Distribution of planktonic foraminifers, d18O and d13C in Globigerinoides sacculifer as well as Cibicidoides kullenbergi at DSDP Holes 90-590A and 90-590B

High biogenic sedimentation rates in the late Neogene at DSDP Site 590 (1293 m) provide an exceptional opportunity to evaluate late Neogene (late Miocene to latest Pliocene) paleoceanography in waters transitional between temperate and warm-subtropical water masses. Oxygen and carbon isotope analyses and quantitative planktonic foraminiferal data have been used to interpret the late Neogene paleoceanographic evolution of this site. Faunal and isotopic data from Site 590 show a progression of paleoceanographic events between 6.7 and 4.3 Ma, during the latest Miocene and early Pliocene. First, a permanent depletion in both planktonic and benthic foraminiferal d13C, between 6.7 and 6.2 Ma, can be correlated to the globally recognized late Miocene carbon isotope shift. Second, a 0.5 per mil enrichment in benthic foraminiferal d18O between 5.6 and 4.7 Ma in the latest Miocene to early Pliocene corresponds to the latest Miocene oxygen isotopic enrichment at Site 284, located in temperate waters south of Site 590. This enrichment in d18O coincides with a time of cool surface waters, as is suggested by high frequencies of Neogloboquadrina pachyderma and low frequencies of the warmer-water planktonic foraminifers, as well as by an enrichment in planktonic foraminiferal d18O relative to the earlier Miocene. By 4.6 Ma, benthic foraminiferal d18O values become depleted and remain fairly stable until about 3.8 Ma. The early Pliocene (~4.3 to 3.2 Ma) is marked by a significant increase in biogenic sedimentation rates (37.7 to 83.3 m/m.y.). During this time, heaviest values in planktonic foraminiferal d18O are associated with a decrease in the gradient between surface and intermediate-water d13C and d18O, a 1.0 per mil depletion in the d13C of two species of planktonic foraminifers, and a mixture of warm and cool planktonic foraminiferal elements. These data suggest that localized upwelling at the Subtropical Divergence produced an increase in surface-water productivity during the early Pliocene. A two-step enrichment in benthic foraminiferal d18O occurs in the late Pliocene sequence at Site 590. A 0.3 per mil average enrichment at about 3.6 Ma is followed by a 0.5 per mil enrichment at 2.7 Ma. These two events can be correlated with the two-step isotopic enrichment associated with late Pliocene climatic instability and the initiation of Northern Hemisphere glaciation.