Description of Variables and Methods

The data file met18.dat (File 6) (see descriptions of data files in Part 2) in this numeric data package (NDP) contains the following variables: station numbers; cast numbers; sample numbers; bottle numbers; CTD pressures, temperatures, and salinities; reversing thermometer readings; potential temperatures; bottle salinities; concentrations of dissolved oxygen, silicate, nitrate, nitrite, and phosphate; TCO₂ and TALK concentrations; and quality flags. The station inventory file met18sta.inv (File 5) contains expedition codes, section numbers, station numbers, cast numbers, sampling dates (i.e., month, day, year), sampling times, latitude, longitude, and bottom depth for each station. The data file uwpco2.dat (File 7) contains sampling dates (i.e., day, month, year), sampling times, latitude, longitude, sea surface salinity, sea surface temperature, and underway pCO₂ measurements.

Water samples were collected in 24 General Oceanics 10-L Niskin bottles mounted on a Neil Brown Mark III CTD instrument (S/N NB3) provided by IFMK. Data were acquired at a rate of 32 ms/cycle by using Oceansoft Rev. 3.1. Further details are given by Meincke (1993), and additional data concerning postcruise and precruise laboratory calibrations of the CTD temperature, pressure, conductivity, and oxygen sensors may be found in Siedler and Zenk (1992) and Ruhsam (1994). ADCP measurements to a depth of 300 m were made nearly continuously (with some breaks for rough weather and minor computer malfunctions) from September 2 to 22 with a hull-mounted system from RD Instruments (San Diego) that used a pulse frequency of 150 kHz.

The rosette systems used with the CTD on this cruise experienced various mechanical and electrical problems such that tripping failures were not uncommon especially at stations 596-613. Repeated checks on board and several careful verifications with the complete bottle data sets were carried out, and the current pressures for each sample are considered correct by the responsible personnel. Reversing thermometers, both electronic (SIS, Kiel) and mechanical (Gohla Precision, Kiel), were also read at the completion of each cast. The processing and quality control of CTD and bottle data performed at BSH followed the guidelines published in the WOCE Operations Manual (WHPO 91-1, 1991). Salinity corrections were made by using bottle salinities measured 1-2 days after collection and determined on a Guildline Autosal model 8400A, which was standardized at each station with reference water (batch P112). Because of temporal conductivity sensor shifts, separate corrections were applied for stations 558-566, 567-602, and 603-622. The final salinity data are expected to be accurate to ± 0.002 on the Practical Salinity Scale (PSS). Bottle oxygen was determined by Winkler titration following the techniques of Carpenter (1965) and Culberson and Will (1991), by using standards and blanks run in seawater. Subsequently, all Winkler results were recalculated and verified by staff of Oceanographic Data Facility at SIO. The concentrations of nitrate, nitrite, phosphate, and silicate dissolved in seawater were determined for samples collected in high-density polyethylene screw-capped bottles by using a Technicon Autoanalyzer according to procedures given in Hager et al. (1972) and Atlas et al. (1971) and by using the spectrophotometric methods of Armstrong et al. (1967) and Bernhardt and Wilhelms (1967). The analyses were completed within 24 h of sampling, including storage at 6^oC for no more than 15 h. Preweighed standards were used to prepare the working standards on board ship.

The TCO₂ concentration was determined by using two SOMMAs described and designed by K. M. Johnson and coworkers (Johnson et al. 1985, 1987; Johnson and Wallace 1992). Along with 158 duplicates, 583 individual samples (total analyzed = 741) from 33 stations (Fig. 2) were collected in 300-mL, precombusted (450^oC for 24 h) BOD bottles and immediately poisoned with HgCl₂, according to DOE's Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water (DOE 1994). Before analysis the BOD bottles were kept in darkness in a cold room until thermally equilibrated to the analytical temperature. Dr. Andrew Dickson of SIO supplied 61 CRMs (DOE 1994), which were also analyzed (37 on BNL I and 24 on BNL II). The CRMs were from Batch 7 (B7), which was a filtered sterile salt solution (S = 37.12) spiked with Na₂CO₃, and analyzed for TCO₂ by vacuum extraction and manometry in the laboratory of C. D. Keeling at SIO. The certified TCO₂ value was 1926.41 ± 0.82 µ mol/kg (n = 13).

