Hydrographic Measurements

During the survey, responsibility for hydrographic and bottle data was divided between ODF and WHOI. Each of these groups uses or may use different procedures. Hence, the hydrographic measurements are described in separate sections. Because the greater number of the cruises were made under the auspices of SIO/ODF, the bulk of the methods description is provided in SIO/ODF Methods and Instrumentation. Information specific to WHOI is given in WHOI Methods and Instrumentations; in this section however, the discussion is limited to significant differences between the SIO/ODF and WHOI operations or methods. Unless otherwise stated in WHOI Methods and Instrumentations, material presented in SIO/ODF Methods and Instrumentation applies to all cruises. Underway Measurements contains a brief description of the underway measurements common to all cruises.

SIO/ODF Methods and Instrumentation

Hydrographic measurements consisted of salinity, dissolved oxygen, and nutrient (nitrite, nitrate, phosphate, and silicate) samples collected from Niskin bottles filled during CTD/rosette casts, and temperature, pressure, salinity, and dissolved oxygen from the CTD. At 5- to 40-nm intervals, depending on the topography, hydrographic casts were made to within 5-20 m of the bottom with a 36-bottle Rosette frame belonging to ODF. This unit consisted of a 36-bottle frame, thirty six 10-L bottles, and a 1016 General Oceanics (GO) 36-place pylon. The GO pylon was used in conjunction with an ODF-built deck unit and power supply. The underwater components comprising the CTD included an ODF-modified Neil Brown Instrument Systems (NBIS) Mark III CTD with conductivity, pressure, oxygen, and temperature sensors. The underwater package also consisted of a SeaTech transmissometer, an LADCP, a Sensormedics dissolved oxygen sensor, a Falmouth Scientific Instruments (FSI) secondary PRT sensor, a Benthos altimeter, and a Benthos pinger. The CTD was mounted horizontally along the bottom of the frame, while the LADCP was vertically mounted inside the bottle rings. The system was suspended from and powered by a three- conductor 0.322-in. electromechanical cable. The Rosette was deployed from the starboard side using either the port side Markey CTD or the starboard side Almon Johnson winch. Standard CTD practices (i.e., soaking the conductivity and O₂ sensors in distilled water between casts and protecting the sensors against sunlight and wind by storing the rosette in the hanger between casts) were observed throughout the cruises. Regular CTD maintenance included the replacement of O-rings when needed, bottle inspections, and a regular cleaning of the transmissometer windows. At the beginning of each station the time, position, and bottom depth were logged. The CTD sensors were powered and control was transferred to the CTD acquisition and control system in the ship's laboratory. The CTD was lowered to within 10 m of the bottom if bottom returns were adequate. Continuous profiles of horizontal velocity from the sea surface to the bottom were made for most CTD/rosette casts using the LADCP.

The CTD's control and acquisition system displayed real-time data [pressure, depth, tem- perature, salinity (conductivity), oxygen, and density] on the video display of a SunSPARC LX computer. A video recorder was provided for real-time analog backup. The Sun computer system included a color display, a keyboard, a trackball, a 2.5-GB disk, 18 RS-232 ports, and an 8-mm cartridge tape. Two additional Sun systems were networked for display, backup, and processing. Two HP 1200 C color ink-jet printers provided hard copy. The ODF data acquisition software not only acquired the CTD data but also processed it so that the real-time data included preliminary sensor corrections and calibration models for pressure, temperature, and conductivity. The sampling depths were selected using down-cast data. Bottles were tripped on the up-cast. Bottles on the rosette were identified with a serial number and the pylon tripping sequence, 1-36, where the first (deepest) bottle tripped was no. 1. For shallow-depth stations, fewer than 36 bottles were closed.

