Latest Quaternary Benthic Oxygen and Carbon Isotope Stratigraphy: Hole 893A, Santa Barbara Basin, California

Abstract

Latest Quaternary oxygen (δ'*O) and carbon (δlλC) isotopic records from benthic foraminifers are presented at relatively high chronologic resolution (-450 to 1000 yr) for Hole 893A, a continuous 200-m sediment sequence from Santa Barbara Basin, Southern California. The oxygen isotope stratigraphy records a continuous ~160-k.y. sequence from isotope Stage 6.4 to the present day, the first of its kind from this region of the Pacific Ocean. The oxygen isotopic record, representing the last two interglacial and glacial cycles, closely resembles the well-dated deep sea reference sequence, and thus provides a detailed chro nologic framework. Site 893 has much potential for ultra-high-resolution stable isotopic investigations because of the very high sedimentation rates (reaching -1.6 mm peryr), and greatly reduced bioturbation. Variability of late Quaternary oxygen isotopic change in Hole 893A is distinctly larger during the last glacial episode (-70 to 11 ka), as for the Greenland Ice Sheet, and is considered to reflect significant climatic instability. Unlike the oxygen isotopic record, carbon isotopic values of each of the benthic foraminifers exhibit relatively little consis tent change during the latest Quaternary, and the 6I3C record does not resemble the typical records of deep-sea sediments. Instead, the carbon isotopic values are inferred to have been dominated by benthic microenvironments. Offsets in carbon isoto pic values between five taxa are maintained throughout the sequence. Uvigerina peregrina curticosta has δ l 3C values ~2%e higher than Bolivina tumida; the other taxa have intermediate values. Our data suggest that Uvigerina peregrina curticosta lives closer to the sediment/water interface in microenvironments inferred to have higher oxygen and organic carbon concentra tions. Bolivina tumida lives deeper in the sediment under lower oxygen concentrations. Carbon isotopic values of Uvigerina are similar in both laminated and nonlaminated sediment intervals that represent dysaerobic/aerobic cycles in Santa Barbara Basin. This suggests that surface sediments of the basin continued to be dominated by local dysaerobic processes, even during glacial maxima when oxygen levels in bottom waters increased sufficiently to support bioturbating organisms.