Geomorphology and benthic cover of mesophotic coral ecosystems of the upper insular slope of southwest Puerto Rico

The upper insular slope of southwest Puerto Rico is defined as extending from the shelf break at ~20 m water depth down to a depth of ~160 m where there is a pronounced change in geomorphic character and the basal slope begins. The upper slope is divided into two geomorphic zones separated by a pronounced break in slope gradient at ~90 m water depth. Descending from the shelf break, these are Zone I (20–90 m) and Zone II (90–160 m). As orientation of the shelf margin changes, geomorphology of Zone I shows systematic variations consistent with changes in exposure to prevailing waves. Within Zone I, exposed southeast-facing slopes have a gentler gradient and lower relief than more sheltered southwest-facing slopes, which are steep and irregular. Mesophotic coral ecosystems (MCEs) are largely restricted to Zone I and concentrated on topographic highs removed from the influence of active downslope sediment transport. Accordingly, MCEs are more abundant, extensive and diverse on southwest-facing slopes where irregular topography funnels downslope sediment transport into steep narrow grooves. MCEs are more sporadic and widely spaced on southeast-facing slopes where topographic highs are more widely spaced and downslope sediment transport is spread over open, low-relief slopes inhibiting coral recruitment and growth. Relict features formed during preexisting sea levels lower than present include deep buttresses at ~45–65 m water depth and a prominent terrace at ~80 m. Based on correlations with existing reef accretion and sea-level records, it is proposed that the 80-m terrace formed during the last deglaciation ~14–15 ka and subsequently drowned during a period of rapid sea-level rise associated with meltwater pulse 1A at ~14 ka and deep buttresses at ~45–65 m formed between ~11.5 and 13.5 ka and then drowned during a period of rapid sea-level rise associated with meltwater pulse 1B at ~11.3 ka.