Carbon Dioxide, Hydrographic, and Chemical Data Obtained During the R/V Maurice Ewing Cruise in the Atlantic Ocean (WOCE Section A17, 4 January - 21 March 1994)
Aida F. Rios,1 Kenneth M. Johnson2, Xose Anton Avarez-Salgado,1 Linda Arlen,3 Andre Billant,4 Linda S. Bingler,5 Pierre Branellec,4 Carmen G. Castro,1 David W. Chipman,6 Gabriel Roson,7 and Douglas W. R. Wallace8
Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
1Consejo Superior de Investigaciones Cientificas, Instituto de Investigaciones Marinas, Eduardo Cabello, 6, 36208-Vigo, Spain
2Department of Applied Science, Brookhaven National Laboratory, Upton, NY, U.S.A. Retired, now at P.O. Box 483, Wyoming, RI, USA.
3James J. Howard Marine Science Laboratory, Highlands, NJ, USA
4Laboratoire de Physique des Ocens, CNRS-IFREMER-UBO, IFREMER/Centre de Brest, B.P. 70, 29280 Plouzane France
5Battelle Pacific Northwest Laboratories, Sequim, WA, USA
6Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA (retired)
7Departamento de Fisica Aplicada, Facultad de Ciencias del Mar, University of Vigo, Campus Lagoas Marcosende s/n, 36200-Vigo, Spain
8Leibniz-Institut fuer Meereswissenschaften an der Universitat Kiel, Kiel, Germany
- ABBREVIATIONS AND ACRONYMS
- BACKGROUND INFORMATION
- DESCRIPTION OF THE EXPEDITION
- DESCRIPTION OF VARIABLES AND METHODS
- HOW TO OBTAIN THE DATA AND DOCUMENTATION
|BNL||Brookhaven National Laboratory|
|CDIAC||Carbon Dioxide Information Analysis Center|
|CICYT||Comisin Interministerial de Ciencia y Tecnologia|
|CNRS||Centre de la Recherche Scientifique|
|CRM||certified reference material|
|CSIC||Consejo Superior de Investigaciones Cientificas, Spain|
|CTD||conductivity, temperature, and depth sensor|
|DIC||dissolved inorganic carbon|
|DOE||U.S. Department of Energy|
|fCO2||fugacity of CO2|
|IFREMER||Institut Francais de Recherche pour L'Exploitation de la Mer|
|IIM.CSIC||Instituto de Investigaciones Marinas, CSIC, Vigo, Spain|
|NSU||Institut National des Sciences de l'Univers|
|GOFS||Joint Global Ocean Flux Study|
|LDEO||Lamont-Doherty Earth Observatory|
|LMCE||Laboratoire de Modelisation du Climate et de l'Environnement|
|LODYC||Laboratoire d'Oceanographie Dynamique et de Climatologie|
|LPO||Laboratoire de Physique des Oceans|
|BS||National Bureau of Standards|
|DP||numeric data package|
|NMFS||National - Marine Fisheries Service|
|NOAA||National Oceanic and Atmospheric Administration|
|NODC||National Oceanographic Data Center|
|NSF||National Science Foundation|
|ORNL||Oak Ridge National Laboratory|
|pCO2||partial pressure of CO2|
|PNEDC||Programme National d'Etude de la Dynamique du Climat|
|PNNL||Battelles Pacific Northwest Laboratory|
|SIO||Scripps Institution of Oceanography|
|SOMMA||single-operator multi-parameter metabolic analyzer|
|TCO2||total carbon dioxide|
|WHOI||Woods Hole Oceanographic Institution|
|WHP||WOCE Hydrographic Program|
|WOCE||World Ocean Circulation Experiment|
Rios, A., K. M. Johnson, X. A. Alvarez-Salgado, L. Arlen, A. Billant, L. S. Bingler, P. Branellec, C. G. Castro, D. W. Chipman, G. Roson, and D. W. R. Wallace,. 2005. Carbon Dioxide, Hydrographic, and Chemical Data Obtained During the R/V Maurice Ewing Cruise in the Atlantic Ocean (WOCE Section A17, 4 January21 - March 1994), ed. A. Kozyr. ORNL/CDIAC-148, NDP-084. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee. doi: 10.3334/CDIAC/otg.ndp084
This documentation discusses the procedures and methods used to measure total carbon dioxide (TCO2), total alkalinity (TALK), and pH at hydrographic stations during the R/V Maurice Ewing cruise in the South Atlantic Ocean on the A17 WOCE section. Conducted as part of the World Ocean Circulation Experiment (WOCE), this cruise was also a part of the French WOCE program consisting of three expeditions (CITHER 1, 2, and 3) focused on the South Atlantic Ocean. The A17 section was occupied during the CITHER 2 expedition, which began in Montevideo, Uruguay, on January 4, 1994 and finished in Cayenne, French Guyana, on March 21, 1994. During this period the ship stopped in Salvador de Bahia and Recife, Brazil, to take on supplies and exchange personnel. Upon completion of the cruise the ship transited to Fort de France, Martinique. Instructions for accessing the data are provided.
TCO2 was measured using a single-operator multiparameter metabolic analyzer (SOMMA) coupled to a coulometer for extracting and detecting CO2 from seawater samples. The overall precision and accuracy of the TCO2 analyses was ±1.6 µmol/kg. A second carbon system variable, TALK, was determined by potentiometric titration with an overall precision of ±1.7 µmol/kg. During the A17 cruise the carbon system was overdetermined because a third carbonate system variable, pH, was also measured potentiometrically with an overall precision of ±0.003. The underway partial pressure of CO2 (pCO2) in surface waters was also continuously measured along the cruise track.
A comparison of A17 TALK with recent data in the South Atlantic Ocean confirms that A17 TALK data need a downward correction of 8 µmol/kg that was integrated in the CDIAC database. The internal consistency study carried out among the four carbon system variables led us to adjust the pH measurements by stations in order to eliminate the difference between TCO2 measured and TCO2 calculated from pH and TALK.
The R/V Maurice Ewing A17 data set is available free of charge as a numeric data package (NDP) from the Carbon Dioxide Information Analysis Center. The NDP consists of three oceanographic data files, one FORTRAN 77 data retrieval routine file, and this printed documentation, which describes the contents and format of all files as well as the procedures and methods used to obtain the data.
