ABSTRACT

Peltola, E., R. Wanninkhof, R. Feely, D. Hansell, R. Castle, D. Greeley, J.-Z. Zhang, F. Millero, N. Gruber, J. Bullister, and T. Graham. 2005. Inorganic and Organic Carbon, Nutrient, and Oxygen Data from the R/V Ronald H. Brown Repeat Hydrography Cruise in the Atlantic Ocean: CLIVAR CO₂ Section A16N_2003a (4 June - 11 August, 2003), ed. A. Kozyr. ORNL/CDIAC-149, NDP-085. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, 36 pp.

This report presents methods and analytical and quality control procedures for nutrient, oxygen, and inorganic carbon system parameters performed during the A16N_2003a cruise, which took place from June 4 to August 11, 2003 aboard NOAA Ship R/V Ronald H. Brown under auspices of the National Oceanic and Atmospheric Administration (NOAA). The first hydrographic leg (June 19 - July 10) was from Reykjavik, Iceland, to Funchal, Madeira, Portugal along the 20°W meridian, and the second leg (July 15 - August 11) continued operations from Funchal, Portugal to Natal, Brazil, on a track southward and ending at 6°S, 25°W. The research was the first in a decadal series of repeat hydrography sections jointly funded by NOAA and the National Science Foundation (NSF) as part of the CLIVAR/CO₂/hydrography/tracer program. Samples were taken from up to 34 depths at 150 stations.

The data presented in this report includes the analyses of water samples for total inorganic carbon (TCO₂), fugacity of CO₂ (fCO₂), total alkalinity (TALK), pH, nitrate (NO₃), nitrite (NO₂), phosphate (PO₄), silicate (SiO₄), and dissolved oxygen (O₂).

The R/V Ronald H. Brown A16N_2003a data set is available free of charge as a numeric data package (NDP) from the Carbon Dioxide Information Analysis Center (CDIAC). The NDP consists of the oceanographic data files and this printed documentation, which describes the procedures and methods used to obtain the data.

Keywords: carbon dioxide, total CO₂, total alkalinity, pH, fugacity of CO₂, carbon cycle, coulometry, potentiometry, hydrographic measurements, CLIVAR, Atlantic Ocean

1. BACKGROUND INFORMATION

The cruise of research vessel (R/V) Ronald H. Brown along section A16N from Reykjavik, Iceland, to Natal, Brazil, was the first in a series of repeat hydrography cruises to measure decadal changes in circulation, heat and fresh water budgets, and carbon inventory in the ocean. The cruises repeat a subset of the World Ocean Circulation Experiment Hydrographic Program (WHP) and Joint Global Ocean Flux Study (JGOFS) lines occupied in each major ocean basin in the 1990s.

The WOCE/WHP program is driven by the need to monitor the changing patterns of carbon dioxide (CO₂) in the ocean and provide the necessary data to support continuing model development that will lead to improved forecasting skills for oceans and global climate. During the 1990s, the WOCE/JGOFS survey provided a full-depth baseline data set against which to measure future changes. By integrating the scientific needs of programs requiring measurement of the full water column, major synergies and cost savings are achieved. These measurements are of importance both for major research programs, such as Climate Variability (CLIVAR) and the U.S. Global Carbon Research Project (GCRP) Carbon Cycle Science Program (CCSP), and for operational activities such as the Global Ocean Observation System (GOOS) and the Global Climate Observing System (GCOS). As outlined in the program documentation one component of a global observing system for the physical climate/CO₂ system should include periodic observations of hydrographic variables, CO₂ system parameters, and other tracers. The large-scale observation component of CCSP has a need for systematic observations of the invasion of anthropogenic carbon in the ocean that is superimposed on a variable natural background. The five topical areas addressed by the CO₂/CLIVAR repeat hydrography program are

1. carbon system studies;

2. heat and freshwater storage and flux studies;

3. deep and shallow water mass and ventilation studies;

4. calibration of autonomous sensors; and

5. data for model calibration.

The Ronald H. Brown cruise consisted of a transit leg from Charleston, South Carolina, USA to Reykjavik, Iceland, during which limited surface-water observations were taken. The first hydrographic leg was from Reykjavik to Funchal, Madeira, Portugal along the 20°W meridian; the second leg continued operations from Funchal, Portugal to Natal, Brazil, on a track southward, ending at 6°S, 25°W (see Fig. 1.1).

