Biostratigraphy and marine paleoenvironments of the Gulf of Mexico, the western Caribbean, and the eastern Equatorial Pacific

Typescript (photocopy).A comparative biostratigraphic and paleoenvironmental analysis was made of the nannofossils in late Neogene sediments from the Gulf of Mexico, the western Caribbean, and the eastern Equatorial Pacific. Species diversity indices and the coccolith assemblages indicate stable surface waters with little physical stress in the Gulf of Mexico from the late Miocene to the very early Pliocene. Thereafter instability and physical stress in surface waters increased dramatically. The diversity record in the western Caribbean is similar but stronger fluctuations of the indices indicate greater instability and physical stress there than in the Gulf of Mexico during the late Miocene, probably caused by interaction of water masses across the central American passage. The diversity indices of the eastern Pacific shows no obvious trend. Cluster analysis shows that vertical distribution of cluster groups in the Gulf of Mexico and the western Caribbean is related to age. The cluster groups indicate that the coccolith assemblages differentiated in the western Caribbean and in the eastern Pacific after the mid-Pliocene, as a result of termination of surface waters exchange between the Caribbean and the Equatorial Pacific. Changes of the entire fossil nannoplankton assemblages suggest 3.6 m.y. B.P. as the best date for emergence of the central American isthmus. Decrease in the dissolution signal extracted from factor analysis, and increase in carbonate content and coarse fraction of sediments during the late Miocene and early Pliocene indicate depression of the lysocline depth in the Gulf of Mexico and western Caribbean, probably as a consequence of increasing restriction on the flow of the more corrosive Pacific intermediate or deep water across the central American passage. The dissolution signal in the western Caribbean indicates more intense dissolution in the early part of the late Miocene than at the Gulf of Mexico site. The productivity signal indicates an increase of productivity in the western Caribbean during the late Miocene, coincident with an increase of carbonate content in the sediments. At the eastern Equatorial Pacific site intense fluctuations of carbonate content result from dissolution as well as dilution with biogenic silica