Geo-Mar Lett (2012) 32:205–225
DOI 10.1007/s00367-011-0257-8
ORIGINAL
The Porcupine Bank Canyon coral mounds: oceanographic
and topographic steering of deep-water carbonate mound
development and associated phosphatic deposition
A. Mazzini & A. Akhmetzhanov & X. Monteys & M. Ivanov
Received: 15 February 2011 / Accepted: 1 September 2011 / Published online: 27 October 2011
# Springer-Verlag 2011
Abstract The head of a canyon system extending along the
western Porcupine Bank (west of Ireland) and which
accommodates a large field of giant carbonate mounds
was investigated during two cruises (INSS 2000 and TTR13). Multibeam and sidescan sonar data (600–1,150 m
water depth) suggest that the pre-existing seabed topography
acts as a significant factor controlling mound distribution and
shape. The mounds are concentrated along the edges of the
canyon or are associated with a complex fault system traced
around the canyon head, comprising escarpments up to 60 m
high and several km long. The sampling for geochemical and
petrographic analysis of numerous types of authigenic
deposits was guided by sidescan sonar and video recordings.
Calcite-cemented biogenic rubble was observed at the top and
A. Mazzini (*)
Physics of Geological Processes (PGP), University of Oslo,
P.O. Box 1048, 0364 Blindern, Norway
e-mail: adriano.mazzini@fys.uio.no
A. Akhmetzhanov
National Oceanography Centre, Southampton, Empress Dock,
Southampton, SO14 3ZH, UK
X. Monteys
Geological Survey of Ireland,
Haddington Road,
Dublin 4, Ireland
on the flanks of the carbonate mounds, being associated with
both living and dead corals (Lophelia pertusa, Madrepora
oculata and occasional Desmophyllum cristagalli). This can
plausibly be explained by dissolution of coral debris
facilitated by strong currents along the mound tops and
flanks. In turn, the dissolved carbon is recycled and
precipitated as interstitial micrite. Calcite, dolomite and
phosphatic hardgrounds were identified in samples from
the escarpment framing the eastern part of the survey area.
The laterally extensive phosphatic hardgrounds represent a
novel discovery in the region, supplying hard substrata for
the establishment of new coral colonies. Based on existing
knowledge of regional oceanographic conditions, complemented with new CTD measurements, it is suggested that
water column stratification, enhanced bottom currents, and
upwelling facilitate the deposition of organic matter, followed
by phosphatisation leading to the formation of phosphateglauconite deposits. The occurrence of strong bottom currents
was confirmed by means of video observations combined
with acoustic and sampling data, providing circumstantial
evidence of fine- to medium-grained sand. Evidently, slope
breaks such as escarpments and deep-water canyon headwalls
are important structural elements in the development of
mature carbonate mounds induced by deep-water coral
growth. Stable isotope data show no evidence of methanederived carbon in the carbonates and lithified sediments of the
Porcupine Bank Canyon mounds.
M. Ivanov
UNESCO Centre for Marine Geology and Geophysics, Moscow
State University,
Moscow 119899, Russia
Introduction
Present Address:
A. Akhmetzhanov
Lukoil Overseas UK, Charles House,
5-11 Regent Street,
London SW1Y 4LR, UK
Carbonate mounds, also called coral build-ups, reefs,
lithoherms or banks, are documented in the fossil record to
have existed since the Proterozoic (Wilson 1975; Neumann
et al. 1977; Bosence and Bridges 1995; Paull et al. 2000;
206
Freiwald 2002). Their dynamics include the growth of living
colonies and the build-up of in situ coral debris combined
with the trapping of externally supplied sediment. This
mixture of sediment and skeletal remains contributes to the
formation of mound structures in up to 300-m-high and 10km-long reefs which can grow in a wide range of water depths
(between 50 and 1,300 m) and temperatures (between 4 and
12°C; Mullins et al. 1981; Frederiksen et al. 1992; Rogers
1999). The dominant cold-water coral species are Lophelia
pertusa and Madrepora oculata; less common are species
such as Desmophyllum cristagalli, Dendrophyllia cornigera,
Enallopsammia rostrata and Solensmilia variabilis.
North Atlantic and Norwegian Sea carbonate mound areas
have been intensively studied during the last two decades in
the course of numerous marine expeditions (Hovland et al.
1994, 1998; Kenyon et al. 1998, 2003a; Henriet et al. 2001;
Huvenne et al. 2003; van Weering et al. 2003; Kano et al.
2007; Wheeler et al. 2007). Various factors have been
suggested to favour the initiation and development of these
cold-water coral banks: (1) oceanographic conditions (mainly
strong current activity, high nutrient supply and low sedimentation rates; Mullins et al. 1981; Kenyon et al. 1998; De Mol
et al. 2002), (2) focused hydrocarbon seepage resulting in the
fertilization of the seafloor (Hovland 1990; Hovland et al.
1994; Hovland and Thomsen 1997; Henriet et al. 1998,
2001; Hovland and Risk 2003) and (3) the availability of
suitable substrates. Commonly, the settlement of coral
colonies (mainly L. pertusa) takes place on hard substrata
such as drop stones and rocky outcrops (e.g. Wilson 1975;
Kenyon et al. 1998, 2003a) or eventually on methanogenic
carbonate deposits which precipitate at seepage sites (Rogers
1999; León et al. 2007). Once initiated, the growth of the
colony can become self-sufficient in terms of primary
substrate; new corals grow on top of dead ones, the activity
of the colony being largely controlled by ambient environmental conditions such as organic matter and sediment
supply, current speed, pH, temperature and salinity. This
fragile system can collapse when the equilibrium between
sedimentation rate and biological growth is disturbed, and
excessive sedimentation leads to a progressive burial of the
coral colony (e.g. Kenyon et al. 2003a). However, if favourable
conditions persist over sufficiently long periods of time, the
carbonate build-ups can reach considerable dimensions.
Conceptually, it seems probable that the growth of the mounds
is further promoted by the presence of carbonate-cemented
coral rubble mixed with trapped foraminifer-rich hemipelagic
sediment. Such solidified deposits provide additional surfaces
on which new coral colonies can develop. So far, this aspect
has been examined only superficially.
The Logachev Mounds on the south-western Rockall
Trough margin (Kenyon et al. 2003a; Mienis et al. 2007)
represent a good example of an advanced stage of mound
development. To date, the information on the internal
Geo-Mar Lett (2012) 32:205–225
structure of these and neighbouring carbonate mounds is
limited (Expedition-Scientists-ODP-307 2005; Williams et
al. 2006; Kano et al. 2007). High-resolution acoustic data
obtained during surveys offshore Ireland revealed the
presence of a strong reflector underneath the mounds (e.g.