This data report focuses on the measurements of total inorganic carbon (TCO₂), fugacity of CO₂ (fCO₂), total alkalinity (TALK), pH, nitrate (NO₃), nitrite (NO₂), phosphate (PO₄), silicate (SiO₄), and dissolved oxygen (O₂).

The methodology, instrumentation, and standardization of these parameters improved significantly during the WOCE/JGOFS era. Notable developments include release of manuals detailing the analytical methods and operating protocols (DOE 1994). Certified reference materials (CRMs) are now available for TCO₂ and TALK, which are run interspersed with samples to determine calibration offsets. For this cruise the TALK values were adjusted to account for the small difference between the CRMs run at sea and the certified value determined at Scripps Institution of Oceanography (SIO). For TCO₂ there were problems with the gas loop calibrations that were attributed to inaccurate temperature sensors. The reference materials were therefore used as primary calibration for both TCO₂ and TALK.

Instrumentation also improved in the last decade. Alkalinity measurements can be done with better precision through automation and close checks of the response of electrodes. Burettes are independently calibrated, and the preparation of titrant (hydrochloric acid) undergoes improved quality control and standardization (Millero et al. 1998). Measurement of pH is now done with extreme precision with spectrophotometric methods (Byrne and Breland 1989). The TCO₂ measurements are done by coulometry, a precise integrative method. During the A16_2003a cruise we utilized two single-operator multiparameter metabolic analyzers (SOMMAs) (Johnson et al. 1999) for analyses, which facilitated a sample throughput of up to 80 samples per day. The fCO₂ measurements were done with an equilibration system described in Wanninkhof and Thoning (1993). For this cruise data reduction and calculation routines were changed. Comparisons of the data with those from a cruise along a similar transect in 1993 show an appreciable bias between results, as is detailed in the section describing the pCO₂ analyses. Oxygen measurements were performed by Winkler titrations (Carpenter 1965) with photometric endpoint detection (Friederich, Sherman, and Codispoti 1984). The titrator worked well, but there were issues with errors in bottle volumes and problems with pipettes used to generate standard curves. Extensive post-cruise troubleshooting and bottle volume redetermination were necessary to reduce the data.

The data underwent careful quality assurance and quality control (QA/QC) both during and after the cruise. The precision of the measurements was determined from duplicate sampling and comparison of data from deep water, where little variability is expected. Outliers in the data were flagged based on several methods utilizing prior knowledge of the trends and known relationships between parameters. Depth profiles for each parameter were scrutinized for outliers. When deviations were observed, other parameters were assessed to determine whether they showed deviations as well. Inorganic carbon system parameters were linked through physical and chemical properties; if two of the four parameters are measured, the other two can be calculated, provided that the silicate composition, the phosphate composition, the temperature, and the salinity of the sample are known. These so-called overdeterminations or internal consistency calculations were used to assess the difference between calculated and measured values. When the difference between the measured TALK and the TALK calculated from TCO₂ and pH or fCO₂ exceeded 10 µmol/kg, the three parameters were scrutinized and compared with other parameters to assess whether the datum should be labeled as questionable. Other techniques, described in detail below, include regional multilinear regressions (MLR) between the inorganic carbon parameters and physical and chemical parameters known to correlate with them [for instance, TCO₂ = f(T, S, AOU, Si, PO₄)]. Again the differences between measured and calculated parameters were inspected. Finally, the parameters were plotted against latitude for narrow depth intervals. Since changes along depth horizons are usually gradual, anomalies can be easily spotted and flagged.

This report describes procedures and methods for hydrographic measurement and the analytical procedures, calculations, and assessment of precision for nutrient, oxygen, TCO₂, TALK, fCO₂, and pH measurements. A description of the QA/QC methods based on internal consistency of these parameters and the MLR technique is also provided.

