Hydrographic Measurements
During the cruises, the responsibility for the hydrographic data was divided between Scripps Institution of Oceanography (SIO) (section A24) and WHOI (sections AR24, A20, and A22). Hence the hydrography description is given in separate sections for SIO and WHOI operations and methods.
SIO/Ocean Data Facility ODF Methods and Instrumentation (Section A24)
The SIO hydrographical procedures are described in detail by Lynne Talley in the chief scientists cruise report for WOCE Section A24, which can be obtained on the WHPO Web site at http://whpo.ucsd.edu/data/onetime/atlantic/a24/index.htm. Navigational data wereas logged automatically at 1-minute intervals on a Sun SPARC station. Underway bathymetry was acquired from the ships SeaBeam system at 5-minute intervals. These data were merged with the navigation data to provide a time-series of underway position, course, speed, and bathymetry, which were used for all station positions, depths, and for bathymetry on vertical sections. A thermosalinograph (Falmouth Scientific Instruments) was mounted on the bow approximately 3 m below the surface for underway salinity.
The hydrographic casts were made with a 36-position 10-L rosette system belonging to the Ocean Data Facility (ODF) at SIO. This unit consisted of a 36-bottle frame, thirty-one 10-L Niskin bottles, an Neil Brown Instrument System (NBIS) Mark III CTD with dual conductivity and temperature sensors, transmissometer, LADCP, altimeter, and pinger. The rosette system was suspended from a three-conductor 0.322-inch electromechanical cable, and power to the CTD and pylon was provided through the cable from the ship. For the first 10 casts, a General Oceanics 1016 36-place pylon was used; thereafter a Sea Bird Electronics (SBE), 36-place pylon, and SBE 33 deck unit were used. The rosette was deployed from the starboard side hangar, and the port side Markey CTD winch was used throughout section A24. At the beginning of each station, the time, position, and bottom depth were logged,; the sensors were powered; and control was transferred to the CTD acquisition and control system in the ships laboratory and the CTD lowered to within 10 m of the bottom. The CTD provided real-time pressure, depth, temperature, salinity (conductivity), oxygen, and density, and these variables were used to select the sampling depths.
At the end of each cast, water samples (full suite) were drawn in the following order: chlorofluorocarbons (CFCs), helium, oxygen, pCO2, TCO2, TALK, tritium, nutrients, and salinity. However, only salinity, oxygen, and nutrients were measured at every station. A log was kept to document the sampling sequence and to note anomalies (e.g., leaks, etc.), and WHP quality flags were assigned to each sample using the analyzed bottle salinities, oxygen, and nutrients. No major problems with the CTD operation were experienced, and the original set of 31 Niskin bottles was used throughout section A24. At the end of the cruise, the CTD and the entire acquisition and control system were packed and shipped back to SIO.
The CTD was calibrated for pressure and temperature at the ODF calibration facility (La Jolla, CaliforniaA) in April 1997, prior to WOCE section A24. At sea, bottle salinity and oxygen were used to calibrate the conductivity and oxygen sensors, respectively, and the temperature calibration was checked with an SBE 35 laboratory-grade reference material. Further details concerning the pre- and post-cruise calibration of the CTD sensors can be found in the cruise report available from WHPO. ADCP and LADCP profiles were made throughout the section using the hull-mounted ADCP system permanently installed on the Knorr and the LADCP mounted vertically inside the rosette frame bottle rings. The quality of the shipboard ADCP to depths of 500 m was good throughout the cruise, as were the LADCP station profiles.
Bottle salinity samples were collected after three rinses in 200-mL Kimax high alumina borosilicate bottles, sealed, and determined after thermal equilibration, usually within 8 to 12 hours of collection. Salinity was determined at 24°C on two Guildline Autosal Model 8400A salinometers located in a temperature-controlled laboratory (21.4 - 24.6°C). The salinometer was standardized with International Association for the Physical Sciences of the Ocean (IAPSO) standard seawater (SSW) batch P-127, using at least one fresh vial per cast. The accuracy of the determination was 0.002 relative to the SSW batch used. Salinity was then calculated for each sample (UNESCO, 1981) and merged into the data base.
Bottle oxygen was determined by filling 125-mL iodine flasks to overflowing (3 x bottle volume) with a draw tube after two bottle rinses. Sample temperature was measured immediately with a thermometer imbedded in the draw tube. The Winkler reagents were added, the flasks stoppered, and shaken upon stoppering and shaken again 20 minutes later to ensure that the dissolved oxygen was completely fixed. Oxygen was determined according to the technique of Carpenter (1965), incorporating the modifications of Culberson et. al (1991) within 4 hours of collection on an SIO-designed automated oxygen titrator using photometric endpoint detection at an ultra-violet wave length of 365 nm. Standards prepared from pre-weighed potassium iodate were run each time the automated titrator was used, and reagent blanks were determined by analyzing distilled water. The final oxygen results have been converted to µmol/kg. Conversion was madedone using the in-situ temperature, not the temperature of the sample from the Niskin bottles, because of due to a software failure. Bottle volumes were precalibrated at SIO. The precision of the analysis calculated from 57 replicate pairs (duplicates drawn from the same Niskin bottle) was ± 0.004 mL/L.
