Hydrographic Measurements

All CTD pressure, temperature, salinity, and oxygen values for the bottle data tabulations 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 data report for the TUNES-1 Expedition. The 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) or the WHP Special Analysis Center.

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 a Scripps Institution of Oceanography (SIO) Oceanographic Data Facility (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). The salinometer was standardized against Wormley P-114 standard seawater, with at least one fresh vial opened per cast. Accuracy estimates of bottle salinities run at sea are usually better then 0.002 relative to the specified batch of standard. Although laboratory precision of the Autosal can be as small as 0.0002 when running replicate samples under ideal conditions, at sea the expected precision was ~0.001 under normal conditions with a stable laboratory temperature.

Samples were collected for dissolved oxygen analyses soon after the rosette sampler was brought on board and after CFC and helium were drawn. Nominal 100- or 125-mL volume iodine flasks were carefully rinsed with minimal agitation, then filled through the use of a drawing tube, and allowed to overflow for at least two flask volumes. Reagents were added to fix the oxygen before stoppering. The flasks were shaken twice -- immediately after drawing and then again after 20 min -- to ensure thorough dispersion of the Mn(OH)2 precipitate. The samples were analyzed within 4-36 h.

Dissolved oxygen samples were titrated in the volume-calibrated iodine flasks with a 1-mL microburet, using the whole-bottle Winkler titration following the technique of Carpenter (1965) with modifications by Culberson and Williams (1991). Standardizations were performed with 0.01 N potassium iodate solutions prepared from preweighed potassium iodate crystals. Standards were run at the beginning of each session of analyses, which typically included from one to three stations. Several standards were prepared. A comparison was then made to ensure that the results were reproducible and to preclude basing the entire cruise on one standard, which would introduce the possibility of a weighing error. A correction was made for the amount of oxygen added with the reagents. Combined reagent/seawater blanks were determined to account for oxidizing or reducing materials in the reagents and for a nominal level of natural iodate or other oxidizers/reducers in the seawater. These latter corrections are contrary to the recommendations of Culberson and Williams (1991), which call for the determination of reagent blanks in distilled water. ODF standard procedures have since been aligned with those recommended by Culberson and Williams (1991).

Oxygen concentrations were converted from milliliters per liter to micromoles per kilogram using the in situ temperature. Ideally, for whole-bottle titrations, the conversion temperature should be the temperature of the water issuing from the Niskin bottle spigot. The temperature of each sample was measured at the time it was drawn from the bottle; however, these values were not used in the conversion from milliliters per liter to micromoles per kilogram because the software was not available. Aberrant temperatures provided an additional flag, indicating that a bottle may not have tripped properly. Measured sample temperatures from middeep water samples were about 4-7°C warmer than the in situ temperature. Converted oxygen values, if this conversion with the measured sample temperature were made, would be about 0.08% higher for a 6°C warming (or about 0.2 µm/kg for a 250 µm/kg sample).

Analyses of nutrients (i.e., phosphate, silicate, nitrate, and nitrite), reported in micromoles per kilogram, were performed on a Technicon AutoAnalyzer®. The procedures used are described in Atlas et al. (1971). Standardizations were performed with solutions prepared aboard ship from preweighed standards; these solutions were used as working standards before and after each cast (approximately 36 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 was used for phosphate analysis, while stannous chloride reduction of silicomolybdic acid was used for silicate analysis. Nitrite was analyzed by using diazotization and coupling to form dye; nitrate was reduced by copperized cadmium and then analyzed as nitrite.

Sampling for nutrients followed that for the tracer gases, CFCs, helium, tritium, Δ14C, dissolved oxygen, TCO2, and TALK. Samples were drawn into narrow-mouth, screw-capped bottles of high-density polyethylene, which were rinsed twice before filling. The samples may have been refrigerated at 2-6°C for a maximum of 15 h. Nutrients were converted from micromoles per liter to micromoles per kilogram by dividing by sample density which was calculated at an assumed laboratory temperature of 25°C.


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