2. DESCRIPTION OF THE EXPEDITION

2.1 R/V Ronald H. Brown: Technical Details and History

The National Oceanic and Atmospheric Administration (NOAA) ship R/V Ronald H. Brown, a state-of-the-art oceanographic and atmospheric research platform, is the largest vessel in the NOAA fleet. With its highly advanced instruments and sensors, R/V Ronald H. Brown travels worldwide supporting scientific studies to increase our understanding of the worlds oceans and climate. Commissioned on July 19, 1997, in its home port of Charleston, South Carolina, Ronald H. Brown has sailed in the Pacific, Atlantic, and Indian Oceans. The ship was named in honor of former Secretary of Commerce Ronald H. Brown, who was killed in a plane crash on April 3, 1996, while on a trade mission to Bosnia. R/V Ronald H. Brown is operated by NOAA Marine and Aviation Operations and carries a complement of 6 NOAA Corps officers, 20 crew members, and a maximum of 33 scientists. Table 2.1 provides a detailed description of the ship.

2.2 R/V Ronald H. Brown A16N_2003a Cruise Information

Parameters measured, institution, and responsible investigators

Participating institutions

3. DESCRIPTION OF VARIABLES AND METHODS

3.1 Nutrient and Oxygen Measurements

The analytical method for determining dissolved oxygen in seawater during the A16N_2003a cruise was based on automated Winkler titration as described by Culberson et al. (1991), and Williams and Jenkinson (1982) and modified by Friederich, Codispoti, and Sakamoto (1991). Dissolved oxygen samples were withdrawn from 10-L Niskin bottles to 145-mL Pyrex brand iodine flasks (Corning 5400, Corning, New York, USA). The exact volume of each flask at room temperature had been gravimetrically calibrated with its ground glass stopper following standard procedures (DOE 1994; WHP Operations and Methods 1991). Quantities of 1 mL of manganese chloride reagent and 1 mL of alkaline iodide reagent were added to each sample in the iodine flasks, and then the stopper was placed in the bottle neck. The bottles were shaken vigorously for about 1 min to completely fix oxygen with manganese hydroxide [Mn(OH)₃]. In this method, dissolved oxygen in the sample reacts with Mn(OH)₃ to form Mn(OH)₃ precipitate. Particulate Mn(OH)₃ dissolves upon the acidification, and the resulting Mn³⁺ ions oxidize iodide to iodine in acidic solution. The liberated iodine complexes with excess iodide, forming I₃, and the latter is titrated with a sodium thiosulfate solution that is standardized by a primary standard potassium iodate. The complex I₃ has a maximum absorbance at 352 nm; this change in absorbance at 352 nm is used to detect the end point. A custom-built automated oxygen titrator with MS DOS interfacing software was used to determine dissolved oxygen concentrations in the samples.

A total of 5011 seawater samples were taken from 150 stations and analyzed for dissolved oxygen concentrations. At the beginning of cruise, a test conductivity, temperature, and depth (CTD) cast was made by sampling 20 Niskin bottles from same depth (170 m). Analysis of these samples indicated a precision of 0.3 µmol/L. Throughout the cruise duplicate samples from the same Niskin bottle were collected at each station to estimate the precision of the overall measurements (sampling and analysis). Analyses of 300 replicate samples indicated that the precision of the shipboard automated Winkler titration was 0.29 µmol/L including all outliers, and 0.24 µmol/L excluding the outliers. An analysis of outliers indicated that most outlying values in the duplicate analysis were due to errors in the volumes of oxygen bottles; a minority were due to problems with Niskin bottles or sampling error. The outliers in vertical profiles of oxygen were also used to identify the bottles that might have errors in volumes. A total of 33 sample bottles were recalibrated; 11 proved to have volume errors greater than 0.3 mL. These accounted for about 5% of the sample bottles used during the A16N_2003a cruise. The volumes of the oxygen bottles identified as questionable were recalibrated after the cruise, and dissolved oxygen concentrations were recalculated for those samples using correct volumes.

At the beginning of leg 2 of the cruise (from stations 72 to 79) a problematic automatic pipette was used to deliver the KIO₃ standard solution for standardization of thiosulfate solution in batch number 14. An unusually high slope was observed, and this pipette was not used in subsequent analyses. A shipboard and post-cruise comparison indicated that there was an error in volume delivery of this automatic pipette. Consequently, dissolved oxygen concentrations from stations 72 to 79 were corrected for errors in volume delivery of iodate solution by this automatic pipette. The correction factor (1.0153) was estimated on the basis of the post-cruise recalibration of the pipette and was applied to data from station 72 to 79.

