A model for the development of a carbonate mound population in the Rockall Trough based on deep-towed sidescan sonar data

A large carbonate mound population is identified on deep-towed TOBI (towed ocean bottom instrument) sidescan data along the eastern margin of the Rockall Trough, west of Ireland. Individual mounds are circular to elliptical in plan view, varying from 50 to 850 m in width and up to about 200 m in height. Strong NE-flowing contour currents at 800 m water depth are inferred from large-scale sedimentary bedforms. Smaller mound arrays are spatially associated with slope-parallel escarpments produced by mass wastage, while around the largest mounds the sharp escarpments may have been smoothed by the vigorous contour currents. Current streamlining effects control the shape of the mounds, which become more elliptical as their size increases, thereby minimising the hydraulic drag force. The frequency distribution of mound size follows a general power law, which is determined by the growth rate of the framework-building coral species and the rate at which they colonised the substrate. Initially, bottom currents support mound growth until the mounds become so large that hydraulic drag forces retard their growth. A model for the evolution of the population predicts that increased hydraulic drag forces on the larger mounds cause a sharp decrease in their number, in agreement with the observations. The model also allows an age structure for the population to be determined and correlations between the growth of the mound population and palaeoclimatic variations in the NE Atlantic to be attempted.