Atlantic fCO₂ Crossovers

System Information

The purpose of this analysis was to determine if any significant systematic offset existed between the various legs of the WOCE/NOAA/JGOFS Atlantic Ocean measurements of fugacity of CO₂ (fCO₂). Three different types of instruments were used to measure discrete fCO₂ samples. With each an aliquot of seawater was equilibrated at a constant temperature of either 4° or 20°C with a headspace of known initial CO₂ content. Subsequently, the headspace CO₂ concentration was determined by non-dispersive infrared analyzer (NDIR) or by quantitatively converting the CO₂ to CH₄ and then analyzing it using a gas chromatograph (GC) with a flame ionization detector. The initial fCO₂ in the water was determined after correcting for the loss or gain of CO₂ during the equilibration process. This correction can be significant for large initial fCO₂ differences between the headspace and the water, and for systems with a large headspace-to-water volume ratio (Chen et al. 1995).

The system used by Takahashi and co-workers on A21/A12 (Chipman et al. 1993; DOE 1994) involved equilibration of a ~50-mL headspace with a ~500-mL sample at either 4°C or 20°C depending on ambient surface water temperatures. The Takahashi values, reported as partial pressure of CO₂ (pCO₂), were converted to fCO₂ using the correction factor (~ 0.996) given by Weiss (1974). Wanninkhof and co-workers utilized two systems during the Atlantic survey cruises. An NDIR-based system with ~500-mL samples was used for analyses during A16S and A16N (Wanninkhof and Thoning 1993). A GC-based system with samples collected in a closed, septum sealed bottle having a volume of ~120-mL of seawater and a headspace of ~10-mL was used for A05R (Neill et al. 1997). Wallace and co-workers used the setup described in Neill et al. on A08 but with bottles having a volume of ~50-mL of seawater and a headspace of ~5-mL.

Detectors were calibrated after every 4-12 samples with gas standards traceable to manometrically determined values of C. D. Keeling at SIO. An assessment of fCO₂ accuracy is difficult to determine because of the lack of aqueous standards. Estimates of precision based on duplicate samples range from 0.1 to 1% depending on fCO₂ and measurement procedure, with higher fCO₂ levels on the infrared based system (>700 µatm) giving worse reproducibility (Chen et al. 1995).

Crossover Analysis

The stations selected for each crossover were those with carbon data that were within roughly 100 km (˜1° latitude) of the crossover point. Data from deep water (>1500 m) at each of the crossover locations were plotted against the density anomaly referenced to 4000 dB (σ-4) and fit with a second-order polynomial. For the crossover compari

son all samples run at 4°C were normalized to 20°C by calculating the TALK from fCO₂ (4°C) and TCO₂, and subsequently calculating fCO₂ (20°C) from the TCO₂ and calculated TALK. The carbonate dissociation constants of Mehrbach et al., (1973) as refit by Dickson and Millero (1987) and ancillary constants listed in DOE (1994) are used for these calculations using the program of Lewis and Wallace (1998). Crossover information is given in the summary table bellow.

Upon examination, it became clear that there may a problem for the crossovers that required a temperature conversion. For example, temperature conversion from 4° to 20°C using the Mehrbach constants yield fCO₂ values in the deep Pacific that are about 50 µatm higher than those derived from use of Roy constants of the conversion. Since the discrepancy in dissociation constants has not been fully resolved, the crossover comparison for fCO₂ data analyzed at different temperatures and for comparisons of measured versus calculated values is problematic.

Data from deep water (>1500 m) at each of the 12 crossovers analyses at five locations, were plotted against the density anomaly referenced to 4000 dB (σ-4) and fit with a second-order polynomial. The difference and standard deviation between the two curves was then calculated from 10 evenly spaced intervals over the density range common to both sets of crossovers. The standard deviation for the 12 fCO₂ crossover comparisons was 13.4 µatm. The average of the absolute value of the differences was 9.8±8.4 µatm.

Summary

Summary Table of Crossover Results (opens a new window)

References

• Chen, H., R. Wanninkhof, R. A. Feely and D. Greeley (1995). Measurement of fugacity of carbon dioxide in sub-surface water: an evaluation of a method based on infrared analysis, NOAA technical report ERL AOML-85, 52 pp. NOAA/AOML.
• Chipman, D. W., J. Marra and T. Takahashi (1993) Primary production at 47N and 20W in the North Atlantic Ocean: A comparison between the 14C incubation method and mixed layer carbon budget observations. Deep-Sea Research II 40:151-169.
• Dickson, A.G., and F.J. Millero (1987) A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Research, 34:1733-1743.
• DOE (1994) Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water; version 2, A.G. Dickson and C. Goyet, eds. ORNL/CDIAC-74 Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee.
• Lee, K., F. J. Millero, R. H. Byrne, R. A. Feely and R. Wanninkhof (2000) The Recommended Dissociation Constants of Carbonic Acid for Use in Seawater. Geophysical Research Letters 27: 229-232.
• Lewis, E., and D. W. R. Wallace. 1998. Program Developed for CO₂ System Calculations. ORNL/CDIAC-105. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee.
• Merhbach, C., C.H. Culberson, J.E. Hawley, and R.M. Pytkowicz (1973): Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnology Oceanography, 18:897-907.
• Neill, C, K.M. Johnson, E. Lewis, and DWR Wallace (1997). Accurate headspace analysis of fCO₂ in discrete water samples using batch equilibration. Limnology Oceanography 42(8), 1774-1783.
• Wanninkhof, R. and Thoning, K. (1993) Measurement of fugacity of CO₂ in surface water using continuous and discrete sampling methods. Marine Chemistry, 44:189-204.
• Weiss, R. F. (1974) Carbon dioxide in water and seawater: the solubility of a non-ideal gas. Marine Chemistry 2:203-215.