Seawater introduced from an automated to deliver pipette into a stripping chamber was acidified, and the resultant CO₂, after drying, was coulometrically titrated on a model 5011 UIC coulometer. In the coulometer cell, the hydroxyethylcarbamic acid that formed from the reaction of CO₂ and ethanolamine was titrated coulometrically (electrolytic generation of OH^-) with photometric end point detection. The product of the time and the current passed through the cell during the titration was related by Faraday's Constant to the number of moles of OH^- generated and thus to the moles of CO₂ that reacted with ethanolamine to form the acid. When possible the SOMMA-coulometer systems were calibrated with pure CO₂ through the use of hardware consisting of an eight-port gas sampling Valve (GSV) with two sample loops connected to a source of pure CO₂ through an isolation valve with the vent side of the GSV plumbed to a barometer. When a gas loop was filled with CO₂, the mass (moles) of CO₂ contained therein was calculated by dividing the loop volume (V) by the Molar Volume of CO₂ at ambient (T) and (P). The molar volume of CO₂ [V(CO₂)] was calculated iteratively from an expression using the instantaneous barometric pressure (P), loop temperature (T), gas constant (R), and the first virial coefficient B(T) for pure CO₂:

The ratio of the calculated mass to the mass determined coulometrically was the gas calibration factor (CALFAC) used to correct the subsequent titrations for small departures from 100% theoretical response (DOE 1994). The volume of the loops was determined gravimetrically with deionized water by the method of Wilke et al. (1993). When possible the standard operating procedure was to make gas calibrations daily or for each new titration cell used (normally one cell per day).

Before the cruise, the to deliver volume (TDV) of the SOMMA sample pipette was determined (calibrated) gravimetrically at 20^oC with milli-Q deionized water, which had been degassed with Helium. The thermostatted sample pipette was filled with water at the same temperature, and then discharged into preweighed 50-mL serum bottles which were reweighed on a model R300S (Sartorius, G ttingen, Germany) balance. The apparent weight (g) of water collected (W_air) was corrected to the mass in vacuo (M_vac) from the following equation:

where a_v is the coefficient of volumetric expansion for pyrex-type glass (1 x 10^-5o C^-1), T₂ is the measurement temperature, and T₁ is the calibration temperature. The corresponding results for the BNL II pipette were 29.6954 and 29.6925 mL, respectively. During the cruise, eight TDV samples were collected at 10.2^oC from the BNL I pipette and sealed for reweighing. The TDV from these weighings was 28.6845 ± 0.0058 mL (0.02%), which differed from the calculated TDV of 28.7080 mL by -0.0235 mL, or -0.082%. For the BNL II pipette, 11 samples were taken at 10.2^oC which gave a TDV of the 29.6712 ± 0.0065 mL (0.02%), which differed from the calculated TDV of 29.6925 mL by -0.0213 mL, or -0.072%. Because the original laboratory calibration took place at 20^oC, and all of the analytical work aboard ship was done at 10.2^oC ± 0.3^oC we have used the latter (shipboard) results for TDV to calculate the TCO₂ values (i.e., for BNL I, TDV = 28.6845 mL at 10.2^oC; for BNL II, TDV = 29.6712 mL at 10.2^oC). These data confirmed the current practice of ensuring identical calibration and analytical temperatures because it appeared that theoretical correction for glass expansion was not adequate to describe the TDV at temperatures significantly different from the calibration temperature (see also DOE 1994).