Kenyon et al. 2003a; van Weering et al. 2003). However,
such profiles are often of little help because they usually
show blank zones within these structures (e.g. Kenyon et al.
1998; Bailey et al. 2003; Huvenne et al. 2003). Thus,
interpretations are overall limited to inferences based on
shallow cores and visual observations.
Within this context, the oceanographic, geomorphological
and biological conditions required for mound initiation and
evolution have been investigated during several recent research
cruises to the western Porcupine Bank offshore south-western
Ireland (Fig. 1a). During the year 2000, the Irish National
Seabed Survey (R/V Bligh) conducted by the Geological
Survey of Ireland (GSI) completed multibeam and CTD
surveys revealing, for the first time, the presence of a large
cluster of inferred carbonate mounds on the western Porcupine
Bank. In 2003, the area was once more visited during the
Training Through Research 13 (TTR13) marine expedition of
the R/V Professor Logachev, in the course of which detailed
sampling, seafloor video recordings, and sidescan sonar and
subbottom profiler surveys were performed. A preliminary
report of this newly discovered deep-water coral mound
province can be found in Kenyon et al. (2004).
This paper provides a detailed presentation and interpretation of the sidescan sonar data from the western Porcupine
Bank, supported by CTD and bathymetric data as well
as selected seafloor video images. This is complemented
by geochemical and petrographic analyses conducted on
authigenic deposits associated with coral colonies sampled
in the region during the TTR13 cruise. The aim of this paper is
to evaluate the significance of the environmental controls in
the growth of the coral colonies and the factors driving the
formation of the authigenic deposits.
Geological and oceanographic setting
Off the southwest coast of Ireland, the Porcupine Bank is
framed by the amphitheatre-shaped Porcupine Seabight to
the south, and the Porcupine Ridge and Slyne Ridge to the
Fig. 1 a Locality map of the study area (red square), and location of
the oceanographic transects (yellow band) in Fig. 5. b Multibeam
bathymetric map of the survey area with sample stations (dots) and the
outline of the sidescan sonar coverage (white dashed line). c
Topographic profile (A–A′) along the edge of the canyon, showing
the carbonate mound distribution. d Magnification of a part of the
bathymetric map showing the locations of the CTD stations. e
Nepheloid layers recorded in the study area (vertical profiles along
B–B′ are from Dickson and McCave 1986), and acoustic single-beam
water column imagery along profile C–C′ shown in b
b
Geo-Mar Lett (2012) 32:205–225
207
208
north. The Porcupine Basin is influenced by strong bottom
currents which have formed channel systems and sediment
waves (McDonnell and Shannon 2001). Three major
mound provinces have been described in the bight, these
being the Belgica Mounds, the Hovland and Magellan
Mounds, and the Pelagia Mounds (Wheeler et al. 2007).
The present study focuses on an area located in the
westernmost part of the Porcupine Bank where a series of
canyons with mound features are located (Fig. 1).
The region is known to be influenced mainly by the
northward-flowing Eastern North Atlantic Water (ENAW)
and Mediterranean Outflow Water (MOW; White 2007) but
is also influenced by minor water masses derived from the
western North Atlantic, which are carried northwards by the
eastern boundary slope current (van Aken and Becker
1996; De Mol et al. 2005; White 2007).
Geo-Mar Lett (2012) 32:205–225
microscope (SEM and BSE) studies were performed by
means of an ISI ABT-55 SEM.
Aliquots for isotopic analyses were collected from various
rock types with a hand-held micro-drill. Bulk portions of
cement where drilled at spots selected to minimise contamination from any bioclasts present in the matrix cement.
Measurements were performed on CO2 liberated by the
standard phosphoric acid technique (MaCrea 1950; Sharma
and Clayton 1965) using a Carbonate Prep System linked to
an AP2003 mass spectrometer. The δ13C and δ18O results are
reported relative to the V-PDB standard (Craig 1957; Coplen
1994); the precision of the analyses is ±0.2‰.
Oceanographic data from 19,801 stations covering
most of the western Irish Margin were retrieved from
the World Ocean Database 2005 (Boyer et al. 2006).
These were analysed using the Ocean Data View software
developed by the Alfred Wegener Institute, Bremerhaven
(Schlitzer 2007).
Materials and methods
In the year 2000, a multibeam survey with the R/V Bligh
investigated the study area, mapping it with a deep-water
multibeam Kongsberg-EM120 (12 kHz, 1°×1°) echosounder
system and a 3.5-kHz subbottom profiler. Bathymetric and
backscatter data were processed by means of the Caris HIPS
6.0 software. From this, bathymetric derivatives such as
digital terrain models, and shaded relief and slope maps were
produced. Multibeam backscatter mosaics were generated by
applying angular and slope correction techniques. Conductivity–
temperature–depth (CTD) measurements were conducted in the
region during the months of July and August. Discussed here are
the results of four stations located within the area outlined in
Fig. 1d.
In the year 2003 (TTR13 cruise), a survey using a MAK
deep-towed sidescan sonar operating at 30 kHz frequency with
a built-in 5-kHz subbottom profiler was performed in the
vicinity of mounds located within the area delimited in Fig. 1b.
Survey lines MAKAT-99 to MAKAT-104 were run parallel to
each other from north–northeast to south–southwest, obtaining a complete coverage of approx. 200–250 km2 of seafloor
at water depths varying from 600 to 1,150 m. Based on the
sidescan sonar images, 21 sites of particular interest were
selected and then surveyed with an underwater video system
(Fig. 1b, Table 1), samples being collected by means of a
dredge, gravity corer, box-corer, and camera-guided grab
sampler.
Semi-quantitative bulk mineralogical compositions of the
authigenic deposits were determined by X-ray diffraction
analyses, using a Dron-3 instrument with CoKα radiation by
step-scanning at 0.02×2Å with fixed counting times (Shlikov
and Kharitonov 2001). The petrography of the hard-rock
samples was studied in thin sections following standard
petrographic and fluorescence techniques. Scanning electron
Results
Acoustic data
Multibeam and sidescan sonar data enabled the identification of a cluster of steep-sided mounds 50 to 220 m high,
with base diameters varying from 300 to 2,000 m along the
western edge and in the central part of the survey area. The
seabed dips gently towards the southwest and, as a result,
the mound tops are located at depths ranging from 550 to
900 m (Fig. 1b, c). The mound area is delimited to the east
by a long escarpment, and to the north and northwest by the
canyon head and a carbonate ridge (see Fig. 2 and profile a′–
a). Between the escarpments and the edge of the canyon, the
slope gradients across the intermound region range between
1–3°. To the northwest of the canyon head, at similar water
depths, the gradients are less than 1° (Fig. 1). These mounds
are characterised by high backscatter commonly associated
with reflections from steep slopes (slope gradients between
25 to 35°; Figs. 2, 3). Some of the mounds have several
peaks. The flanks of some mounds appear to have smooth
surfaces, whereas others appear to be hummocky, presumably
due to the presence of coral colonies. Transparent acoustic
wedges found at the bases of some mounds probably represent
aprons of coral debris (Fig. 4), as observed also on video
images. Sidescan and multibeam records show patches of
high and irregular backscatter at the base of some mounds,
which are interpreted as small landslides, hardgrounds or
coral patches covered by sediment (Fig. 4).
