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Alkalinity and Dissolved Inorganic Carbon (DIC) measurements during ARK-XXII/2

Data Originator

PI: Steven van Heuven (RuG) and Hein de Baar (NIOZ-Royal Netherlands Institute for Sea Research)

Originator's Protocol for Data Acquisition and Analysis

Work at sea

Water samples were collected during the ARK XXII-2 (July 31- October 4, 2007) expedition on RV Polarstern. Samples were sampled from GoFlo-samplers of the UCCTD-system prior to the sub-sampling for the other parameters.

The Total Alkalinity (TA) analysis was performed with the 'standard' titration, using curve fits along modified Gran plots (Gran, 1952; Bradshaw 1981; Dickson, 1993). Analysis of DIC was performed using a coulometric method (Johnson et al., 1993; DOE, 1994). Both analyses were performed on a single integrated system: the VINDTA system (Versatile Instrument for Determination of Titration Alkalinity, MARIANDA: Marine Analytics and Data, Kiel, Germany), which also controlled the coulometer. Analyses accuracy was assured through the use of certified reference material (CRM, supplied by Dr. A. Dickson, Scripps Institute of Oceanography).

For the coulometric determination of DIC (a slight adaptation of the method described by Johnson 1993), an accurately known amount of sample (20 ml) is dispensed with an automated, thermostated pipette into a stripper vessel. The sample is acidified, converting all carbonate and bicarbonate species into CO2(aq). The evolving CO2 is rapidly removed from the sample by using N2 as a carrier gas. The CO2- enriched N2 stream is led through the solution in the coulometric cell, which absorbs the CO2 and becomes more transparent. The coulometer subsequently electrically titrates the solution back to its original opacity. The required amount of charge is a direct and linear measure of the amount of CO2 absorbed. With knowledge of the sample's volume and density, the concentration of DIC (the sum of CO2-species in the sample) is easily calculated. Total alkalinity is mathematically derived from a fourth-order curve fit along a modified Gran plot of electrode potential versus volume of acid added to an accurately known amount of sample (100 ml), dispensed with an automated thermostated pipette. Titration is performed in a thermostated cell. Samples are brought to the calibration temperature of the pipettes.

Before samples were analyzed the VINDTA was stabilized by running 4 to 6 dummy samples. As soon as the VINDTA was stable a CRM was applied (in duplicate). After a batch of samples was measured a second CRM was applied to check for drift of the VINDTA.

References

Bradshaw, A.L., Brewer, P.G., Shafer, D.K., Williams, R.T., 1981. Measurements of total carbon dioxide and alkalinity by potentiometric titration in the GEOSECS programme. Earth and Planetary Science Letters 55, 99-115.

DOE, 1994. Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water. Version 2, A. G. Dickson and C. Goyet, eds. ORNL/CDIAC-74

Gran, G., 1952. Determination of the equivalence point in potentiometric titrations. Analyst, 77, 661-671.

Johnson, K.M., Wills, K.D., Butler, D.B., Johnson, W.K., Wong, C.S., 1993. Coulometric total carbon dioxide analysis for marine studies: maximizing the performance of an automated gas extraction system and coulometric detector. Marine Chemistry, 44, 167-187.

BODC Data Processing Procedures

Data were submitted to BODC in csv format. Sample metadata provided (Station, lat, lon, depth, bottle number and date) were checked against information held in the database and the cruise report, no discrepancies wee found. The originators quality control flags were changed to BODC quality control flags no other changes were made. The mapping between the originator's channels and BODC parameter codes is detailed in the table below.