Total Alkalinity Measurements
TALK samples were collected in 250-mL standard borosilicate glass, screw-cap bottles and poisoned with 50 µL of saturated HgCl2 solution. The samples were stored at room temperature and measured on board, generally within 18 h. TALK was determined by potentiometric titration; the method used was derived from one first described by Dyrssen (1965) and later modified by Bradshaw et al. (1981). The automated titration was performed in a closed cell maintained at constant temperature (25 ±0.1°C); the ionic strength of the hydrochloric acid solution (0.1 N) was adjusted with NaCl to increase its similarity to seawater. The ratio of the acid normality to the cell volume was calibrated before and after the sample analysis. Calibration consisted of preparing solutions of known TALK concentration and measuring them as described by Brewer et al. (1986). The precision of the measurements was estimated to be better than 0.1%.