Position and depth were manually logged every 10 minutes on the P6 Section. A thermosalinograph (Falmouth Scientific Instruments) was mounted on the bow ~ 3 m below the surface and operated on all legs except the latter stages of Leg P6C. An underway fluorometer was operated on Legs P6E and P6C, until it also failed toward the end of Leg P6C and was not used again. Water samples were collected using a 36-position underwater frame and 10-L sample bottles designed and constructed by the Ocean Data Facility (ODF) at SIO. Modified Neil Brown MkIII CTD instruments mounted on the 36-bottle frame were used for data acquisition. The CTD (Nos. 7, 9, and 10) were supplied by the WHOI Group with No. 10 being used for the bulk of the work. CTDs 7 and 9 were used only very sparingly on Leg P6C when CTD 10 required electronic repairs. On the other legs their use was largely restricted to test stations. Small shifts between the pre- and post-cruise pressure and temperature CTD calibrations were found, but the P6 CTD data have been corrected according to procedures given by Millard et al. (1992), and the CTD salinity data have been empirically corrected to conform to the bottle salinity. The prescribed WOCE sample order was as follows: CFCs, helium, oxygen, CO2, nutrients, tritium, and salinity. Surface currents were measured continually during the cruise with a hull-mounted ADCP, and current profiles were also made during the CTD casts with an ADCP mounted on the rosette frame.
Some problems were experienced with the CTD/rosette systems. The CTD oxygen sensor functioned poorly on Leg P6E particularly in the top several 100 m probably because of cavitation of a specially installed pump used to circulate water past the sensor. This pump was not used on Legs P6C or P6W. Problems with the data acquisition software were noted and corrected on Leg P6E. CTD No. 10 failed on Leg P6C at Station 75, and CTD No 9. was used through Stations 76-85 while CTD 10 was repaired. Careful post-cruise verifications using the complete bottle data sets have been carried out, and the sample pressures and salinity assigned for each sample are to our knowledge correct.
Bottle salinity was measured on every water sample using 2 Guildline Autosal Model 8400A salinometers. The instruments (No. 10 and 11) were furnished by WHOI. The measurements were made in a climate-controlled portable laboratory secured to the deck of the ship. The temperature of the laboratory was kept at 22 ± 1°C. Salinity samples were the last water samples drawn from the rosette. The bottles and caps were rinsed twice and filled to 1/2 inch of the neck to leave an air space for expansion. The samples were thermally equilibrated in the laboratory before measurement (5-6 h). The salinometers were standardized with International Association for the Physical Sciences of the Ocean (IAPSO) Standard Sea Water (Batch P116), and a description of the salinity measurement is given by Knapp et al. (1990). Salinometer 11 was used until June 18 when it began to give intermittently higher results during the standardization. From this point on salinometer 10 was used. The precision of the salinity determination was the mean difference between duplicate salinity samples. For samples taken at less than and greater than 3000 m the precision was 0.0012 (n = 107) and 0.0011 (n = 23), respectively.
Bottle oxygen was measured on 50-mL aliquots of all P6 water samples by a modified Winkler titration technique (Knapp et al. 1990) using a computer-controlled titrator with amperometric end-point detection in a constant-temperature laboratory. Oxygen bottles were rinsed twice with sample water and carefully filled to overflowing to avoid air bubbles. Next the reagents were added (1 mL each of the MnCl2 and NaI-NaOH ), and the bottles shaken. Following thermal equilibration they were titrated with 0.01 sodium thiosulphate. The precision of the oxygen determination calculated from the mean difference and the standard deviation of the mean difference for 121 pairs of duplicate oxygen samples was ± 0.70 to ± 0.87 µmol/kg (n = 98) for depths <3000 m and ± 0.52 to ± 0.39 ±mol/kg (n=23) for depths >3000 m.
Phosphate, nitrate, nitrite, and silicate were determined on every bottle drawn from Stations 3 through 257. The nutrient concentrations were determined on samples collected in high-density polyethylene 30-mL tubes that were directly transferred to an autoanalyzer (AlpKem, Model 300 Rapid Flow Analyzer) according to procedures given by anonymous (1985) and Gordon et al. (1992; 1994). Samples were transferred to a climate-controlled laboratory and were analyzed within a few hours of collection. Standards and reagents were provided by the OSU group, and working standards (i.e., solutions having nitrate, nitrite, phosphate, and silica concentrations similar to those of the Pacific deep and bottom waters) were prepared from stock solutions every 4 to 7 days. Standard and stock solutions were kept refrigerated. Precision was measured by the difference between duplicate samples taken from the same rosette bottle analyzed one after the other (not separated in time) or at the start and end of the run (separated in time). Together the differences between replicate analyses is the short-term precision, which includes short-term instrumental drift as well as random error. The mean standard deviations or short-term precision for the replicate analyses made on the three P6 Legs are: silicic acid ±0.16 to ±0.035 µmol/L; nitrate/nitrate ±0.05 to ±0.01 µmol/L; phosphate ±0.011 to ±0.006 µmol/L. Long-term precision was estimated by comparing "old" working standard solutions made on the previous station with freshly made working standards (i.e., "new" standard solutions made for the current station). The long-term precision for the three P6 Legs are: silicic acid ±0.21 to ±0.028 µmol/L (n = 284); nitrate ±0.087 to ±0.019 µmol/L (n=284); phosphate ±0.015 to ±0 .0006 µmol/L (n = 241); and nitrate ±0.015 to ±0.007 µlmol/L (n = 284).
Problems with the nutrient analyses included nonlinearity for the nitrate/nitrite on Leg P6E through Station 112 on Leg P6C at which time a plumbing error was discovered and corrected. Post-cruise corrections have been applied to the nitrate/nitrite data through Station 112. In addition, the phosphate analysis was lost from Stations 171 through 188 on Leg P6C when noise rendered the phosphate channel unusable. This was corrected in Auckland prior to Leg P6W when the air injection phasing board was replaced.