Abstract:
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.
