The importance of the permanent thermocline to the cold water coral carbonate mound distribution in the NE Atlantic

Earth and Planetary Science Letters 296 (2010) 395–402

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

The importance of the permanent thermocline to the cold water coral carbonate mound distribution in the NE Atlantic Martin White a,⁎, Boris Dorschel b a b

Department of Earth and Ocean Sciences, National University of Ireland, Galway, Ireland Department of Geology, Environmental Research Institute, University College Cork, Ireland

a r t i c l e

i n f o

Article history: Received 3 December 2009 Received in revised form 14 May 2010 Accepted 20 May 2010 Available online 23 June 2010 Editor: M.L. Delaney Keywords: carbonate mounds permanent thermocline benthic dynamics

a b s t r a c t A prominent feature of the NW European continental slope is the presence of numerous cold water coral carbonate mounds that are clustered in a number of provinces. These provinces occupy a relatively narrow depth range along the continental slope: 95% of all coral carbonate mounds identified on the Irish seabed have their mound bases between 500 and 1000 m water depths, with a peak in distribution at ∼ 650 m water depth. The distribution in mound base depths is skewed with a tail extending from the maximum at 650 m to deeper depths. This distribution brackets the depth of the permanent thermocline in the NE Atlantic (600– 1000 m) formed below the base of the winter mixed layer. It is shown that the permanent thermocline is associated with the strongest residual near seabed current flow, with typical residual current speeds up to 2– 3 times larger at the thermocline depth compared to other depths. The strong vertical density gradient associated with the permanent thermocline, together with the steep continental slope at those depths, also enhances the energy of certain periodic motions such as internal waves and baroclinic tidal currents. These dynamic conditions favour mound growth through the promotion of significant along-slope sediment transport and also provide large across-slope sediment movement and organic matter fluxes. The stability of the thermocline structure is likely the key in providing favourable conditions over long time scales that allow mound growth through sediment baffling processes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction A prominent feature of the continental margin of the NE Atlantic is the occurrence of numerous cold water coral carbonate mounds, located at the continental slopes of the Rockall and Porcupine Banks and the Porcupine Seabight (e.g. de Mol et al., 2002; Kenyon et al., 2003; van Weering et al., 2003; Roberts et al., 2006). These biogenic seabed structures are composed of open frameworks of scleractinian corals (mainly Lophelia pertusa or Madrepora oculata) filled with hemipelagic sediments and dead coral fragments. They can reach heights in excess of 250 m and may have a base of up to 3 km in diameter (Freiwald, 2002; Kenyon et al., 2003; Roberts et al., 2003; van Weering et al., 2003; Wheeler et al., 2005) but mostly only elevate tens of meters above the surrounding seafloor. These cold water coral carbonate mounds occur clustered in so called mound provinces and provide a significant proportion of the cold water coral occurrences in the NE Atlantic (Roberts et al., 2003). A characteristic of the mound distribution is that the majority of these mounds at the continental margin fall within a relatively narrow depth range between 600– 1000 m (de Mol et al., 2002; Kenyon et al., 2003; Roberts et al., 2003; ⁎ Corresponding author. Tel.: + 353 353 91 493214; fax: +353 353 91 494533. E-mail addresses: [email protected] (M. White), [email protected] (B. Dorschel). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.05.025

van Weering et al., 2003; Fig. 1a), thus suggesting that oceanography and/or local hydrology may likely be responsible for the mound distribution characteristics. At the ocean basin scale, the scleractinian corals L. pertusa and M. oculata are generally found over a large depth range and wide range of hydrographic conditions, including temperature and salinity. As an azoothanthellate coral, requiring an external energy source, there is a natural relationship of their occurrence in regions with large overlying surface productivity (Freiwald, 2002; Roberts et al., 2006). In the NE Atlantic, several carbonate mound clusters are present along the flanks of the Rockall and Porcupine Bank. It has been suggested that elevated surface productivity over these banks, driven by increased nutrient levels there, may be a significant contributory factor to the presence of the mounds at these locations (White et al., 2005). At intermediate spatial scales in the order of 10–100 km, there has been much speculation on the environmental control of carbonate mounds in terms of their setting, depth distribution and growth (Freiwald, 2002; Roberts et al., 2003). Generally the initiation of mound growth at the continental margin has occurred at hard erosional surfaces associated with dynamic boundary currents (van Weering et al., 2003; Mienis et al., 2007, 2009; van Rooij et al., 2007; Dorschel et al., 2009). The interplay of coral growth and sediment input result in the formation of these mounds (Wheeler et al., 2007;

