Description of Variables
Data file m115.dat (File 5) (see description of data files in Part 2) in this numeric data package contains the following variables: station numbers; cast numbers; sample numbers; bottle numbers; CTD pressures; CTD temperatures; CTD salinities; potential temperatures; bottle salinities; concentrations of dissolved oxygen, silicate, nitrate, nitrite, phosphate; total CO2 concentrations; partial pressures of CO2 at 20oC; potential densities at 0 dbar; and quality flags. Station inventory file m115sta.inv (File 4) (Part 2) contains section numbers; station numbers; latitude, longitude, sampling date (i.e., day, month, and year), and bottom depth for each station.
The temperature and pressure readings of the Neil Brown IIIB CTD unit were corrected through the use of 4-6 pairs of reversing thermometers; the electrical conductivity readings were corrected by using the salinity values determined aboard the ship for all 24 Niskin samplers. A Guildline Autosal 8400A salinometer and the Wormley Salinity Standards were used for the determination of salinity in the discrete water samples. The precision of the measurements obtained by the CTD unit has been estimated to be ± 0.002oC for temperature and ± 0.002 PSS for salinity. Potential temperature and potential density values were computed through the use of the potential temperature algorithm of Fofonoff (1980), the International Equation of State for Seawater (Millero et al. 1980), and Bryden's (1973) formulation for the adiabatic temperature gradient.
The concentration of dissolved oxygen was determined by means of the Winkler titration method. A molar volume at STP of 22.385 liter/mole (Kester 1975) was used to convert oxygen concentrations from milliliter per liter to micromoles per kilogram of seawater at the in situ temperature.
The concentrations of nitrate, nitrite, phosphate, and silicate dissolved in the seawater samples were determined through the use of standard calorimetric methods with an Auto-Analyzer. Determinations were generally made within 6 hours of collection. The water samples were stored in a refrigerator at 4oC before analysis.
All of the concentration values are expressed in units of per kilogram of seawater, although analytical samples were isolated by volumetric means. For the conversion from the volume to the mass of seawater sample, the density of each water sample was computed by using the International Equation of State for Seawater (Millero et al. 1980) and the measured salinity and the temperature at which the volumetric measurements were made.
The total CO2 concentration in approximately 1300 seawater samples and the partial pressure of CO2 in approximately 870 seawater samples collected at 76 (Fig. 2) stations were determined aboard the ship.
The TCO2 concentration in seawater samples was determined by the use of a coulometric system, which was modified from that described by Johnson et al. (1985). For analysis, the seawater was introduced into the stripping chamber using fixed-volume syringes. The sample was acidified with 1 ml of 8.5% phosphoric acid while it was in the stripping chamber, where the evolved CO2 gas was swept from the sample and transferred with a stream of CO2-free air into the electrochemical cell of the CO2 coulometer (UTC-Coulometric Model-5011). In the coulometer cell, the CO2 was quantitatively absorbed by a solution of ethanolamine in dimethylsulfoxide (DMSO). Reaction between the CO2 and the ethanolamine formed the weak hydroxyethylcarbamic acid. The pH change of the solution associated with the formation of the acid resulted in a color change of the thymophthalein pH indicator in the solution. The color change, from deep blue to colorless, was detected by a photodiode, which continually monitored the transmissivity of the solution. The electronic circuitry of the coulometer, on detecting the change in the color of the pH indicator, caused a current to be passed through the cell generating hydroxyl (OH-) ions from a small amount of water in the solution. The OH- that was generated titrated the acid, returning the solution to its original pH (and hence color); the circuitry then interrupted the current flow. The product of current passed through the cell and time was related by the Faraday constant to the number of moles of OH- generated to titrate the acid and hence to the number of moles of CO2 absorbed to form the acid.
The volumes delivered by the constant-volume syringes were determined by repeatedly weighing distilled water dispensed in the same manner as a sample; the volume was calculated from the delivered weight by using the density of pure water at the temperature of the measurement and a buoyancy correction for the air displaced by the water (amounts to approximately 0.1% of the weight of the water). The density of the seawater in the pipet was calculated at the temperature of injection by using the International Equation of State (Millero et al. 1980).