An IBM-compatible personal computer with two RS232 serial ports, one 24-line digital input/output port, and one analog-to-digital port was used to control the coulometer, barometer, solid state control relays, and temperature sensors, respectively. The temperature sensors (model LM34CH, National Semiconductor, Santa Clara, California), with a voltage output of 10 mV/^oF built into the SOMMA, were calibrated against thermistors certified to 0.01^oC (PN CSP60BT103M, Thermometrics, Edison, New Jersey) by using a certified mercury thermometer as a secondary standard. These sensors monitored the pipette, gas sample loop, and the coulometer cell temperatures. The barometer, model 216B-101 Digiquartz Transducer (Paroscientific, Inc., Redmond, Washington), was factory-calibrated for pressures between 11.5 and 16.0 psia. The SOMMA software was written in GWBASIC Version 3.20 (Microsoft Corp., Redmond, Washington), and the instrument was driven from the computer.

The analytical method for determination of TCO₂ concentration in seawater used during R/V Meteor Cruise 18/1 differed from the technique described in an earlier data report (Johnson et al. 1995) for R/V Meteor Cruise 15/3 (March 1991). During Cruise 18/1 an electronic calibration procedure was used to check the theoretical response of the coulometers's voltage to frequency converter (VFC) as described in Johnson et al. (1993) and DOE (1994). At least two levels of current (usually 50 and 2 mA) were passed through an independent and very precisely known resistance (R) for a fixed time. The voltage (V) across the resistance was continuously measured, and the instantaneous current (I) across the resistance was calculated from Ohm's law and integrated over the calibration time. Then the number of pulses (counts) accumulated by the VFC during this time was compared with the theoretical number computed from the factory calibration of the VFC [frequency = 105 pulses (counts) generated per second at 200 mA] and the measured current. If the VFC was perfectly calibrated, electronic calibration yielded a straight line passing through the origin (intercept = 0) with a slope of 1. Calibrations and titrations were done with the coulometer in the counts mode (the total charge passed during a titration was displayed as the number of counts accumulated by the VFC). From the factory calibration of the VFC and the value of the Faraday (96489 Coulomb/mol), a scaling factor of 4.82445 x 10³ counts/µ mol was derived, and the theoretical number micromoles of carbon titrated (M) was determined by the following equation:

This was the first cruise during which two SOMMA systems were used side-by-side to analyze samples from the same profile and measurements of system precision and bias were made in addition to the CRM analyses. The system precision data are given in Table 2. For these data, "within-sample" precision was the average difference between two replicates analyzed from the same sample bottle, "between-sample" precision was the average difference between duplicate sample bottles taken from the same Niskin bottle, "between-Niskin" precision was the average difference between single sample bottles taken from two Niskin bottles closed at the same depth, and S_p² was the pooled standard deviation calculated from multiple "between-sample" replicates (n > 2; stations 557, 581, and 608) analyzed on the same instrument (instrument specific) or from replicates of the same sample analyzed on both instruments (method specific). The S_p² was the square root of the pooled variance, according to Youden (1951):

(7)

where K is the number of samples analyzed and are the degrees of freedom for the calculation.

^aThe mean precision is given as the mean of the absolute differences between two duplicates analyzed on the same instrument i.e., where n is the number of comparisons between duplicate analyses x₁ and x₂. See text for explanation of S_p².

^binstrument specific.

^cmethod specific.

For the instrument specific S_p², K is the number of samples where more than two replicates were analyzed on the same instrument, and n is the total number of replicates analyzed from K samples. The method specific S_p² was calculated from 34 samples (K) for which at least one replicate was analyzed on each instrument [93 replicates (n) analyzed between the two instruments]. If more than one replicate was analyzed on the same instrument, the mean was used to calculate S_p² according to the equation from Youden (1951), (Eq. 7). Thus, for the method specific calculation n_j = K x 2 (68 instead of 93). This treatment reduces the degrees of freedom (n_j - K) to 34 from 59 (93 - K) and yields the most conservative estimate of precision for a single measurement, irrespective of the instrument it was made on. Overall sample precision (method specific S_p²) was ± 1.65 µ mol/kg which agreed very well with the precision of the CRM analyses measured on both instruments (± 1.58 µ mol/kg, n = 61, Table 1). Note that BNL I, as a rule, gave slightly better precision than BNL II and that the other precision estimates were consistently better than S_p². However, the higher value of ± 1.65 µ mol/kg was considered to be the most conservative estimate of analytical precision because it includes all sources of error random and systematic encountered over several days.

