During the planning phase of WOCE, the accelerator mass spectrometry (AMS) technique for measuring 14C was still relatively new in the United States. The general procedures had been worked out; however, no laboratory was prepared to handle the large number of samples expected from the WOCE program, nor had it been demonstrated that the AMS technique could deliver the required precision on a routine basis. The National Ocean Sciences AMS Facility (NOSAMS) at Woods Hole Oceanographic Institution (WHOI) was established in 1989 to serve this purpose. In planning the WOCE Pacific field work, it was recognized that sample collection would begin well before NOSAMS could deliver the high precision offered by conventional beta counting techniques. Therefore, both techniques were utilized.
On those WOCE legs that included both large-volume (LV) and small-volume (SV) sampling, the LV stations were spaced at an average interval of once every five degrees (~300 nautical miles). LV stations normally included two casts of nine Gerard barrels each, covering the water column from ~1000 m to the bottom. The upper kilometer of an LV station was covered by 16 SV samples taken from the CTD/rosette cast. One to three SV stations were placed between each LV station. At SV stations, only the upper thermocline region was sampled. Sixteen SV samples were taken at these stations.
Radiocarbon was extracted from the LV samples at sea as 14CO2, absorbed on excess NaOH and returned to shore in well-sealed glass bottles using a modification of the technique described by Fonselius and Östlund (1959). Once ashore the samples were sent to researchers at one of two laboratories for analysis: G. Östlund, Tritium Laboratory, University of Miami, Miami; or M. Stuiver, Quaternary Isotope Laboratory, University of Washington, Seattle. A short description of the measurement procedure and a cross-check between these two laboratories are available in Stuiver et al. (1974). Stuiver reports an error estimate for each analysis that ranges from 2.5 to 4.0 per mille (°/°°), while Östlund reports a uniform sample error of 4°/°°. In both cases the reported uncertainty is primarily counting error and does not include any error due to sample collection.
All SV 14C samples were collected from standard CTD/rosette casts into 500-mL glass bottles
fitted with high-quality ground-glass stoppers. The samples were poisoned with HgCl2 immediately
after collection and then returned to the United States for extraction and analysis at NOSAMS. Details
of the extraction, counting, etc., are available from Key (1991), McNichol and Jones (1991), Gagnon
and Jones (1993), and Cohen et al. (1994). The standard used for the 14C measurements is the National
Bureau of Standards oxalic acid standard for radiocarbon dating. All results are reported as 14C,
which is the deviation (in °/
13C value of -25°/
13C was calculated from
13C = 1000 x [13C/12C]sample - [13C/12C]reference
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[13C/12C]reference
to correct for fractionation and dilution by anthropogenic CO2. For more information on standards and
calculation methods, refer to the papers by Broecker and Olson (1961), Stuiver and Robinson (1974),
and Stuiver (1980). As measurements were completed, the results were communicated from the
analytical laboratory to the principal investigator responsible for the cruise via periodic data reports.
R. Key gathered the 14C data; merged it with hydrographic data supplied by either the chief scientist
or WHPO; added WOCE quality-control flags; and finally submitted the data to WHPO along with
a final report for the WOCE Section P17C, Cruise TUNES, Leg 1 (Key 1996a, b). All of the LV
samples collected in the Pacific Ocean will be processed by 1997, and the Pacific SV samples will be
completed by 1998.
During the GEOSECS Program, the precision of the LV technique was established as 2-4°/
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