Hydrographic Measurements

The hydrographic measurements and water sampling were conducted by the staff of the SIO Oceanographic Data Facility (ODF). Water samples were collected through the use of Niskin 10-L sampling bottles mounted on an ODF-constructed 24-bottle rosette sampler. The rosette was equipped with an ODF-modified NBIS Mark IIIb CTD for in situ measurements of conductivity, temperature, pressure, and dissolved oxygen.

Salinity samples were drawn into 200-mL Kimax high-alumina borosilicate glass bottles with custom-made plastic insert thimbles and Nalgene screw caps, which provided low container dissolution and sample evaporation. These bottles were rinsed three times before filling, and measurements were usually made within 8-36 h after collection. Salinity was determined on the basis of electrical conductivity measured by an ODF-modified Guildline Autosal Model 8400A salinometer, and the values were obtained according to the equations of the Practical Salinity Scale of 1978 (UNESCO 1981).

Water samples for oxygen analyses were collected shortly after the rosette sampler was brought on board and after the samples for CFCs and helium were drawn. Sampling flasks (100-125 mL), calibrated before the expedition, were carefully rinsed and then filled using a drawing tube (after being allowed to overflow for at least two flask volumes). Reagents were added to fix the oxygen before the flasks were sealed with stoppers. The flasks were shaken immediately after being sealed and again after 20 min to ensure thorough dispersion of the manganous hydroxide [Mn(OH)2] precipitate. The oxygen concentration in these solutions was determined within 4-36 h using the Winkler titration methods of Carpenter (1965) with modifications by Culberson and Williams (1991). The titrator was calibrated with 0.01 N potassium iodate standard solutions prepared using preweighed potassium iodate crystals. Oxygen concentrations were converted from milliliters per liter to micromoles per kilogram of seawater using the in situ temperature. A molar volume (at standard temperature and pressure) of 22.3914 L/mol (Kester 1975) was used for this purpose.

Nutrient analyses were performed by analysts from SIO using a Technicon AutoAnalyzer II provided by ODF. The procedures used are described in Gordon et al. (1992). Standardization was performed with solutions prepared aboard the ship from preweighed standards. These solutions were used as working standards before and after each cast (~24 samples) to correct for instrumental drift during analyses. Sets of 4-6 different concentrations of shipboard standards were analyzed periodically to determine the linearity of colorimetric response and the resulting correction factors. Hydrazine reduction of phosphomolybdic acid, as described by Bernhardt and Wilhelms (1967), was used for phosphate analysis, while stannous chloride reduction of silicomolybdic acid was used for silicate analysis. Nitrite was analyzed by use of diazotization and coupling to form dye. Nitrate was reduced by copperized cadmium and then analyzed as nitrite. The last three analyses used the methods of Armstrong et al. (1967).

A full cruise report, which includes details about processing the hydrographic data, and the final CTD data are available from the WOCE Hydrographic Programme (WHP) Office (WHPO) at SIO or the WHP Special Analysis Center in Germany.

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