Since the Dosimat titrators have demonstrated high precision and accuracy in volume delivery of titrants (0.05 and 0.2%, respectively, at delivery of 10 mL solution), we recommend use of a Dosimat or a similar positive-displacement burette to quantitatively dispense the iodate standard solution during future cruises. This procedure can improve the accuracy of shipboard oxygen analysis.

Nutrient samples were collected from Niskin bottles in acid-washed 25-mL linear polyethylene bottles after at least three complete seawater rinses and analyzed within 2 hours of sample collection. Measurements were made in a temperature-controlled (20 ± 2°C) bioanalytical laboratory aboard the R/V Ronald H. Brown. Concentrations of nitrite (NO₂), nitrate (NO₃), phosphate (PO₄³), and silicic acid (H₄SiO₄) were determined using a modified Alpkem Flow Solution Auto-Analyzer coupled with a modified RFA 301 autosampler. Sample and wash time for the auto sampler were set at 120 and 5 seconds, respectively.

Nitrite content was determined by diazotizing the samples with sulfanilamide and bonding with N-1 naphthyl ethylenediamine dihydrochloride to form an azo dye. The color produced is measured at 540 nm (Zhang, Ortner, and Fischer 1997). Samples for nitrate analysis were passed through a laboratory-crafted cadmium column (Zhang, Fischer, and Ortner 2000) to reduce nitrate to nitrite. Total nitrite, mostly from reduction of nitrate with a small amount of nitrite present in the original samples, was then determined as described above. Nitrate concentrations in seawater samples were calculated by difference.

The amount of phosphate in the samples was determined by reacting the samples with molybdenum (VI) in an acidic medium to form a phosphomolybdate complex. This complex was subsequently reduced with hydrazine at a temperature of 55°C to form phosphomolybdenum blue (Zhang, Fischer, and Ortner 2001). An AAII detector with an 880-nm filter was used to measure the absorbance during the cruise.

Silicic acid in the samples was analyzed by reacting samples with molybdate in a acidic solution to form b-molybdosilicic acid. The b-molybdosilicic acid was then reduced by ascorbic acid to form molybdenum blue (Zhang and Berberian 1997). The absorbance of the molybdenum blue was measured at 660 nm.

The low-nutrient water used for the preparation of working standards, determination of blank, and the wash between samples was filtered seawater obtained from the surface of the Gulf Stream. Stock standard solutions were prepared by dissolving high-purity standard materials (KNO₃, NaNO₂ , KH₂PO₄ and Na₂SiF₆ ) in deionized water. Working standards were freshly made at each station by diluting the stock solutions in low-nutrient seawater. Standardizations were performed prior to each sample run with working standard solutions. Two or three replicate samples were collected from a Niskin bottle that tripped at the deepest depth at each cast. The relative standard deviation from the results of these replicate samples was used to estimate the overall precision for the sampling and analytical procedures. The precision of analyses was 0.08 µmol/kg for nitrate, 0.01 µmol/kg for phosphate, and 0.1 µmol/kg for silicic acid, respectively.

3.2 Total Inorganic Carbon Measurements

Samples were drawn from the Niskin bottles into cleaned, precombusted 540-mL Pyrex bottles using Tygon tubing according to procedures outlined in DOEs handbook of methods for CO₂ analysis (DOE 1994). Bottles were rinsed once and filled from the bottom, overflowing half a volume. Care was taken not to entrain any bubbles. The tube was pinched off and withdrawn, creating a 5-mL headspace; and 0.2 mL of saturated HgCl₂ solution was added as a preservative. The sample bottles were sealed with glass stoppers lightly covered with Apiezon-L grease and were stored at room temperature for a maximum of 12 hours prior to analysis.

The TCO₂ analytical equipment was set up in a seagoing laboratory van. The analysis was done by coulometry with two analytical systems (AOML-1 and AOML-2) used simultaneously on the cruise. Each system consisted of a coulometer (UIC, Inc.) coupled with a SOMMA inlet system developed by Ken Johnson, formerly of Brookhaven National Laboratory (Johnson 1992; Johnson, King, and Sieburth 1985; Johnson et al. 1987, 1993). In the coulometric analysis of TCO₂, all inorganic carbon is converted to CO₂ (gas) by the addition of excess hydrogen ions (acid) to the seawater sample, and the evolved CO₂ gas is swept into the titration cell of the coulometer with compressed nitrogen, where it reacts quantitatively with a proprietary reagent based on ethanolamine to generate hydrogen ions. These are subsequently titrated with coulometrically generated OH. Carbon dioxide is quantified by integrating the total charge required to achieve this.