Most of the mounds are elongated in shape and have
near-symmetrical slopes (Fig. 2b). Individual conical
mounds were also recognised in the south of the study
area along the edge of the canyon (Fig. 2). Towards the
Geo-Mar Lett (2012) 32:205–225
209
Table 1 Sampling stations on the western Porcupine Bank with
geographic positions, water depths, and short descriptions of the
geographical settings and sedimentary characteristics. Letters after the
station numbers indicate the sampling method: D gravity core, B box-
core, Gr remote-controlled TV grab (double coordinates refer to start
and end of the TV survey). Note that stations TTR13AT502B and
TTR13AT503G were collected outside the mound area and are not
shown in Fig. 1
Station no.
Lat. (N)
Long. (W)
Depth (m)
Geographical setting
Sedimentary summary
TTR13AT489B
TTR13AT490Gr
15°00,725
14°57,828
14°57,823
15°04,638
773
694
693
900
Edge of canyon
Carbonate mound
TTR13AT491B
51°54,300
51°50,884
51°50,883
51°43,701
Top of carbonate mound
Hemipelagic sediments
Living corals, coral debris,
hemipelagic sediments
Coral debris, hemipelagic sediments
TTR13AT492G
51°43,697
15°04,642
897
Top of carbonate mound
Coral debris
TTR13AT493G
TTR13AT494G
51°45,837
51°48,597
15°02,153
15°05,224
737
1,588
Top of carbonate mound
Canyon
Coral debris
Coral and shell debris, calcite-cemented
TTR13AT495G
51°47,053
14°59,620
740
Saddle on top of carbonate mound
Alternated foraminiferal and coccolith
ooze, coral and shell debris,
terrigenous sediment
TTR13AT496G
51°46,523
14°55,650
651
Upper part of escarpment
Sand
TTR13AT497G
51°46,271
14°55,863
715
Lower part of escarpment
Sand
TTR13AT498D
51°46,408
51°46,544
14°55,746
14°55,639
697
650
Steep slope along escarpment
Authigenic deposits: calcite, dolomite,
phosphatic hardgrounds+drop stones
TTR13AT499D
51°45,840
51°45,860
51°46,302
51°46,337
14°55,518
14°55,395
14°55,642
14°55,476
684
646
677
646
Steep slope along escarpment
14°57,730
14°57,929
14°45,895
726
640
497
Slope of carbonate mound
TTR13AT502B
51°51,003
51°50,776
51°46,201
Authigenic deposits: calcite,
phosphatic hardgrounds
Authigenic deposits: calcite
glauconite-rich, calcite coccolith-rich,
phosphatic hardgrounds
Calcite-cemented bioclasts, coral rubble
Pockmark area
Hemipelagic sediments
TTR13AT503G
TTR13AT504G
51°43,971
51°46,730
14°47,996
14°57,504
535
762
Scour mark area
Intermound area
Hemipelagic sediments, sand
Sand
TTR13AT505G
51°47,795
14°58,260
787
Intermound area
Sand, clay
TTR13AT506G
TTR13AT507G
TTR13AT508D
51°47,706
51°47,874
51°43,650
51°43,565
14°59,044
14°59,902
15°06,373
15°06,260
828
852
1,212
1,133
Intermound area
Intermound area
Escarpment
Sand, shell debris
Sand
Corals
TTR13AT509D
51°43,568
51°43,731
15°04,420
15°04,656
884
932
Slope of carbonate mound
Calcite-cemented bioclasts, coral rubble
TTR13AT500D
TTR13AT501D
Steep slope along escarpment
north, closer to the canyon head, mound distribution gets
denser. Here the mounds have lower topography and form
E-W-elongated ridge-like features.
The seafloor between these ridges is often characterised
by sinuous bands of low backscatter, interpreted as pathways of bottom currents (Figs. 2, 3). This was corroborated
by sediment samples collected during the cruise (Table 1,
samples AT504G to 507 G) and by underwater video
observations. The majority of the individual mounds and
ridges are concentrated in two elongated clusters extending
for 8 and 10 km in north–south and southwest–northeast
directions. These clusters contain the largest mounds
recorded in the area and are separated by one of the main
sand transport pathways (Fig. 3a). A prominent E–W
carbonate ridge rises up to 120 m above the surrounding
seafloor and extends for more than 5 km along the western
flank of the canyon head in the northwest of the study area
(Fig. 3a). On the acoustic data the ridge is seen to have high
backscatter levels suggesting a hard-bottom seabed. This
high backscatter signature extends northwards of the ridge
for hundreds of metres onto the flat seabed, becoming
gradually irregular and patchier. Beyond this patchy zone,
the seabed is characterised by mostly lower backscatter
which is attributed to a sandy bottom. This ridge is
interpreted to represent a carbonate build-up similar to the
ones seen along the eastern flank of the canyon head. The
distribution of the mounds, however, differs from that along
the eastern flank as the ridge represents the only major
topographic feature running predominantly along the edge
of the canyon head. Small E-W-aligned clusters of low
mounds (25–50 m high) can be found a few km north of the
ridge (Figs. 1, 2). The eastern flank, by contrast, is
210
Fig. 2 Combination of bathymetry, slope gradients, and reflectivity
data from the study area. a Bathymetry: 25-m grey-shaded relief. The
topographic control of the mound province is clearly delimited. The
line annotated FE indicates the faulted escarpment. Lines a–a′ and b–b′
indicate the locations of two profiles through the canyon and the
carbonate mound field (see top and bottom of figure; m mound). The
profile reproduced in Fig. 4 is marked by a thick dashed line. b Slope
map in 25-m grey scale: light grey high slope (>1°), dark grey low slope
(<1°). The N-S-oriented escarpment fault and the mound crests are
Geo-Mar Lett (2012) 32:205–225
clearly visible. Inset Rose diagram for mound crest orientation, white
dashed line MAK-1 sidescan sonar coverage (cf. Fig. 1). c Reflectivity
map (EM120 data) highlighting the patchy signature of the mound area,
the very high reflectivity of the fault escapement, the low reflectivity of
the intermound areas (loose fine sand, FS), the higher reflectivity
of the iceberg-ploughed seabed and the more compacted sand (S) in
the east, and the low reflectivity of mobile sands around mound
clusters (e.g. arrow)
Geo-Mar Lett (2012) 32:205–225
211
Fig. 3 a Bird’s eye view of the carbonate mound field (vertical
exaggeration 1:4) based on multibeam echosounder and sidescan sonar
data. It shows the pathways of bottom currents and sand transport, the
locations of sediment drifts, the E-W-oriented carbonate ridge in the
northern sector, and the two major carbonate mound clusters in the central
sector. b Visualisation of the fault-related escarpment (vertical exaggeration 1:4) based on multibeam echosounder and sidescan sonar data. It
shows the high degree of bedrock exposure (high acoustic backscatter)
and small individual carbonate mounds growing along its edge. Also
indicated are five sampling sites
characterised by the presence of several mound clusters
oriented predominantly normal to the canyon edge (Fig. 3b).
