University of California at Santa Cruz, Santa Cruz, CA, United States

Prior work with marine sedimentary sections exposed on Jamaica had been interpreted as evidence for a curious pattern of diachrony between the tropics and subtropics in the ages of certain biostratigraphic zonal boundaries of middle to late Miocene age. We offer an alternative to the Jamaican low-latitude nannofossil calibration, using a combination of paleomagnetic, biostratigraphic, and lithostratigraphic data derived from tropical sections recovered by the Ocean Drilling Program (ODP) in the eastern Pacific and western Atlantic oceans. Our nannofossil calibration is based on magnetic polarity stratigraphy obtained from the Pacific Ocean and is consistent with the notion that Milankovitch periodicities governed cyclic changes in the lithology of sediments from the Ceara Rise, a sea-floor high in the Atlantic Ocean located offshore of the Amazon Delta. Our results thus appear to be representative of the tropics and so call into question the ages that had been assigned to nannofossil zonal boundaries based on results from Jamaican sections.