Cold-water coral mounds in an erosive environmental setting: TOBI side-scan sonar data and ROV video footage from the northwest Porcupine Bank, NE Atlantic B. Dorschel a, ⁎, A.J. Wheeler a , V.A.I. Huvenne b , H. de Haas c a Department of Geology / Environmental Research Institute, University College Cork, Lee Road, Cork, Ireland b Geology and Geophysics, National Oceanography Centre Southampton, European Way, Southampton SO14 3ZH, United Kingdom c Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, The Netherlands abstract article info Article history: Received 31 October 2008 Received in revised form 16 April 2009 Accepted 1 June 2009 Communicated by D.J.W. Piper Keywords: cold-water coral mound erosion environmental window coral facies iceberg ploughmarks side-scan sonar remotely operated vehicle Porcupine Bank NE Atlantic Cold-water coral mounds are common features in certain regions along the Atlantic margin. They occur in mound provinces in various settings, characterised by specific environmental conditions that steer and influence coral mound initiation, growth and demise (e.g. bottom current intensity, sediment input and food supply). In order to add detail to the diversity of environmental conditions described in relation to these structures, this study investigates mounds in a generally erosive setting on the northwest Porcupine Bank (NE Atlantic)—in contrast to previous studies in less hydrodynamically active settings. TOBI (Towed Ocean Bottom Instrument) side-scan sonar data revealed abundant erosive features in the study area. They occur in the form of erosional scarps, erosional pits and locally scoured seabed. Furthermore, two large (several hundred metres high) and 101 small (tens of metres high) cold-water coral mounds were identified on the TOBI images. Most of the mounds are located on top of erosional scarps pointing to the cold- water corals' preference for areas with enhanced current intensities. Within the study area, a general trend of northward decreasing mound heights and increasing abundance of erosive features can be observed. High-resolution ROV (Remotely Operated Vehicle) video observations at four sites within the TOBI coverage illustrate that only the southernmost mound shows signs of active mound growth while the other mounds represent relict structures with limited coral cover opportunistically taking advantage of available hard substrate in the form of consolidated/lithified sediments. The distribution of erosion features, seabed features and seabed facies on the northwest Porcupine Bank indicates an environmental window in which the interplay of coral growth, sediment input and sediment preservation leads to the formation of mound sediments and therefore mound growth. Whereas too weak bottom currents do not support thriving coral thickets, too strong bottom currents hamper any deposition and, thus, mound growth. As this environmental window varies over time, mound sizes on the Porcupine Bank (and probably also in other mound settings) most likely reflect the duration in time of such an environmental window of optimal growth conditions around a given mound, rather than an indication of the overall age of the mound. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Cold-water corals, mainly the species Lophelia pertusa and Madrepora oculata for the north Atlantic, have the potential to develop cold-water coral reefs and mounds with successive reef development over geological time periods leading to the development of cold-water coral mounds (Roberts et al., 2006). Studies of mound sediments have shown that these mounds are true coral build-ups from initiation, composed of loose coral frameworks embedded in a matrix of hemipelagic sediments (De Mol et al., 2002; Williams et al., 2006; Dorschel et al., 2007b). Recent research indicates that cold-water coral mounds have developed as the result of complex interaction of hydrological, biological and geological processes (Freiwald, 2002; Dorschel et al., 2005, 2007a; Wheeler et al., 2008). L. pertusa preferably dwells in bottom waters with temperatures ranging from 4 to 12 °C and with typical marine salinities (Mortensen et al., 1995; Rogers, 1999). The corals require a hard substrate for settlement and a sufficient food supply. In the northeast Atlantic, hard substrates are widely available in the form of rock outcrops, glacial erratic boulders and gravel, many of which are not colonised suggesting that suitable substrates are not a significant limiting factor for coral growth in this area (Bett, 2001). More significantly, areas of enhanced bottom current velocities also appear to be generally favoured by the corals (Frederiksen et al., 1992; Mortensen et al., 1995; Freiwald, 2002; Dorschel et al., 2007a; Mienis et al., 2007) with further possible con- nections between coral distribution and surface productivity (White et al., 2005). Marine Geology 264 (2009) 218–229 ⁎ Corresponding author. Tel.: +353 21 4901948. E-mail address: b.dorschel@ucc.ie (B. Dorschel). 0025-3227/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2009.06.005 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo