A PRELIMINARY MODEL OF HOLOCENE CORAL-REEF DEVELOPMENT IN THE ENRIQUILLO VALLEY, SW DOMINICAN REPUBLIC

Rapid evaporation of Bahia Enriquillo after closure ca. 4000 ybp has exposed well-developed Holocene reefs in which all modern coral zones are represented. At Cañada Honda, a 400-m long outcrop spans a depth range of over 40 m, providing a rare opportunity to view modern reef structure. Six coral facies occur in the outcrop (branching, mixed, 3 massive-coral zones, platy). They reflect early deepening and transgression as sea level rose rapidly, followed by progradation of the shallowest facies over their deeper, massive counterparts as sea-level rise slowed - probably around 6000 years ago. Molluscan zonation generally follows that of corals.

Inferred species abundance and diversity seem high when compared to cover data from reefs in similar, sediment-stressed environments today. Based on 53 quadrats from 21 vertical transects, 63% of the reef was occupied by in-situ colonies (n=6400). Corals and molluscs from 27 and 56 species, respectively, were found in the outcrop. However, the overall importance of live coral may be inflated by comparison with monitoring data from modern reefs. Over 50% of the corals immediately beneath a storm-debris layer in the massive-coral facies were already dead at the time they were buried. If this reflects conditions on the reef over time, then live-coral cover was less than half of what might be inferred from the raw coral data alone.

Within the dominant coral genera (Montastraea and Siderastrea) corals vary from hemispheres to conical to columnar forms as sedimentation presumably increased. Siderastrea spp. are dominantly conical, while Montastraea spp. tend to form columns. If this relationship can be confirmed, then it may be a useful sedimentation proxy.

This project builds on pioneering work started in the 1980’s and will hopefully provide a detailed depositional model that can be extrapolated to reef development all around the valley. Because the topographic context of the Bahia Enriquillo reefs can be readily discerned, it is easier to relate what is seen in outcrop to changing oceanographic conditions around the bay. The ability to compare outcrop measurements to monitoring and core data within a known depositional context will be increasingly important as we try to use the recent geologic past to better understand present reef decline in the face of increasing anthropogenic stress.