Stable isotope stratigraphy and paleoceanography of the last 170 k.y.: Site 1014, Tanner Basin, California

Abstract

Late Quaternary oxygen (δ 18 O) and carbon (δ 13 C) 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 Borderland. Sedimentation rates are relatively constant and high (~11.5 cm k.y. –1 ). The planktonic foraminiferal record is well preserved 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‰ 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 δ 18 O 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‰ higher. This is unlikely to reflect a cooler Eemian but is instead the result of preferential dissolution of thin-shelled (low δ 18 O) specimens during this interval. In this mid-depth basin, a large benthic δ 18 O shift during Termination I suggests dramatic temperature and salinity changes in response to switches in the source of North Pacific Intermediate Water. Although δ 13 C values of the planktonic foraminifer G. bulloides are in disequilibria with seawater and hence interpretations are limited, the G. bulloides record exhibits several negative δ 13 C excursions found at other sites in the region (Sites 1017 and 893). This indicates a response of G. bulloides δ 13 C to regional surface water processes along the southern California margin. A general increase in benthic carbon isotopic values (–1.75‰ to –0.75‰) 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 δ 13 C values, which correlate with global δ 13 C shifts. The opposite relationship is exhibited during the last interglacial before 85 ka, when lower benthic δ 13 C 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 δ 13 C record. These results suggest that a delicate balance in intermediate water δ 13 C has existed between the relative influences of global δ 13 C and regional ventilation changes at the 1165-m water depth of Site 1014.