Oligocene-Miocene extinction and geographic restriction of Caribbean corals; roles of turbidity, temperature, and nutrients

About half the Caribbean hermatypic corals died out in the Caribbean during the latest Oligocene through Early Miocene, about 24-16 Ma. The majority of those corals that died out in the Caribbean are extant in the Indo-Pacific, i.e., they suffered geographic restriction, rather than extinction. The coral and coral associate faunas of three Upper Oligocene and three Middle Miocene fossil reefs in western Puerto Rico were compared. Coral genera at these sites suffered 50% regional extinction (extinction or restriction), equivalent to earlier reports of this Caribbean coral extinction/restriction event. Nearly all coral genera tolerant of both turbidity and cool water survived; those tolerant of cool water or turbidity alone survived in much lesser proportions. Corals occurring on both patch reefs and shelf edge reefs survived in much greater proportions than those occurring on only patch reefs or only shelf edge reefs. There are no shelf edge barrier reef complexes documented from the Early or Middle Miocene in the Caribbean; the uppermost Miocene shelf edge reefs of Mona Island are the only known Miocene shelf edge reef deposits in the Caribbean. Coral associates, the endolithic sponges, bivalves, worms, and barnacles that live in coral skeletons, were almost completely unaffected by this event. Likewise, reef and off-reef gastropods, bivalves, and echinoids suffered only insignificant reductions in diversity. Only corals and large benthic foraminifera were strongly affected by the extinction. It is significant that zooxanthellate organisms were the primary victims of this extinction. Miocene endolithic sponge borings are significantly larger than their Oligocene counterparts, suggesting more intense bioerosion on Miocene than Oligocene reefs. Bioerosion is generally correlated with nutrient levels, and the apparently more intense bioerosion of Miocene corals may indicate enhanced nutrient availability on Miocene Caribbean reefs. Extensive Miocene phosphorites throughout the Caribbean indicate enhanced upwelling in the region during the time of the coral extinction/restriction. Biogeographic evidence from corals, coral associates, and molluscs corroborates this pattern, along with the evidence from bioerosion levels. Enhanced upwelling could account for the extinction/restriction by generally increasing nutrient levels and cooling Caribbean costal surface waters, thus restricting reef development to on-shelf patch reefs, where corals would be subject to more intense sedimentation. Modern reefs of the Eastern Pacific may provide a modern analogue to Miocene Caribbean reefs. This regional extinction was important in dividing a previously cosmopolitan reef fauna into several modern provincial faunas. This biogeographic separation was completed in the mid-Pliocene with the rise of the isthmus of Panama. Coral associates, which universally survived the Oligocene-Miocene event, also have much more cosmopolitan distributions than do corals.