THE MIDDLE PLEISTOCENE REEF TRANSITION: CHANGES IN CORALS AND REEF ARCHITECTURE AS RECORDED IN THE DOMINICAN REPUBLIC CORE TRANSECT

The Dominican Republic Drilling Project was initiated to provide temporal constraints on the emergence of modern Caribbean reefs after a peak in regional coral extinctions (~2-1 Ma). This emergence partly coincides with, and follows, a protracted climatic deterioration and cooling between 2.0 and 0.8 Ma. A transect of 7 core borings (~700 m total) across the south coast terraces, constrained in age by U/Th and U/Pb geochronometry, radiogenic Sr isotopes, and several diagnostic calcareous nannofossil biomarkers, indicate that most extinction had occurred prior to ~1 Ma (MIS 31). Following this extinction, fringing reef margins of the Caribbean display a characteristic zonation in which Acropora palmata dominates shallow high-energy reef crests and Acropora cervicornis calmer fore-reef slopes and backreef lagoons. In the two seaward cores, 7 sequences were recovered associated with sea level fluctuations (Marine Isotope Stage 5e at ~128 ka to MIS 17 at ~700 ka). A U-Pb age of 0.646±0.013 Ma (2σ) for the shallow reef deposits at -110 m record deposition during sea level lowstand MIS 16. The highstand flooding at MIS 11 marks a sedimentological transition. Reef development in the shallow shelf resulted in more carbonate production, steeper slopes and significant carbonate shedding into the forereef. The sequences of MIS 11 to MIS 5 show an overall shallowing-upward trend, where transgressive cycles are less evident as the margin prograded seaward. The terraced nature of the DR reefs (and other middle and late Pleistocene reefs) signals the final stage in the progression from late Pliocene-to-early Pleistocene platform-forming (pre-extinction) reef units, to a modern predominantly fringing-type reef motif driven by high-amplitude sea level cyclicity. The dominance of acroporids across the modern reef zonation is attributed to growth rates 5-100 times faster than other corals.