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Fig. 1. The distribution of (a) carbonate mounds in the NE Atlantic region, indicated by red dots, and (b) location of current meter measurements used in the analysis presented here (dots). In (a) the letter denote the mound provinces; H—Hovland, B—Belgica, WP—West Porcupine, NP—North Porcupine (Pelagia), L = Logachev, and WR—West Rockall. In (b), the cross indicates the location of the CTD data used in Fig. 3. The bathymetry and elevation data are generated from GEBCO data.

Dorschel et al., 2009). As a sessile filter feeder, cold water corals require a hard substrate on which to settle, a dynamic environment with little sedimentation and large organic fluxes (Freiwald, 2002; White et al., 2005; Roberts et al., 2006). For example the depth distribution of corals has been related to regions where internal waves cause both enhanced productivity in the upper layers through vertical mixing and nutrient fluxes, and by generating enhanced organic fluxes in the benthic boundary layer due to wave-generated, large near seabed shear stresses (Frederiksen et al., 1992). Other processes that have been proposed are enhanced sub-inertial tidal period currents near the seabed (White, 2007; White et al., 2007), or local flow acceleration around smaller scale topographic features such as seamounts (Genin et al., 1986). The region associated with carbonate mounds of the NE Atlantic encompasses the region between the two main Atlantic gyres and the eastern portion of the sub polar gyre. The water masses associated with the depth range where carbonate mounds are found are mainly the northward spreading Eastern North Atlantic Water (ENAW) at intermediate depths (200–700 m), and below these depths, there is significant influence of Mediterranean Outflow Water (MOW,) most influential to the south. Also Sub Arctic Intermediate Water (SAIW) is present at depths (400–800 m) which has a source from the west, under the North Atlantic Current (NAC), and generally has diminishing influence as the continental slope is approached (van Aken and Becker, 1996). A prominent feature of the margin circulation is the presence of the poleward flowing slope or shelf edge current (SEC, e.g. Huthnance, 1981; Pingree and LeCann, 1990; Pingree et al., 1999). This flow is driven principally by the north–south density and associated zonal and cross-slope sea level differences. Residual flow speeds of between 5 and 20 cm s− 1 have been reported and flows are concentrated as relatively narrow cores located over the mid slope (e.g. Pingree and LeCann, 1990; Pingree et al., 1999; White, 2007). The current plays a significant role in driving the benthic boundary layer (BBL) at the continental margin — the turbulent near seabed frictional layer of direct importance to the resident benthic fauna, including the cold water corals. In addition to the SEC, tidal currents and various wave motions also determine the spatial and temporal variation in the BBL

turbulent structure on the slope (e.g. Thorpe et al., 1990; White, 1994, 2007). Another characteristic of the dynamics at the NE Atlantic margin is the relatively large amount of energy that is transferred from the barotropic tide to periodic, baroclinic, motions (Baines, 1974). These baroclinic motions take two forms; i) bottom trapped baroclinic waves or ii) freely propagating internal waves. Oscillations with a frequency ranging between the buoyancy frequency (N, Eq. (2)) and the local inertial frequency (f, the Coriolis parameter) will be freely propagating and generate internal waves, principally at the forcing period (e.g. Pingree et al., 1986; New and Pingree, 1990; Thorpe, 1992). An example would be the deformation of the seasonal thermocline by tidal excursions across rapidly varying topography such as the continental slope (e.g. Sharples et al., 2009). Of particular relevance for internal waves is the characteristic angle (to the horizontal, β) of energy propagation (group velocity) along the internal wave beams which, according to, for example Cacchione et al. (2002), depends on the wave frequency (σ), the vertical stratification (N) and the Coriolis parameter (f): 2