The coulometer was calibrated by introducing research-grade CO2 gas (99.998%) into the carrier gas line upstream of the extraction tube, using a pair of fixed-volume sample loops on a gas-sampling valve and measuring the gas pressure in the loops as the gas was vented to the ambient atmosphere, and determining the barometric pressure by means of the electronic barometer used with the pCO2 system. The loop temperature was measured to ± 0.05oC with a thermometer calibrated against one traceable to the National Institute of Standards and Technology (NIST), and the non-ideality of CO2 was incorporated in the computation of the loop contents. The volume of the calibration loop had previously been determined by weighing empty loops and then loops filled with mercury. The volumes of these loops have additionally been checked by comparing the amount of CO2 introduced by them with the amount derived from gravimetric samples of calcium carbonate and sodium carbonate. They were found to be accurate to within 0.1%. During the expedition, the coulometer was calibrated several times daily by using the calibrated loop and pure CO2 gas.
In order to evaluate the long-term reproducibility and precision of the coulometric determination of CO2 in seawater, a number of sample bottles were filled with a homogeneous sample of surface water and deep water. Bottles made of Pyrex glass and PET plastic (500 ml and 1000 ml, respectively) were used. Bottled samples were poisoned with mercuric chloride solutions (200 µ l for each 500-ml water sample) and analyzed for total CO2 during the expedition. On the basis of these measurements (Fig. 3), the precision of TCO2 measurements during this expedition was estimated to be approximately ± 1 µ mol/kg. Additional details on the TCO2 measurements are discussed in Chipman et al. (1992).
A fully automated equilibrator-gas chromatograph system was used during the expedition to determined the pCO2 exerted by the seawater samples. Prior to analysis, the sample flasks were brought to 20oC in the thermostated water bath, and approximately 45 ml of seawater was displaced with air that had a known CO2 concentration. The air in the flasks and in the tubing connecting the flasks to the sample loop of the gas chromatograph was recirculated continuously for approximately 20 minutes; the gas disperser about 1 cm below the water surface provided a large contact area between the water and air bubbles. At the end of the equilibration period, the circulation pump was switched off, and the air pressure throughout the system was allowed to equalize. A 1-ml aliquot of the equilibrated air was isolated from the equilibration subsystem and injected into the carrier gas stream of the gas chromatograph by cycling the gas sampling valve to which the sample loop was attached. After chromatographic separation, the CO2 was converted into methane and water vapor through a reaction with the hydrogen carrier in the catalytic converter. The methane produced by this reaction was then measured with a precision of ± 0.05% (one standard deviation) by the flame ionization detector. The concentration of CO2 in the sample was determined through comparison with the peak areas of known amounts of CO2 from injections of three reference gas mixtures, which were calibrated against the World Meteorological Organization standards created by C. D. Keeling. The reference gas mixtures were injected into the gas chromatograph by means of the same sample loop used for the equilibrated air samples; the pressure of the gas in the sample loop at the time of injection was determined by venting the loop to atmospheric pressure and measuring that pressure by means of a high-accuracy electronic barometer (Setra Systems, Inc., Model 270, accuracy ± 0.3 millibar; calibration traceable to the NIST provided by the manufacturer). The sample loop was located within the well-controlled temperature environment of the column oven of the gas chromatograph; hence, all injections were made at a constant temperature.
The equilibrated air samples were saturated with water vapor at the temperature of equilibration and had the same pCO2 as the water sample. By injecting the air aliquot without removing the water vapor, the partial pressure of CO2 was determined directly, without the need to know the water vapor pressure (Takahashi et al. 1982). However, was necessary to know the pressure of equilibration, which was controlled by keeping the equilibrator flask at atmospheric pressure. The atmospheric pressure was, in turn, measured with the electronic barometer at the time each equilibrated air sample was injected into the gas chromatograph. Corrections were required to account for the change in pCO2 of the sample water as a result of the transfer of CO2 to or from the water during equilibration with the recirculating air. The overall precision of the pCO2 measurement is estimated to be about ± 0.10%, based on the reproducibility of replicate equilibrations. Greater details on the pCO2 measurements are discussed in Chipman et al. (1992).