HIGH-PRECISION AMS RADIOCARBON MEASUREMENTS OF CENTRAL ARCTIC-OCEAN SEA WATERS

We report on the first sub-5 parts per thousand precision radiocarbon dataset measured on single targets using accelerator mass spectrometry (AMS). A 13 sample water column profile collected in the Canada Basin (74-degrees-N, 150-degrees-W, 3850 m water depth) of the central Arctic Ocean in September 1992 has been analyzed in duplicate and the average total precision achieved for the 26 targets is +/-3.2 parts per thousand. The reproducibility of the 13 paired analyses averages +/-4.8 parts per thousand as determined by a chi-square fit minimization for a quality factor of unity, and +/-7.8 parts per thousand using ANOVA. Eliminating two of the 13 paired analyses because of apparent outlier behavior in one of the two analyses comprising the pair results in a total precision of +/-3.2 parts per thousand, a chi-square fit of +/-3.5 parts per thousand, and ANOVA precision of +/-3.5 parts per thousand. Comparison with a recently published AMS C-14 profile from the same basin suggests these data are accurate as well. Results show that the deep waters of the Canada Basin have a renewal rate of 430 years, in comparison with 250 years estimated for the deep waters of the Eurasian Basin. Although the major requirement of the World Ocean Circulation Experiment (WOCE) for a radiocarbon analysis precision of +/-3 to 4 parts per thousand for deep water samples can now be met with the AMS technology available at the National Ocean Sciences AMS Facility at the Woods Hole Oceanographic Institution for single-target analyses, a careful program of duplicate analyses should be included to insure the highest quality in the WOCE DELTAC-14 dataset.