Phosphate, nitrate, nitrite, and silicate were determined on virtually every Niskin bottle closed from stations 1 through 153. The samples were collected in 45-mL high-density polypropylene, narrow-mouth, screw-capped centrifuge tubes whichthat were rinsed with HCL and then rinsed three times with sample before filling. The samples were analyzed on an ODF-modified four-channel Technicon AutoAnalyzer II, usually within 1 hour of the cast, in a temperature-controlled laboratory. If the samples were stored for longer than 1 hour prior to analysis, they were stored at 4°C (no more than 4 hours). The Auto Analyzer instrumentation which incorporates the method of Armstrong, Stearns, and Strickland et al. (1967) for silicate, the method of Armstrong et al. (1967) as modified for nitrate and nitrite, and the method of Bernhardt and Wilhelms (1967) for phosphate is described by Gordon and co-workers (Atlas et al., 1971; Hager et al., 1972; Gordon et al., 1992). Standards were analyzed at the beginning and end of each group of sample analyses with a set of secondary intermediate concentrations prepared by diluting pre-weighed primary standards. The primary standard for silicate was Na2SiF6; and for nitrate, nitrite, and phosphate, KNO3, NaNO2, and KH2PO4 were used, respectively. Chemical purities ranged from 99.97% (NaNO3) to 99.999% (KNO3). Deep seawater was also used as a substandard. Some 3439 nutrient samples were analyzed during section A24.
WHOI Methods and Instrumentations (Sections AR24, A20, and A22)
Unless otherwise stated, procedures remain as in Section above. The underway-Doppler speed log did not function during sampling of section A22, and the hull-mounted transducer had to be removed for repair. Otherwise, navigational and meteorological data wereas logged by the shipboard data acquisition system (dubbed Athena) at 1-minute intervals. These data include heading, time and date, geographic positioning system (GPS), wind speed and direction, sea surface temperature and conductivity, and additional meteorological data. A secondary suite of data calculated or derived from these data was also logged at 1-minute intervals. These files were also the source of event-specific data (e.g., station location). Underway salinity was calibrated with bottle salinity.
For the WHOI sections, the Knorr was outfitted with equipment belonging to WHOI. For details see the chief scientists report for Sections A20 and A22 on the WHPO Web site: http://whpo.ucsd.edu/. The CTD used was a WHOI-modified NBIS Mark-III CTD with oxygen sensor, conductivity cell, LADCP, pressure transducer, temperature sensor, and pinger mounted on a 33-position 10-L Niskin bottle rosette frame. The AR24 section differed from the above A20 and A22 sections and virtually all other CO2 survey work because a 24-bottle rosette was used with 4-L sampling bottles, instead of the usual 10-L Niskin bottles, in order to reduce sampling time at the rosette when in late fall when seas were expected to be rougher. A General Oceanics-1016 pylon was used and was controlled using an SIO-furnished Surface Control Interface, a dedicated PC, and software provided by SIO/ODF. Temperature and pressure calibrations were performed at WHOI before section A20. The CTD was re-calibrated after its return to WHOI in December 1997. For details concerning the pre-cruise, at-sea, and post-cruise CTD calibrations, see the cruise report and Millard and Yang (1993).
Oxygen, nutrients, and salts were analyzed from virtually every Niskin bottle closed during the cruise. Bottle salinity samples were collected after two rinses in 8-ounce glass bottles. Then they were thermally equilibrated to 22°C and analyzed on a Guildline Autosal Model 8400B salinometer. The salinometer was standardized once a day using IAPSO SSW Batch P-131 (dated October 10, 1996). The accuracy was ± 0.002.
Bottle oxygen was determined by filling 15-mL brown glass tincture bottles. Oxygen was determined using a modified Winkler technique similar to that described by Strickland and Parsons (1972). The titration is automated using a PC controller and a Metrohm Model 665 Dosimat buret. The endpoint is determined amperometrically using a dual -plate platinum electrode, with a resolution of better than 0.001mL. The accuracy of the method is 0.02 mL/L with a standard deviation of replicate samples of ± 0.005. Standardization of the thiosulphate titrant was madedone daily. No problems were noted during the cruises. This technique is described further by Knapp, Stalcup, and Stanleyet al. (1990).
The nutrient samples were collected in 30-mL high-density polypropylene bottles whichthat were loaded directly into the autosampler tray. The bottles were soaked in 10% HCl every other day, and rinsed three times with sample before filling. The samples were analyzed on the same SIO/ODF-modified four-channel Technicon AutoAnalyzer II used on A24, except that an Alpkem Model 303 autosampler was substituted for the ODF autosampler. The analytical methods are described in Gordon et al. (1994).
