Radiocarbon Measurements

During the R/V Thomas G. Thompson expedition along WOCE Section P10, 588 accelerator mass spectrometry (AMS) delta ¹⁴C samples were collected at 38 stations. In addition to the AMS samples, LV Gerard samples were also collected during this cruise. The LV measurements will be published in a separate report.

Sampling of ¹⁴C during the cruise was carried out by R. Key of the Ocean Tracer Laboratory at Princeton University. Sample extraction, δ¹³C analyses, and ¹⁴C analyses were performed by the National Ocean Sciences AMS Facility (NOSAMS) at Woods Hole Oceanographic Institution. Key collected the data from the originators, merged the files, assigned quality control flags to the ¹⁴C results, and submitted the data files to the WOCE office in April 1998.

All SV 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 were sent for extraction and analysis at NOSAMS after the cruise. 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 Δ¹⁴C values reported here were originally published in a NOSAMS data report (NOSAMS, March 13, 1998). That report included results that had not been through the WOCE quality control procedures.

All 588 of the AMS samples from this cruise have been measured and presented in this report. Replicate measurements were made on 21 water samples. These replicate analyses are tabulated in Table 2.

Table 2. Summary of Replicate Analyses

*Error-weighted mean reported with data set.
**Larger of the standard deviation and the error-weighted standard deviation of the mean.

Table 2 shows the error-weighted mean and uncertainty for each set of replicates. Uncertainty is defined here as the larger of the standard deviation and the error-weighted standard deviation of the mean. For these replicates, the simple average of the tabulated uncertainties for the replicates is 4.0‰ (equal weight for each replicate set) This precision is typical for the time frame over which these samples were measured (February-October 1997). Note that the errors given for individual measurements in the final data report (with the exception of the replicates) include only counting errors and errors due to blanks and backgrounds. The uncertainty obtained for replicate analyses is an estimate of the true error that includes errors due to sample collection, sample degassing, etc. For a detailed discussion of this see Key (1996).

Figures 10-14 summarize the Δ¹⁴C data collected on the P10 Section. Only dΔ¹⁴C measurements with a quality flag value of 2 ("good") or 6 ("replicate") are included in each figure. Figure 10 shows the Δ¹⁴C values with 2 sigma error bars plotted as a function of pressure. The mid-depth Δ¹⁴C minimum occurs around 2000 to 2400 m, but is weak in this data set relative to the eastern North Pacific. Measurements in the thermocline region fall into two distinct groups with the higher values being from the southern end of the section and the extreme northern end while the lower grouping is from the central portion (see Fig. 11 and Fig. 12).

Figure 11 shows the Δ¹⁴C values plotted against silicate. The straight line shown in the figure is the least squares regression relationship derived by Broecker et al. (1995) on the basis of the GEOSECS global data set. According to their analysis, this line (Δ¹⁴C =-70 - Si) represents the relationship between naturally occurring radiocarbon and silicate for most of the ocean. Broecker et al. interpret deviations in Δ¹⁴C above this line to be due to input of bomb-produced radiocarbon; however, they note that the interpretation can be problematic at high latitudes. Samples collected from shallower depths at these stations show an upward trend with decreasing silicate values reflecting the addition of bomb-produced ¹⁴C. As in Fig. 10, two distinct trends are apparent. Here the upper grouping is from the northern end of the section and the lower from the southern end.

Another way to visualize the ¹⁴C silicate correlation is as a section. Figure 12 shows Δ¹⁴C as contour lines in silicate-latitude space for samples having a potential density greater than 26.9, which corresponds to ~500 m. In this space, shallow waters are toward the bottom of the figure. The density cutoff was selected to eliminate those samples with a very large bomb-produced ¹⁴C component. For this data set, Broecker's hypothesis does not work very well. The Δ¹⁴C isolines trend upward to the north, and the spacing between the isolines decreases northward, for contours that fall below the depth of bomb-radiocarbon contamination. The upward curvature of the isolines at the northern end of the section is due to the addition of bomb-produced radiocarbon via ventilation or due to an "anomalous" silicate signal (Talley and Joyce 1992).

Figure 13 and 14 show Δ¹⁴C contoured along the section. Figure 13 is a normal section in latitude-depth space whereas Fig. 14 shows the same data set in potential density-latitude space. The depth section was gridded by means of LeTraon's (1990) objective technique, and the density section was gridded using the "loess" methods described in Chambers et al. (1983), Chambers and Hastie (1991), Cleveland (1979), and Cleveland and Devlin (1988).

In Fig. 13, the primary structure of the isopleths is due to the presence of the Pacific North Equatorial Current which flows westward across the southern end of the section and the Japan current that flows northeastward across the far northern end of the section. Upwelling near the equator is not particularly evident in Fig. 13 but is the source of most of the structure seen in the isopleths in Fig. 14 in the low-latitude zone. The deep and bottom water AMS results are too sparse to contour.