Late Pleistocene Planktonic Foraminiferal Trends Off Oregon: ABSTRACT

Abstract

Oscillations in radiolarian-planktonic foraminiferal ratios with increasing depth in several deep-sea sediment cores from the southern Cascadia basin and the adjoining Blanco fracture zone off Oregon are thought to be the result of climatic fluctuations. Greater production of planktonic foraminifers compared with radiolarians seems to have occurred during glacial advances of the Pleistocene, whereas radiolarians predominated during glacial retreats. At least four intervals are evident in the deep-sea sediment record of the last 50,000-70,000 years. Based on radiocarbon dates, these intervals correspond approximately with late Wisconsin glacial advances and retreats as defined for the Puget lowlands of Washington. Five species generally make up 90 per cent or more of the planktonic foraminiferal assemblage. Globigerina pachyderma and Globigerina bulloides together compose more than half of the assemblage. In grain-size fractions of less than 177 microns, Globigerina quinqueloba and Globigerinita uvula predominate. A few specimens of Globoquadrina hexagona, Globigerina digitata, and Globigerinoides ruber have been observed. The species composition of planktonic Formaminifera does not appear to reflect the change from glacial to post-glacial climates. Neither do the coiling habits of Globigerina pachyderma. This species is dominantly left-handed throughout the sections examined. Right-handed forms reach a maximum of 13 per cent and average 2.7. End_of_Article - Last_Page 464------------