4.1 Hydrographic Measurements
The ODF CTD/rosette casts were carried out with a 36-bottle rosette sampler of ODF manufacture using General Oceanics pylons. An ODF-modified NBIS Mark 3 CTD, a Benthos altimeter, a SensorMedics oxygen sensor, and a SeaTech transmissometer provided by Texas A&M University were mounted on the rosette frame. Seawater samples were collected in 10-L PVC Niskin and ODF bottles mounted on the rosette frame. A Benthos pinger was mounted separately on the rosette frame; its signal was displayed on the precision depth recorder (PDR) in the ship's laboratory. The rosette/CTD was suspended from a three-conductor electromagnetic cable that provided power to the CTD and relayed the CTD signal to the laboratory.
Each CTD cast extended to within approximately 10 m of the bottom unless the bottom returns from both the pinger and the altimeter were extremely poor. Subsets of CTD data taken at the time of water sample collection were transmitted to the bottle data files immediately after each cast in order to provide pressure and temperature at the sampling depth and to facilitate the examination and quality control of the bottle data as the laboratory analyses were completed.
After each rosette cast was brought on board, water samples were drawn in the following order: CFC-11 and CFC-12, helium-3, oxygen, pCO2, TCO2, and 14C. Tritium, nutrients (silicate, phosphate, nitrate and nitrite), and salinity were drawn next and could be sampled in arbitrary order.
All CTD pressures, temperatures, salinities, and oxygen concentrations for the bottle data tabulations on the rosette casts were obtained by averaging CTD data for a brief interval at the time the bottle was closed on the rosette.
A single ODF-modified Guildline Autosal Model 8400A salinometer (Serial Number 57-396), located in a temperature-controlled laboratory, was used to measure salinities. Analyses and data acquisition were controlled by a small computer through an interface board designed by ODF. The salinometer cell was flushed until successive readings met software criteria, then two successive measurements were made and averaged for a final result.
Salinity samples were analyzed for the rosette casts and the large-volume casts from both the piggyback bottle and the Gerard barrel. Salinity samples were drawn into 200-mL Kimax® high alumina borosilicate bottles, after 3 rinses, and were sealed with custom-made plastic insert thimbles and Nalgene screw caps. This assembly provides very low container dissolution and sample evaporation. If loose inserts were found, they were replaced to ensure an airtight seal. Salinity was determined after sample equilibration to laboratory temperature, usually within 8-36 hours of collection. Salinity was calculated according to the equations of the Practical Salinity Scale of 1978 (UNESCO 1981).
The salinometer was standardized for each cast with IAPSO standard seawater using at least one fresh vial per cast.
The estimated accuracy of bottle salinities run at sea is usually better than 0.002 relative to the particular standard seawater batch used. Although the laboratory precision of the Autosal can be as small as 0.0002 when running replicate samples under ideal conditions, at sea the expected precision is about 0.001 under normal conditions, with a stable lab temperature.
Dissolved oxygen analyses were performed with an SIO-designed automated oxygen titrator using photometric end-point detection based on the absorption of 365-nanometer wavelength ultraviolet light. Thiosulfate was dispensed by a Dosimat 665 buret driver fitted with a 1.0-mL buret. ODF used a whole-bottle Winkler titration following the technique of Carpenter (1965) with modifications by Culberson and Williams (1991), but with higher concentrations of potassium iodate standard (approximately 0.012N) and thiosulfate solution (50 gm/L). Standard solutions prepared from pre-weighed potassium iodate crystals were run at the beginning of each session of analyses, which typically included from one to three stations. Several standards were made up during each cruise and compared to assure that the results were reproducible and to preclude the possibility of a weighing error. Reagent/distilled water blanks were determined to account for oxidizing or reducing materials in the reagents. The auto-titrator generally performed very well. A decrease in voltage output led to changing the UV source lamp during the cruise.
Samples were collected for dissolved oxygen analyses soon after the rosette sampler was brought on board and after CFCs and helium were drawn. Nominal 125-mL volume-calibrated iodine flasks were rinsed twice with minimal agitation, then filled via a drawing tube, and allowed to overflow for at least 3 flask volumes. The sample temperature was measured with a small platinum resistance thermometer embedded in the drawing tube. Reagents were added to fix the oxygen before stoppering. The flasks were shaken twice (immediately after drawing and then again after 20 minutes), to assure thorough dispersion of the MnO(OH)2 precipitate. The samples were analyzed within 4-36 hours of collection. Oxygen data were converted from milliliters per liter to micromoles per kilogram using the in-situ temperature.
Nutrient analyses (phosphate, silicate, nitrate, and nitrite) were performed on an ODF-modified AutoAnalyzer II, generally within a few hours of the cast, although some samples may have been refrigerated at 2-6°C for a maximum of 12 hours. The procedures used are described in Gordon et al. (1992).
Silicate is analyzed using the basic method of Armstrong et al. (1967). Ammonium molybdate is added to a seawater sample to produce silicomolybdic acid which is then reduced to silicomolybdous acid (a blue compound) following the addition of stannous chloride. The sample is passed through a 15-mm flow cell and measured at 820 nanometers. This response is known to be nonlinear at high silicate concentrations; this nonlinearity is included in ODF's software.
A modification of the Armstrong et al. (1967) procedure is used for the analysis of nitrate and nitrite. For nitrate analysis, a seawater sample is passed through a cadmium column where the nitrate is reduced to nitrite. This nitrite is then diazotized with sulfanilamide and coupled with N-(1-naphthyl)-ethylenediamine to form an azo dye. The sample is then passed through a 15-mm flow cell and measured at 540 nanometers. A 50-mm flow cell is required for nitrite. The procedure is the same for the nitrite analysis less the cadmium column.
Phosphate is analyzed using a modification of the Bernhardt and Wilhelms (1967) method. Ammonium molybdate is added to a seawater sample to produce phosphomolybdic acid, which is then reduced to phosphomolybdous acid (a blue compound) following the addition of dihydrazine sulfate. The sample is passed through a 50-mm flow cell and measured at 820 nanometers.
Nutrient samples were drawn into 45-mL high-density polypropylene, narrow mouth, screw-capped centrifuge tubes that were rinsed three times before filling. Standardizations were performed at the beginning and end of each group of analyses (one cast, usually 36 samples) with a set of an intermediate concentration standard prepared for each run from secondary standards. These secondary standards were in turn prepared aboard ship by dilution from dry, pre-weighed standards. Sets of 4 to 6 different concentrations of shipboard standards were analyzed periodically to determine the deviation from linearity as a function of concentration for each nutrient.
Nutrients, reported in micromoles per kilogram, were converted from micromoles per liter by dividing by sample density calculated at zero pressure, in-situ salinity, and an assumed laboratory temperature of 25°C.