After the CTD was on deck, the acquisition system, the CTD, the pylon, and video recording were turned off and the sensor protective measures were completed before sampling began. If a full suite of samples was drawn, the sampling order was CFCs, ³He, O₂, TCO₂, TALK, ¹⁴C, ³H, nutrients, and salinity. Only salinity, O₂, and nutrients were measured at every station. A deck log was kept to document the sampling sequence and to note anomalies (e.g., status of bottle valves, leaks, etc.). One member of the sampling crew was designated the "sample cop," and it was his or her responsibility to maintain this log and to ensure that the sampling order was followed. Oxygen sampling included measurement of the temperature, which proved useful for determining leaking or mis-tripped bottles. Following the cruises, WHP quality flags were assigned according to the WOCE Operations Manual (Joyce and Corry 1994) to each measured quantity.

The principal ODF CTD (no. 1) was calibrated for pressure and temperature at the ODF Calibration Facility (La Jolla, Calif.) in December 1994 prior to the five consecutive WOCE Indian Ocean sections beginning with I9N and ending with I7N. The CTD was also calibrated postcruise in September 1995 prior to the I10 cruise. Pre- and postcruise laboratory calibrations were used to generate tables of corrections, which were applied by the CTD data. At sea, bottle salinity and oxygen data were to calibrate or check the CTD sensors. Additional details concerning calibration and the CTD data processing can be obtained from the chief scientists' cruise reports at the WHPO Web site: http://whpo.uscd.edu/.

Bottle salinity samples were collected in 200-mL Kimax high alumina borosilicate bottles, sealed with custom-made plastic insert thimbles and Nalgene screw caps. Salinity was determined after equilibration in a temperature-controlled laboratory, usually within 8-20 h of collection. Salinity was measured with two ODF-modified Guildline Autosal Model 8400A salinometers, normally at 21 or 24°C, depending on the prevailing temperature of the salinometer laboratory. The salinometers included interfaces for computer-aided measurements (e.g., acquiring the measurements, checking for consistency, logging results, and prompting the analyst). The salinometers were standardized with International Association for the Physical Sciences of the Ocean (IAPSO) Standard Seawater (SSW) Batches P-124, P-126, or P-128 using at least one fresh vial per cast (usually 36 samples). The accuracy of the determination was normally 0.002 relative to the SSW batch used. PSS-78 was then calculated for each sample (UNESCO 1981). On some stations (e.g., on Section I5EI8N), bottle salinity exhibited small offsets (0.002-0.004) compared to the corresponding CTD results and bottle salinity from nearby stations, and corrections of this magnitude need to be applied to the bottle salinity. Errors of this magnitude have no practical effect on the calculated TCO₂ or TALK values. Hence, bottle salinity is sufficiently accurate to express inorganic carbon results in µmol/kg.

Bottle oxygen was determined by rinsing 125-mL iodine flasks twice and then filling to overflowing (3x-bottle volume) with a draw tube. Sample temperature was measured immediately with a thermometer imbedded in the draw tube. The Winkler reagents were added; and the flask was stoppered, shaken, and then shaken again 20 min later to ensure that the dissolved O₂ was completely fixed. Oxygen was determined within 4 h of collection using a whole-bottle modified Winkler titration following the technique of Carpenter (1965) and incorporating the modifications of Culberson et al. (1991) on an SIO/ODF-designed automated oxygen titrator. A Dosimat 665 burette driver fitted with a 1.0-mL burette was used to dispense thiosulfate solution (50 g/L). Standards prepared from preweighed potassium iodate (0.012N) were run each time the automated titrator was used, and reagent blanks were determined by analyzing distilled water. The final oxygen results were converted to µmol/kg using the in situ temperature. Bottle volumes were precalibrated at SIO. Laboratory temperature stability during the sections was considered poor, varying from 22 to 28°C over short time periods; and therefore, portable fans were used by ODF analysts to maintain temperature.

