Cold-water coral banks and submarine landslides: a review

ORIGINAL PAPER

Cold-water coral banks and submarine landslides: a review

Ben De Mol Æ Veerle Huvenne Æ Miquel Canals

Received: 8 May 2007 / Accepted: 20 September 2008 / Published online: 8 October 2008

� Springer-Verlag 2008

Abstract This paper aims to review the relation between

cold-water coral bank development and submarine land-

slides. Both are common features on continental margins,

but so far it has not been reviewed which effect—if at all—

they may have upon each other. Indirect and direct rela-

tions between coral banks and landslides are evaluated

here, based on four case studies: the Magellan Mound

Province in the Porcupine Seabight, where fossil coral

banks appear partly on top of a buried slide deposit; the

Sula Ridge Reef Complex and the Storegga landslide both

off mid-Norway; and the Mauritania coral bank province,

associated with the Mauritanian Slide Complex. For each

of these locations, positive and negative relationships

between both features are discussed, based on available

datasets. Locally submarine landslides might directly

favour coral bank development by creating substratum

where corals can settle on, enhancing turbulence due to

abrupt seabed morphological variations and, in some cases,

causing fluid seepage. In turn, some of these processes may

contribute to increased food availability and lower sedi-

mentation rates. Landslides can also affect coral bank

development by direct erosion of the coral banks, and by

the instantaneous increase of turbidity, which may smother

the corals. On the other hand, coral banks might have a

stabilising function and delay or stop the headwall retro-

gradation of submarine landslides. Although local

relationships can be deduced from these case studies, no

general and direct relationship exists between submarine

landslides and cold-water coral banks.

Keywords Cold-water coral banks �Submarine landslides � Storegga Slide �Mauritanian Slide complex � Porcupine Seabight

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 rela-

tionships between cold-water coral bank development and

submarine landsliding from a number of case studies. Both

cold-water coral bank development and submarine land-

sliding 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 Euro-

pean margin demonstrates that only at a few locations a

direct spatial relation is observed between the two. How-

ever, the development of coral banks can also be influenced

by landslides that occur at a distance, for example through

B. De Mol (&)

GRC Geociencies Marines, Parc Cientıfic de Barcelona,

Campus Diagonal, Universitat de Barcelona, Adolf Florensa 8,

08028 Barcelona, Spain

e-mail: [email protected]

V. Huvenne

Geology and Geophysics, National Oceanography Centre,

Southampton, European Way, Southampton SO14 3ZH, UK

M. Canals

GRC Geociencies Marines, Departament d’Estratigrafia,

Paleontologia i Geociencies Marines, Facultat de Geologia,

Universitat de Barcelona, Campus de Pedralbes,

08028 Barcelona, Spain

123

Int J Earth Sci (Geol Rundsch) (2009) 98:885–899

DOI 10.1007/s00531-008-0372-6

temporary enhancements of the sediment load in the water

column.

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 indica-

tions 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 (Mor-

tensen 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

Fig. 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)

886 Int J Earth Sci (Geol Rundsch) (2009) 98:885–899

123

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;

Fossa 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 trans-

port 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 resi-

dence 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 eleva-

tions 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 sedi-

ment. 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 fis-

sures. 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 sedi-

ments 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 sedi-

ment 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 opti-

mum 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

Int J Earth Sci (Geol Rundsch) (2009) 98:885–899 887

123

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 geo-

logically 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 ter-

race-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 un-

derconsolidation, 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 indi-

cate 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

(Bunz et al. 2003; Hovland et al. 2002; Lastras et al. 2004).

Their presence suggests that discontinuities and/or uncon-

formities 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 develop-

ment of coral banks.

Erosion along the pathway of a slope failure clearly is

the most lethal effect on cold-water coral bank develop-

ment, 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 cor-

als, 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.


123

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)con-

solidated 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). How-

ever, 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 signi-

ficant N/S elongation in the mound morphology. The

Magellan mounds are on average about 62-m high

and 340 9 415-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 indi-

cates 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.

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 iden-

tified 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).

The observed features are similar to those in polygonal

fault systems (Cartwright and Lonergan 1996), and there-

fore, 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

Fig. 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


123

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.

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 pock-

marks 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

Fig. 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. Reddiamonds 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)


123

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 sedi-

mentary 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 tu-

ffaceous 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).

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 abra-

ded Late Pleistocene benthic foraminifera (Freiwald et al.

Fig. 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)


123

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).

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 poten-

tial 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 sed-

iment 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 lead-

ing to high instantaneous sedimentation.

Fig. 5 Overview map of the

cold-water corals (Fossa et al.

2002) and submarine landslides

(Weaver et al. 2000) occurrence

along the Norwegian margin


123

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 south-

west 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

Fig. 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 Morten-

sen 1999; Hovland 2008)


123

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 ± 25014C 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 col-

umn, 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 Norwe-

gian 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-Nor-

way (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 over-

pressured 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, Fossa 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 (Fossa

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 indi-

cating 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.


123

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�300N to 19�000Nand 16�000W to 17�000W) 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).

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

Fig. 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)


123

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 sub-

marine landslides but also with other seabed irregularities.

The coral banks seem to have delimited the eroding pro-

cesses 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 oceano-

graphic 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 sedi-

ment 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

Fig. 8 3D seismic reflection

profile illustrating the mounds

and shallow seabed slope failure

on the Mauritanian margin (data

courtesy of Woodside Ltd)


123

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 nega-

tively 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.

Acknowledgments This study was originated in the framework of

the EC FP5 RTN EURODOM and EC FP6 HERMES (contract

GOCE-CT-2005-511234-1). GRC Geociencies Marines (GRCGM)

is funded by ‘‘Generalitat de Catalunya’’ excellence research grants

program (ref. 2005 SGR-00152). GRCGM also acknowledges

the support received from Landmark Graphics Corporation via

the Landmark University Grant Program, and from SMT Inc. via the

educational User License for Kingdom Suite interpretation software.

The authors like to thank the reviewers, Martin Hovland and Andre

Freiwald for their comments.

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Effects of cold-water coral banks on landsliding

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