ࡱ> HJGq` bjbjqPqP .4::@@@TD $T[PPPPPkkk$hy!n@@gk@@PP@P@P@0@PD 0l$$z$+0[!:!!@k"kkkkkk[@@@@TTT XTTTXTTT CRUISE FICARAM VI R/V Hesprides Cruise: He091 Dates: 8 to 24 April 2003 Chief scientist: Aida F. Rios (IIM-CSIC) Alkalinity and pH responsible (A.F. Rios) DIC analysis responsible (coulometric method): Rik Wanninkohf (NOAA/AOML) Nutrients analysis responsible: Carmen G. Castro (IIM-CSIC) During the cruise FICARAM VI, seven stations were performed. Alkalinity and pH were measured on board. The variables of carbon system, pH and alkalinity, in surface waters together with the fCO2 measured, were used to study the internal consistency of the measurements. pH was measured spectrophotometrically following Clayton and Byrne (1993). Total Alkalinity (TA) was measured using potentiometric titrations with hydrochloric acid to a final pH of 4.40 (Prez and Fraga, 1987). The electrodes were standardised using a buffer of pH 4.42 made in CO2 free seawater (Prez et al., 2002). Dissolved oxygen was determined by Winkler potentiometric titration. The estimated analytical error was (1 mol(kg-1. Oxygen saturation was calculated following Benson and Krause equation (UNESCO, 1986). Nutrient samples were frozen before the analysis. Nutrient concentrations were determined by segmented flow analysis with Alpkem system, following Hansen and Grasshoff (1983) with some improvements (Mourio and Fraga, 1985; lvarez-Salgado et al., 1992). The analytical error was (0.05 mol(kg-1 for nitrate, (0.05 mol(kg-1 for silicic acid and (0.01 mol(kg-1 for phosphate. To check the accuracy of the pH and TA measurements, samples of CO2 reference material (CRM, batch 54 distributed by A.G. Dickson from the Scripps Institution of Oceanography) were analysed during the cruises. The uncertainties of pH and alkalinity were 0.002 and 0.7 mol kg-1, respectively. The corresponding theoretical pH25T value (7.918) for this batch was calculated using the dissociation constants from Lueker et al. (2000), which was in agreement with the theoretical value (0.0002). Figure 1 compares the CO2 fugacity (fCO2) values measured at every station sampled during FICARAM VI cruise and those calculated from pHT and total alkalinity with the Mehrbach et al. (1973) dissociation constants refitted by Dickson and Millero (1987) dissociation constants. The agreement between both fCO2 is excellent (r2 =0.94), confirming the good internal consistency of our measurements. The average and standard deviation of the differences between both calculated and measured CO2 was -0.53.1 atm. Figure 2 compares the DIC measured using coulometric method with those calculated from pH and total alkalinity with the Mehrbach et al. (1973) dissociation constants refitted by Dickson and Millero (1987). The agreement between both DIC is excellent (r2 =0.995). The average and standard deviation of the differences between both calculated and measured DIC was -5.0 4.7 mol/kg.  FIGURE 1- Relationship between measured CO2 fugacity and that calculated as a function of TA and pH measured at the surface of the FICARAM VI stations.  FIGURE 2- Relationship between measured DIC and that calculated as a function of TA and pH References Alvarez-Salgado X.A., Fraga F., Perez F.F., 1992. Determination of nutrient salts by automatic methods both in seawater and brackish waters: The phosphate blank. Marine Chemistry, 39, 311-319. Clayton T.D., Byrne R.H., 1993. Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale concentration scale calibration of m-cresol purple and at-sea results. Deep-Sea Res. I, Vol. 40, 10, 2115-2129. Dickson A.G., Millero. F.J., 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res., 34, 1733-1743 Hansen H.P., Grasshoff K., 1983. Automated Chemical Analysis. In: K. Grasshoff, M. Ehrhardt and K. Kremlig (Editors), Methods of Seawater Analysis. Verlag Chemie, Weinheim, 419 pp., 1983. Mehrbach C., Culberson C.H., Hawley J.E., Pytkowicz,R.M., 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnology and Oceanography, 18, 897-907 Mourio C., Fraga F., 1985: Determinacion de nitratos en agua de mar. Investigacion Pesquera, 49, 81-96. Prez F.F., Fraga F., 1987. A precise and rapid analytical procedure for alkalinity determination. Marine Chemistry, 21, 169-182. Prez F.F., Ros A.F., Relln T., lvarez M., 2000. Improvements in a fast potentiometric seawater alkalinity determination. Ciencias Marinas, 26, 463-478. 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