Phosphate, nitrate, nitrite, and silicate samples were collected in 45-mL high-density polypropylene, narrow-mouth, screw-capped centrifuge tubes which were cleaned with 10% hydrochloric acid (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 h of the cast, in a temperature-controlled laboratory. If the samples were stored for longer than 1 h prior to analysis, they were stored at 2-6°C (for no more than 4 h). The AutoAnalyzer incorporates the method of Armstrong, Stearns, and Strickland (1967) for silicate, this same method as modified for nitrate and nitrite, and the method of Bernhardt and Wilhelms (1967) for phosphate. The last method is described by Gordon and coworkers (Atlas et al. 1971; Hager et al. 1972; and 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 preweighed primary standards. Replicates were also drawn at each station for measurement of short-term precision. For reagent blanks, deionized water (DIW) from a Barnstead Nanopure deionizer fed from the ship's potable water supply was analyzed. An aliquot of deep seawater was run with each set of samples as a substandard. The primary standard for silicate was Na₂SiF₆; and for nitrate, nitrite, and phosphate the standards were KNO₃, NaNO₂, and KH₂PO₄, respectively. Chemical purity ranged from 99.97% (NaNO₂) to 99.999% (KNO₃).

Most hydrographic data sets met or exceeded the WHP requirements. Some exceptions for silicate were noted when differences between overlapping stations on I3 (Station 548) and I4I5W (Stations 705 and 574) approached 3%; these silicate data (Stations 702-707) were corrected by adding 3% to the original results. Instrument problems also caused difficulties for the nitrite and silicate analyses on many of the I2 cruise stations. Silicate problems were noted at some 30% of these stations, with errors typically being on the order of 2-4%. This required considerable post- cruise evaluation and workup before the desired between-station precision for deep water values of 1% was attained. However, users of the I2 silicate data are urged to use caution or to contact the analysts for assistance. Because of the difficulties with the nutrient analyses on the I2 cruise, the post-cruise I2 precision is given in Table 7 as a "worst case" for comparison with the WHP standards shown in Table 6. Short-term precision is the absolute mean difference between replicates analyzed within a sample run; the standard deviation of the differences is also shown. The authors know of no remaining CTD problems, that would affect the quality of the carbonate system data.

WHOI Methods and Instrumentations

Unless otherwise stated procedures are as decribed in SIO/ODF Methods and Instrumentation. For the hydrographic work on I8SI9S, I1, and I2, the R/V Knorr was outfitted with equipment belonging to both WHOI and SIO/ODF. For the I8SI9S section a NBIS CTD was used. For I1, four CTDs were available. The primary sensors were two new FSI CTDs belonging to WHOI with a Sensormedics oxygen sensors, a titanium pressure transducer, and a temperature monitor. The secondary sensors were two NBIS Mark-III CTDs (WHOI Nos. 9 and 12) also with a Sensormedics oxygen sensor, a titanium pressure transducer, and a temperature monitor. The MKIII CTDs experienced failures early during I1 (Stations 858 and 864), and the bulk of the hydrography was carried out using the FSI (Nos. 1338 and 1344) CTDs . Usually, the frame was set up with the two CTDs one configured to send data up the wire and one configured to record data internally. Electrical modifications had to be made to the CTDs and the deck controllers before CTD data dropouts were eliminated and the confirmation of bottle closure from the pylon was restored.

For the CTDs, a FSI DT-1050 deck unit was initially used to demodulate the data, but this unit was replaced for most of the cruise with an EG&G MK-III deck unit. These units fed serial data to two personal computers (PCs) running EG&G CTD acquisition software, with one displaying graphical output and the other a running data listing. After each station, the CTD data were forwarded to another set of PCs running EG&G postprocessing and software modified by WHOI (Millard and Yang 1993) in which the data were centered into 2 dbar bins for data quality control, which included fitting to bottle salinity and oxygen results.

The CTDs were calibrated before and after the cruise for temperature and pressure at WHOI by M. Swartz and M. Plueddemann. Both calibrations were consistent, but the data set for I1 was considered to be only of fair quality because noise levels in the data set are somewhat larger than typical for other CTDs. For example, this data set has a salt noise level of 0.002 which is 2 times larger than the norm. Residuals between the bottle and profile data range from 0.001 to 0.004. For a detailed discussion of the CTD calibration and problems experienced at sea during I1, consult the chief scientist's cruise report on the WHPO Web site.

