Genesis and morphology of cold-water coral ridges in a unidirectional current regime

Abstract

Cold-water coral ridges with waveform morphology are commonly observed in mound surfaces in the Northeast Atlantic Ocean and interpreted as sediment waves overgrown by corals. This work documents the first cold-water coral ridges that are waveform but lack sand dune cores. These coral ridges are situated at the base of the Miami Terrace in the Straits of Florida (depths of 630 to 870m). They are entirely biogenic in origin and are unrelated to antecedent topography. Five lines of evidence, based on 27km2 of high-resolution geophysical, oceanographic, and ground-truthing data, support this interpretation. First, the waveform coral ridges are aligned perpendicular to a southward flowing bottom current and juxtaposed to an active sand dune field devoid of corals. Both the coral ridges and sand dunes are asymmetric in profile with respect to the current. However, the ridges have their steep sides facing the current, whereas the dunes exhibit a steep leeside. Second, the coral ridge matrix consists of very fine sand to mud particles with no bedload sand transport on the ridge flanks or troughs. In contrast, active sand ripples superimpose the juxtaposed sand dunes indicating that coarse sand is transported via traction in this domain. Third, bedload sand transport occurs in linear streaks at the shallowest part of the surveyed area. These streaks orthogonally crosscut some coral ridges, but coral colonies are not aligned along these streaks. Instead, the bedload sand smothers the coral colonies and thereby segments the ridges. Fourth, the sand dune and coral ridge acoustic responses are different. Below the dunes, there is no sub-bottom reflection due to energy attenuation. In contrast, a continuous reflection below the coral ridges proves the absence of attenuating sand cores. Fifth, this continuous sub-bottom reflection is not correlated with the coral ridge topography, indicating that antecedent topography does not control the waveform morphology and spatial distribution of the ridges. The differences in depositional profiles, sediment dynamics and features, as well as acoustic facies between the coral ridges and the juxtaposed dunes show that the coral ridges are entirely bioconstructional and not a veneer of corals covering paleo-sand dunes. This field of biogenic ridges at the Miami Terrace and the distribution of sand streaks as well as active sand dunes provide new insights how the lateral variability of current velocities and sediment transport conditions control the morphology, distribution and genesis of cold-water coral features.