PALEOCEANOGRAPHY OF THE EASTERN EQUATORIAL PACIFIC DURING THE PLIOCENE: A HIGH-RESOLUTION RADIOLARIAN STUDY

Pliocene sediments from hydraylic piston cores of Deep Sea Drilling Project sites 572 and 573 in the eastern equatorial Pacific provide material for a high-resolution stratigraphic and paleoceanographic study during a period of time from 2.4 to 3.7 Ma. Radiolarian stratigraphy of these two sites reveals two major faunal events. The older event involves the gradual disappearance of five species and the appearance of three equatorial surface-dwelling radiolarian species and occurs at the Gauss/Gilbert paleomagnetic reversal boundary which is coincident with the time of the closing of the Isthmus of Panama around 3.5 Ma [Berggren and Hollister, 1974; Keigwin, 1978, 1982a, b]. The younger event involves a disappearance and appearance of two mainly Subarctic species and occurs at 2.9 Ma, prior to the onset of northern hemisphere glaciation as dated in North Atlantic sections. Analysis of quantitative radiolarian data for extant species divides the Pliocene fauna into two assemblages which are composed of radiolarian species that are abundant in the surface sediments of central and eastern equatorial regions. From the time series variations of these fauna, the variability of these factor has remained constant over the time interval at the eastern site but underwent a stepwise decrease at the western site. Paleo-temperature transfer function techniques provided quantitative comparisons between Pliocene and Pleistocene paleooceanographic variability. At the more western site, a shift in paleotemperature estimates occurred. Using the modern day observation that the slope of the thermocline is related to the magnitude of westward wind stress, this change corresponds to an increase in wind stress. This event has been dated at about 2.8 Ma and may represent an important oceanic and atmospheric precursor to the major onset of northern hemisphere glaciation at 2.4 Ma. From the modern relationship between seasonal differences in temperatures at each site the Pliocene gradient in seasonality has been compared with the Pleistocene (and modern) gradients. Relative to the Pleistocene, seasonal differences in temperature were the same for the more eastern site but lower at the western site. The Pliocene gradient in seasonality was thus located farther east toward the South American continent than during the Pleistocene. No influence of the final closure of the Isthmus of Panama is seen in the extant radiolarian data for sites 572 and 573 during the time interval from 2.4 Ma to 3.7 Ma.