South Pacific Planktonic Foraminiferal Biogeography and Late Cenozoic Paleo-Oceanography: ABSTRACT

Distinct changes in the character of South Pacific planktonic foraminiferal faunas occur at the 2 major water-mass boundaries. The southern limits of at least 10 species occur at or near the Subtropical Convergence which marks the southern limit of subtropical water. An additional 8 species have their southern limits approximately coincident with the Antarctic Convergence, leaving only 1 species characteristic of Antarctic waters. Micropaleontologic and sedimentologic studies of South Pacific deep-sea cores have enabled partial definition of the climatic and glacial history of Antarctica and the South Pacific. Pliocene and Pleistocene cores can be divided into zones on the basis of upward sequential appearance of planktonic Foraminiferida, upward sequential disappearance of Radiolaria, and stratigraphic succession of calcareous nannofossils. A chronology of South Pacific Pliocene and Pleistocene climatic oscillations has been established, based on paleomagnetic and thorium-230 dating. Alternations of cold and warm water planktonic Foraminiferida distinguish 6 intervals of climatic warming during the last 700,000 years (Brunhes epoch) and 10 between 2.4 m.y. and 700,000 years ago (Matuyama epoch). The relative m gnitudes of climatic warmings were considerably greater during the last 500,000 years than between 2.4 m.y. and 500,000 years ago. Cooling was at times as intense during the Matuyama epoch as during the Brunhes epoch. This determination conflicts with previous paleotemperature determinations based on radiolarians which suggest somewhat warmer conditions throughout the Matuyama. Several present subtropical radiolarian species, however, apparently lost their environmental tolerance for subpolar temperatures about 0.7 m.y. ago. Climatic curves for planktonic forams and radiolarians are essentially the same for the late Pleistocene, confirming the usefulness of both groups in paleoclimatic studies. Fluctuations of certain calcareous nannofossils species closely follow radiolarian and foramin feral paleotemperature oscillations for the last 400,000 years but diverge strongly in older core sections. Abundances of the silicoflagellate genera, Dictyocha and Distephanus, clearly mark warm- and cold-water intervals respectively in late Pleistocene cores. The first appearance of ice-rafted quartz in the sub-Antarctic, about 5 m.y. ago, coincides with increases in bottom-transported quartz, suggesting a relation between increased bottom-water activity and Antarctic glaciation. Distinct changes in the radiolarian assemblages also occur at this time. Studies of land-based marine sections in New Zealand and of deep-sea cores have shown that the first major late Cenozoic cooling occurred during the late Miocene and earliest Pliocene when south-central sub-Antarctic planktonic foraminiferal faunas (approximately 55°S equivalent lat.) spread over central New Zealand. A further major cooling during the middle Pliocene spans the Gauss-Matuyama boundary (t = 2.43 m.y. ago). This cooling was followed by more fluctuating climatic conditions in the late Pliocene and early Pleistocene (lower and middle Matuyama; t = 2.43 to 1.60 m.y. ago). Evidence of possibly synchronous late Miocene cooling has also been reported in California, Italy, Japan, and the equatorial Pacific. End_of_Article - Last_Page 631------------