Besides the described clusters, a system of escarpments
was observed extending from south to north for up to 10 km in
212
Geo-Mar Lett (2012) 32:205–225
Fig. 4 Slope-perched sediment drift: the area of low backscatter with
seabed striations observed on the sonograph corresponds to the
stratified, pinching-out units seen on the deep-tow subbottom profiler
record. Sediment collapse on the mound slope can be observed on
both the sonograph and the profiler record as thin, chaotic, wedgingout units (for profile location, see Figs. 2a and 3a)
the north-eastern part of the study area at 600–700 m water
depth (Fig. 1). These features are up to 60 m high and are
separated by terraces characterised by moderate backscatter
levels. The main lineament appears segmented into a number
of curvilinear escarpments with high backscatter ascribed to
their steepness and the presence of bedrock outcrops (Fig. 3b).
In some places the high backscatter is disrupted by areas of
low relief and patchy backscatter signatures. Multibeam data
show that these discontinuities are associated with individual
small carbonate build-ups growing along the edge of the
escarpment. Trends of individual ridges within the main
clusters (Fig. 2b) suggest that they also grow along a series
of characteristic crescent-shaped escarpments. The crescentshaped geometry indicates that, as in the case of the other
escarpments, these are the seabed expression of a regional
listric fault system.
On multibeam and sidescan sonar images, the south-eastern
part of the study area is generally characterised by a smooth
relief and homogeneous low backscatter levels (Fig. 2),
implying a transitional thickening of the soft sedimentary
drape towards the south. The 5-kHz subbottom profiler
records show that the sedimentary cover often thickens at
these locations. Here, the seabed has a convex morphology
which onlaps onto the underlying sediments. Where the
thickness is lower, erosional features are visible. Such
sediment distributions and acoustic signatures indicate the
presence of several small, fine-grained drifts formed by the
northward-flowing bottom current (Fig. 4).
Oceanographic data
The regional, E-W-oriented temperature and salinity profiles
across the area indicate the existence of a major interface
between Eastern North Atlantic Water and Mediterranean
water masses at about 1,100 m water depth (Fig. 5). CTD
profiles taken from the southern (000730) and northern parts
of the study area (000727) have a similar structure, showing
a salinity peak associated with MOW between 900 and
1,150 m (Fig. 6). This corresponds to the lower limit of
mound distribution. Furthermore, in the vicinity of the
mound region, an irregular salinity pattern is observed over
the depth range of the mounds (600–1,000 m). This could be
caused by MOW mixing with the ENAW, but may also be
induced by the locally irregular topography within the deepwater canyon system. Such irregular water mass patterns are
not detected in the profiles of stations in deeper water to the
Geo-Mar Lett (2012) 32:205–225
213
-1000
-2000
-3000
Water depth (m)
0
Porcupine
Seabight
Porcupine
Bank
Phosphate (µmol/l)
Salinity (psu)
-4000
34.5
0 .2 5
3 5 .5
35
1.25
0.75
Rockall Trough
-5000
-1200000
-1000000
-800000
-1200000
Easting (m)
-1000000
-800000
Easting (m)
0
-1000
-2000
-3000
Silicate (µmol/l)
Temperature (°C)
-4000
10
5
15
10
20
30
40
-5000
-1200000
-1000000
-800000
-1200000
Easting (m)
-1000000
-800000
Easting (m)
0
-1000
-2000
-3000
Oxygen (ml/l)
Nitrate (µmol/l)
-4000
5
6
5
7
10
15
20
-5000
-1200000
-1000000
-800000
-1200000
Easting (m)
-1000000
-800000
Easting (m)
Fig. 5 Oceanographic transects across the study area (see location in Fig. 1, yellow band) based on information extracted from the World Ocean
Database 2005, GEBCO and GSI multibeam bathymetry data (cf. BODC 2003). Note the depth range of coral mound distribution
southwest (000722) and west (000804) of the study area.
Salinity plots of the latter two stations show a smoother
transition between the ENAW and the MOW, suggesting that
the MOW signature becomes more mixed in deeper water.
Profiles of oxygen, nitrate, phosphate, salinity and temperature show another interface at about 600 m water depth,
which corresponds to the upper limit of mound occurrence
(Fig. 5). The oxygen profile also reveals a marked oxygen
minimum zone (OMZ) between the two interfaces.
Authigenic deposits: morphology and petrography
At many of the sampling stations (Table 1, Fig. 1), mostly
rounded gravels and boulders (up to 0.5 m in diameter) of
different lithologies were retrieved, indicating that the entire
region has in the past been exposed to the discharge of icerafted material. More detailed sampling on selected carbonate
mounds and along the north-eastern escarpment (Table 1)
revealed the presence of two main rock types (carbonate,
phosphate) associated with outcrops of authigenic deposits
(Table 2). These rocks are all heavily bored, a feature ascribed
to annelids and crustaceans coexisting with sponges and other
encrusting organisms.
Carbonate deposits
Carbonate deposits were retrieved in three distinct settings: the
escarpment in the eastern part of the survey area, the upper
parts of carbonate mounds, and the flanks of carbonate
mounds.