System bias was also checked by comparing the calibration station (station 581) samples from a depth of ~2033 m on both instruments over a period of 3-5 days. These date are shown in Table 3. For BNL I, the mean result was 2159.07 ± 0.61 µ mol/kg (n = 5, analyzed between September 12 and 16), and the corresponding result for BNL II was 2158.26 ± 1.18 µ mol/kg (n = 12, analyzed between September 11 and 13). The absolute value of the difference was 0.81 µ mol/kg with BNL I giving a slightly higher result. The mean difference and the absolute value of the mean difference between duplicate analyses for the 34 samples used for the calculation of the method specific S_p² in Table 2 are also shown in Table 3.

As a final estimate of data quality, duplicate samples from seven Niskin bottles at five stations were collected for later shore-based reference analyses of TCO₂ by vacuum extraction/manometry performed in the laboratory of Dr. Charles Keeling at SIO. The results are given in Table 4, in which the BNL data are compared with the SIO results (Guenther et al. 1994). All samples except the shallow sample from station 580 are clearly consistent with our estimate of accuracy and precision, given previously. Temperature sensors were not included in the shipping crates (as is now standard operating procedure), so the temperature history of these samples between cold storage aboard ship and their arrival at SIO was not known. Table 4 supersedes Tables 3e and 5e from Guenther et al. (1994).

^a The difference between the ship s CTD sample salinity and the salinity measured at SIO.
^b Calibration station. The BNL result is the mean of 17 analyses on the two systems between September 11 and 16.

Note that six of the seven differences were within the analytical precision of the methods, and salinities agreed to within 0.005, which ruled out evaporative losses.

Total Alkalinity samples were collected in 500-mL bottles with the same precautions as for TCO₂. Samples were stored in the dark at 4^oC and analyzed within 24 h. Samples were transferred into a closed titration cell with a volume of ~120 mL and titrated at 25^oC ± 0.1^oC with 0.1 M HCl containing 0.6 M NaCl. The titration cell was based on the systems described by Bradshaw and Brewer (1988) and Millero et al. (1993). The potential was followed with an electrode pair consisting of a ROSS (Orion Inc.) glass pH electrode and a ROSS AgCl reference electrode connected to a high-precision digital voltmeter. The titration was controlled by a computer, which waited for stable emf-readings before adding the next acid increment. The titration curve was analyzed with a modified GRAN-plot method described by Stoll et al. (1993), using the carbonic acid constants of Goyet and Poisson (1989), and taking into account the silicate and phosphate concentrations of the sample to obtain the titration alkalinity. The precision of the method was ± 2.0 µ mol/kg, which was determined by replicate analysis of samples. Standardization was accomplished with NaCO₃ standards in NaCl solutions corrected for the blank arising from impurities in the salt.

Underway pCO₂ was measured by using the method of Schneider et al. (1992). Surface seawater was continuously pumped along the cruise track (Fig. 4) at a rate of 200-300 mL/min into a glass equilibrator with a volume of ~300 mL. The seawater was equilibrated with continuously circulating air entering the bottom of the equilibrator through a frit from a closed- loop system. This system included a heat exchanger to keep the air at sample temperature, a filter and water trap, and an infrared (IR) analyzer (Siemens, Ultrmat 5F) for the determination of the CO₂ content of the equilibrated air. The infrared analyzer and equilibrator temperature sensor were connected to a computer or to an analog recorder for data display and preservation. The time constant for the equilibration was ~3 min, which corresponded to a spatial resolution of 0.5 mile with the ship steaming at 10 knots. Atmospheric air was periodically measured, and the system was calibrated every 12 h through the use of calibration gases with CO₂ mixing ratios of 252.5 and 412.8 ppm (v). Pressure corrections were made for the effect of water vapor on total pressure in the equilibrator and the pressure at the inlet of the IR analyzer; the correction for the difference between in situ temperature and measuring temperature was made according to Gordon and Jones (1973). Figure 5 presents the plots of the underway sea surface salinity, temperature, and pCO₂ measured during R/V Meteor Cruise 18/1.

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