Description of Variables and Methods
Each station consisted of the lowering of a CTD/rosette package, upon which 10-L standard Niskin® bottles were suspended. Pressure, temperature, and CTD salinities for the tabulated data were taken from the calibrated CTD data. The discrete water samples were drawn from the Niskin® bottles upon retrieval of the rosette on deck. Samples were collected for analyses of oxygen, chlorofluorocarbons (CFCs), helium, tritium (Leg 1 only),TCO2, 13C, pH (Leg 1 only), nutrients, and salinity. In addition, underway partial pressure of CO2 (pCO2) was measured throughout the cruise. This report addresses the temperature, salinity, TCO2, and nitrate data.
CTD data were collected using a Neil Brown (Mark III) Instrument System. Pressure, temperature, and conductivity were recorded on the downtrace, and the discrete water samples were collected on the upcast with an electronically fired rosette sampler. The bottle salinity samples were analyzed with a Guideline Autosal, which was calibrated at the beginning of each day's run with a vial of Wormley standard seawater, and again after each case of samples was analyzed with another vial. An accuracy of 0.002 Practical Salinity Scale (PSS) and a precision of 0.001 PSS were achieved. A more detailed description of the CTD data processing has been published as a NOAA Data Report (McTaggart et al. 1993).
Nutrient analyses included nitrate, silicate, and phosphate. Due to accuracy and precision problems for silicate and phosphate, only nitrate data are included in this report.
Nitrate samples were collected from each Niskin® bottle in aged 60-mL linear polyethylene bottles and analyzed for dissolved inorganic nitrate (NO3--N). Analyses were performed on samples from all CTD casts with a five-channel Technicon Auto-Analyzer (AA-II) aboard ship. The analytical procedures and methodologies used in the analysis of nitrate are similar to those described by Armstrong et al. (1967), with modifications described in Atlas et al. (1971) and in a Technicon Corporation (1977) technical report. The detection limit for nitrate was 0.39 µmol/kg with a standard deviation of ±0.1 µmol/kg. The precision of duplicate measurements was ±0.25%, full scale. The accuracy was assumed to be 1% because no absolute standards were available. Nitrate measurements are reported in µmol/kg at 1 atm and an assumed laboratory temperature of 25oC. Calibration standards ranged between 0 and 30 µmol/kg; therefore, only samples within that concentration range are reported in the data file.
Upon retrieval of the CTD-rosette package on deck, samples for TCO2 were collected in 500-mL glass-stoppered bottles and poisoned with 0.5 mL of saturated HgCl2 solution to decrease bacterial oxidation of organic matter prior to analysis. The samples were analyzed immediately when possible, but always within 24 h after collection.
The coulometric technique for TCO2 analyses in seawater was originated by Ken Johnson (Johnson et al. 1985, 1987). UIC, Inc. supplied the coulometic/extraction system (Models 5011 and 5030, respectively). The following changes were made to the standard extraction system:
- Both the pipet and seawater sample bottle were jacketed (the sample bottle was placed in a jacketed beaker) and were connected to a circulating bath set at 25oC.
- Ultra-pure N2 was used for both carrier gas and sample delivery; prior to hook-up to glassware, the N2 was sent through an in-line NaOH (Malcosorb) scrubber to remove any CO2.
- The condensing column was connected to a circulating bath filled with antifreeze and run at 1oC.
- An ORBO (activated Si gel from Supelco) tube was placed in-line between the glassware and titration cell to eliminate any excess moisture.
- Standardization of instrumentation was modified (as described later in this section). The computer interfaced to the system was a Zenith ZBF-2339-BK.
The program supplied by UIC was modified significantly, to accommodate our particular needs. In coulometric analysis of TCO2, all carbonate species in seawater (CO2(aq), H2CO3, HCO3- and CO32-) are converted to CO2 by addition of excess acid. The evolved CO2 is then moved into the titration cell by N2 carrier gas where it is titrated potentiometrically by reacting quantitatively with ethanolamine to form hydroxyethyl carbamic acid; this is titrated with OH- ions electrogenerated by the reduction of H2O at a platinum cathode:
CO2 + HO(CH2)2NH2 --> HO(CH2)2NHCOO- + H+ Ag(s) --> Ag+ + e- H2O + e- --> 0.5H2(g) + OH- H+ + OH- --> H20
The equivalence point is detected photometrically with thymolphthalein as an indicator. The cell solution is blue at the equivalence point of 10.5 pH and colorless at pH 9.3 after the addition of CO2 in aqueous solutions. CO2 drives down the pH and raises percent transmittance. As the acid is titrated, pH increases (hence, the blue color returns) and percent transmittance decreases, thus causing the titration current to pass from high to low to zero as the equivalence point is approached and sensed by the optical detector. The CO2 level is calculated based on the quantity of electricity required to reach the equivalence point and the time of passage. The entire sequence takes from 8 to 11 min.
The volume of the pipet was ~50 mL, and was calibrated in the laboratory before and after the cruise. The pipet was cleaned by drawing a 25% solution of NaOH into the cell and allowing it to soak overnight. This eliminated any organic film inside the pipet and ensured a clean delivery. Pipet calibrations were conducted to be within the measured pipet temperature range during the cruise (24.5oC-25.5oC) utilizing a circulating bath. Milli-Q water was drawn into the pipet in the exact manner that a liquid standard or seawater sample was handled. The water was delivered by N2 (flow rate is 200 mL/min) into a tared, ground-glass stoppered mixing flask, and drained for an additional 5 s (monitored by a stop-watch) to allow the droplets of water to be delivered; the flask was then immediately stoppered and weighed on a Mettler AE240 balance. Approximately 15-20 samples were collected per experiment. The following references were used to correct the weighings:
- Volume Properties of Ordinary Water.