The coulometers were calibrated by injecting aliquots of pure CO₂ (99.995%) by means of an eight-port valve outfitted with two sample loops that had been calibrated by Kelly Brown, CCN Consulting (Wilke, Wallace, and Johnson 1993). Due to large temperature variations of the gas loops, the calibration factors obtained from gas loop measurements were of poor quality. Instead of using an average of the small and large loop values as recommended in DOE (1994), we used a constant value for each analyzer throughout the entire cruise. The constant calibration value used for AOML-1 was 1.00532 and for AOML-2 1.00650. The CO₂ gas volumes bracketed the amount of CO₂ extracted from the water samples for the two AOML systems. All TCO₂ values were corrected for dilution by 0.2 mL of HgCl₂ used for sample preservation. The correction factor used for dilution was 1.00037. A correction was also applied for the offset from the Certified Reference Material (CRM) Batch 59, supplied by Dr. A. Dickson (SIO). This correction was applied for each cell using the CRM value obtained for the cell at the beginning. To check the stability of the coulometer and coulometer solutions, the CRMs were measured at the beginning, middle, and end of each coulometer cell solution. The coulometer cell solution was replaced after 25 mg of carbon was titrated, typically after 912 hours of continuous use. Sample titration times were 916 minutes.

Replicate seawater samples were taken from Niskin sample bottles at the surface, at 1000 m, and at the sea bottom and were run at different times during the cell. The first replicate of the surface water was used at the start of the cell with fresh coulometer solution; the second surface replicate and the first one of the 1000 replicates were run in the middle of the cell after about 12 mg of carbon were titrated. The second one of the 1000 m replicates and the first one of the bottom replicates were run at the end of the cell after about 25 mg of carbon were titrated, while the second one of the bottom replicate samples was run using a new coulometer cell solution. No systematic difference between the replicates was observed. The trends do not suggest any systematic dependency of results with amount of carbon titrated for a particular cell. The results of the duplicate samples are shown in Fig. 3.1 and Table 3.1.

Fig. 3.1. Results of the duplicate TCO₂ samples collected during the R/V Ronald H. Brown cruise along the Atlantic Ocean section A16N_2003a.

The concentration of TCO₂, designated as [CO₂], in the samples was determined according to

, (3.1)

where Cal. factor is the calibration factor fixed for this cruise because of the malfunctioning of the gas loops, Counts is the instrument reading at the end of the analysis, Blank is the counts per minute determined from blank runs performed at least once for each cell of the solution, Run time is the length of coulometric titration (in minutes), and K is the conversion factor from counts to mol, which is dependent on the slope and intercept relation between instrument response and charge. For a unit with an E_cal slope of 1 and an intercept of 0, the conversion factor is 2.0728 X 10^-4.

The blank values for AOML-1 were in the range of 12.0 to 33.3 counts/min, with an average value of 19.6 counts/min and a standard deviation of 6.8 counts/min. For AOML-2 they were in the range of 12.0 to 30.0 counts/min, with an average value of 21.7 counts/min and a standard deviation of 6.1 counts/min.

The pipette volume was determined by taking aliquots of distilled water at a known temperature from the volumes prior to the cruise. The weights with the appropriate densities were used to determine the volume of the pipettes (AOML-1: 28.726 cm³ @ 19.96C, AOML-2: 22.623 cm³ @ 22.63C).

Calculations of pipette volumes, density, and final CO₂ concentrations were performed according to procedures outlined in the DOE CO₂ handbook (DOE 1994).

3.3 Fugacity of CO₂ Measurements

correction algorithms. One empirical correction was established before the cruise, and one was determined from running wet versus dry standards after the cruise. The results show differences in the range from 7 to 17 µatm.

During the cruise a total of 1515 Niskin samples were analyzed for fCO₂, compared with 2500 TCO₂ samples. This was because only one full-time and a part-time operator were available for the work, while two full-time analysts were involved in TCO₂ analysis. A summary of the analysis statistics is given in Table 3.3.