HOLOCENT REEF DEVELOPMENT IN THE CARIBBEAN: GEOLOGIC PERSPECTIVES REVISITED

Ian Macintyre and Walter Adey laid to rest earlier presumptions that emergent Caribbean reefs were “feeble novices” (W.M. Davis) forming thin veneers over older substrates (N. Newell). However, reef ridges along deeper shelf margins were still considered as antecedent Pleistocene features with only thin, Holocene caps. Their new paradigm presumed that 1) reefs near sea level reflect maximum accretion (and, therefore, thickness), and 2) shelf-edge reefs were left behind by rapidly rising sea level as turbid water from the flooding bank-top discouraged reef development. Today's emergent reefs generally formed at ca. 8.0-7.5 CalBP as reef development back-stepped from abandoned shelf-edge reefs. Initially dominated by massive species (especially Montastrea spp.), they gradually caught up to slowing sea level and were capped by branching A. palmata that dominated Caribbean reef crests until the mid-1980s. Studies over the past two decades have built on these seminal works while, at the same time, challenging some of their foundations. Deeper reefs that formed while sea level was rising rapidly are not limited to thin veneers. Shelf-edge reefs in 12-16m of water off Puerto Rico and St. Croix exceed 15m in thickness, on par with their shallow counterparts. The thickest Holocene coral sequence is not a shallow, protected Alacran reef, but a highly exposed shelf-margin feature 75 m below sea level off Barbados. Early shelf-edge reefs dominated by Acropora palmata formed ca. 12,000 – 10,000 years ago, shortly after shelf flooding. While re-suspension of remnant soils delayed reef initiation, the role of turbid bank-top waters in discouraging subsequent reef development, as sea level continued to rise rapidly, is less important than was previously considered. On St. Croix, shelf-edge reefs flourished through most of the “reef gap” attributed to “inimical waters” by earlier studies. Similarly, shelf-edge reefs off Puerto Rico and the Florida Keys remained active as the adjacent shelf flooded; higher wave action near the platform margin ameliorated the impact of turbid bank waters. Reef development ceased at the shelf-edge 5,800 years ago not due to turbidity, but rather a sudden collapse of A. palmata throughout the region - for unknown reasons. A similar gap in the record of A. palmata in shallower reefs occurred 3,000 years ago.