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 Department of Earth and Ocean Sciences, National University of Ireland, Galway, Ireland b Department of Geology, Environmental Research Institute, University College Cork, Ireland abstract article info 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 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; 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; Earth and Planetary Science Letters 296 (2010) 395–402 ⁎ Corresponding author. Tel.: + 353 353 91 493214; fax: + 353 353 91 494533. E-mail addresses: martin.white@nuigalway.ie (M. White), b.dorschel@ucc.ie (B. Dorschel). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.05.025 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl