Stable isotope stratigraphy of ODP Site 167-1014

Late Quaternary oxygen (d18O) and carbon (d13C) isotopic records for the benthic foraminifer Uvigerina and the planktonic foraminifer Globigerina bulloides are presented for the upper 20 meters composite depth sediment sequence of Ocean Drilling Program Site 1014, Tanner Basin, in the outer California Borderland province. The benthic oxygen isotopic record documents a continuous >160-k.y. sequence from marine isotope Stage (MIS) 6 to the present day. The record closely resembles other late Quaternary North Pacific benthic isotope records, as well as the well-dated deep-sea sequence (SPECMAP), and thus provides a detailed chronologic framework. Site 1014 provides a useful record of the California response to climate change as it enters the southern California Border-land. Sedimentation rates are relatively constant and high (~11.5 cm/k.y. ). The planktonic foraminiferal record is well pre-served except during marine isotope Substages 5b and 5d, when normally high G. bulloides abundance is strongly diminished as a result of dissolution. The planktonic oxygen isotopic shift of ~3 per mil between the last glacial maximum and the Holocene suggests a surface water temperature shift of <7°C, similar to estimates from Hole 893A (Leg 146) to the north. Unlike Santa Barbara Basin, G. bulloides d18O values during the last interglacial (MIS 5) at Site 1014 were significantly higher than during the Holocene. In particular, marine isotope Substage 5e (Eemian) was ~0.8 per mil higher. This is unlikely to reflect a cooler Eemian but is instead the result of preferential dissolution of thin-shelled (low d18O) specimens during this interval. In this mid-depth basin, a large benthic d18O shift during Termination I suggests dramatic temperature and salinity changes in response to switches in the source of North Pacific Intermediate Water. Although d13C values of the planktonic foraminifer G. bulloides are in disequilibria with seawater and hence interpretations are limited, the G. bulloides record exhibits several negative d13C excursions found at other sites in the region (Sites 1017 and 893). This indicates a response of G. bulloides d13C to regional surface water processes along the southern California margin. A general increase in benthic carbon isotopic values (-1.75 per mil to -0.75 per mil) in Tanner Basin during the last 200 k.y. is overprinted with smaller fluctuations correlated with climate change. The coolest intervals during the last glacial maximum (MISs 2 and 4) exhibit lower benthic d13C values, which correlate with global 13C shifts. The opposite relationship is exhibited during the last interglacial before 85 ka, when lower benthic d13C values are associated with warmer intervals (marine isotope Substages 5c and 5e) of the last interglacial. These time intervals were also marked by decreased intermediate water ventilation. Increased dissolution and organic accumulation during Substages 5b and 5d are anticorrelated with the benthic d13C record. These results suggest that a delicate balance in intermediate water d13C has existed between the relative influences of global 13C and regional ventilation changes at the 1165-m water depth of Site 1014.

Sedimentation rate, bulk density, accumulation rate of terrigenous material and grain sizes of ODP Leg 167 sites

The terrigenous mineral fraction of sediments recovered by drilling during Ocean Drilling Program Leg 167 at Sites 1018 and 1020 is used to evaluate changes in the source and transport of fine-grained terrigenous sediment and its relation to regional climates and the paleoceanographic evolution of the California Current system during the late Pleistocene. Preliminary time scales developed by correlation of oxygen isotope stratigraphies with the global SPECMAP record show average linear sedimentation rates in excess of 100 m/m.y., which provide an opportunity for high-resolution studies of terrigenous flux, grain size, and mineralogy. The mass flux of terrigenous minerals at Site 1018 varies from 5 to 30 g/(cm**2 x k.y.) and displays a general trend toward increased flux during glacials. The terrigenous record at Site 1020 shows a similar pattern of increased glacial input, but overall accumulation rates are significantly lower. Spectral analysis demonstrates that most of this variability is concentrated in frequency bands related to orbital cycles of eccentricity, tilt, and precession. Detailed grain-size analysis performed on the isolated terrigenous mineral fraction shows that sediments from Site 1018 are associated with higher energy transport and depositional regimes than those found at Site 1020. Grain-size data are remarkably uniform throughout the last 500 k.y., with no discernible difference observed between glacial and interglacial size distributions within each site. X-ray diffraction analysis of the <2-µm clay component suggests that the deposition of minerals found at Site 1020 is consistent with transport from a southern source during intervals of increased terrigenous input.

Carbon and oxygen isotopic composition of benthic foraminifers from ODP Site 167-1014 and Hole 167-1020C

Benthic foraminiferal stable isotope data are presented for Sites 1014 (Tanner Basin, 1176 m) and 1020 (Gorda Ridge, 3040 m) to constrain past changes in Pacific deep- and intermediate-water nutrient chemistry associated with the onset of large-amplitude 100-k.y. climate cycles after ~900 ka. The Site 1014 data were based on analyses of separate species of Cibicidoides, whereas only Cibicidoides wuellerstorfi was used to generate the Site 1020 record. The present data span 380-920 and 620-950 ka at Sites 1014 and 1020, respectively.