The Santa Barbara Basin
During Leg 146, the first continuously-cored Quaternary sequence was obtained from the southern
California borderland province (Site 893). The sequence represents deposition at high
sedimentation rates in suboxic to oxic conditions and contains sufficient diatoms, radiolarians,
foraminifers, and pollen to allow high-resolution paleoclimatic/paleoceanographic investigations for
the late Quaternary. Stable isotopic, geochemical, and micropaleontological studies will provide
information critical to the analysis of global climate change and the role of the ocean in the global
Changing oxygen conditions in the basin, inferred from the degree of bioturbation and the strength
and persistence of laminations, correspond with distinct changes in the benthic foraminifer
assemblages; laminated sequences are associated with low-diversity benthic foraminifer
assemblages and reflect very low oxygen levels at the ocean floor, well-homogenized sediments are
associated with more diverse benthic foraminifer assemblages, and intermittently laminated
sequences are associated with assemblages intermediate in character. The sedimentary cycles and
associated changes in the benthic foraminifer assemblages appear to correspond closely with
glacial/interglacial oscillations during the last 150 k.y. Each sedimentary cycle represents a
progressive dominance of fully oxygenated basin waters, culminating in a period of sustained basin
oxygenation (glacial maxima Stages 6 and 2). The fully oxygenated intervals are then terminated by
an abrupt return to a sustained period of low-oxygen conditions that promote lamina preservation
during interglacial Stages 5 and 1. Thus, global paleoclimatic cycles are reflected by lithofacies and
biofacies changes that are largely modulated through changes in oxygen concentrations. The
changes in basinal oxygen concentrations resulted from changing interplay between biogenic
productivity, from the depth and strength of the Oxygen Minimum Zone in relation to the basin sill
depth, and from changing oxygen concentrations in Pacific Intermediate Waters at the southern
California continental margin. Preliminary evidence suggest that, during the Quaternary, the latter
was largely responsible for controlling the cycles of oxygenation/dysaerobia in the basin.
Glacial/interglacial fluctuations in the planktonic foraminifer assemblages range from cool temperate
during the interglacial episodes to lower diversity subarctic assemblages during the glacial episodes.
Surface-water temperatures in the Santa Barbara Basin during glacial maxima were as low as ~8 deg C.
Cooler conditions during the glacial episodes are reflected by a dominance of coniferous pollen and
warmer, interglacial episodes by an abundance of oak pollen.
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