2

2

2

1=2

c = tanðβÞ = ½ðσ – f Þ=ðN –σ Þ

ð1Þ

and the vertical stratification is given by the expression; 2

N = −ðg = ρo Þ⁎ dρ = dz

ð2Þ

Here g is the acceleration due to gravity, ρ is the density, ρo a mean density and z the vertical coordinate, such that dρ/dz represents the vertical variation (gradient) in density. Furthermore, this angle is maintained upon wave reflection, so that the angle relative to the seabed will remain the same for a flat seabed but changes for reflection off a sloping seabed (of angle α), due to the invariance of β. Reflection from a sloping seabed, therefore, changes the wave number of the internal wave and hence the energy density within the wave, with the likelihood of increased currents and vertical mixing (Thorpe, 1987). Of particular importance is the condition when β = α which corresponds to “critical” conditions at

M. White, B. Dorschel / Earth and Planetary Science Letters 296 (2010) 395–402

locations where internal waves can be generated and where impinging internal waves will be reflected along the seabed. If the period of oscillation is longer than the local inertial period (2π/f), the wave motion may be trapped in the vicinity of the topographic feature that they have been generated from, e.g. the continental slope (e.g. diurnal period waves for latitudes greater than 30° (e.g. Rhines, 1970)). Two characteristics of this type of wave are i) that the tidal motion may become rectified, i.e. a residual current will be generated directed along the slope and ii) that under certain conditions of vertical stratification both the residual and tidal period motion may be amplified (Huthnance, 1981). The extent to which any periodic tidal current, and associated residual rectified flow, will be amplified will depend on a coupling of the forcing frequency (tidal period) and the natural oscillation period within the water column. This is determined by the vertical density stratification (N) and the bottom seabed slope (α). According to Huthnance (1981), the maximum amplification, if any exists, will be expected at a bottom depth where N ⁎ sinðαÞ = maximum:

ð3Þ

The degree of amplification will be dependent on the degree of resonance between tidal forcing and the response of the water column stratification to the forcing. This resonant period (Tres) of the density oscillation can be found from the expression Tres = N ⁎ sinðαÞ⁎ sinðγÞ

ð4Þ

Here γ is the angle of the baroclinic wave to the orientation of the isobaths defining the continental slope, where γ = 0° is directed along the isobaths. If the waves are directed across the slope then sin(γ) = 1 and the maximum resonance frequency for the response to the periodic forcing will be found, or correspondingly, the minimum wave period (Rhines, 1970). In this paper we investigate the local importance of the thermocline structure for the generation of dynamic hydrographic environments near the seabed and correlate our findings with the vertical and spatial distribution of cold water coral carbonate mound provinces in the Rockall Trough and Porcupine Sea Bight, NE Atlantic. We highlight the correlation of this distribution to the depth of the permanent thermocline, the depth range where the vertical temperature (and density) gradients are largest (with the exception of the shallower seasonal thermocline). This is achieved through an analysis of historical current meter data from the region along the NE Atlantic margin in comparison to simple theoretical considerations of baroclinic wave dynamics generated by the presence of the thermocline. 2. Methods

397

critical slope angle of 6° provided the best mound-detection verse noise ratio. In irregular terrain, both slope-inclination and bathymetric data were used to map mound outlines. 2.2. Hydrographic data In order to model the position of the permanent thermocline in the NE Atlantic on a regional scale, oceanographic data from different sources have been collected and collated. Vertical profiles of temperature, salinity and density have been collected at the deep ocean regions adjacent to the continental margin west of Porcupine Bank (Fig. 1b). These have been measured during Irish surveys on the R.V. Celtic Explorer using a SeaBird Sbe11 CTD system during hydrographic surveys carried out in the southern Rockall Trough, details of which can be found in Ullgren and White (2010). To calculate the vertical density stratification at different mound provinces, CTD measurements made during the EU funded research projects ACES (Atlantic Coral Ecosystem Study) and ECOMOUND (Environmental Controls on Mound Formation along the European Margin) have also been utilised (see White, 2007). In addition, a large number of historical records of Eulerian measured currents along the continental margin have been analysed as part of numerous recent EU funded projects (e.g. OMEX — Ocean Margin EXchange, ACES — Atlantic Coral Ecosystem Study, ECOMOUND — Environmental Control of Carbonate Mounds). A summary of an earlier analysis of the data at the time of the OMEX project has been reported (Huthnance et al., 2001), but the data quantity has increased significantly since that time. For this study, time series of current data have been extracted from the NUI, Galway, Earth and Ocean Sciences database that have a length of at least 4 weeks and which occur within the region delimited by 48–58°N, 8–18°W. These data sets have been taken to determine the characteristic residual flow speeds and magnitudes of the periodic (tidal) current speeds within the region where the carbonate mounds occur. The residual current speeds were determined at a depth corresponding to a height of 10–50 m above the seabed (10– 50 mab). The residual speed was determined for the total measurement period, irrespective of length but with a minimum of 4 weeks of observations. The height range 10–50 mab above the seabed was chosen to exclude those measurements close to the seabed but not within the main bottom frictional layer. We note that the frictional layer may well exist above 10 m height, but its influence at those heights will be less significant. The cut-off was chosen to allow a suitable size data set for analysis, given that a large number of past measurements have been made between 10–12 mab. In addition, no account of the current direction has been taken as it is assumed that the scalar current speed is more important than the vector velocity for coral carbonate mound occurrence.

