Graphite in various forms has become the standard target for accelerator 14C dating. Graphite has been made by catalytic graphitization of charcoals (Lowe,1984). Thin films of graphite have also been produced by thermal cracking (Beukens & Lee, 1981), electric discharge (Andree et al, 1984;Wand, Gillespie & Hedges, 1984). Vogel et al (1984) pointed out the ease of graphite formation on iron from CO2 and H2 mixtures at ca 600°C. The deposition reactions of carbon from the CO, H2, and CO2 equilibria are well known (Wagman et al, 1945) and well studied. Formation of graphite from CO2 was discussed extensively by Boudouard (1902) and Schenck and Zimmerman (1903), and was known to chemists in France in 1851. We have used a related method, where graphite forms away from the iron, by using a higher temperature, and reduction of CO2 to CO over Zn in the presence of H2 (,Dull et al, 1986) as an alternative to the use of Fe alone. The object of this paper is to point out an even simpler graphite preparation system, which eliminates hydrogen. The decomposition reaction of CO (Boudouard, 1902) takes place according to reaction (1). 2C0CO2 + C (gr) (1) If carbon is introduced as C02, and is reduced to CO by hot Zn and any C02 formed due to reaction (1) is reduced to CO by the hot Zn, then the reaction proceeds in one direction. This method uses a small-volume system, which can be of any size but is typically 5 to 10cc. Using a 5cc system, reduction of CO samples of as small as 0.7cc has been achieved. The apparatus is shown in Figure 1. As mentioned, two reactions occur in the system. CO2 is reduced to CO over -30mg Zn at ca 500°C, and the CO disproportionates to graphite over the iron (1 mg) mg) at typically 700°C. Deposition occurs at temperatures as low as 450°C. The graphite is separate from the iron, suggesting a vapor-phase intermediate.The gas pressure is monitored, and 100% reaction is easily observable, as we wait until the gas pressure is zero. This takes from 2 to 10 hours, depending on several factors. It is clear that the reaction is pressure-and temperature-dependent; however, variations in reaction rate appear to be complex, and apparently dependent on the catalyst. The amount of Fe catalyst used can be very small (< 1 mg) without measurable effect on the reac-

ABSTRACT. We present here the method we use to convert to radiocarbon ages (14C/13C) ratios measured in the Arizona Accelerator Mass Spectrometer facility. We describe the procedures we use to convert sample and standard isotope ratios to values appropriate for calculation of radiocarbon ages. We also discuss, in some detail, corrections to account for sample contamination.

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Radiocarbon measurements on a 175‐year (A.D. 1800 to 1974) growth of the coral Montastrea annularis from The Rocks reef off the Florida Keys reveal the rate of local uptake of fossil fuel CO2 and bomb 14C by surface ocean waters of the Gulf Stream. In the nineteenth century, the pre‐bomb, pre‐industrial Δ14C value of surface ocean waters as seen in these corals of the Gulf Stream in the Florida Straits was −51 ± 2‰. By 1955, uptake of industrial CO2 by these waters had lowered the Δ14C values to about −61‰. The results can be used to make predictions regarding anthropogenic CO2 that can be expected to enter the oceans in future decades. Bomb‐produced 14C is found to be present in the corals in comparable concentrations to that found in the dissolved inorganic carbon (DIOC) of the North Pacific and North Atlantic Oceans.

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