MIOCENE STABLE ISOTOPIC STRATIGRAPHY AND MAGNETOSTRATIGRAPHY OF BUFF BAY, JAMAICA

Previously reported biostratigraphic relationships from middle-upper M iocene sections exposed near Buff Bay, Jamaica (18-degrees-N, tropical bioprovince), differ from the subtropical North Atlantic (Sites 563 a nd 558). Time scales for this interval rely on correlations establishe d at these subtropical sites, and the differences with the tropical se ction have implications to global correlations. Planktonic foraminifer al Zones N13 and N15 are thick at Buff Bay but are virtually absent at Sites 563 and 558; nannofossil Zone NN9 is associated with Zone N15 a nd uppermost Zone N14 at Buff Bay but is associated with Zone N16 at t he other sites. Magnetostratigraphic data presented here further compl icate the interpretation: Zone NN9 is associated with a thick normal m agnetozone at Sites 563 and 558; at Buff Bay, it is associated with a thick reversed magnetozone. Although a secondary magnetization at Buff Bay makes it difficult to identify confidently Miocene normal magneto zones, the thick reversed magnetozone most likely represents the paleo magnetic field and correlates with Chron C5r. The magnetobiostratigrap hic relationships require either diachrony of taxa or two mutually exc lusive hiatuses in Jamaica and the North Atlantic. We address this pro blem by analyzing benthic foraminiferal deltaO-18 and deltaC-13 from t he Buff Bay section. These isotopic data allow us to evaluate three hy potheses that reconcile the magneto-, bio-, and isotopic stratigraphic data and conclude that the first and last occurrences of five taxa we re diachronous by approximately 0.3-0.5 m.y. between tropical and subt ropical locations. This requires revised age estimates for late middle to early late Miocene biostratigraphic datum levels. We suggest that the ranges of several taxa are useful for endemic tropical or subtropi cal zonations, but correlations between the low and midlatitudes were affected by an increase in latitudinal thermal gradients during the la te middle Miocene. However, we admit that further studies are needed b efore this issue is resolved.