Keywords: carbon dioxide, TCO2, total alkalinity, pH, partial pressure of CO2, carbon cycle, coulometry, potentiometry, hydrographic measurements, World Ocean Circulation Experiment, meridional section, South Atlantic Ocean.
The World Ocean Circulation ExperimentWorld Hydrographic Program (WOCE-WHP) was a major component of the World Climate Research Program. The primary goal of WOCE was to understand the general circulation of the global ocean well enough to be able to model its present state and predict its evolution in relation to long-term changes in the atmosphere. The need for carbon system measurements arose from the serious concern over the rising atmospheric concentrations of carbon dioxide (CO2). Increasing atmospheric CO2 may intensify the earths natural greenhouse effect and alter the global climate.
Although CO2-related measurements - specifically, total CO2 (TCO2), total alkalinity (TALK), partial pressure of CO2 (pCO2), and pH - were not official WOCE measurements, a coordinated effort was supported as a core component of the Joint Global Ocean Flux Study (JGOFS). This effort received support in the United States from the U.S. Department of Energy (DOE), the National Oceanic and Atmospheric Administration (NOAA), and the National Science Foundation (NSF), and in Spain from the Comisin Interministerial de Ciencia y Tecnologia (CICYT), for WOCE cruises through 1998 to measure the global spatial and temporal distributions of CO2 and related parameters. Goals were to estimate the meridional transport of inorganic carbon in a manner analogous to oceanic heat transport (Bryden and Hall 1980; Roemmich and Wunsch 1985; Brewer et al. 1989; Holfort et al. 1998; Alvarez et al. 2003; Rosn et al. 2003), and to build a database suitable for carbon-cycle modeling and the estimation of anthropogenic CO2 increase in the oceans. To obtain a reliable database, Wanninkhof et al. (2003) made a comparison of inorganic carbon system parameters measured in the Atlantic Ocean from 1990 to 1998, recommending small adjustments for consistency among other cruises in the zone. The CO2 survey took advantage of the sampling opportunities provided by the WOCE cruises during this period, and the final data set covered on the order of 23,000 stations. Wallace (2002) reviewed the goals, conduct, and initial findings of the global CO2 survey, and recently Sabine et al. (2004) estimated a global oceanic anthropogenic CO2 sink between 1800 and 1994.
This report discusses results of the research vessel (R/V) Maurice Ewing expedition along the WOCE Section A17, from 4 January to 21 March, 1994 (Fig. 1.1). The cruise, designated as CITHER2_12, was a part of the French WOCE program consisting of three expeditions focusing on the South Atlantic Ocean: CITHER 1 (1993), 2 (1994), and 3 (1995). TCO2 analysis personnel and support for this expedition were from Brookhaven National Laboratory (BNL), Lamont-Doherty Earth Observation (LDEO), and Battelle Pacific Northwest National Laboratory (PNNL). Analyses of TALK, pH, and nutrients were performed by Spanish scientists from the Consejo Superior de Investigaciones Cientificas (CSIC), Instituto de Investigaciones Marinas of Vigo. The hydrographic work was carried out by French scientists under the direction of Laurent Memery Laboratoire d'Oceanographie Dynamique et de Climatologie (LODYC), University of Pierre et Marie Curie, Paris, France.
The A17 section work will yield a map of the large-scale three-dimensional distribution of temperature, salinity, and chemical constituents, including the carbon system variables. This map will be combined with the results of the remaining French WOCE South Atlantic sections (A6, A7, A13, and A14) and the other South Atlantic WOCE sections measured by CO2 survey participants (A8, A9, A10, and A11) to provide an extensive reference data set. Knowledge of the measured variables and their initial conditions allow determination of heat and water transports as well as carbon transport and elucidate regional sources and sinks of carbon and fossil fuel carbon. Studies estimating the carbon transport and establishing the anthropogenic CO2 sources and sinks based on these data have already appeared in the literature (Holfort et al. 1998; Rios et al. 2003). An understanding of anthropogenic CO2 uptake and transports contributes to the understanding of processes relevant to climate change. The South Atlantic A17 section was especially relevant to CO2 transport because it focused on the western boundary sections and currents, and provided a description of the water masses and their meridional evolution between 50°S and 10°N (Memery et al. 2000).
Fig. 1.1. Cuise track during the R/V Maurice Ewing Atlantic Ocean survey expedition along WOCE section A17.
The work aboard the R/V Maurice Ewing was supported by the Institut Francais de Recherche pour L'Exploitation de la Mer (IFREMER; Grant 210161), the Institut National des Sciences de l'Univers (INSU), and the Centre de la Recherche Scientifique (CNRS), in the framework of the Programme National d'Etude de la Dynamique du Climat (PNEDC) and its WOCE/France subprogram. The carbon dioxide and nutrients work was supported by DOE (DE-ACO2-76CH00016) and CICYT (Grant ANT93-1156-E). We would like to thank the master, officers, and crew of R/V Maurice Ewing and all the participants on the cruise CITHER-2. Special thanks go to M. Arhan, coordinator of the WOCE-France program CITHER, and L. Memery, chief scientist of cruise CITHER 2. The authors are also especially grateful to the Sonderforschungsbereich 460 (SFB) at the University of Kiel (Dr. F. Schott, Leader), funded by the Deutsche Forschungsgemeinschaft, for their support and assistance in completing the written documentation.
The R/V Maurice Ewing is a research vessel owned by the National Science Foundation (USA) and operated by the Lamont-Doherty Earth Observatory (LDEO) of Columbia University. It is classified by the America Bureau of Shipping as an A-1 and Baltic Ice Class IA ship. It was originally constructed as a seismic vessel in 1983, but it was acquired by Columbia in 1989, modified, and outfitted for tasks required of a general-purpose oceanographic research vessel. The vessel does, however, incorporate extensive and unique geophysical capabilities into its hardware; these include a Syntrak 480-24 seismic recording system, hydrophones, and sound source arrays. The vessel operates in the Atlantic, Indian, and Pacific Oceans. Table 2.1 provides a detailed description of the ship.
Table 2.1. Specifications of R/V Maurice Ewing
|US NODC code||3230|
|Gross registered tonnage||1978|
|Overall length||73.0 m|
|Draught (maximum)||5.30 m|
|Fuel capacity||604 m3|
|Service speed||11.0 kn|
|Maximum speed||13.5 kn|
|Freeboard to working deck||2.5 m|
|Personnel||Crew: 22; scientists: 28|
|Main engine (s)||4 X Diesel El at 5200 bhp at 1200 rpm|
|Maximum cruise duration||60 days|
|Nautical equipment||Integrated navigation system with radar, loran, SatNav; 3- and 12-kHz echosounders (hull-mounted) for scientific research; geological sonar; 4 oceanographic winches carrying 6,000 m of 9/16-in. 3 X 19, 0.68-in. coaxial cable, 0.322-in. CTD wire, or 1/4in. 3 X 19 wire. Hull-mounted Atlas deep ocean multibeam swath bottom mapping system and electronic data processing equipment (SUN computer). Ship has 35-ton- capacity gantry, 4-ton-capacity crane, and other winches for instruments or sampling|
|Science quarters||Dark room, 465 m3 of cargo storage space, 65 m2 of wet laboratory space, 208 m2 of dry laboratory space, 30 m2 of free working deck area, science office, vehicle staging room, and a small amount of container space|
|Ship Name||Maurice Ewing|
|Ports of call||Montevideo, Uruguay; Salvador de Bahia and Recife, Brazil; Cayenne, French Guyana|
|Dates||January 4March 21, 1994|
|Funding support||CITHER cruise: INSU, CNRS, PNEDC, France
Alkalinity, pH, and nutrients: CICYT, Spain
|Chief scientist||Dr. Laurent Memery, LODYC, Paris, France|
Parameters measured, institution, and responsible investigators
|CTD, salinity, XBT||LODYC||L. Memery, M. Arhan, H. Mercier|
|Nutrients||IIM.CSIC||X. Alvarez-Salgado, C. G. Castro|
|Tritium, He, 14C||LMCE||P. Jean Baptiste|
|TCO2||BNL/PNNL||L. Bingler, L. Arlen|
|Total alkalinity, pH||IIM.CSIC||A. F. Rios, G. Roson|
|Underway pCO2||LDEO||D. Chipman|
|Brazilian observer||RB||J. A. Fontainha|
|BNL||Brookhaven National Laboratory|
|IIM.CSIC||Instituto de Investigaciones Marinas, CSIC, Vigo, Spain|
|LDEO||Lamont-Doherty Earth Observatory|
|LMCE||Laboratoire de Modelisation du Climate et de l'Environnement|
|LODYC||Laboratoire d'Oceanographie Dynamique et de Climatologie|
|LPO||Laboratoire de Physique des Oceans|
|PNNL||Pacific Northwest National Laboratory|
|RB||Rio de Janeiro, Brazil|
Although the cruise took place aboard a U.S. ship, the R/V Maurice Ewing, the A17 section was a part of a three-year-long French hydrographic expedition (CITHER) in the South Atlantic. In order to make the TCO2 measurements on A17, four U.S. institutions had to combine forces to complete the work. The single-operator multi-parameter metabolic analyzer (SOMMA) and coulometer analytical system came from the Lamont-Doherty Earth Observatory (LDEO) at Columbia University; the TCO2 group leader L. Bingler was from Battelles Pacific Northwest Laboratory (PNNL); the assistant TCO2 analyst L. Arlen came from the NOAAs National Marine Fisheries Service (NMFS) Laboratory in Sandy Hook, New Jersey. The training and financial support of the analysts was carried out at and provided by BNL. In addition, PNNL paid for a barometer, which was installed in the LDEO system, and supported the production of a revised instrument manual for the SOMMA-coulometer systems. The TALK was measured by A. F. Rios and G. Rosn of CSIC, Instituto de Investigaciones Marinas of Vigo, Spain. The latter group also measured pH, so that as a result of the A17 cooperative scientific effort, the carbonate system was overdetermined. Underway pCO2 was also measured by David Chipman, who installed the pCO2 equipment in Montevideo, just before the start of the expedition
The Maurice Ewing departed Montevideo on January 4, 1994, and headed generally south in direction of the Falkland Islands, where measurements for the main section were to begin just to the north of the islands. On the way measurements for two test stations were taken. Measurements for the main south-north section were started on January 10 beginning on the Falklands Plateau, with station intervals normally of 30 nm decreasing to 9 nm depending upon the topography. The station work was interrupted by a storm for a day during the period of January 14-15. On January 21 the ship turned northwest toward Brazil and then east to sample the Porto Alegre western boundary section, work which was completed on January 26. A storm and problems with the CTD wire and rosette interrupted the work for two days, but by January 31 the ship reached the Vema channel between the Argentine and Brazil basins, thereafter moving northward to continue the A17 section until February 10, when work at station 115 was completed. The ship then transited to Salvador de Bahia, Brazil, making two additional test stations before arriving in port on February 13. In Salvador de Bahia, French CTD and hydrographic personnel were exchanged. The CO2 and nutrients measurement groups remained on board, however.
The Maurice Ewing departed Salvador de Bahia on February 17, 1994, and commenced sampling the Salvador western boundary section. However, difficulties were again experienced with the CTD and rosette, so that the completion of measurements for the latter section was delayed by two days. Due to these problems a decision was made to transit to Recife to pick up the expert Jean Pierre Gouillou, arriving from France, who was tasked with repairing the CTD and performing the software modification. The personnel transfer was completed during the period February 24-26, and the ship continued with the station samplings. With the removal of the first 500 m of the CTD wire, the CTD and rosette were restored to function, and after February 28 no additional problems were noted. By March 14 the main north-south section had been completed (station no. 210), and by March 15 the last section sampling between the Mid-Atlantic Ridge and Cayenne had begun, with sampling for the last station of the cruise (no. 235) taken just out from Cayenne on March 20. Some of the scientific personnel disembarked in Cayenne on March 21, 1994, whereupon the ship left immediately for Fort de France, Martinique, where the remainder of the scientific and crew disembarked.
A SOMMA (S/N 007) with CO2 detected by coulometry was used to determine TCO2 on the A17 section. TALK was determined by potentiometric titration using an automatic potentiometric titrator, Titrino Metrohm, with separate glass and reference electrodes. The pH was determined potentiometrically using a Metrohm Model 654 pH meter, a combination glass electrode, and National Bureau of Standards (NBS) buffers for standardization. The CO2 samples from more than 50% of the 235 CTD stations occupied during the A17 cruise were always drawn in conjunction with tracer samples (CFCs, tritium, etc.) and the standard WOCE variables (salinity, oxygen, temperature, and nutrients). As on previous cruises, not all stations could be sampled for TCO2 and TALK because of the time required for analysis of the samples (see Table 3.1 for inorganic carbon sample distribution). However, pH and the WOCE standard variables were measured on all samples.
Water samples were collected using a 32-position rosette with 8-L Niskin bottles developed at the Laboratoire de Physique des Oceans, IFREMER, Brest, France. The rosette was equipped with a Neil-Brown Mark-III CTD-O2 (see Brown and Morrison 1978). In order to check the pressure measurements and temperature of the CTD on board, inverse thermometers and pressuremeters, type SIS, were mounted in the Niskin bottles to be fired at the bottom. The signal of the CTD was transmitted to the hydrographic data acquisition system of the LPO. This new system, created around a UNIX work station, allowed the user to see in real time the vertical profiles of the variables measured and calculated in order to check the quality of the signal transmitted by the CTD. The set of data transmitted by the CTD with a cadence of 32 cycles per second was recorded on a diskette. After each station, the data profiles were plotted vs pressure following the procedure of Billant (1985).
At the end of each cast, a full suite of water samples were drawn in the following order: CFCs, helium, oxygen, TCO2, TALK, pH, nutrients, tritium, and salinity.
During the cruise 6778 samples were analyzed for salinity within 12 days of collection using a Guildline PORTASAL salinometer that was calibrated with standard seawater (Batch P123, K15 = 0.99994) produced at Wormley and dated June 10, 1993. The temperature of the thermostat was fixed at 21°C until station 134, and at 22°C from stations 135 to 235. The precision of the salinity determination was 0.002 from 181 pairs of samples taken from two rosette sampling bottles closed at the same depth. The accuracy of the bottle salinity data was 0.001.
Dissolved oxygen was determined by Winkler titration after the technique of Culberson and Huang (1987). The operational conditions and the analytical method, including the calculation of oxygen concentrations, followed the standard WOCE procedure and recommendations given by Culberson et al. (1991) in the WOCE Manual of Operations and Methods. Appropriate corrections for sample density, blanks, and volumetric expansion have been included. The precision of the analyses was 0.78 µmol/kg from 196 pairs of samples taken from two rosette sampling bottles closed at the same depth. In total, 6756 oxygen analyses were completed during the A17 section.
The nutrients nitrate, nitrite, phosphate, and silicate were determined on every bottle closed on the A17 section by segmented flow analysis with a Technicon II Autoanalyzer. The combined nitrate and nitrite were determined after reduction of nitrate to nitrite in a Cd-Cu column according to the procedure of Mourio and Fraga (1985). The method was calibrated by diluting concentrated primary standards of dried salts (KNO3, KH2PO4, and SiF6Na2) dissolved in Milli-Q water with aged, filtered, and low-nutrient seawater and analyzing these substandards for each run of samples analyzed on the autoanalyzer. Phosphate was determined according to the procedure of Hansen and Grasshoff (1983) as modified by Alvarez-Salgado et al. (1992). The accuracy of the method was 0.01 µmol/kg. Silicate was determined according to Hansen and Grasshoff (1983) using ascorbic acid as the reducing agent. The accuracy of the method was 0.25 µmol/kg. Quality control and consistency of nutrient measurements can be seen in Groupe CITHER-2 (1996).
Table 3.1 summarizes the carbonate system variables measured on WOCE Section A17.
|Parameter||Total stations on a section||No. of stations sampled for carbonate system||% of total stations sampled||CSP samples analyzed|
aCertified reference material.
As on previous cruises, TCO2 was determined using an automated SOMMA dynamic headspace sample processor (S/N 007) with coulometric detection of the CO2 extracted from acidified samples. A description of the SOMMA-coulometry system and its calibration can be found in Johnson et al. 1987; Johnson and Wallace 1992; and Johnson et al. 1993. Further details concerning the coulometric titration can be found in Huffman (1977) and Johnson et al. (1985). The methods used for discrete TCO2 on WOCE sections have been extensively dealt with in previous reports (Johnson et al. 1998a) and only need to be briefly summarized.
Seawater samples were collected in 300-mL ground-glass stoppered bottles and poisoned with 100 L of a 100% saturated solution of HgCl2 to prevent biological alterations. Prior to analyses, the samples were stored in the dark and thermally equilibrated to within 23°C of the thermostatted SOMMA system (sample pipette and sample bath), which was kept at a constant temperature of approximately 20°C. The analysis of the TCO2 samples was usually completed within 14 h of collection (see DOE 1994). Duplicate samples were usually collected on each cast at the surface and from the bottom waters and analyzed within the run of cast samples from which they originated. Following standard procedure, certified reference material (CRM) was routinely analyzed during the sample analyses (approximately one CRM for every 30 samples) according to DOE (1994). The CRMs were supplied by Dr. Andrew Dickson of the Scripps Institution of Oceanography, and the A17 cruise analysts were supplied with batch 18. The certified values for batch 18 were S = 35.298 and TCO2 = 2115.15 µmol/kg. The CRM TCO2 concentration was determined by vacuum-extraction and manometry in the laboratory of C. D. Keeling at Scripps Institution of Oceanography (SIO).
The SOMMA injected an accurately known volume of seawater from an automated to-deliver (TD) pipette into a stripping chamber. Following acidification of the seawater and continuous gas extraction, the resultant CO2 was dried and coulometrically titrated on a model 5011 UIC coulometer with a maximum titration current of 50 mA in the counts mode [the number of pulses or counts generated by the coulometers voltage-to-frequency converter (VFC) during the titration was displayed]. In the coulometer cell, the acid (hydroxyethylcarbamic acid) formed from the reaction of CO2 and ethanolamine was titrated coulometrically (electrolytic generation of OH–) with photometric endpoint detection. The product of the time and the current passed through the cell during the titration (charge in Coulombs) was related by Faradays constant to the number of moles of OH– generated and thus to the moles of CO2, which reacted with ethanolamine to form the acid. The age of each titration cell was logged from its birth (time that electrical current is applied to the cell) until its death (time when the current is turned off). The age was measured in minutes from birth (chronological age) and in mgC titrated since birth (carbon age).
The system was controlled with an IBM-compatible PC equipped with two RS232 serial ports (coulometer and barometer), a 24-line digital input/output (I/O) card (solid state relays and valves), and an analog-to-digital (A/D) card (temperature, conductivity, and pressure sensors). The cards were manufactured by Real Time Devices (State College, PA 16803). The SOMMA temperature sensors (model LM34CH, National Semiconductor, Santa Clara, CA), with a voltage output of 10 mV/°F, were calibrated against thermistors certified to 0.02°F prior to the cruise using a certified mercury thermometer. These sensors monitored the temperature of SOMMA components, including the pipette, gas sample loops, and the coulometer cell. The SOMMA software was written in GWBASIC Version 3.20 (Microsoft Corp., Redmond, WA), and the instruments were driven from an options menu appearing on the PC monitor. With the coulometer operated in the counts mode, conversions and calculations were made using the SOMMA software rather than having the programs and the constants hardwired into the coulometer circuitry.
The SOMMA-coulometry systems were calibrated with pure CO2. The calibration hardware consisted of an eight-port gas sampling valve (GSV) with two sample loops of known volume (determined gravimetrically by the method of Wilke et al. 1993) connected to the calibration gas through an isolation valve with the vent side of the GSV plumbed to a barometer. When a gas loop was filled with CO2 at known temperature and pressure, the mass (moles) of CO2 contained therein was calculated, and the ratio of the calculated mass to that determined coulometrically was the calibration factor (CALFAC). The CALFAC was used to correct the subsequent sample titrations for small departures from 100% recoveries (DOE 1994). The standard operating procedure was to make gas calibrations daily for each newly born titration cell (normally, one cell per day). Normally, two or three sequential gas calibrations were run per cell between the carbon ages of 39 mgC with the last CALFAC used for calculation of TCO2 if it was consistent with the preceding CALFAC (i.e., agreement to ±0.1% or better). The mean CALFAC and the standard deviation of the mean are shown in Table 3.2. The CALFAC for system 007 remained very stable throughout the A17 section (the change in TCO2 concentration due to change in CALFAC was 0.05% or 1.0 µmol/kg) over the period November 1993 through March 1994. The mean carbon age for the mean CALFAC shown in Table 3.2 was 8.9 ± 5.1 mgC titrated (N = 73).
The to-deliver volume (Vcal) of the sample pipettes was determined (calibrated) gravimetrically in November 1993 prior to the cruise. The calibration was checked periodically (for A17, once weekly) by collecting aliquots of deionized water dispensed from the pipette into preweighed serum bottles. The serum bottles were crimp-sealed and weighed immediately during the on-shore laboratory calibrations, or returned to shore where they were reweighed on a model R300S (Sartorius, Gttingen, Germany) balance as soon as possible. The apparent weight (g) of water collected (Wair) was corrected to the mass in vacuo (Mvac) and the calibrated TD pipette volume (Vcal) was calculated by dividing Mvac by the density of the calibration fluid at the calibration temperature (tcal). For A17, Vcal was 28.9315 ± 0.0033 mL at a tcal of 19.81°C (N = 47). The sample volume (Vt) at the pipette temperature was calculated for all A17 samples from the expression
Vt = Vcal [1 + av (t − tcal)] , (3.1)
where av is the coefficient of volumetric expansion for Pyrex-type glass (1 X 105/°C), and t is the temperature of the pipette at the time of a measurement. The mean pipette temperature or analytical temperature (t) on the A17 section was 19.70 ± 0.29°C.
The factory-calibrated coulometer was electronically calibrated independently in the laboratory in November 1993, prior to the cruise as described in Johnson et al. (1993, 1996) and DOE (1994); and the terms INTec and SLOPEec were obtained and entered into the software for system 007. The micromoles of carbon titrated (M), whether extracted from water samples or the gas loops, was
M = [Counts / 4824.45 − (Blank X Tt) − (INTec X Ti)] / SLOPEec , (3.2)
where 4824.45 (counts/mol) is a scaling factor obtained from the factory calibration, Tt is the length of the titration in minutes, Blank is the system blank in µmol/min, INTec is the intercept from electronic calibration in µmol/min, Ti is the time in minutes during the titration where current flow was continuous, and SLOPEec is the slope from electronic calibration. Note that the slope obtained from the electronic calibration procedure applied for the entire length of the titration, but the intercept correction applied only for the period of continuous current flow (usually 34 min) because the intercept can only be calculated from calibrated levels of current flowing continuously. The coulometer electronic calibration should not change over the duration of the cruiseshown for earlier cruises although not without some exceptions (Johnson et al. 1998b)and system 007 was not electronically recalibrated during the A17 section. The electronic and gas calibration coefficients for system 007 are summarized in Table 3.2.
|Period||SLOPEec||INTec (µmol/min)||CALFAC (n)||St. Dev.||Rel. st. dev. (%)
|Nov. 1993||0.994635||0.000840||1.005434 (2)||0.000230||0.02|
|A17 section||0.994635||0.000840||1.005049 (73)||0.000466||0.05|
For water samples, the discrete TCO2 concentration in µmol/kg was calculated from:
TCO2 = M x CALFAC x [1 / (Vt x p;)] x dHg (3.3)
where p is the density of sea water in g/mL at the measurement temperature and sample salinity calculated from the equation of state given by Millero and Poisson (1981), and dHg is the correction for sample dilution with bichloride solution (for A17 dHg = 1.000333).
Quality control and quality assurance (QC-QA) were assessed from the results of the 163 CRM analyses made during the A17 section. The mean and standard deviation of the differences between the measured and the certified TCO2 (measured certified) are given in Table 3.3, and the temporal distribution of the differences is plotted in Fig. 3.1
|System||∆TCO2 (µmol/kg)||St. dev.(µmol/kg)||n|
The overall accuracy of the CRM analyses was better than 1 µmol/kg on system 007 for the A17 section. The precision of the CRM determination is the standard deviation of the differences between the measured and certified CRM TCO2 (±1.64 µmol/kg, N = 163). The outlier results are summarized in Table 3.4. Because six of the CRMs analyzed on A17 were considered to be outliers—meaning that the analytical difference (∆TCO2) between the measured and certified TCO2 exceeded ±5.0 µmol/kg (measured - certified)—these data are not included in Table 3.4.
Throughout the WOCE work, care was taken to titrate a limited number of samples in each coulometer cell to avoid excessive cell carbon ages and coulometer-cell-solution exhaustion or failure. In actual practice, this has meant that, on average, no cell was used to titrate more than a single 36-bottle station (a cell age of 35 mgC titrated), and experience has confirmed this practice (Johnson et al. 1998b).
Fig. 3.1. Temporal distribution of differences between measured and certified TCO2 for analyzed on SOMMA-coulometry system 007 during WOCE section A17. The differences were calculated by subtracting the certified TCO2 from the measured TCO2.
This convention was not followed on the A17 section because, at this point in the program, experimental evidence was needed concerning the actual lifetime of the cells. Hence, the A17 cells were run so that their carbon ages (mgC titrated) routinely exceeded the 35 mgC limit by factors of 1.5 - 2.5. Based on thousands of CRM analyses made during the CO2 survey and an overall precision of 1.6 µmol/kg for the coulometric determination of TCO2, an empirical definition of "cell failure" was proposed. Failure was defined as two successive CRM analyses with a difference >5 µmol/kg on a cell whose carbon age exceeded 35 mgC. The 5 µmol/kg limit was chosen because it was equivalent to three standard deviations in precision. These "failures" have been designated as outliers (see Table 3.4). Table 3.4 indicates that two of the A17 cells (on 2.9 and 3.17) exhibited outliers, but that the second CRM analysis at a later carbon age with these cells was accurate. Hence, the sample data obtained with them were not flagged. For failed cells (2.24, 3.19, and 3.22), a quality flag of 3 the WHPO questionable measurement flag was assigned to those samples analyzed between the carbon age at the time of the last accurate CRM analysis and the carbon age at failure or cell death. However, based on WHPO criteria, the flagged measurements could be correct but may be open to interpretation; we have no direct evidence that they are not correct. The data shown in Table 3.4 also suggest that the original decision to set a conservative limit on cell lifetimes of 35 mgC was sound because failures or outliers become more frequent after 35 mgC.
The second phase of the QC-QA procedure was an assessment of precision, which is presented in Table 3.5. The single-system precision was determined from samples with duplicates analyzed on system 007.
|Date||CRM no.||Carbon age (mgC)||∆TCO2 (µmol/kg)||Comments|
|2.9.94||596||34.4||+11.47||CRM OK at 53.2 mgC; no sample data flagged|
|2.24.94||261||59.4||−6.99||Sample data flagged between 40 and 71 mgC|
|3.17.94||140||64.8||+7.12||CRM OK at 86.8 mgC; no sample data flagged|
|3.19.94||47||92.3||+6.46||Sample data flagged between 73 and 92 mgC|
|3.22.94||441||65.9||+5.91||Sample data flagged between 45 and 71 mgC|
|sbs (µmol/kg)||St. dev. (µmol/kg)||K|
Single-system precision has been assessed in Table 3.5 as "between-sample" precision (σbs), which is the mean absolute difference between duplicates (n = 2) drawn from the same Niskin bottle, where K is the number of samples with duplicates analyzed.
Although the single-system sample precision (±0.73 µmol/kg) was excellent, it cannot be taken as the precision of the TCO2 determination for the A17 section for two reasons unique to this cruise:
During section A17, the replicate samples were always analyzed one right after the other. On other WOCE sections, replicate analyses were spaced such that the interval between replicates was >3 but <12 h. This was done to provide a measure of drift (change in system response) during a sequence of sample analyses on the assumption that drift would be reflected in the single-system precision by an increase in the imprecision of the duplicate analyses. Running the duplicates in sequence eliminated the possibility of detecting drift, and sample precision consequently was probably overestimated.
An evaluation of the samples for which duplicates were taken indicated that 10 duplicate pairs exhibited very poor precision (absolute difference between replicates from 7 to 280 µmol/kg). These samples were flagged when the data set was submitted, and they are not included in the precision given in Table 3.5. Further study indicated that 9 of the 10 pairs originated from the surface rosette sample bottle (stations 8, 13, 25, 51, 61, 155, 177, 188, and 230) from 0 to 5 m, and that only one pair originated from a deep bottle (5334 mat station 157). If the flagged results were used to calculate precision, then sbs was 3.34 ± 18.70 µmol/kg (K = 115) for the rosette surface bottle duplicates and 0.82 ± 1.41 µmol/kg (K = 121) for the nonsurface bottle duplicates. These data suggested that the observed imprecision did not lay with the TCO2measurement system. The cause was probably due to an occasional but undetected mechanical problem with the rosette, especially when the rosette bottles were closed at or near the surface. Alternatively, the on-deck sampling procedure at the rosette could have caused the degassing of CO2 into the Niskin bottle headspace during the time it took to draw the duplicate samples.
For the above reasons, the precision of the TCO2 determination on the A17 section was taken to be the standard deviation of the CRM differences (measured − certified) or ±1.64 µmol/kg (Table 3.3) instead of the single-system precision of ±1.02 µmol/kg given in Table 3.5.
The final step in the QC-QA procedure was the ship-to-shore comparison. Here, sample duplicates (commonly called the Keeling samples) were analyzed in real time at sea by continuous gas extraction/coulometry and later, after storage, on shore by vacuum extraction/manometry. The Keeling samples were collected in specially provided, threaded 500-mL glass bottles with 4 mL of headspace volume, poisoned with 100 µL of a saturated HgCl2 solution, and then sealed airtight with a greased ground-glass stopper that was secured to the bottle with a threaded plastic screw cap. The cap was bored out to fit over the top of the stopper and mated to the bottle threads. The airtight seal was made by gently tightening the cap until a secure seal between the stopper and bottle was attained. Overtightening caused the bottles to break immediately or during transit so that considerable care and practice were required to prepare a sample that would survive the journey back to SIO. The manometric analyses for 21 samples collected from 14 stations during section A17 were completed by December 1994 in the SIO laboratory of C. D. Keeling. The results of the comparison are given in Table 3.6. The mean ship-to-shore analytical difference (ship − shore) and the standard deviation of the differences was −0.09 ± 1.50 µmol/kg (N = 21). This was the best agreement between the ship and shore duplicate sample analyses made by any measurement group with or without BNL-supported equipment during the entire CO2 survey. Prior to and subsequent to the A17 section, the ship-to-shore comparisons had and have consistently yielded slightly lower TCO2 values (~2 µmol/kg) for samples analyzed in real time aboard ship compared to the reference analyses made at SIO (Wallace 2002).
Table 3.6 is particularly useful in view of the problems with surface bottle precision, which suggested the possibility of mechanical or chemical problems during rosette sampling during the A17 section. Inspection of the data in the table indicates excellent agreement between surface sample duplicates analyzed on ship and on shore and indicates that the incidence of poor precision, for whatever reason, probably did not compromise the accuracy of the A17 TCO2 data. Indeed, Tables 3.3, 3.5, and 3.6 show that the TCO2 data set for the A17 section was internally consistent and highly accurate and precise with respect to the both the CRM, the seawater duplicate samples, and the ship-to-shore comparison seawater samples. Hence, no correction for CRM differences has been applied to the data, and the TCO2 data clearly met survey criterion for accuracy (4 µmol/kg) and precision. The reader is also referred to a recent assessment of TCO2 data quality in the Atlantic Ocean resulting from comparisons of TCO2 analyzes from crossover points sampled by different cruises between 1990 and 1998 (Wanninkhof et al. 2003).
TALK was determined with a Titrino Metrohm automatic potentiometric titrator using separate glass working and reference electrodes. Potentiometric titrations were carried out in a covered but not completely closed (headspace present) titration flask to a final pH of 4.4 as described by Perez and Fraga (1987a). The electrodes were standardized using an NBS buffer of pH 7.413, checked using an NBS buffer of 4.008, and acclimated in a seawater solution buffered to a pH of 4.4. To determine the systematic errors produced by variations of the electrode residual liquid-junction potential, titration curves were performed each week in CO2-free seawater acidified to pH 4.0 with hydrochloric acid as described by Culberson (1981). The titration curves were linearized, and the inverse slope was taken to represent the apparent hydrogen ion activity coefficient. The decimal logarithm difference (ranging from 0.01 to 0.06) between the apparent activity coefficients of the electrode and those given by Mehrbach et al. (1973) at the same salinity and temperature with their electrode was the pH difference added to the final pH of the sample alkalinity titration to make our results equivalent with theirs using the constants of Mehrbach et al. (1973).
|Station||Date||Niskin no.||Depth (m)||TCO2 ship (µmol/kg)||TCO2 shore (µmol/kg)||ΔTCO2 Ship − shore< (µmol/kg)|
aThe surface samples are usually the mean of two analyses. The SIO results are always mean of two analyses.
bThe precision of the method was ±1.64 µmol/kg.
During the cruise, the TALK of 146 CRMs from batch 18 was determined by this method. The TALK for batch 18 was not known at the time of the cruise because it was not measured during the original TCO2 certification. Subsequently, TALK was measured at SIO on archived samples from batch 18 with the value TALK = 2297.77 µmol/kg. The mean and standard deviation of the differences between the measured and the certified TALK (measured − certified) are given in Table 3.7, and the temporal distribution of the differences is plotted in Fig. 3.2.
|Mean ∆TALK (µmol/kg)||St. dev. (µmol/kg)||n|
Fig. 3.2. Temporal distribution of differences between measured and certified TALK for CRM analyzed during WOCE section A17. The differences were calculated by subtracting the certified TALK from the measured TALK
The precision of the method was assessed from 59 pairs of samples taken from two rosette sampling bottles closed at the same depth. The mean difference or precision of the TALK determination was 1.2 ± 1.1 µmol/kg, or approximately 0.1%.
The TALK values were checked with CRMs and according to the data presented in Fig. 3.2 and Table 3.7, the mean analytical difference between measured and certified CRM was 2.1 ± 1.7 µmol/kg. Normally, the correction of 2 µmol/kg should be applied to the TALK values measured on WOCE A17 section. On the other hand, in the comparison of carbon system variables measured in the Atlantic Ocean, Wanninkhof et al. (2003) showed a deviation of 7 µmol/kg with respect to A09 in the crossover analysis, but the multiple-parameter linear regression suggested that the TALK data were 56 µmol/kg higher compared with data from other cruises in the tropical and southern regions. Wanninkhof et al. (2003) argued that the internal consistency suggested that the TALK was higher by 8 µmol/kg; therefore, a decrease in TALK of 6 µmol/kg would bring the values in better agreement. However, because a decrease was suggested for A09, the bias between A17 and A09 remains of the same magnitude. Taking into account that the offset of 6 µmol/kg is the recommended correction, no adjustment was recommended.
In order to clarify this apparent offset, a comparison of A17 TALK data was made with WOCE A14 data obtained during the CITHER-3 cruise carried out in January and February 1995 in the eastern South Atlantic, and with recent cruises carried out following approximately the same line A17 (FICARAM II and FICARAM IV) in March and April 2001 and 2002, respectively. Figure 3.3 shows the comparison of the normalized TALK (NTA) data with regard to the silicate content among the four cruises. The upper graph presents the data comparison corresponding to the zone between 3°S and 29°S at 1000-2000 dbar.
Fig. 3.3. Relationship between normalized TALK (NTA) and silicate, both in µmol/kg, for the cruises CITHER-2, CITHER-3, FICARAM-2, and FICARAM-4. (A) Between 3°S and 29°S at 1000-2000 dbar. (B) Between 9°N and 25°S at 2000-3000 dbar.
The lower graph exhibits the same kind of comparison but along the latitude 9°N to 25°S at 2000-3000 dbar. In both cases, the slopes NTA vs silicate of the three cruises CITHER-3, FICARAM-2 and FICARAM-4 are coincident, and the slope NTA vs silicate for CITHER-2 is higher and parallel to the others. Taking a concentration of silicate of 30 µmol/kg, we find a difference of 8.0 ± 0.6 µmol/kg in the zone 3°S-29°S and 7.7 ± 1.1 µmol/kg in the zone 9°N-25°S.
The comparison made with recent cruises is coincident with the internal consistency made by Wanninkhof et al. (2003) that suggested that A17 TALK was higher by 8 µmol/kg. Therefore, a downward correction of 8 µmol/kg was applied to all the TALK values.
For pH measurements, a Metrohm 654 pH meter with a Metrohm 6.0233.100 combination glass electrode was used. The pH electrode was standardized in the same way as the alkalinity electrodes [NBS buffer at pH 7.413 to calibrate, NBS buffer at pH 4.008 to check calibration according to Perez and Fraga (1987b), acclimatization in a pH 4.4 seawater buffer]. The latter was made up in 1 kg of CO2-free seawater with 4.0846 g of C8H5KO4 and 1.52568 g of B4O7Na2-10H20 (borax). The temperature measurement for each pH sample was done with a Pt-100 probe, and pH values were normalized to 15°C. Changes in electrode response were corrected in the same manner as for alkalinity using titration curves generated at the end of the cruise in seawater (S = 34.655) with HCl at 25.7°C. The resulting correction factor for pH was 0.026 ± 0.001, which was added to the pH analyses. The precision of the pH determination was assessed from 186 pairs of samples collected from two rosette bottles closed at the same depth in the same way as for TALK. The precision of the determination was 0.002 ± 0.003.
Surface pCO2 was measured continuously using a shower-type equilibrator with detection of CO2 by an infrared analyzer according to a design and techniques reported by Broecker and Takahashi (1966). Partial pressures of CO2 in the surface seawater have been computed from the CO2 concentration measured in dried equilibrated air in the following manner. The pressure of equilibration, reduced by the vapor pressure of water (computed at the equilibrator temperature) was applied to the CO2 concentration to yield the pCO2 at equilibrator temperature. This value was then adjusted to the sea surface temperature using the relationship given by Takahashi et al. (1993) and expressed in units of microatmospheres (atm). The R/V Maurice Ewing was not equipped with a thermosalinograph at the time of this cruise. Surface temperature was measured by means of a pair of thermistors attached to the keel. These thermistors were calibrated in place against a thermometer traceable to NIST. The resolution of the device used to read the thermistor was 0.1°C. These data will likely be combined with other surface pCO2 data from the Atlantic Ocean to form a separate report and will not be discussed further here.
The pH values have a good precision, as shown by the reproducibility (0.002 ±0.003) of the 186 pairs of samples collected from two rosette bottles closed at the same depth. During the cruise, surface seawater stored in 25-L plastic containers was used as pH quasi-steady seawater sub-standard (SSS). At each station, the pH of this SSS was measured before and after each series of samples, but samples of CRM were not analyzed. Therefore, the pH data could be displaced with respect to one station to other. The internal consistency comparison made in Wanninkhof et al. (2003) between measured and calculated (from TALK and pH) TCO2 showed a bias. They suspect that the calculation involving pH is the culprit.
Consequently, to correct the deviations of pH between stations, we carried out an internal consistency check using the CRM-referenced TCO2 and TALK data and the fugacity of CO2 (fCO2) in surface waters. The first step to check if there is an offset is to compare the variations of fCO2 (calculated - measured) in surface waters (ΔfCO2) with the variations of the average TCO2 (calculated - measured) of water column data (ΔTCO2). Figure 3.4 shows a negative and significant correlation (r2 = 0.39) between ΔfCO2 and ΔTCO2, which means that there is a pH bias between stations.
Fig. 3.4. Relationship between variations of fCO2 (calculated - measured) in surface waters (ΔfCO2) and variations of the average TCO2 (calculated - measured) of water column data (ΔTCO2).
The correction of pH was made using the surface data of calculated pH from measured fCO2 and TCO2. In all cases the dissociation constants of Mehrbach et al. (1973) as modified by Lueker et al. (2000) were used. After pH values were corrected, the relationship between variations of fCO2 (calculated measured) and variations of the average TCO2 (calculated measured) did not show any correlation (r2 = 0.002), indicating that the bias had disappeared.
Once pH values were corrected, TCO2 was calculated from pH and and compared with the measured TCO2. Figure 3.5 is a plot of the coulometrically measured TCO2 vs TCO2 derived from the TALK and pH measurements made on the A17 section. This plot shows a high regression (r2 = 0.997; p < 0.0001) with an average error of the estimate of 2.8 µmol/kg and a slope very close to unit (1.0088 0.0011). The average difference between calculated and measured TCO2 during the cruise was 4.3 ± 2.9 µmol/kg.
Fig. 3.5. Relationship between measured and calculated TCO2 from TALK and pH using the dissociation constants given by Lueker et al. (2000).
Figure 3.6 compares the fCO2 underway measured and fCO2 calculated from surface TALK and pH data waters obtained during the A17 cruise. There is a high regression (r2 = 0.992, p < 0.0001) with an average error of the estimate of ±3.1 µatm and a slope close to unity (1.0055 ± 0.0066). The average difference between calculated and measured pCO2 was -2.1 ± 3.1 µatm.
After the TALK and pH corrections, the regressions between both measured and calculated TCO2 and fCO2 are higher (r2 = 0.997, r2 = 0.992, respectively) than those obtained by Rios and Perez (1999) using the original dissociation constants of Mehrbach et al. (1973) [r2 = 0.990, r2 = 0.966, respectively]. Also, their slopes are closer to unity in both cases (1.00088 vs 1.024 for TCO2 and 1.0055 vs 0.899 for fCO2), and the average error of their estimates decreases from ±4.4 to ±2.8 µmol/kg for TCO2 and from ±6 to ±3.1 µatm for fCO2.
In the light of the comparison of the TALK data with other cruises and the analysis of the internal consistency among the four variables of carbon system measured during the A17 section, we would suggest to apply the following corrections for the data in the CDIAC database:
According to the comparison between TALK measured on A17 and recent cruises (CITHER-3, FICARAM-2, and FICARAM-3), a downward correction of 8 µmol/kg is proposed for TALK.
Based on the internal consistency check carried out among the four carbon system variables, we propose to modify the pH measurements adding by stations the values gathered in Table 3.8.
Fig. 3.6. Relationship between fCO2 measured and fCO2 calculated from TALK and pH using the dissociation constants given by Lueker et al. (2000).
This database (NDP-084) is available free of charge from CDIAC. The data are available from CDIAC's anonymous file transfer protocol area. The complete documentation and data can be obtained from the CDIAC oceanographic Web site.
For additional information, contact CDIAC.
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