Leg 146B

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 carbon cycle.

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.