HOLOCENE UPPER PLEISTOCENE RADIOLARIAN BIOGEOGRAPHY AND PALEOECOLOGY OF THE EQUATORIAL PACIFIC

Polycystine radiolarians were investigated in 66 samples from 5 box-cores spanning the last ca. 40,000 years from the western (approx. 160-degrees-E) and the central (136-degrees-W) equatorial Pacific. The assemblages investigated show clear differences associated with the geographic locations of the sites: the central Pacific is characterized by higher specific diversities, much higher absolute abundances of shells per gram of dry bulk sediment, and by conspicuously better radiolarian preservation. The differences involved also include significant changes in the relative proportions of several radiolarian species (these accounting for approx. 25% of total individuals), families and orders. These dissimilarities are chiefly attributed to differences in the primary production of the two zones and to enhanced advection of colder-water species characteristic of the California Current to the central equatorial Pacific. In both areas, downcore faunal changes, if present at all, are so weak that background noise from random sample-to-sample variations almost completely masks them. There do not seem to be any noticeable shifts associated with the 18 k.y. B.P. level. Inconclusive evidences of environmental changes at 30-20 k.y. B.P. are suggested in some of the cores by the increase in the proportions of several species characteristic of colder northeastern Pacific waters and by lower radiolarian accumulation rates; these changes seem to be paralleled by shifts in the isotopic composition of planktonic Foraminifera.