- Reductions of Weighings in Air to Vacuo for Brass Weights and a Water Density of 1.00. Density of air used was 0.0012.
- Temperature Correction for Glass Volumetric Apparatus.
The corrected volumes were then linearly regressed with temperature, and a calibration curve was established (typical r2 = 0.80).
Schott-Duran glass 500-mL bottles were annealed at 450oC for 1 h, then cleaned in a dishwasher with commercial grade dishwashing detergent. Prior to collection of samples, the solid ground-glass stoppers were coated with Type M Apiezon grease.
Acid used to convert carbonate species to CO2 was a 1:10 solution of Baker reagent grade H3PO4. All coulometric chemicals (cathode solution, anode solution, and KI) were purchased from UIC, Inc.
Liquid standards were made up in a 0.7 M solution of KCl. The standards were treated just as a seawater sample and were delivered through the pipet under the same conditions. The standard used was Na2CO3 (Ultrex, Lot 935113); the KCl was reagent grade from Mallinkrodt.
The Na2CO3 was prepared in the laboratory by baking at 260-270oC for 0.5 h, and desiccated overnight. The standards were weighed into pre-cleaned ground-glass stoppered vials with weights ranging from 0.20 to 0.25 g. They were immediately stored in an evacuated desiccator with fresh Si gel until prepared. The KCl was baked in a muffle furnace for 0.5 h at 260-270oC, and cooled in a desiccator overnight.
The KCl solution and liquid standards were prepared in the following manner: Milli-Q water was boiled in a 3-L boiling flask for 20 min to drive off CO2, then cooled overnight with a NaOH column attached to the neck of the flask. A glove-box was purged with ultra-pure N2 for about 20 min; in the glove box, the KCl was mixed with the CO2-free water in a clean 2-L volumetric. Half of this solution was stored in a 1-L sample bottle with siphon tube and clamp and was used to determine blank values for the KCl (see following discussion). The other half was used to make the Na2CO3 in a 1-L volumetric. After the standard equilibrated, it was poured into a 1-L bottle with siphon tube and clamp. This work was performed in an N2 environment in the glove box.
The KCl solution was analyzed to determine a mean blank for the standard. Being careful not to expose the KCl solution to the atmosphere, it was drawn into the pipet in the same way as a sample. When handled in this way, the KCl blank was very constant, usually with a mean of around 6.0 ±0.3 µg C for an individual batch and an "over the cruise" mean of 6 ±1 µg C.
The standards yielded a mean calibration factor of 99.6588% ±0.0600 (n = 25).
A Certified Reference Material (CRM) was prepared and bottled by Dr. Andrew Dickson of the Scripps Institution of Oceanography (SIO). The TCO2 concentration of the CRM was determined to be 2020 ±0.009 µmol/kg by manometric technique in the laboratory of Dr. Charles Keeling of the SIO. Bottles of the CRM were taken on the cruise and analyzed frequently to determine the accuracy and precision of the coulometric method; the results are reported in Table 1. Throughout the cruise, the accuracy was determined to be within 0.15% and the precision was within 0.12%. Replicates analyzed at three different stations throughout the cruise yielded a precision of < or = 0.05%. The reported TCO2 data have been corrected to reflect the difference in accuracy to the CRM; the correction applied is +3 µmol/kg TCO2.
Samples were analyzed with the same method as standards, that is, 4.5 mLs of acid were dispensed into the reaction vessel, and 2-3 minutes were allowed to pass to purge CO2 from the acid. Following that, the pipet was rinsed twice with the sample, and the third fill was isolated and used for analysis. The sample was emptied into the reaction vessel, and allowed to drain for an additional 5 seconds (monitored via stopwatch) to allow droplets of water to be delivered.
Table 1. Certified Reference Material (batch 1) analyzed for total CO2 during R/V Malcolm Baldrige CGC-90 Cruise
|24 Feb. 1990||2012|
|25 Feb. 1990||2024|
|27 Feb. 1990||2014|
|02 Mar. 1990||2015|
|05 Mar. 1990||2012|
|05 Mar. 1990||2016|
|06 Mar. 1990||2019|
|07 Mar. 1990||2019|
|08 Mar. 1990||2019|
|10 Mar. 1990||2015|
|11 Mar. 1990||2016|
|12 Mar. 1990||2016|
|13 Mar. 1990||2017|
|15 Mar. 1990||2017|
|16 Mar. 1990||2017|
|20 Mar. 1990||2017|
|29 Mar. 1990||2017|
|30 Mar. 1990||2014|
|02 Apr. 1990||2018|
|03 Apr. 1990||2014|
|05 Apr. 1990||2016|
|06 Apr. 1990||2017|
|07 Apr. 1990||2018|
|07 Apr. 1990||2018|
|11 Apr. 1990||2020|
|12 Apr. 1990||2017|
Sigma-t and sigma-theta were calculated using standard UNESCO algorithms (Fofonoff and Millard 1983), and the CTD measured in situ temperature and bottle salinities. When no bottle salinities were available or when they were defined as a questionable or unacceptable measurements, CTD salinities were used in the calculation.