The escarpment along the eastern side of the study area
(Fig. 3b) was initially surveyed with an underwater video
camera, which revealed irregularly shaped carbonate blocks
with bored surfaces. Occasional small colonies of living
0.4
0.0
3.5
0.3
1.1
2.2
2.9
18.2
1.9
1.4
1.3
1.7
72.2
61.6
87.3
81.5
3.3
98.9
63.4
66.3
3.6
81.8
3.6
6.4
1.2
0.5
5.9
0.5
12.8
40.2
35.7
33.4
1.1
0.8
2.3
3.1
0.5
0.7
7.1
15.5
1.6
1.2
3.3
4.3
14.6
15.5
−1.0
1.7
−4.1
4.3
1.1
0.9
2.0
1.7
4.6
3.0
11.2
8.9
Qz
Plag
Micr
MgCal
Ill
Hal
4.1
Dolomite block
Phosphatic hardground (groundmass)
Phosphatic hardground (light-coloured intraclast)
Phosphatic hardground (dark-coloured intraclast)
Calcite, glauconite-rich
Coral and shell debris, calcite-cemented
Coral and shell debris, calcite-cemented
Coral and shell debris, calcite-cemented
AT498D-3
AT500D
AT501D
AT509D-1
AT509D-2
corals were observed on these surfaces. These blocks consist
of porous micritic calcite cementing foraminiferal (planktonic
and benthic) and coccolith ooze. Some better lithified samples
(e.g. station AT500D, Fig. 7a–d) show broadly distributed
glauconite peloids and bryozoan remains; others samples
(stations AT498D, AT499D, AT500D, Fig. 7e, f) are less
lithified and lack any signs of glauconite.
Large reddish-pink elongated blocks of dolomite were
also collected from the escarpment (Fig. 7g; e.g. station
AT498D). Although borings have affected the outer
surfaces of these samples, epibenthic biota were rarely
observed. Thin sections and XRD analyses revealed the
presence of a fine-grained dolomite groundmass with
coarser-grained crystals occupying the voids. The observed
microfossil ghost features are consistent with re-cementation
of older hemipelagic deposits now exposed on the seafloor
(Fig. 7h).
AT498D-2
AT498D-3
AT498D-3
Fig. 6 Salinity profiles from four stations (see locations in Fig. 1d)
collected during 2000 (last four digits of each station represent the
month and day of sampling) showing the distribution of different
water masses in the study region; the stations deployed in deeper
waters to the southwest (000722) and west (000804) of the study area
show an irregular water column structure in the depth range of mound
occurrence (cf. main text for details)
2.9
Salinity (psu)
34.0
84.4
35.55
3.8
35.45
0.9
35.35
Coral and shell debris, calcite-cemented
Porous and burrowed calcite
35.25
AT494G
AT498D-1
35.15
δ18O (‰)
1200
Goe
MOW
Flap
1000
Dol
mounds
800
Chl
ENAW
Cal
600
000730
Arag
Depth (m)
400
Alb
000727
Isotopes
000804
Content (%)
200
Rock sample description
000722
Station
0
δ13C (‰)
Geo-Mar Lett (2012) 32:205–225
Table 2 Summary of XRD and isotope analyses on cementing groundmass samples. Alb Albite, Arag aragonite, Cal calcite, Chl chlorite, Dol dolomite, Flap fluorapatite, Goe goethite, Hal halite,
Ill illite, MgCal magnesium calcite, Micr micrite, Plag plagioclase, Qz quartz
214
Geo-Mar Lett (2012) 32:205–225
Video images from the upper parts of carbonate mounds
showed dense thickets of living white and pink corals
abundantly colonized by various species of echinoderms
and crustaceans up to an approx. water depth of 1,125 m
(Fig. 8a). Thickets of L. pertusa, M. oculata and occasional
D. cristagalli were observed to populate well-cemented
carbonate blocks dredged at station AT509D (Fig. 8b).
Petrographic observations are similar to those made on
rock samples from the escarpment region, micritic calcite
being the main cement binding together coccolith and
foraminiferal ooze but, in this case, with high contents of
sponge spicules and partly dissolved coral branches. Organic
matter and aggregates of pyrite framboids are concentrated in
defined areas of the rock samples (Fig. 8c). A similar type of
carbonate lithology was retrieved at station AT494G,
representing the deepest site sampled in the canyon.
The video surveys run on the flanks of carbonate mounds
revealed the presence of irregular blocks protruding from
fine-grained sediment together with scattered bioclasts and
ice-rafted debris. Dredge hauls from this area (stations
AT509D, AT501D) retrieved rocks similar to those described
from the tops of the mounds, except that the corals show
evidence of much stronger dissolution (Fig. 8d–f). Noé et al.
(2006) and Mienis et al. (2009) possibly describe similar
authigenic deposits from carbonate mounds of the NE
Atlantic, which contain carbonate-cemented layers with
dissolved coral branches.
Phosphatic hardgrounds
The video record run along the escarpment shows widespread slabs populated by abundant fauna, in places
including small colonies of living corals. The large slabs
retrieved from dredge hauls have a yellowish-red colour,
and are very well lithified and heavily bored (stations AT498D,
AT499D, AT500D; Fig. 9a). Freshly broken surfaces reveal a
greenish internal structure consisting of cm-sized, subrounded
brownish intraclasts and large (>0.5 cm) bioclasts supported
by a yellowish-green groundmass (Fig. 9b).
Petrographic analyses (Table 2) show the cement of the
microfossil-rich groundmass to consist of collophane binding
together hemipelagic sediment, microfossils, apatite and
sparsely distributed glauconite aggregates, the latter conferring the greenish colour to the samples (Fig. 9c). A similar
composition is recorded for the lighter-coloured intraclasts
(Mg-calcite 33.4%, fluorapatite 66.6%) as well as the darker
ones (Mg-calcite 18.2%, fluorapatite 81.8%; Fig. 9c). The
intraclasts typically contain higher amounts of fluorapatite,
and occasionally also hemipelagic clay, hence the darker
colour in comparison to the groundmass. Bioclasts (e.g.
bivalves, corals, fish teeth, microfossils) are scattered
throughout the sample. Microfossils are mostly disrupted
within the intraclasts, in contrast to their good preservation in
215
the groundmass. In addition, the contact intraclast–groundmass shows truncations of shells and glauconite, indicating
prior transportation before their final deposition (Fig. 10a, b).
In some instances the edges of irregularly shaped intraclasts
reveal the presence of cross-cutting glauconite pellets on the
intraclast margins (Fig. 10c, d), which is consistent with
continued local in situ diagenesis. These intraclasts have
sharp margins defined by a zoned rim of collophane cement
(Fig. 10e, f).
Microfossil identification suggests a relatively recent age
for these hardgrounds (Early Pliocene), as indicated by the
presence of foraminifers such as orbulinids and numerous
thin-walled Globigerinoides.
