High-accuracy 14C measurements for atmospheric CO2 samples by AMS

In this paper, we investigate how to achieve high-accuracy radiocarbon measurements by accelerator mass spectrometry (ANIS) and present measurement series (performed on archived CO2) of (CO2)-C-14 between 1985 and 1991 for Point Barrow (Alaska) and the South Pole. We report in detail the measurement plan. the error sources, and the calibration scheme that enabled us to reach a combined uncertainty of better than +/- 3 parts per thousand. The delta C-13 correction and a suggestion for a span (or 2-point) calibration for the C-14 scale are discussed in detail. In addition, we report new, accurate values for the calibration and reference materials Ox2 and IAEA-C6 with respect to Ox1. The atmospheric (CO2)-C-14 records (1985-1991) are presented as well and are compared with other existing records for that period. The Point Barrow record agrees very well with the existing, Fruholmen (northern Norway) record from the same latitude. The South Pole record shows a small seasonal cycle but with an extreme phase with a maximum on January 1st (+/- 13 days). Together with its generally elevated C-14 level compared to the Neumayer record (coastal Antarctica). this makes our South Pole data set a valuable additional source of information for global carbon cycle modeling using (CO2)-C-14 as a constraint.