Carbon Isotope Measurements

Sections P16S_2005 and P16N_2006 were sampled for carbon isotopes. On average, full depth samples were collected every 5 degrees of latitude, and the upper water column was additionally sampled at the midpoint between full depth stations. Generally, 16 samples were collected for the upper water column stations and 32 samples for full depth stations. The sample collection and analysis procedures were identical to those used for WOCE and previous CLIVAR cruises. Briefly, ~500 mL samples were collected in Pyrex bottles fitted with high precision ground-glass stoppers. Before being used, the bottles were acid washed and annealed. The collected water samples were poisoned with 100µL of saturated HgCL2, sealed, and returned to National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) at WHOI where they were analyzed with the accelerator mass spectrometry radiocarbon technique described in McNichol et al. (1994). The analytical procedure yields δ13C values as part of the method. The routine estimated 1 sigma counting error for these samples was 23 ‰, but analysis of a very large number of replicate samples indicates the true radiocarbon error is between 3 and 4 ‰ (Elder et al. 1998). The mean reproducibility for duplicates collected on these two sections was 3.5 ‰ (24 pairs) for DI14C and 0.02 ‰ (21 pairs) for DI13C.

After the analyses were completed, NOSAMS produced a final report for each set of samples (NOSAMS Data Report #08-001 and #08-003). The data were then transferred to Princeton for final QC and submission to the data centers (CDIAC and CCHDO).

Figure 7 shows the radiocarbon concentration results for these two cruises; dots represent sample locations. Topography is taken from the bottom depth measurement at each station. The North Pacific Deep Water minimum is located well south of the Alaskan slope. Waters with concentration greater than approximately 100‰ contain some bomb-produced radiocarbon contamination. At first glance, these data are quite similar to the WOCE occupation results for this section. The highest concentrations are found in near-surface waters in the subtropics. In these waters, the radiocarbon concentration generally mimics the density distribution. In deep water, the minimum concentration is found in North Pacific Deep Water with the extreme centered near 2500 m around 35°N. In the upper water column, the maximum concentrations are often at the surface as during WOCE, but more often now, maximum concentrations are found as deep as 250 m (not visible in figure).

While the overall distribution pattern measured on these two cruises is remarkably similar to that measured during WOCE, there are easily measured changes. Figure 8 shows the change in radiocarbon concentration in the upper 1500 m between WOCE and CLIVAR occupations along Section P16. Colors indicate the change in Δ14C (‰). Only the zero (no change) contour line is shown as a heavy line. The light lines are contours of potential density along the section. In general, the change pattern follows the density distribution. The change at the far southern end of the section is not shown here, but implies a significant increase. This increase at the southern edge and the strong increase at the northern edge may be due to minor changes in the density structure rather than invasion of the bomb signal. Further investigation is needed in these areas. This figure was prepared by simply gridding the data from each occupation and then subtracting the WOCE results from the CLIVAR results. As in the concentration figure, the change distribution generally follows the density distribution. In the upper water column, the concentrations show a strong decrease while values in the thermocline generally show strong increases. The large increase adjacent to the Alaskan slope was unexpected and may be an artifact due to small changes in station locations and the density structure. Similar large increases at the southern end of the section are not shown in this figure since they could be an artifact of either the gridding or a small movement of the Circumpolar Current. The strong increases at both ends of this section will require much more careful analysis.

The overall distribution of 13C is similar to that measured previously; the observed patterns differ from that of 14C because 13C is a tracer of biological as well as physical processes. Values in the southern hemisphere are enriched relative to those in the north due to the increased input of 13C-depleted carbon from the oxidation of organic matter. Measurement of DI13C in the full water column of the northern section added detailed coverage to the deep waters, which were not measured during WOCE. The biggest changes over time are expected in the surface waters where there should be changes due to the equilibration of the surface ocean with the changing atmosphere. At present we are unable to show these differences because of a systematic calibration issue affecting some of the DI13C data; future versions of this document will include a figure detailing the changes when the issue has been resolved.

akozyr 05/26/2009