2.1. Carbonate mound-detection

3. Results

To date, various clusters of mound features have been groundtruthed and identified as cold water coral carbonate mound provinces (e.g. de Mol et al., 2002; Kenyon et al., 2003; van Weering et al., 2003; Roberts et al., 2006; de Haas et al., 2008). This study only considers mound features from these groundtruthed cold water coral carbonate mound provinces. To determine the water depth in which coral carbonate mounds are located, the outlines of these mounds (the contact between mound and adjacent seafloor) were mapped. This was done on high resolution multibeam bathymetry data collected during the Irish National Seabed Survey (INSS). The slopes of coral carbonate mounds are generally steeper than the adjacent seafloor. Therefore, the outlines of coral carbonate mounds were not mapped directly from bathymetry data but on the base of slope-inclination data sets generated as first derivative of the bathymetry data sets. A

3.1. Distribution of cold water coral carbonate mounds in the study area In total, 7 coldwater coral carbonate mound provinces (including 1013 individual carbonate mounds) have been identified, located in water depths between 525 and 1650 m. The deepest of the mounds occur in a canyon that intersects with the SW Porcupine Bank which is not resolved by the bathymetric contours shown in Fig. 1a while the shallowest mounds are those located on the west Rockall Bank. A look at the vertical distribution of the identified mounds (Fig. 2a) indicates that 95% of all carbonate mounds are located within the depth range of 500–1000 m. The vertical distribution is skewed, with a rapid increase in mound numbers from 500 to 700 m water depth and a longer tail in distribution at depths greater than 700 m. The peak in mound abundance, based on 10 m depth intervals, occurs

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Fig. 2. Depth distribution of (a) all carbonate mounds in the NE Atlantic binned into 10 m vertical depth bins, and (b) the same data averaged into 100 m thick depth intervals starting at 200 m.

typical for the region (e.g. van Aken and Becker, 1996). Below this layer there is a relatively large vertical temperature gradient, or the permanent thermocline. As temperature determines the density to a larger degree than salinity changes, the vertical profiles of density matches closely that of temperature (Fig. 3b). The base of the winter mixed layer represents the depth to which the atmosphere can act directly on the surface ocean in winter. This layer is capped during the summer heating period by the shallower (∼75 m) seasonal thermocline, leaving the interval between 100 and 600 m water depth with low vertical density stratification. The seasonal thermocline partially isolates this weakly stratified region layer from atmospheric heat input. This is apparent in the buoyancy frequency profiles, which show a maximum value of N at the seasonal thermocline in October, and the lowest values of N in the upper 600 m in February. The permanent thermo/pycnocline separates this low stratification layer between 100 and 600 m with the deeper ocean regimes, also of relatively low vertical density stratification. The permanent thermocline is characterised by the increased buoyancy frequency which occurs over a broad vertical depth range and showing a maximum between 700 and 900 m both February and October profiles (Fig. 3c). 3.3. Currents

between 640 and 650 m and 36% of the identified mounds are located between 600 and 700 m water depth (Fig. 2b). 3.2. Vertical water column structure Vertical profiles of temperature and density from a location in the southern Rockall Trough for February and October 2004 can be used to characterise the basic features of the water column structure in both winter and summer (Fig. 3). The profiles shown are representative of the water mass conditions that occur in late winter and autumn in the region. A winter mixed layer depth (MLD) of ∼ 700 m is apparent as is

