DEVELOPMENT, DECLINE, AND TRANSITION IN A MID-HOLOCENE FRINGING CORAL REEF COMPLEX, DOMINICAN REPUBLIC

An extensive fringing coral reef system existed in mid-Holocene time (7.2 to 4.2 ka) along the margins of a marine embayment of the Caribbean Sea that occupied the fault-bounded Enriquillo Valley of the southwestern Dominican Republic. Subaerial exposures below present sea level contain pristinely preserved fossils and permit high-resolution examination of facies changes revealed within a coral reef system in transition from a high-diversity, "healthy" fringing reef to an essentially monospecific Acropora cervicornis reef. Final transition to a serpulid worm-tube dominiated facies marks the termination of coral growth. Primary causes for decline of the coral reef are attributed to changes in rate of sea-level rise and salinity of the paleo-seaway.

Large colonies of Montastraea annularis species complex and other domal coral species, commonly with platy growth forms, were dominant in the early reef system, and enormous thickets of Acropora cervicornis, now forming beds up to 15 m thick, dominated the later stages of reef development and flourished between 5.2-4.2 ka. During this time interval, the connection to the Caribbean Sea began to close, generating conditions of fluctuating salinity that initiated decline of the coral reef system. As sea level rise slowed and the embayment connection became restricted, serpulid worms began to form large aggregates. Mounded aggregates extend to elevations of 2+ m above present sea level. Individual mounds can be up to 2 m high and 3 m in diameter. The mounds commonly coalesce to form prominent ridges and have a framework of serpulid tubes coated with a layer of porous carbonate tufa up to 30 cm thick. Serpulid layers of 1-3 cm in thickness also occur within the A. cervicornis beds, first appearing ~18.5 m below sea level. The stratigraphy and stable isotope geochemistry of the Enriquillo corals and serpulid layers and mounds suggest mulitple transitions in salinity of the seaway. Changing composition of the microfossil assemblages preserved in the reef deposits supports this interpretation of variable salinity conditions during decline of the reef complex.