Carbon and oxygen stable isotopes
The majority of the analysed samples (Table 2) have small
variations in stable isotope composition, with δ13C values
between 0 and 2‰ and δ18O values between 3 and 4.6‰.
The dolomite samples all have negative δ13C (−1.0‰) and
δ18O (−4.1‰) values.
Discussion
Interrelationships between carbonate precipitation
and oceanography
Sonar images and sediment samples taken around the
mound area suggest that development of a small sediment
drift is currently taking place. This is consistent with the
observations of bottom current activity on the video footage
of the seafloor surrounding the mounds. Furthermore,
measurements conducted by a current-meter array a few
kilometres farther south revealed average bottom current
speeds of 30–50 cm/s (Dickson and McCave 1986), which
are capable of transporting up to medium-grained sand. The
interpretation of the acoustic data agrees with that of
previous studies in other carbonate mound areas along the
Irish margin (Kenyon et al. 1998, 2003a; Wheeler et al.
2005), where the low backscatter has been ascribed to the
accumulation of fine- to medium-grained sand along the
bottom current pathways (Kenyon et al. 2004). This
interpretation indicates that the current from the south is
responsible for the accumulations of lag sands and perhaps
also for the transport of nutrients necessary for coral
growth. It is suggested that the colonisation started from
isolated symmetrical build-ups which then spread along the
edge to form elongated complexes of merged mounds, the
growth of which was facilitated by strong geostrophic flow
along the escarpment and by tidal currents running in an E–
W direction. Outcrops of basement rock, which occur along
such escarpments, provide the necessary substratum for
216
coral settlement (Wilson 1975). Mound growth is then
facilitated by bottom currents and the hydrodynamic regime
in the canyon where local current speed enhancement can
be expected to occur due to the irregular topography around
the edge of the canyon.
According to the sand transport pathways identified on
the MAK-1 sonographs, prominent carbonate mounds also
contribute to the topographic steering of the current.
Abundant linear irregularities of high backscatter recognized on the sonographs are interpreted as half-buried
escarpments located under the drift sequence along the
southern extension of the previously described fault system
(Fig. 2). The gradual northward variation in mound
alignment from NW–SE to E–W, their shape evolution
from conical to elongated ridge, and their increase in
abundance emphasise the importance of the canyon head in
enhancing the hydrodynamic conditions which then favour
mound growth. Similar conditions within canyons funnelling nutrients and sediments to the benefit of coral growth
have been reported by de Stigter et al. (2007) and Orejas et
al. (2009). These observations, in combination with video
surveys, also suggest that numerous coral colonies were
initially emplaced along regional fault-related escarpments
which provided a hard substratum suitable for later growth
and preferential southward expansion into the current. A
similar type of accretion was reported by Paull et al. (2000)
from the Florida–Hatteras Slope.
Different mound distribution patterns along the western
and eastern flanks of the canyon head may indicate the
presence of cascading bottom flow (Shapiro and Hill 1997),
which should be more intense on the eastern flank, thereby
favouring mound growth. The western flank is less affected
by the current, and the dominant process promoting coral
growth there is probably the edge-related enhancement in
turbulence.
The presence of several interfaces within the water
column—e.g. between the MOW marked by the salinity
anomaly, the OMZ and the ENAW—facilitates the widespread development of nepheloid layers in the depth
interval between 300 and 1,000 m (Dickson and McCave
1986; Rice et al. 1991; De Mol et al. 1998). These
nepheloid layers are an important source of detrital organic
particles which increase the food availability for bottom
fauna (Frederiksen et al. 1992). These layers spread out
along the interfaces between the different water masses in
the form of bottom-attached and intermediate nepheliod
sheets (BNL and INL; Dickson and McCave 1986).
The development of nepheloid layers can be facilitated
by a number of processes, including localised downwelling
events following slack periods of tides (Davies et al. 2009,
Mingulay Reef off NW Scotland), tidally pulsed Ekman
drainage (driven by cascading of denser trapped winter
water on the bank) which flushes the nutrient- and organic
Geo-Mar Lett (2012) 32:205–225
Fig. 7 Escarpment area. a Hand sample of lithified carbonate rock„
(station AT500D); the greenish colour denotes a high glauconite
content. b Thin section plane light image showing glauconite
aggregates often coated by organic matter in a foraminifer- and
organic-rich micritic calcite groundmass. c SEM image showing some
glauconite aggregates in micritic calcite groundmass. d Thin section
plane light image showing bryozoans (arrow) in micritic, organic-rich
groundmass with benthic and planktonic foraminifers and larger
bioclasts. e Hand sample of less-lithified carbonate rock; note the
greyish colour and the numerous borings. f SEM image of the internal
part of the sample in e showing coccolith-rich groundmass devoid of
glauconite. g Large block of dolomite (dredged). h Thin section plane
light image showing fine-grained dolomite cement with microfossil
ghost features (centre)
matter-rich ‘soup’ from shallow waters downslope into the
bathyal realm (White et al. 2005), and others. To identify
the process which is most active in the area lies beyond the
scope of this paper, although prominent tidal influence can
be inferred from analogy with other mound areas in the
region (Dorschel et al. 2007; Mienis et al. 2007).
Both INL and BNL layers were initially described for
the study area on the basis of CTD profiles (Dickson and
McCave 1986; Fig. 1e). More recent single-beam surveys
have provided acoustic images of three such layers in the
north of the study area (Fig. 1e). These layers occur in the
same depth range as the mounds (600–1,000 m; Fig. 1) and
are therefore likely to be responsible for the enhanced coral
growth.
The present findings are consistent with strong current
activity facilitating the dissolution of aragonitic bioclasts
and the precipitation of calcitic micrite. The observed
aragonite dissolution (mainly from corals) operates as
carbonate source for the precipitation of micritic calcite (e.
g. Mazzullo 1980; Lasemi and Sandberg 1984). This
process is favoured where coarser bioclasts are present
and where a higher flux of water can be flushed through the
pores to improve ion transport under oxic conditions. van
der Land et al. (2010, 2011) have also demonstrated that
carbonate precipitation is greatly enhanced by the presence
of coccoliths which form the nucleation sites for calcite
precipitation. In contrast to the previous sampling by
Mienis et al. (2009), where carbonate-cemented intervals
were cored in the subsurface, the present findings show that
these rocks actually crop out on the seafloor. The model
proposed herein is thus consistent with observations at the
top and along the flanks of the carbonate mounds where
coarse coral debris are present and where the currents are
stronger, favouring better cementation and enabling the
cemented blocks to remain exposed. The existence of such
localized hard substrata would have an active role in
promoting the continuous growth of the mounds.