3.3.1. Residual flows The historical records show that the magnitude of residual currents (i.e. the long-term drift speeds) include values up to 5 cm s− 1 throughout the entire depth range (Huthnance et al., 2001). Between water depths of 500–1000 m, however, a significant number of records show elevated residual current magnitudes in excess of 8 cm s− 1 and up to 20 cm s− 1 (Fig. 4a). In particular the largest residual near seabed flows occur between 700 and 800 m, within the depth range of the permanent thermocline (Fig. 3). This is more clearly indicated in Fig. 4b, where the individual records are binned into 100 m thick depth

Fig. 3. Typical vertical Profiles of (a) temperature (°C), (b) density (σt, kg m− 3), and (c) buoyancy frequency, (N, s− 1), for the Southern Rockall Trough during (P) February and (– – –), October, 2004.

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Fig. 4. Vertical variation of residual current speed in the NE Atlantic from historical current meter observations, showing (a) individual observations between 10 and 50 mab, and (b) same data averaged into 100 m thick depth intervals (thick line). The data are taken from within the depth range 50 mab, up to a depth corresponding to 20% of water depth. In (a) the open slope data sets are from west Porcupine Seabight at 51.729 N, 12.911 W (1200 m), and west Porcupine Bank at 50.555 N, 14.697 W (2947 m) and 51.712 N, 15.185 W (1700 m).

Fig. 5. Time exceedance plots for (a) selected current meter records characterising the Rockall/Porcupine margin: (―) Logachev and Pelagia, (–––) Hovland mounds, and 3 open slope locations at water depths of (P) 1200 m, (–.–.–) 1700 m, and (…..) 2947 m; and (b), 3 current meter records within the region of the Belgica mounds characterising (P) mound, (–––) province margin and (…..) open slope locations. In (b) data is courtesy of Murray Roberts and Robin Pingree.

intervals centred at 200 m and every 100 m thereafter. The maximum residual near seabed flow speed occurs at 700 m water depth and occurs as a definite peak in distribution. A similar plot for mid and upper level residual flows, i.e. for current meter measurement depths of N50 mab or within a depth range of 0.2–0.8× the water depth, is also shown for comparison. (Here we assume that the upper 20% of the water column may be influenced by atmospheric forcing such as wind stress so it is not included here). These residual currents also show elevated values between 500 and 1500 m, relative to deeper waters, but the enhancement is much smaller, broader in vertical extent, and with no definite peak observed.

and LeCann, 1990). The most dynamic currents were found at the Galway mound with significant magnitude currents measured at the fringe of the mound province. Both these sites are where baroclinic diurnal tidal motions are the dominant signal (Pingree and LeCann, 1990; White et al., 2007), and where one may expect a localised enhancement close to the mound summit (e.g. Genin et al., 1986). The weakest currents are found outside the province where no enhance tidal signal has been measured (Pingree and LeCann, 1990). Corresponding current speeds to an exceedance of 20% were 12 cm s− 1 for the non province record, 25 cm s− 1 within the province but off mound and 40 cm s− 1 for the mound site. The Galway mound observations were not made at the same time as the other 2 locations, which were coincident. Some temporal variability may be present in the records, therefore, which may cause some error for a direct comparison. The large differences within the 3 curves, however, indicate high variability in current dynamics at scales 10–100 km, due to local processes.

