef environment. Therefore, in the case of the Storegga headwall reefs it appears that transported boulders, exposed consolidated sediments and rocky materials provided a firm substratum for coral settlement in a setting known for its high current speed and reduced sedimentation.

Cold-water coral banks influencing submarine landslides: the Mauritania slope failures and carbonate mounds

Observations

Colman et al. (2005) described a shelf edge parallel mound province offshore central Mauritania (16°30′N to 19°00′N and 16°00′W to 17°00′W) where L. pertusa occurs (Figs. 1, 7, 8). This province comprises a series of large mounds located at approximately 450–550 m water depth that form a 190-km-long quasi-continuous linear feature on a 3–4° average gradient continental slope (Fig. 8). The largest mound measures ~500 m across its base and rises about 100 m above the surrounding seafloor. Subparallel mound ridges have been observed, often associated with basal moats (Figs. 7, 8). Video surveys of the mounds illustrated that most coral colonies are dead, probably due to fishing activities (Colman et al. 2005).

Fig. 7
figure 7

Known extent of the carbonate mounds off Mauritania based on 2D and 3D seismic surveys. Bathymetric map extracted from 3D seismic data showing carbonate mounds, the submarine canyon to the north and slope failures to the south. The location of the Banda and Chingueti hydrocarbon fields are shown (modified from Colman et al. 2005). The inset shows a detailed shaded relief map illustrating mound shapes, associated moats and slope failures. Locally, the mounds developed in two parallel ridges (modified from Colman et al. 2005)

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Fig. 8
figure 8

3D seismic reflection profile illustrating the mounds and shallow seabed slope failure on the Mauritanian margin (data courtesy of Woodside Ltd)

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Buried mounds identified in seismic reflection profiles extend eastward or shoreward from the seabed mounds (Fig. 8). In some areas, buried mounds also appear to extend in a north–south direction beneath the gaps in the mound alignment exposed on the seafloor (Fig. 7). The oldest of the buried mounds are estimated to be 1–2 million years old (Colman et al. 2005).

The coral bank province is cut by a deeply incised submarine canyon, and by a complex set of landslides known as the Mauritania Slide Complex (MSC) (Fig. 7). The seismic reflection profiles from the area reveal that the mounds are older than the slope failures. The MSC results from a series of retrogressive failure events caused by excess pore pressure that created widespread weak layers. The failures may have been triggered by earthquakes and diapiric uplift (Antobreh and Krastel 2007). These failures deposited huge debris flow units towards the Gambia Abyssal Plain and adjacent fracture zone valley (Wynn et al. 2000). In general, the central Mauritanian margin is not affected by sliding at water depths shallower than 600 m. To the south of the study area, a stretch of the coral bank province coincides with the MSC headwall scarp at this depth (Antobreh and Krastel 2007). Where this occurs, the seafloor is characterised by an intricate morphology composed of slide scars, detached blocks and gullies. Upslope of the mound ridge, several local bathymetric features form an arc-shaped feature indicative of seabed crown cracking that might correspond to the initial stages of a new retrogressive slide phase (Antobreh and Krastel 2007; Colman et al. 2005).

Discussion

As no large failures are observed upslope of the coral bank ridge (Antobreh and Krastel 2007), as the mounds are rooted on a lower reflector than the slide plane, and as the headwall scarp and the mound ridge are closely related in shape and location, it is suggested here that the mound ridge may have stopped the retrogression of the complex landslide (Fig. 7). In other words, the coral banks may have contributed to stabilise the continental slope in that margin segment. The coral framework clogged with sediments introduces a discontinuity in the weak layers as coral banks are considered as bodies with low internal fluid flow (Ferdelman et al. 2006), and may have been more resistant against instability than the surrounding parallel stratified drift sediments; therefore, causing the slide to break off at the location of the mound ridge.

Cracks in the seabed upslope of the main mound ridge express the potential for new instabilities that might destroy the coral banks in the future. It seems also plausible that in the past retrogressive landsliding may have destroyed mounds located downslope of the present ones. However, within the bathymetric maps and the seismic reflection profiles available, no additional indication has been found to prove or disprove such hypothesis.

Besides the headwall of the submarine landslides, mounds are also observed in areas without indication of landsliding and at the flanks of the northern canyon, which divides the coral bank province. This illustrates that coral banks in the area are not only associated with the submarine landslides but also with other seabed irregularities. The coral banks seem to have delimited the eroding processes associated with canyon and gully widening.

While seismic and geochemical observations indicate active hydrocarbon seepage downslope of the mounds, especially between 650 and 700 m depth, no hydrocarbon seepage evidence has been detected in the immediate vicinity of the coral mound system. Therefore, Colman et al. (2005) suggest a close relation between oceanographic conditions, low sedimentation rates and increased food supply in the coral mound bathymetric interval due to interaction of different water masses. Although current speeds at present are relatively weak, low amplitude sediment waves near the coral mounds may indicate higher palaeo-current speeds. Buried coral banks suggest that past environmental conditions were more favourable for coral bank development over a larger and more proximal area. Based on the available data set, no clear cause for the stopping of coral colonisation on the coral banks could be put forward. A change in oceanographic conditions is suggested, but also the landslides might have played a role. In the latter case, the sudden turbidity increase triggered by landsliding might have stopped the coral bank development in an early stage as the rain of resuspended fine-grained sediment could have suffocated corals. Hovland 2008 argues for a seepage relationship for these mounds.

Summary

  • This literature review illustrates that, in general terms, no direct relationship exists between submarine landslides and cold-water coral banks, although both are continental margin process which may occur at similar water depths.

  • Locally and punctually submarine landslides might indirectly help cold-water coral reefs to develop:

    • The headwalls of submarine landslides might favour coral growth by the outcropping of firm layers and the local enhancement of turbulence and current speed due to the roughness in seabed morphology resulting from landsliding. This might be the case offshore Mauritania and at the headwall of the Storegga Slide, yet in the latter case only small coral build-ups have been observed.

    • Landslides are often associated with fluid seepage, which might, according to some authors, favour coral bank development. No direct evidence for this hypothesis has been observed so far.

  • Landslides might have a negative effect on coral bank development due to the temporal increase in turbidity and to the direct destruction of existing coral growths. This could be the case of the Sula Ridge Reef Complex offshore Norway.

    • A landslide can destroy reefs growing within the sliding area and along the pathway of the sliding sediment mass.

    • A landslide-generated tsunami could mechanically destroy cold-water coral reefs at great distances.

    • Increased turbidity could influence the reefs negatively by suffocation, destroying polyps and causing food shortage.

  • Landslides might be stopped in their retrogressive upslope development if they encounter coral banks on their pathway. As suggested, the building up of fluid overpressure might be altered by the mound, due to difference in permeability and porosity of the mound body with the surrounding sediments and the more stable construction of a coral framework clogged with sediments and the formation of hardgrounds. This might be the case along the Mauritanian margin. Coral growth might interrupt the lateral continuity of weak layers, while coral framework might stabilise the slope sediments. Table 1.

    Table 1 Possible interactions between submarine landslides and cold-water coral banks
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