¹⁴C and ¹³C Measurements

A small number of samples (34 surface and 3 depth profiles) were collected directly from the Niskin bottles for measurement of radiocarbon in dissolved inorganic ¹⁴C using the procedures described at http://nosams.whoi.edu/clients/submissions_sample_prep.html#dic. Samples were poisoned with mercuric chloride and shipped to National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) for analysis. For radiocarbon analyses, the procedures used are described in McNichol et al. (1994), McNichol and Jones in WHP 91-1 (accessible at http://whpo.ucsd.edu/manuals.htm), and McNichol et al. 2000, 2001. Briefly, the carbon is first extracted in a sample as CO₂ and then reduced from a gas to solid carbon. Samples are acidified with phosphoric acid and CO₂ gas is stripped out with a nitrogen carrier gas. After conversion to CO₂, samples are reduced to "graphite" on Fe powder with hydrogen gas as the reducing agent. Sample graphite compacted into target cartridges is loaded into a multi-sample wheel mounted in the accelerator mass spectrometry (AMS) Cs-sputter ion source. The graphite (pure carbon) derived from a sample is compressed by the target press and inserted into the cathode of the AMS ion source where the ratio of ¹⁴C to ¹²C is measured and compared to that of known standards. Shipboard duplicates, i.e., two separate AMS samples collected from the same water depth but measured separately, continue to show a high level of precision (Elder et al. 1998). Fifteen paired analyses from samples collected along 150°W in the North Pacific demonstrate reproducibility of ± 3.3% for dissolved inorganic ¹⁴C and ± 0.04% for dissolved inorganic ¹³C, consistent with results we have obtained in the past. This precision is obtained through careful preparation in the laboratory, increased counting statistics on the accelerator, and careful quality screening after analysis. Long-term reproducibility of results over time is demonstrated by periodically re-running entire water profiles from randomly selected stations.

Comparing the surface values obtained in 1997 with those measured here (Fig. 21, and Fig. 22 for ¹⁴C and Fig. 23, and Fig. 24 for ¹³C) shows no obvious patterns of change. There is a hint of an increase near the equator, but the sparse data set makes it challenging to consider this a robust feature.