3.3.2. Current speeds In addition to residual vector speed magnitude, the magnitude of mean/max/min scalar flow speeds will be of importance to advection and turbulent processes within the benthic boundary layer. It is those quantities that may determine short periodic organic matter fluxes, sediment re-suspension and settlement processes. The typical variation of current speeds across the continental margin is illustrated by a plot of scalar current speed exceedance for some example sites at the NE Atlantic margin (Fig. 5a). This shows the percentage time a certain current speed is exceeded during the measurement period. Generally the highest currents were measured at water depths associated with both the thermocline and also the location of the carbonate mounds. Typically a current of 15 cm s− 1 was exceeded in excess of 50% of the measurement period at the thermocline depth. For greater depths, between 1200 and 3000 m, for 15% of the time currents exceeded 15 cm s− 1. The value of 15 cm s− 1 has been suggested as a typical threshold speed for the re-suspension of fresh detritus material along the NE Atlantic margin (Thomsen and Gust, 2000). Variation in the continental margin topography (seabed slope, presence of canyons etc.) will cause a variation in both residual and scalar current speeds. Data from the continental margin of extent ∼100 km and encompassing the Belgica mounds (Fig. 3a), and located within a narrow depth range all in the depth range of 860–1000 m, provides an example of this (Fig. 5b). All the currents were measured between 10 and 13 m above the seabed at contrasting locations: near the top of Galway carbonate mound (Dorschel et al., 2007), at the edge of and to the south of, the Belgica carbonate mound province (Pingree

3.3.3. Wave motions Vertical profiles of the parameter Nsin(α) can be used to assess if, and at what depth, trapped baroclinic waves may be a significant presence at the margin (Eq. (3)). The maximum in the expression Nsin (α) at the Pelagia, Belgica and Logachev mound provinces is found close to a water depth of 850 m (Fig. 6a). This coincides with both the maximum in N (c.f. Fig. 3) and also the depth where the continental margin is generally steep. The only exception to the character of a peak in Nsin(α) values at a mid depth region of the continental margin occurs at the Hovland mound province in the northern Porcupine Seabight. This is principally due to the smaller bottom slopes here and the generally shallower depths here, above the main permanent thermocline, where N is small. The corresponding vertical profile of the minimum resonant wave period (Eq. (4)) displays also a minimum at 850 m water depth. More importantly this minimum period is less than the diurnal period in a depth range of 600–1200 m (Fig. 6b). This is important as the diurnal period is the principal forcing period at sub-inertial frequencies, so allows for the possibility of a diurnal period resonance. Only at the Hovland mound province is the period of the resonant wave period longer than the diurnal period; indeed it is N40 h at all depths, far from any natural forcing period. Profiles of the ratio of seabed slope/characteristic slope for regional M2 period internal waves, i.e. α/β (Eq. (1)), may quantify the possibility

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4. Discussion

Fig. 6. Vertical profile of (a) the parameter N sin(α) and (b) the associated minimum wave period for typical seabed slope (a) and vertical density stratification (N) at (―) Belgica, (P) Pelagia, (–––) Logachev, and (….) Hovland mound provinces.

of internal wave activity at any particular mound province (Fig. 7). The α/β profile indicates critical conditions at 1200 m and 600 m, where evidence of large internal wave oscillations has been found (e.g. Dickson and McCave, 1986; Mienis et al., 2007). Between 600 and 1200 m, supercritical conditions exist where β b α and some enhancement of energy will occur due to the combination of large N or α. A comparison of the α/β profile with the corresponding profile of Nsin(α) for the Pelagia mound province in Fig. 6a shows large similarities due to the dominance of N in the formulation. Other mound regions will therefore likely show similar comparisons and supercritical conditions exist at the Belgica and Logachev mound province within the depth range of the permanent thermocline. Again similar to the arguments for the trapped diurnal motions, the small slope angles of the seafloor associated with the Hovland mound province means one may not expect significant enhancement of internal wave energy there.

Fig. 7. Vertical profile of α/β for the Pelagia mounds where (α) is the seabed slope and β the characteristic angle of wave energy propagation for internal waves (Eq. (1)).

