Radiocarbon Chronology and Planktonic-Benthic Foraminiferal 14C Age Differences in Santa Barbara Basin Sediments, Hole 893A

Abstract

Radiocarbon dating by accelerator mass spectrometry of planktonic and benthic foraminifers separated from sediments from Ocean Drilling Program Hole 893A, Santa Barbara Basin (576.5 m water depth), were used to provide the chronology for the upper 70 m of the core. Radiocarbon ages were corrected to calendar ages using two calibration techniques, which gave similar results. One was based on tree-ring calibration and the other based on U-Th dating of corals. The maximum reliable planktonic foraminiferal radiocarbon age in the core, at a corrected depth of 43.18 mbsf, was 25,470 ± 240 radiocarbon yr, with a corrected age of 28,896 yr before present. The youngest age obtained for the core was from 1.55 mbsf, with an age of 1850 radiocarbon yr, and corrected age of 950 yr, assuming a reservoir age, R(t), of 825 yr. A total of 43 samples was analyzed between these two levels, exhibiting a near linear sedimentation rate of 14.53 cm/100 yr. The Younger Dryas cooling event, represented in Santa Barbara Basin by a climatic change based on oxygen isotopic changes of benthic and planktonic foraminifers, occurs in a non-laminated interval within laminated sediments (17.60 to 20.40 mbsf). The event was dated using six planktonic foraminifer samples. Radiocarbon ages for the event are 11,980 to 10,630 yr. The corrected age (calendar age) for the initiation of the Younger Dryas is 12,970 yr BP, which is similar to those determined in North Atlantic sediments, glacial deposits in New Zealand, and Greenland ice cores. The corrected age for the termination of the Younger Dryas in Santa Barbara Basin is poorly constrained, due to its occurrence during the 1300-yr radiocarbon plateau, and falls between 10,900 and 12,300 yr BP. We have used an interpolated age of 11,200 yr BP. Pairs of planktonic and benthic foraminifers separated from the same depth were also measured to determine radiocarbon age differences, presumably reflecting the age of upper intermediate waters entering the basin. The age differences vary between 40 and 740 radiocarbon yr, with smallest age differences occurring within the non-laminated Younger Dryas (based on three samples with an average I4C age difference of 90 yr) and the last glacial maximum at -20 ka (100 yr). Paired samples in laminated sediments older and younger than the YD have an average I4C age difference of 470 yr. The decrease in surface to bottom age difference during the Younger Dryas and the last glacial maximum reflects a change in source of intermediate waters, with a greater proportion originating from a more proximal source.