Carbon Measurements

To measure the TCO2 concentration in seawater, a coulometric analysis system was used during all cruises. This system has been described by Chipman et al. (1993) and consists of a coulometer (Model 5011), manufactured by UIC, Inc. (Jolliet, Ill.), and a sample introduction/CO2 extraction system of LDEO design. The TCO2 concentration in 4419 water samples was analyzed. In addition, 758 determinations were made at sea for 260 bottles of the Certified Reference Material (CRM) (batch nos. 12 and 13) yielding an average value of 1983.0 ± 1.5 µmol/kg for 166 analyses during leg P16A/P17A, 2013.7 ± 2.1 µmol/kg for 233 analyses during leg P17E/P19S, and 2015.3 ± 1.9 µmol/kg for 359 analyses during leg P19C. These compare with the SIO manometric values of 1984.0 ± 0.7 µmol/kg (N=7), 2015.1 ± 0.6 µmol/kg (N=7), and 2015.1 ± 0.6 µmol/kg (N=7) respectively. The CRMs were prepared by Dr. Andrew Dickson of SIO and analyzed manometrically by Dr. C. D. Keeling of SIO. The mean difference between the shipboard analyses by the LDEO group and the manometric analyses by SIO (LDEO SIO) for CRM has been estimated to be 1.0 ± 1.7 µmol/kg for Section P16A/P17A, 1.4 ± 2.2 µmol/kg for P17E/P19S, and 0.2 ± 2.0 µmol/kg for P19C. The overall precision of all TCO2 measurements is estimated to be ~±2 µmol/kg.

To measure the pCO2 in seawater, a fully automated equilibrator-gas chromatograph system was used during the cruises. This system has been described by Chipman et al. (1993). The pCO2 in 4419 water samples was measured. Because pCO2 is strongly affected by temperature changes, the equilibration flasks were kept in a constant-temperature water bath of 20 or 4°C depending on latitude of sampling. The precision of the pCO2 measurements has been estimated to be ~±0.12% for a single station based on the reproducibility of replicate equilibrations. However, the station-to-station reproducibility was about ±0.5%.

A full description of methods and instrumentation used to perform the TCO2 and pCO2 measurements during the R/V Knorr expeditions along WOCE Sections P16A/P17A, P17E/P19S, and P19C is provided in Takahashi et al. (1998), which is reprinted in the Appendix of this documentation.

Figures 2, 3, and 4 show the sampling density and depth along the WOCE Sections P16A/P17A, P17E/P19S, and P19C.


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