The vast majority of the carbonate mounds located at the continental margin of the NE Atlantic are located in a relatively well ventilated near-thermocline region between 600 and 900 m water depth (e.g. Frank et al., 2009), which also is well within the temperature tolerance range for cold water corals (4–12 °C, Freiwald, 2002). An analysis of historical current meter data suggests that the relationship between the depth interval of mound occurrences and thermocline depth is associated with enhancement of energy associated with stronger tidal and residual currents (Figs. 4 and 5). A number of studies have indicated the likely role of the dynamic physical setting at cold water ecosystems, in terms of long-term erosional currents (Dorschel et al., 2007; van Rooij et al., 2007), internal waves (Frederiksen et al., 1992) and baroclinic tidal waves (White, 2007; White et al., 2007). The initiation of carbonate mounds in the NE Atlantic has occurred in a number of phases since the Pliocene (e.g. van Weering et al., 2003; van Rooij et al., 2007) with a common dynamic feature of these periods being the existence of erosional currents at the margin generating regional unconformities (Kenyon et al., 2003; van Weering et al., 2003). Initiation and growth of cold water reefs and mound structures require an initial hard substrate for larval settlement and subsequently high organic matter fluxes to supply food, and in the case of carbonate mound growth, a significant sediment supply to baffle within the coral framework (Dorschel et al., 2009; Mienis et al., 2009). Residual flow along the margin is maximum at mid depths where the thermocline is present i.e. between 500–1000 m (Fig. 4). The continental margin is the region of large depth change (200– 2000 m) and this allows for stability of any current flowing along the margin as currents tend to follow contours of f/water depth. Due to conservation of potential vorticity in a flow along the margin, any deviation of the flow from the margin causes a change in the vorticity which tends to “push” the current back to the mid slope depths. This is a process known as “topographic steering”. In addition, tidal rectification processes will also enhance residual along-slope flow at the margin (e.g. Huthnance, 1981). The fact that there is cross slope variation in the residual current strength has important implications for the flow that enters/leaves BBL close to the seabed. Similar to the surface wind influenced frictional layer, the bottom frictional layer is driven at sub-tidal timescales by the overlying, essentially geostrophic, flow (of magnitude, u). As the frictional layer exerts a stress on this flow, currents (in the northern hemisphere) are rotated anticlockwise as the bottom is approached. Therefore, for a flow with shallow water to the right, a downslope transport in the BBL is generated, one that is proportional to the stress (τ) generated by the overlying flow (u), with τ proportional to u2. A cross slope variation in overlying boundary flow at the margin will be associated with a change in downslope BBL transport with associated convergences/divergences requiring a compensating in/output flow through the top of the BBL. This has been explored by Thiem et al. (2006) in a modelling study of the Norwegian margin and it was found that increased flow into the top of the BBL occurred where the greatest abundance of cold water coral reefs were found. The conclusions were that this flow into the BBL may increase the flux of organic matter to the seabed. For the case of the NE Atlantic margin, maximum residual flow at thermocline depths results in a divergence in downslope BBL transport immediately above this depth and an associated influx of water to the BBL. Below the thermocline region, a convergence in BBL flow would be expected and ejection from the BBL. Therefore there appears to be a quasipermanent “conveyor belt” system of flow, with any associated organic matter contained within it, which enters the BBL at the shallowest end of the mound distribution depth range. This may also be a contributory factor to facilitate mound growth and coral ecosystem development through increased particle fluxes/encounter

