Total CO₂ Measurements

The total carbon dioxide (TCO₂) or dissolved inorganic carbon analytical equipment was set up in a seagoing container modified for use as a shipboard laboratory. The analysis was done by coulometry with two analytical systems (PMEL-1 and PMEL-2) used simultaneously on the cruise. Each system consisted of a coulometer (UIC, Inc.) coupled with a SOMMA inlet system developed by Ken Johnson (Johnson et al. 1985, 1987, 1993; Johnson 1992) of Brookhaven National Laboratory. In the coulometric analysis of TCO₂, all carbonate species are converted to CO₂ gas by addition of excess hydrogen to the seawater sample, and the evolved CO₂ gas is carried into the titration cell of the coulometer, where it reacts quantitatively with a proprietary reagent based on ethanolamine to generate hydrogen ions. These are subsequently titrated with coulometrically generated OH^-. CO₂ was measured by integrating the total change required to achieve this.

The coulometers were each calibrated by injecting aliquots of pure CO₂ (99.995%) by means of an 8-port valve outfitted with two sample loops. The instruments were calibrated at the beginning and end of each station with a set of the gas loop injections.

Secondary standards were run throughout the cruise on each analytical system. These CRMs of seawater are poisoned and filtered and ultra-violet-irradiated. The CRMs, supplied by Dr. A. Dickson of SIO, have been certified in SIO's shore-based facility to have a known concentration of TCO₂. Although there were numerous small equipment problems during the first third of the cruise on section A20, the overall accuracy and precision of the at-sea analyses of the CRMs on both instruments for this section was 1±1.7 µmol/kg (n=88) for both systems combined and -0.05±1.01 µmol/kg (n=77) for both systems on section A22. TCO₂ data reported to the database have been recalculated using the bottle salinities, and, although insignificant for this cruise, corrected to the Batch 61 CRM value on a per-instrument basis.

Samples were drawn from the Niskin-type bottles into cleaned, precombusted 500-mL Pyrex bottles using Tygon tubing. Bottles were rinsed once and filled from the bottom, overflowing half a volume, and care was taken not to entrain any bubbles. The tube was pinched off and withdrawn, creating a 5-mL headspace, and 0.2 mL of saturated HgCl₂ solution was added as a preservative. The sample bottles were sealed with glass stoppers lightly covered with Apiezon-L grease and were stored at room temperature for a maximum of 12 h prior to analysis.

In all, 1,640 samples were analyzed for TCO₂ during section A20 and 1,600 samples were analyzed during section A22: full profiles were completed on the even-numbered stations, with replicate samples taken from the surface, oxygen minimum, and bottom Niskin-type bottles. At a minimum, replicate surface samples were taken at every odd-numbered station, and when time permitted, additional depths were sampled. Approximately 120 replicates were collected in total. The replicate samples were run at different times during the station analysis for quality assurance of the integrity of the coulometer cell solutions. No systematic differences between the replicates were observed and the standard deviation of the differences was ~ 1.2 µmol/kg on both systems during sections A20 and A22.