Plankton and surface sediment samples from the Gulf of California
were analyzed to determine the present geographic distribution of silicoflagellate
species in this area. Variations in the composition of the
silicoflagellate assemblage are related to water mass distributions.
Seven species and three forms were identified in these samples. Octactis
pulchra is associated with high levels of primary productivity in the
surface waters and is found in greatest abundance in the central Gulf
of California. Dictyocha messanensis dominates the silicoflagellate
assemblage in stations outside the Gulf of California and increases in
relative abundance with decreasing amounts of 0. pulchra. D. calida and
D. spec. 1 forms are associated with equatorial waters and have the
highest relative abundance near the mouth of the Gulf. D. epiodon and
Distephanus speculum are associated with cold California Current Water
and Dictyocha epiodon is present in minor abundance in Gulf samples.
D. spec. 2 has a patchy distribution with low relative abundance in these
samples.
Downcore sediment samples were analyzed from six locations in the
southern Gulf of California. The results suggest that major variations
in the relative composition of the silicoflagellate assemblage have not
occurred over the past 1000 years.
Sediment material obtained from the basin slopes in the central Gulf
of California exhibit alternating light and dark laminations. A laminae
couplet equals one year of deposition. The composition of the silicoflagellate
assemblage was determined in one hundred twenty individual
laminae samples from the top section of a box core from the central Gulf.
There is no indication of a characteristic assemblage composition associated
with either light or dark laminations in these samples. A high
relative abundance of 0. pulchra is present in the well laminated sections
of the core. D. spec. 1, D. epiodon, D. calida, and D. spec. 2
are periodically in great abundance spanning more than one laminae sample.
Variations in the relative abundance of silicoflagellate species in
the central Gulf of California during the last 38,000 years were determined
from DSDP Leg 64 Site 480 samples. The results indicate that 0.
pulchra was in lower relative abundance and Distephanus speculum,
Dictyocha epiodon, and D. spec. 1 forms were in greater abundance during
the glacial period compared to the present. This suggests that there was
a decrease in primary production, an increase in Pacific water influence,
and a decrease in surface water temperatures in the central Gulf of
California during the last major glacial interval compared to the present
conditions.
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