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rates as envisaged by Thiem et al. (2006). Strong residual alongmargin flow may also be important for the propagation of the coral larvae between mound provinces. Indeed, de Mol et al. (2005), have suggested that larval propagation via the commencement of Mediterranean Outflow Water may have initiated the coral colonisation of the NE Atlantic margin. Furthermore, the persistence in an alongmargin flow that drives a downward flux of organic material from about 500 m depth and below, may be factor in determining the skewed distribution of mounds observed in Fig. 2. Given the likely source of organic matter is the shelf edge, the predominant cross margin flux driven by the residual current is downslope so that one may expect the mound density to tail off downslope also. Enhanced baroclinic tidal motions are apparent at the continental margin associated with the thermocline region. These periodic motions provide conditions for local sediment re-suspension, cross slope sediment transport and the generation of intermediate nepheloid layers, which may also contribute to cross slope sediment transport fluxes (e.g. Dickson and McCave, 1986; Thorpe and White, 1988; White et al., 2007). These enhanced currents will be concentrated over a depth range where the value of Nsin(α) is large. Mienis et al. (2009) has shown large diurnal variability of 2–3 °C at the Logachev province, corresponding to vertical excursions of 250–400 m centred at about 800–900 m water depth. This is likely to correspond, therefore, to the a vertical range where enhanced cross slope sediment fluxes will exist and perhaps leads to a constraint for the depth range for the concentration of mounds. In addition, the vertical water column extent of the trapped, enhanced motions may play a role in the vertical extent of mound growth. However, there is presently insufficient current data to support such speculation. It is clear, however, that the action of baroclinic waves act to enhance sediment transport and may also be significant in shaping carbonate mound growth (White et al., 2007; Mienis et al., 2009), generating wave features in channels between mounds (Wheeler et al., 2005), as well as the continental margin slope themselves over long periods (Cacchione et al., 2002). Of course, not all locations within 600– 1200 m water depth will have dynamical conditions conducive to carbonate mound development. Patchiness in the dynamical conditions will result in variability in sediment fluxes and perhaps also constrain mound province location. We note that the mound provinces are not present as a continuous feature along the margin, but in distinct provinces with length scale of order 100 km. Similar variability in the dynamic conditions along the permanent thermocline was suggested by Fig. 5b, although has to be treated with a little caution as further data would be required to provide conclusive results. Frederiksen et al. (1992) have suggested that internal waves may not only produce local sediment re-suspension, but also help to contribute to organic sediment fluxes through promoting increased primary productivity at the shelf edge via vertical mixing and enhanced nutrient fluxes (e.g. Sharples et al., 2007). Frederiksen et al. (1992) argue for a direct link in the depths of internal wave criticality (α N β) and the depth of peak coral abundance. It is more likely, however, that the distributions of both are somewhat spread vertically as there is likely a broad vertical band of enhanced energy present near the seabed at mid slope depths. Furthermore, Sharples et al. (2009) suggest that whilst there is some modest increase of surface productivity at the shelf edge, the phytoplankton community structure varies across the shelf edge region due to changes in water column structure. It may be that organic matter quality may also play a significant role in the pelagic–benthic coupling at the shelf edge that requires further investigation. In the present geological and oceanographic setting, the presence of the thermocline and associated strong currents are highly conducive for mound development. It would also seem likely that the long-term stability in vertical location of the thermocline has helped the carbonate mounds to grow. These periods of stability are

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punctuated by less conducive conditions when the vertical stratification and other processes (surface productivity) are different, such as periods of ice sheet growth at the NW European margin. For example, during the Last Glacial Maximum, the NE Atlantic was more stratified with lower sea surface salinities through increased freshwater influxes and with reduced deep water formation (e.g. de Vernal et al., 2000; Peck et al., 2006). It is also likely that the E Atlantic region would be a region of reduced surface productivity under glacial conditions, which together with reduced deep water formation, results in a poorly aerated intermediate water mass with lower organic fluxes (Rasmussen et al., 2002). It is interesting to note that in the present time, the Hovland Province is partially buried and with far less live coral growth and is located above the main thermocline. A deepening thermocline since the last major European ice sheet, together with the increased sedimentation at that time, has now likely resulted in the Hovalnd Province being displaced to a location of less conducive mound growth condition above the main thermocline “hotspot” depth. 5. Conclusions Analysis of the vertical distribution of carbonate mounds in the NE Atlantic and the hydrography associated with the continental margin show a distinct relationship between the peak mound abundance and the depth of the permanent thermocline. This relationship is likely due to the increased energy in the near seabed dynamics associated with the permanent thermocline. Both enhanced residual and periodic motions promote large organic matter fluxes required by the coral and carbonate mound growth. Stability in the thermocline depth is conducive to the long-term growth of the carbonate mounds at the continental margin. Periodic motions also produce intermediate length scale (10–100 km) “hotspots” of enhanced sediment dynamics associated with the location of the individual mound provinces.

Acknowledgements This study has been undertaken within the context of the EU 6th Framework project HERMES project, EC contract no GOCE-CT-2005511234, funded by the EC's Sixth Framework Programme under the priority “Sustainable Development, Global Change and Ecosystems”, whose financial support is gratefully acknowledged. We are grateful to the INSS for the supply of the bathymetry data used in this study, to Murray Roberts and Robin Pingree for the use of the current meter data displayed in Fig. 5b, and to 2 anonymous reviewers for comments that have enhanced the paper.

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