For I2, WHOI CTD No. 9, a WHOI-modified NBIS MK-IIIb, was used. The CTD incorporated a Sensormedics oxygen sensor, titanium pressure transducer, and temperature sensor, which were calibrated in November 1995 immediately before the cruise. On most stations, one of the FSI CTDs was used in the memory mode and downloaded after station sampling to provide independent or backup CTD traces. An FSI Ocean Temperature Module was also attached to the MK-III and CTDs. The Mark-III CTD data were acquired using an NBIS Mark-III deck unit/display that provided demodulated data to two PCs, as described for the Section I1 cruise. A PC was also devoted to recovering the data from the FSI CTDs. Post-cruise calibration, including dunk tests of the CTDs, was completed in April and May of 1996 in the WHOI calibration laboratory. The procedure of Millard and Yang (1993) was used to correct the pressure temperature sensor calibration post-cruise to eliminate down/up pressure historesis. Multiple regression fits of the CTD data to the bottle data were used to calibrate the oxygen and conductivity sensors. See the chief scientist's report on the WHPO Web site for further details.

Bottle salinity samples were collected in 200-mL glass bottles with removable polyethylene inserts and caps. Then they were removed to a temperature-controlled van at 23°C and analyzed on a Guildline Autosal Model 8400B salinometer (WHOI No. 11). The salinometer was standardized once a day using IAPSO SSW (128, dated July 18, 1995). The accuracy was ~0.002. A complete description of the WHOI measurement techniques is given by Knapp, Stalcup, and Stanley (1990).

Bottle oxygen was determined according to procedures given by Knapp, Stalcup, and Stanley (1990). WHOI used a modified Winkler technique similar to that described by Strickland and Parsons (1972). The oxygen reagents and bi-iodate standard were prepared at WHOI in August 1994. There was no evidence that the reagents or standard deteriorated during the 17 months they were aboard the Knorr. Standardization of the thiosulphate titrant was made daily. The accuracy of the method was 0.5%, or approximately 1.0 µmol/kg. The nutrients were analyzed as described in SIO/ODF Methods (see also Gordon et al. 1994).

Underway Measurements

Navigational data (heading, speed, time, date, and position) were acquired from the ship's Magnavox MX global positioning system (GPS) receiver via RS-232 and logged automatically at 1-min intervals on a SunSPARC station. Underway bathymetry was logged manually at 5-min intervals from the hull-mounted 12-kHz echo sounder and a Raytheon recorder corrected according to methods described by Carter (1980). These data were merged with the navigation data to provide a time-series of underway position, course, speed, and bathymetry data that were used for all station positions, depths, and vertical sections. The Improved METeorology (IMET) sensors logged meteorological data which included air temperature, barometric pressure, relative humidity, sea surface temperature, and wind speed and direction at 1-min intervals. Underway shipboard measurements were made throughout the work to document the horizontal velocity structure along the cruise tracks using a 150-kHz hull-mounted acoustic Doppler current profiler (ADCP) manufactured by RD Instruments. The ADCP was mounted at a depth of 5 m below the sea surface.

Underway chemical measurements in water and air included salinity, pCO₂ (PU and SIO), pN₂O (SIO), and CH₄ (SIO). Two different systems were used for pCO₂; the PU group used a rotating disk equilibrator and infrared detector, while the Scripps group used a shower type equilibrator and gas chromatograph for the detection of CO₂. The pCO₂ measurements, including a comparison of the shower and disk equilibrator results, were described by Sabine and Key (1998). A thermosalinograph (manufactured at FSI) was mounted on the bow approximately 3 m below the surface for underway salinity, which was calibrated against surface CTD and bottle salinity values after the cruise (Sabine and Key (1998)). The CFC groups periodically analyzed air for CFCs using sampling lines from the bow and stern of the ship.