Total CO₂ Measurements

During the TUNES-1 Expedition, 1022 seawater samples were analyzed for TCO₂ concentrations in seawater. The sampling frequency for measurements of the carbonate parameters was reduced to a complete depth profile (36 samples) at approximately every third hydrographic station (Fig. 2). This reduction in sampling was implemented not on the basis of any prearranged geophysical criterion but to accommodate the time constraints of the two analysts on board who performed CO₂ sampling and measurements. In other words, the CO₂ sampling strategy adopted was to measure as many samples as was technically and humanly possible.

For TCO₂ measurements, the seawater samples were drawn into 250-mL standard borosilicate glass, screw-cap bottles, poisoned with 50 µL of a saturated solution of mercuric chloride (HgCl₂), stored at room temperature, and analyzed on board (generally within 18 h). TCO₂ concentration was measured by semiautomated coulometry, using an improved version of the instrument earlier described by Johnson et al. (1985, 1987). This early "SOMMA-type" system did not have gas loops for calibration. Consequently, plans were to calibrate the system with standard solutions prepared as described in Goyet and Hacker (1992); however, these standard solutions could not be prepared on board during the cruise. The certified reference materials were used as standards to calibrate the TCO₂ extraction/coulometer system. The latter worked consistently well throughout the cruise. Precision of the measurements was estimated to be better than ±3 µmol/kg; the desired accuracy was better than ±4 µmol/kg.

The automated coulometric system forced the sample into the pipette using a pressurized headspace gas. Pure nitrogen (N₂) headspace gas was used for measurements of standards, and CO₂ headspace gas (290 ppm in air) was used for measurements of seawater samples. The volume of the pipette was calibrated with distilled water and seawater (volume was ~30 mL, depending on the individual pipette used), and there was no significant difference in the delivery volume as a result of possible differences in surface tension at different salinities. The sample was drained from the pipette into a stripper containing 1.5 mL of 8.5% phosphoric acid.

This chamber and the added acid were purged of any CO₂ with pure N₂ carrier gas before the sample was added. In the stripper, the CO₂ gas was extracted from the acidified sample by a continuous flow of pure N₂ gas through a frit at the bottom of the stripper. The gas (mainly CO₂, N₂, and water vapor) was passed through a condenser thermostated with 4°C water and magnesium perchlorate [Mg(ClO₄)₂] to remove water vapor. It was then passed through silica gel to remove residual aerosols and traces of hydrogen sulfide (H₂S) and phosphoric acid (H₃PO₄) before being bubbled into a commercially available coulometric solution containing ethanolamine [NH₂(CH₂)₂OH], dimethyl sulfoxide [(CH₃)₂SO], and thymolphthalein dye (made by UIC, Inc., Joliet, Illinois). A coil made from glass tubing with thermostated water flowing through it was placed in the cell to maintain the solution at 24°C. The pH of the solution was monitored on total CO₂ coulometer (UIC, Inc.) by monitoring the thymolphthalein-absorbance indicator at ~610 nm. Hydroxide (OH^-) ions were generated by the coulometer circuitry to maintain absorbance of the solution at a constant value. The analytical procedure was controlled by a microcomputer that also recorded the coulometric titration and computed the total CO₂ extracted from the sample based on the amount of OH^- generated to reach the endpoint.