Spatial distribution and morphometry of the Namibian coral mounds controlled by the hydrodynamic regime and outer-shelf topography

Cold-water corals mounds develop over millennial timescales as a result of sustained coral growth and concurrent with sediment deposition within their coral frameworks. So far, coral mounds have been primarily investigated as deep-sea biodiversity hotspots and geo-biological paleo-archives, whereas their morphological appearance and spatial arrangement have received much less attention. Here, we analysed the spatial distribution and the morphometry of coral mounds that developed on the Namibian shelf during a single short period dating back to the Early. The spatial distribution of these “early-stage” mounds and their morphological characteristics revealed a hierarchy of three different patterns. These comprise an alongslope mound distribution at a regional scale (first-order pattern), a topography-steered downslope alignment of mounds at a local scale (second-order pattern), and a hydrodynamic-controlled downslope orientation of the individual mounds at a mound scale (third-order pattern). In addition, because the Namibian mounds rarely exceed 20 m in height, key steps in the development of early-stage coral mounds (e.g. elongation, merging, limited gain in height compared to lateral extension) have been identified. With increasing size, coral mounds are more elongated, parallel to the prevailing tidal system, which is interpreted to reflect the transition from an “inherited” to a “developed” mound morphology. Besides supporting this earlier hypothesis on mound development, we could show that this transition takes place when the Namibian coral mounds reach ~150 m in length and ~8 m in height. This study reveals that the spatial-morphological appearance of coral mounds, often treated as a descriptive information, can provide valid information to understand their formation.