ALBERT

Description: The oxygen- and carbon-isotope compositions of planktic and benthic foraminifera and calcareous nannofossils from Middle Oligocene-Early Miocene Equatorial Atlantic sediments (DSDP Site 354) indicate two important paleoceanographic changes, in the Late Oligocene (foraminiferal Zone P.21) and in the Early Miocene (foraminiferal Zone N.5). The first change, reflected by a delta18O increase of 1.45‰ in Globigerina venezuelana, affected only intermediate pelagic and not surface, deep or bottom waters. The second change affected surface and intermediate waters, whereas deep and bottom waters showed only minor fluctuations. In the case of the former the isotope effect of the moderate ice accumulation on the Antarctic continent is amplified in the Equatorial Atlantic by changes in the circulation pattern. The latter paleoceanographic change, reflected by a significant increase in 18O in both planktic and benthic forms (about 1.0‰ and 0.5‰, respectively), may have been caused by ice volume increase and temperature decrease. Both oxygen- and carbon-isotope compositions indicate a marked depth-habitat stratification for planktic foraminifera and calcareous nannofossils. Three different dwelling groups are recognized: shallow Globigerinoides, Globoquadrina dehiscens, Globorotalia mayeri and nannofossils; intermediate Globigerina venezuelana; and deep Catapsydrax dissimilis. The comparison of foraminifera and calcareous nannofossils suggests that the isotopic compositions of nannofossils are generally controlled by the same parameters which control the isotopic composition of shallow-dwelling foraminifera, but the former are more enriched in 18O.