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 d uring 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 t han extinction. The coral and coral associate faunas of three Upper Ol igocene and three Middle Miocene fossil reefs in western Puerto Rico w ere compared. Coral genera at these sites suffered 50% regional extinc tion (extinction or restriction), equivalent to earlier reports of thi s Caribbean coral extinction/restriction event. Nearly all coral gener a tolerant of both turbidity and cool water survived; those tolerant o f cool water or turbidity alone survived in much lesser proportions. C orals occurring on both patch reefs and shelf edge reefs survived in m uch greater proportions than those occurring on only patch reefs or on ly shelf edge reefs. There are no shelf edge barrier reef complexes do cumented from the Early or Middle Miocene in the Caribbean; the upperm ost Miocene shelf edge reefs of Mona Island are the only known shelf e dge reefs 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 of f-reef gastropods, bivalves, and echinoids suffered only insignificant reductions in diversity. Only corals and large benthic foraminifera w ere strongly affected by the extinction. It is significant that zooxan thellate organisms were the primary victims of this extinction. Miocen e endolithic sponge borings are significantly larger than their Oligoc ene counterparts, suggesting more intense bioerosion on Miocene than O ligocene reefs. Bioerosion is generally correlated with nutrient level s, and the apparently more intense bioerosion of Miocene corals may in dicate enhanced nutrient availability on Miocene Caribbean reefs. Exte nsive Miocene phosphorites throughout the Caribbean indicate enhanced upwelling in the region during the time of the coral extinction/restri ction. Biogeographic evidence from corals, coral associates, and mollu scs corroborates this pattern, along with the evidence from bioerosion levels. Enhanced upwelling could account for the extinction/restricti on by generally increasing nutrient levels and cooling Caribbean costa l surface waters, thus restricting reef development to on-shelf patch reefs, where corals would be subject to more intense sedimentation. Mo dern reefs of the Eastern Pacific may provide a modern analogue to Mio cene Caribbean reefs. This regional extinction was important in dividi ng a previously comsmopolitan reef fauna into several modern provincia l faunas. This biogeographic separation was completed in the mid-Plioc ene with the rise of the isthmus of Panama. Coral associates, which un iversally survived the Oligocene-Miocene event, also have much more co smopolitan distributions than do corals.