Abstract:
Three high-resolution (av. 1 sample per about 7 ky) stable oxygen-isotope records for planktonic (Globigetina bulloides and Orbulina universe) and benthonic (Uvigerina sp.) foraminiferal species are presented for the late Neogene interval (6.40-0 Ma) of foraminifer-bearing nannofossil oozes from DSDP Site 593, Challenger Plateau, southern Tasman Sea. The curves all show moderate- to high-amplitude isotope fluctuations and intervals of high-frequency oscillations, commonly with periodicities of about 40 ky. For descriptive purposes they are subdivided into several alphanumeric time intervals having distinctive isotope characteristics compared to adjacent sections. The ten benthonic time intervals (MioUvi1 to QuatUvi4) monitor significant variations in the delta 18O composition and temperature of Antarctic Intermediate Water, while the eight planktonic intervals (MioGlob1 to QuatGlob3) record Subantarctic/Subtropical Surface Water compositional and temperature changes. A particularly notable feature of the curves in the latest Miocene interval (6.4-5.1 Ma) is the occurrence of previously unresolved high-amplitude, high-frequency delta 18O fluctuations associated with ice-volume changes on Antarctica and having at least two-thirds the magnitude of those associated with the late Quaternary glacial-interglacial cycles. Climatic conditions ameliorated and were relatively warm and stable during the about 2 my interval from the early to early late Pliocene (5.1-3.0 Ma) as indicated by a shift to more depleted, lower-variability delta 18O values. At -3.0 Ma another climatic deterioration took place with the intensity of glacial-interglacial cycles in both the benthonic and planktonic isotope records once again increasing and culminating in the well documented series of delta 18O enrichment events between about 2.42 and 2.30 Ma which fall -40ky apart. These post-3 Ma delta 18O variations coincide with the growth of large continental ice sheets in the Northern Hemisphere, and of further ice accumulation on Antarctica, and mark the transition of the cryosphere and global-climate system from a unipolar to a bipolar one. With the Quaternary a distinctive bimodal variation in the delta 18O record occurs, involving relatively depleted, low-variability values prior to about 1.0 Ma followed by enriched, high-variability delta 18O values, which probably coincides with a major change in global ice budget due to Northern Hemisphere ice-sheet expansion.