Hydrographic Measurements

A total of 96 hydrographic stations were occupied during the TUNES-2 Expedition. Stations 124 through 179 (WOCE Section P17S) were located approximately along the 135oW meridian between 6oS and 33oS; and stations 180-220 (Section P16S) were located along the 150oW meridian between 17.5oS and 37.5oS. Figure 2 shows the station locations and numbers. The hydrographic measurements and water sampling were conducted by the staff of the SIO Oceanographic Data Facility (ODF). During the expedition, a 24/12 double-ring 36-bottle CTD/rosette sampler was used for sampling. An ODF-modified Neil Brown Mark III CTD, a Benthos altimeter, and a Sea Tech transmissometer were mounted on the rosette frame. Seawater samples were collected in PVC 10-L Niskin and ODF bottles.

Pressure and temperature: All pressure and temperature values were obtained by averaging CTD data for a brief interval at the time the bottle was closed on the rosette. All reported CTD values were calibrated with reference to the International Temperature Scale of 1990 and processed with the methodology described in the documentation accompanying the final CTD report.

Salinity: Salinity samples were drawn into 200 ml Kimax high alumina borosilicate glass bottles with custom-made plastic insert thimbles and Nalgene screw caps. This provided low container dissolution and sample evaporation. These bottles were rinsed three times before filling, and measurements were made usually within 8 to 36 hours of collection. Salinity was determined on the basis of electrical conductivity measured with 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). For the samples from Stations 124 through 140, the salinometer was calibrated using a batch of Wormley IAPSO standard seawater, P-114, with at least one fresh vial opened per cast. For the samples from Stations 141 through 220, the standard seawater from batch P-108 was used. A single comparison at sea during TUNES-2 yielded a salinity value for P-114 about 0.002 PSS higher than that of P-108, when standardized against P-108. This compares with the estimated precision of about 0.001 PSS of this instrument operated at sea under normal conditions.

Oxygen: Water samples for oxygen analyses were collected shortly after the rosette sampler was brought on board and after the samples for CFC's and helium-3 were drawn. Sampling flasks (100 to 125 ml), whose volumes (with a stopper in place) were calibrated prior to the expedition, were rinsed carefully with minimal agitation, then filled using a drawing tube after allowing to overflow for at least two flask volumes. Reagents were added to fix the oxygen before stoppering. The flasks were shaken immediately after the stoppers were placed to seal them, and then again after 20 minutes, to assure thorough dispersion of the Mn(OH)2 precipitate. The oxygen concentration in these solutions were determined within 4 to 36 hour using the Winkler titration methods of Carpenter (1965) with modifications by Culberson and Williams (1991). The titrator was calibrated with 0.01N potassium iodate standard solutions prepared using pre-weighed potassium iodate crystals. Standards were run at the beginning of each session of analyses, which typically included samples from one to three stations. Several batches of standard solutions were made up and compared to assure that the results were reproducible. A correction was made for the amount of oxygen added with the reagents, and combined reagent/distilled water blanks were determined to account for oxidizing or reducing materials in the reagents.

Oxygen concentrations were converted from milliliter per liter to micromoles per kilogram of seawater using the in-situ temperature. A molar volume at STP of 22.385 liter/mole (Kester, 1975) was used for this purpose. Ideally, the temperature at which a liter of seawater is converted to kilograms should be the temperature of the water when samples were collected in the sampling flasks. However, in-flask temperatures were measured and found to be 4 to 7oC warmer than the in-situ temperatures for mid-depths water samples. Accordingly, the magnitude of errors induced by using about 6oC colder temperature would be about + 0.08% or + 0.2 µ mol/kg.

Nutrients: The concentrations of phosphate, nitrate, nitrite and silicate in seawater were determined using a Technicon AutoAnalyzer based on the procedures described by Hager et al. (1972) and Atlas et al. (1971). The analyzer was calibrated using the standard solutions prepared aboard ship from pre-weighed standards. These solutions were used before and after the analyses for each cast consisting of about 36 samples to correct for instrumental drift. Sets of 4-6 different standard solutions having different concentrations were used periodically to determine the non-linear colorimeter response and the resulting corrections. The phosphate concentration was determined using the hydrazine reduction of phosphomolybdic acid (Bernhardt and Wilhelms, 1967). The silicate concentration was determined using stannous chloride reduction of silicomolybdic acid (Armstrong et al., 1967). The nitrite concentration was determined using diazotization and coupling to form dye; the nitrate concentration was determined as nitrite after nitrate was reduced by copperized cadmium (Armstrong et al., 1967). The precision of measurements has been estimated to be ± 0.1 µ mol/kg for nitrate and silicate and ± 0.01 µ mol/kg for phosphate.

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