Influence of benthic sediment transport on cold-water coral bank morphology and growth: the example of the Darwin Mounds, north-east Atlantic

The Darwin Mounds are small (up to 70 m in diameter), discrete cold-water coral banks found at c. 950 m water depth in the northern Rockall Trough, north-east Atlantic. Formerly described in terms of their genesis, the Darwin Mounds are re-evaluated here in terms of mound growth processes based on 100 and 410 kHz side-scan sonar data. The side-scan sonar coverage is divided into a series of acoustic facies representing increasing current speed and sediment transport/erosion from south to north: pockmark facies, ‘mounds within depressions’ facies, Darwin Mound facies, stippled seabed facies and sand wave facies. Mound morphometric changes are quantified and show a south-to-north divergence from an inherited morphology, reflecting the outline of coral-colonized fluid escape structures, to developed, downstream elongated, elevated mound forms. It is postulated that increasing current speeds and bedload sand transport favour mound growth and development by a process of enhanced sand sedimentation within mounds due to current deceleration by frictional drag around coral colonies. Comparisons are made with similar growth processes attributed to comparably sized cold-water coral mounds in the Porcupine Seabight, offshore Ireland.