Abstract

The drilling vesselSubmarex of Global Marine was used to drill and core sediments to a depth of 56.4 m on the Nicaragua Rise, between Walton Bank and Jamaica, in 610 m of water. Seismic reflection profiles revealed thick accumulations of layered sediments with some fossil reefs. The sediments consist of undisturbed layers rich in planktonic microfossils alternating with turbidite layers. Absence of older coccoliths indicates that the redeposited material was not appreciably older than the time of redeposition, and oxygen isotopic analysis of benthonic elements shows that this material was derived from a depth not much shallower. The lower portion of the cored section correlates with the Manchioneal Formation of Jamaica. Taxonornic analysis of the calcareous nannoplankton indicates that the level at 2354 cm correlates with the midportions of the eastern equatorial Pacific cores 58 and 62; with the “Nebraskan-Aftonian” boundary of the Gulf Coast; and with the appearance of Hyalinea baltica at Le Castella, southern Italy. This level, therefore, represents the Plio-Pleistocene boundary as officially designated, and an age of about 700,000 years is estimated for the bou ndary. Oxygen isotopic analysis shows important oscillations, with a full glacial-interglacial amplitude, occurring both above and below the Plio-Pleistocene boundary.

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