- Micropaleontological data have been analyzed through mathematical
and statistical procedures, in order to: (1) establish the distribution
pattern of radiolarian assemblages in the surface sediments,
(2) establish the relationship of these assemblages with oceanographic
variables, (3) determine the faunal composition down-core with
respect to the faunal surface pattern, and (4) estimate water temperatures
and 1OO M PO₄ within the geological past. This, together with
the analysis of opal and quartz content of the sediments, has permitted
the inference of the paleo-oceanography of the subtropical southeastern
Pacific during the last 75,000 years. Factor analysis shows
that five major radiolarian assemblages (factors), accounting for
93.6% of the variance, reflect the mixed layer circulation and its
associated water masses. According to their position in the area,
these factors are called: (1) the Subtropical factor, (2) the Equatorial
factor, (3) the Peru (current) factor, (4) the Chile (current) factor,
and (5) the Backwater factor. The subtropical factor is correlated
with "warm" water temperatures, the Equatorial factor with opal
production, the Peru factor with coastal upwelling, the Chile factor
with "cool" water temperatures, and the Backwater factor with
mixing of water masses. The distribution pattern of quartz reflects
the position of the southeast trade winds which largely control the
oceanographic conditions of the subtropical southeastern Pacific. The
opal distribution resembles the patterns of surface primary productivity.
Opal content is "high" below the Equatorial Undercurrent. However,
it is "low" along the South American coast probably due to
dilution by terrigenous input.
Cores RC11-230 (8°48'S-110°48'W), V19-29 (3°35'S-83°56'W),
and Y71-6-12 (16°26'S-77°23'W) were used in the stratigraphic study.
Their stratigraphic control (correlation) is made through their δ ¹⁸O
records. The changes of quartz, opal and radiolarian assemblages
contents down-core are considered to be the result of shifts or
changes in intensity of the atmospheric and oceanic circulation.
Oceanographic changes have occurred in the subtropical southeastern
Pacific during the last 75,000 years, but they are not manifested at
each location in the same form. These changes have not been synchronous
with changes of global ice volume; rather, during pronounced
climatic changes, a sequence which is initiated by changes in the wind
circulation is observed. The oxygen isotope stage 4 is characterized
by mixing of water masses, "deep" thermocline and low concentration
of nutrients. The lower interval of isotope stage 3 is characterized
by weakened wind-driven circulation and "high" water temperatures.
The middle of isotope stage 3 is a time in which wind-driven conditions
become dominant in the region. Oxygen isotope stage 2 is
"cool" and has strong wind-driven conditions. The shift to the
present oceanographic conditions in the subtropical southeastern
Pacific occurred 11,000 years ago.