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

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.