The formation of carbonate deposits at the base of the
mounds is unlikely to be due to high sedimentation rates or
the supply of large amounts of coral debris from the upper
part of the mounds. These conditions would not be
Geo-Mar Lett (2012) 32:205–225
217
a
b
m
c
d
m
e
g
5 cm
f
10 cm
h
m
218
favourable for carbonate dissolution and further precipitation, as confirmed by core AT495G (see log in Kenyon et
al. 2004) recovered from the moat of a multi-peaked
carbonate mound. The 5-m-long core penetrated a sequence
rich in coral debris intercalated with clay or sand beds and
occasional pebble layers. Similar cores from the bases of
other mounds along the Irish margin are documented in
Kenyon et al. (1998, 2003b). This implies that periods of
fast growth (and higher rates of debris production) of the
coral colony alternate with periods of dominant hemipelagic and terrigenous sedimentation (and perhaps erosional events). The presence of water-saturated coral debris
layers suggests rapid burial, presumably occurring when the
angle of repose of the mounds is exceeded, causing the
collapse of the mounds accompanied by reduced coral
growth. The absence of lithified hemipelagic sediment in
the core strengthens the contention that carbonate precipitation occurs only at specific sites.
Interrelationships with phosphatic hardgrounds
As stated in the Introduction, the presence of phosphate
deposits in carbonate mound regions of the Irish Margin
has been poorly documented so far. Phosphatic hardgrounds
were particularly common during the Cretaceous and are
today cropping out in southern England (e.g. Kennedy and
Garrison 1975a, 1975b), in Northern Ireland (MarshallNeill and Ruffell 2004), and at various places along the
Irish west coast (Wilson 1975). More recently, Haughton et
al. (2005) documented the stratigraphy of three borehole
sites on the western slope of the Porcupine High. At
borehole 83/20-sb01, a thin phosphatised crust was cored
between Maastrichtian chalky micrites and the overlying
Miocene silty clays. However, it is currently unknown
whether deposition of phosphate-rich sediments recurred in
the recent past. The textural and petrographic characteristics
of the hardgrounds in the present study area are very
similar to those described by Haughton et al. (2005). By
contrast, the Early Pliocene age estimated here indicates
that they are much younger. It cannot be excluded that
these laterally extensive hardgrounds correspond to the
erosional unconformity observed at the base of mounds in
the Porcupine Seabight and on the Rockall Bank (e.g.
Stoker et al. 2005), but more data are required to verify
this hypothesis.
Based on the present findings, the phosphatic hardgrounds could be the result of two different depositional
processes. The rounded intraclasts, which contain more
fragmented fossils and broken shells and minerals, represent the initial deposition on the upper part of the
escarpment. These poorly lithified clasts were then locally
moved along the seabed before they became fully cemented
within the lighter-coloured groundmass. The well-preserved
Geo-Mar Lett (2012) 32:205–225
Fig. 8 a–c Top of coral mounds. a Dense thicket of corals (scale: ca.„
1 m). b Hand sample consisting of micritic calcite cementing
foraminiferal ooze; note the numerous borings and attached living
corals. c Thin section plane light image showing micrite-cemented
foraminiferal ooze with locally high concentrations of organic matter
(dark colour). d–g Flank of coral mounds. d Extensive carbonate
deposits rich in coral debris associated with attached thickets of living
corals (scale: ca. 1 m). e Detail of a partly dissolved coral fragment
extracted from carbonate deposits. f Hand sample of micritic calcite
cementing hemipelagic sediments rich in coral and shell debris. g Thin
section plane light image showing micritic calcite cementing hemipelagic sediments rich in microfossils (mostly planktonic) and
bioclasts (e.g. shell fragment to right)
microfossils within the collophane groundmass suggest that
cementation occurred at or close to the sediment–water
interface.
In most cases, this type of hardground occurs as a
consequence of the deposition of large amounts of organic
material, this being frequently related to upwelling currents.
The onshore (UK, Ireland) record of such deposits in the
geological history indicates a recurrence of such events.
Dinoflagellate blooms associated with major benthic and
pelagic mortalities related to small-scale coastal upwelling
events have indeed been documented along the southwest
coast of Ireland (e.g. O’Boyle 2002; Silke et al. 2005),
although of considerably smaller magnitude than those
occurring, for example, off south-western Africa (e.g. Carr
2002; Weeks et al. 2004; Santana-Casiano et al. 2009).
Such events occur where abrupt bathymetric changes take
place (e.g. escarpments, continental shelf breaks). They
generally induce high organic productivity and phytoplankton growth, resulting in the deposition of organic-rich
sediments in the neighbouring escarpment area. Low
sedimentation rates and bacterial decay of organic
matter liberate phosphates which can then induce the
phosphogenesis of hardgrounds containing glauconite.
High-energy conditions are required to allow these
reactions and to erode the hemipelagic capping sediment, or
to prevent its deposition, in order to expose the hardgrounds at
the seafloor.
This scenario seems to apply to the Porcupine Bank
Canyon escarpment region where the presence of strong
currents was confirmed by sidescan sonar data and the
medium- to coarse-grained sand with foraminifers and shell
debris retrieved at sampling station AT496G (Mazzini et al.
2004). Moreover, the water depth in this area coincides with
the limits of the previously described OMZ. The regional
oceanography also provides complementary arguments to
explain the presence of phosphate deposits. Glenn et al.
(1994) describe phosphate precipitation as a widespread
phenomenon in marine sediments, but emphasise that the
accumulation of such deposits requires the maintenance of
the phosphogenic system for extended periods of time
coupled with prolonged concentration of phosphatic particles by current winnowing and transport. In the Porcupine
Geo-Mar Lett (2012) 32:205–225
219
a
2 cm
b
m
c
2 cm
d
f
e
5 cm
g
m
220
Geo-Mar Lett (2012) 32:205–225
4 cm
a
c
b
m
Fig. 9 Escarpment area, phosphatic hardgrounds. a Large slab; note
the intense boring and fauna colonizing the external surface. b Freshly
broken surface of a slab showing yellowish-green groundmass and
subrounded cm-sized light- and dark-coloured intraclasts. c Thin
section plane light photo mosaic showing groundmass and intraclasts
both consisting of collofane containing glauconite, bioclasts and clay
Bank Canyon area, the water column stratification and the
circulation pattern can provide the mechanism for efficient
trapping and concentration of organic matter within narrow
zones along the continental margin. The bottom nepheloid
layers described by Dickson and McCave (1986) and Rice
et al. (1991) can evidently supply large volumes of
suspended organic matter, and the long-term persistence of
these conditions along the Irish continental margin would
Geo-Mar Lett (2012) 32:205–225
221
a
b
c
d
e
f
Fig. 10 SEM-BSE images of phosphate deposits. a Contact between
nodule (top) and groundmass; sharp truncations of the glauconite and
microfossils (arrowed) suggest fracturing and transportation of this
nodule. b Groundmass and nodule (bottom right, dotted line); the
truncated glauconite and foraminifers (arrowed), the fragmented
microfossils within the nodule, and the better preserved microfossils
in the groundmass suggest that the nodule was not fully lithified when
transported to its final place of deposition. c, d Contact between
nodule (left) and groundmass with well-preserved glauconite crosscutting the margin between the two features. e Magnification of the area
framed in d; note the lighter backscatter of the diagenetic rim framing
the nodule. f Magnification of the area framed in e; zonations within the
rim suggest variations in diagenetic conditions during rim formation
thus promote the formation of phosphate crusts which, in turn,
provide the hard substrata required for the establishment of
deep-water coral bioherms. These conditions persist to the
present day, as indicated by the thriving coral colonies and by
the glauconite aggregates observed within the carbonate
deposits of the escarpment (e.g. station AT500Gr). This
222
implies constant slow accumulation rates, which is a
precondition for the in situ formation of glauconite at the
water–sediment interface.
The association of phosphatic hardgrounds with corals has
also been widely documented in the literature. For example,
Jarvis (2006) described deep-water colonial coral thickets
(Diblasus arborescens) in conjunction with phosphate-rich
hardgrounds in the Santonian–Campanian phosphatic chalks
of England and France. Phosphate ‘nodules’ and intraclasts
were drilled close to the base of the Magellan carbonate
mound (Expedition-Scientists-ODP-307 2005). Furthermore,
glauconite has been observed to occur in the upper bathyal
coral zone of the Rockall Bank (Scoffin et al. 1980), in the
hardgrounds near Connacht Mound (Noé et al. 2006), in
bathyal gorgonian skeletons from the Bay of Biscay margin
(Noé et al. 2007), on the Sula Reef of the mid-Norwegian
shelf (Freiwald et al. 2002), and in Gonicorella thickets of
the Chatham Rise, New Zealand (Dawson 1984; Kudrass
and von Rad 1984).
The data available to this study are insufficient to determine
to what extent the localised upwelling events known to occur
in the area have contributed to the development and
maintenance of the deep-water coral ecosystem. The distribution of the mounds along the escarpments along the canyon
head suggests that the role of upwelling is substantial.
However, the increased current-induced turbulence at the
escarpment edges is probably also sufficient to create the
enhanced nutrient fluxes needed for coral growth. A combination of both factors would, of course, also be plausible.
Whichever the case, the formation of phosphate crusts greatly
enhances the probability of widespread coral settlement. The
high backscatter recorded in the escarpment area together with
the phosphate deposits suggest that it would not be unrealistic
to expect much more extensive deposits of this kind in the
Porcupine Bank region.
Insights from isotopic signatures
The isotopic values show that seawater was the initial source
of the carbon in the cements and bioclasts of the authigenic
deposits in the study area. With δ18O=0.6‰SMOW as the
present-day value for the Porcupine region bottom waters (F.
Mienis, personal communication), and applying the equations of, for example, Friedman and O’Neil (1977) and
Grossman and Ku (1986), the oxygen isotopes of the
analyzed samples indicate that these carbonates precipitated
in equilibrium with the actual seafloor conditions (~7°C).
Moreover, the δ13C values show no significant depletion
which could be related to seepage of hydrocarbon-rich fluids.
This is consistent with the findings of Noé et al. (2006) who
obtained similar δ18O and δ13C values for carbonate hardgrounds from the Propeller Mound in the Porcupine Seabight.
The association of coral reefs, carbonate deposits and
Geo-Mar Lett (2012) 32:205–225
methane seepages therefore remains a speculative issue at
this stage.
Conclusions
The western margin of the Porcupine Bank is characterised by
pronounced bathymetric variations and a high variety of
depositional environments. In the course of this multidisciplinary study, a large province of carbonate coral mounds was
examined together with the environmental controls and
depositional processes involved in their formation. The
mounds are located in a distinctive geomorphological and
oceanographic setting when compared to other large mound
provinces in Irish waters, and thus represent an important case
study for research on mound development.
The data presented in this study show that particular
oceanographic processes are the predominant factor controlling mound distribution and growth. Water column
properties indicate mixing of water masses across the
mound depth interval. In addition, a distinct salinity peak
at about 1,000 m associated with the MOW coincides with
the lower limit of the mounds. The available geophysical
evidence indicates that the distribution of the mounds
follows the pre-existing seabed topography, being located
on the almost flat seabed along the edges of the canyon
system framed by fault escarpments in the easternmost part.
Furthermore, mound density increases towards the canyon
head where the specific hydrodynamic regime effectively
influences the shape and orientation of mound morphology.
Different types of authigenic deposits (carbonate and
phosphatic) have been identified. On the top and along the
flanks of the mounds, calcite and coral-rubble deposits are
present. The model proposed in this study implies that strong
currents facilitate the dissolution of coral debris and the
precipitation of micritic calcite. Phosphatic and carbonate
deposits have been sampled from the eastern escarpment.
These phosphatic hardgrounds may extend well beyond the
eastern part of the survey area. The bottom currents in this area
and localised upwelling favour the formation of these
phosphate-rich deposits, at the same time preventing their
burial.
Sampling and video observations show that all the
described authigenic deposits represent a hard substratum
enhancing the growth of the mounds and promoting the
establishment of new coral colonies along the escarpment.
The available geochemical data do not support any
relationship between methane seepage and coral colony
emplacement.
Acknowledgements The Geological Survey of Ireland (GSI) provided funding and scientific guidance for the TTR-13 study of the
Porcupine Canyon mounds. The authors are thankful to GSI and its
Geo-Mar Lett (2012) 32:205–225
director for access to the multibeam and CTD data and for allowing
their publication. We are grateful to Felix Gradstein for his assistance
in microfossil identification, J. Still for help during microprobe and
SEM analyses, and also J. Parnell for his support. We would like to
acknowledge the crew and the Scientific Party of the TTR-13 cruise
and the UNESCO-MSU Centre for Marine Geosciences. The
UNESCO, and in particular Alexei Suzyumov, are thanked for their
support during the preparation of this manuscript. M. Badali, F.
Mienis, B.T. Cronin, A. Freiwald, as well as the editors M.T.
Delafontaine and B.W. Flemming are acknowledged for their
suggestions and critical reviews.
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