Late Quaternary Changes in the Oceanography of the Eastern Tropical Pacific

Factor analysis of radiolarian species from the tops of 34 gravity cores in the area extending from 5°N to 45°N, 95°W to 155°W, yields five factor assemblages which are related to the present surface cir­culation: 1) a Pacific Equatorial Water assemblage; 2) a California Current assemblage; 3) a North Equatorial Current assemblage; 4) a Tropical Surface Water assemblage; 5) an oxygen minimum layer assem­blage. A sixth assemblage appears to be the resu1t of dissolution. The variation of these factors through the late Pleistocene is based on the analysis of samples from two cores. Transfer functions which re1ate core top assemblages to temperature at the sea surface and at 130 m depth were used to determine paleotemperatures. Stratigraphie control was provided by OxYgen isotope ana1ysis of planktonic foramini­fera from one core. Synchronous levels in the other core were deter­mined from biostratigraphic correlation of the variations in the abundance of three species of radiolaria between cores. A reconstruc­tion of the paleoceanography based on the results of the above analyses indicates that during the last glacial maximum (18,000 yrs BP): 1) sea surface temperatures were slightly lower than today; 2) the areal extent of warm Pacific Equatorial water and Tropical Surface water was diminished; 3) the California Current, North Equatorial Current, and the Equatorial Current were stronger than at present; 4) there was an increase in the thermal gradient in the thermocline and/or an increase in the depth of the mixed layer; 5) air and sea surface circulation patterns were constricted and experienced minimal north-south oscillation