| Summary: | A total of 57 core top samples from the Panama Basin were used in a quantitative study of complete radiolarian thanatocoenoses to determine whether surface oceanographic conditions are reflected in the microplankton faunas deposited onto and buried in the sea floor. Information obtained from this study was used to resolve the sequence of temporal fluctuations in oceanographic and climatologic conditions in the eastern equatorial Pacific. Four piston cores from within the basin were sampled on the basis of carbonate stratigraphies to levels no older than 130,000 years B.P. Although the sample intervals are irregular and samples are not closely spaced, this procedure allowed reasonably good stratigraphic resolution. Because of high diversity at low latitudes, the 131 radiolarian taxa recognized in this study accounted for only 37-61% of the individuals encountered in counts of >1000 specimens. Q-mode factor analysis of the surface sediment samples yielded three end member samples, which allowed recognition of three different assemblages. The dominant assemblage can be identified with a tropical fauna and the least important assemblage is considered to be associated with the Peru Current fauna, The distribution of the tropical assemblage does not reflect surface oceanographic conditions whereas the distribution of the Peru Current assemblage shows a weak association with surface conditions, The other assemblage shows no coherent distribution pattern. The nature of this assemblage is best explained by considering it to be a residual of the tropical fauna resulting from chemical and mechanical modification in the water column and at the sea floor. All evidence suggests that solution, winnowing and lateral transport are major processes in determining the distribution of the faunal assemblages in the sediments of the Panama Basin. The signal of the Peru Current assemblage in the surface study was used for a quantitative analysis of the subsurface samples. This analysis yielded the following results: 1) During most of the last glacial, assemblages associated with the Peru Current.were deposited in the Panama Basin, indicating a northward shift of current boundaries, This has important climatological implications since such a shift must be associated with a shift of the major wind belts and of the South Pacific high pressure cell towards the equator. 2) At least once during, the last glacial, from approximately 50,000 to 40,000 years B.P., warmer conditions are indicated by a re-appearance of tropical assemblages. 3) The previous interglacial appears to have been slightly cooler than present interglacial conditions. 4) The warming trend towards present conditions began between 15,000 and 20,000 years B.P. This is earlier than observed in the North Atlantic. These results are correlative with records of oceanographic and climatic changes in the southeast Pacific, northwestern South America and the Galapagos Islands.
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