Catherine Goyet

New determination of carbonic acid dissociation constants in seawater as a function of temperature and salinity

The apparent dissociation constants of carbonic acid (K1∗, K2∗) were determined at atmospheric pressure by potentiometric titrations in artificial seawater. Constants (K1∗, K2∗) were extracted from the experimental titration curves both by a non-linear curve-fitting procedure, and by using the Gran functions. There is good agreement between the two mathematical calculations for both constants. Throughout the salinity range of 10 < S < 50 and the temperature range of −1°C < T < 40°C this new data set agrees closely with the previous set of Hansson (1972). His results, combined with ours and fitted as a function of temperature and salinity, satisfy the following relationships: pK1∗=812.27T+3.356−0.00171 x S x ln(T)+0.000091 x S2 pK1∗=1450.87T+4.604−0.00385 x S x ln(T)+0.000182 x S2 where the constants K1∗ are expressed in moles per kilogram of solution.

Temporal variations of pCO2 in surface seawater of the Arabian Sea in 1995

As part of the JGOFS program in the Arabian Sea we continually measured the partial pressure of CO2 gas in surface seawater (pCOsea2) and in the atmosphere above the sea surface (pCOair2) along six similar cruise tracks in January, February, March, July, November, and December of 1995. The observed variations of pCOsea2 reached over 260 μatm close to the Omani coast in January–February and July–August. Offshore the pCOsea2 variations with time were relatively small (<40 μatm). The pCOair2 shifted by 15 μatm between the northeast and southwest monsoons. In order to estimate monthly and annual mean of CO2 flux in the Arabian Sea, we used sea surface temperature and wind speed data, given on a 2.5°×2.5° resolution, from FNMOC and interpolated the observed pCOsea2 data both in time and space. Close to the Omani coast, the estimated monthly mean CO2 efflux across the air–sea interface varies from 8.3 to 44 mmol m-2 month-1 during most of the year and up to 312.8 mmol m-2 month-1 during the southwest monsoon season (July/August). Overall, the annual mean CO2 flux from the northern Arabian Sea to the atmosphere is estimated to be 0.46 mol m-2 yr-1 or 7.0 Tg C yr-1.

Consistency and synthesis of Pacific Ocean CO2 survey data

Between 1991 and 1999, carbon measurements were made on twenty-five WOCE/JGOFS/OACES cruises in the Pacific Ocean. Investigators from 15 different laboratories and four countries analyzed at least two of the four measurable ocean carbon parameters (DIC, TAlk, fCO2, and pH) on almost all cruises. The goal of this work is to assess the quality of the Pacific carbon survey data and to make recommendations for generating a unified data set that is consistent between cruises. Several different lines of evidence were used to examine the consistency, including comparison of calibration techniques, results from certified reference material analyses, precision of at-sea replicate analyses, agreement between shipboard analyses and replicate shore based analyses, comparison of deep water values at locations where two or more cruises overlapped or crossed, consistency with other hydrographic parameters, and internal consistency with multiple carbon parameter measurements. With the adjustments proposed here, the data can be combined to generate a Pacific Ocean data set, with over 36,000 unique sample locations analyzed for at least two carbon parameters in most cases. The best data coverage was for DIC, which has an estimated overall accuracy of ∼3 μmol kg−1. TAlk, the second most common carbon parameter analyzed, had an estimated overall accuracy of ∼5 μmol kg−1. To obtain additional details on this study, including detailed crossover plots and information on the availability of the compiled, adjusted data set, visit the Global Data Analysis Project web site at: http://cdiac.esd.ornl.gov/oceans/glodap.

Surface water carbon dioxide in the southwest Indian sector of the Southern Ocean: a highly variable CO2 source/sink region in summer

Abstract Measurements of partial pressure of carbon dioxide (pCO2), total dissolved inorganic carbon (TCO2), total alkalinity (TA) and chlorophyll a (Chl a) have been made in surface water in the southwestern Indian sector of the Southern Ocean (20–85°E) in the austral summer (INDIVAT V cruise, January-February 1987). Between Antarctica and Africa, pCO2 distribution was linked to the oceanic frontal zones and Chi a variations. The pCO2 spatial structure was very close to that explored in summer 1967 in the same region but the pCO2 differences between the ocean and the atmosphere were smaller in 1987 than 20 years ago. At all latitudes we found strongly contrasting surface pCO2 characteristics between eastern (around 80°E) and western (around 25°E) regions; C02 sources were mainly in the west and CO2 sinks in the east. South of 60°S, the contrast could be due to biological activity. Between 60°S and the Antarctic Polar Front, intensification of upwelling might be responsible for the higher pCO2 values in the west.

Variability of C02 distributions and sea-air fluxes in the central and eastern equatorial Pacific during the 199–1994 El Nin˜o

Abstract As part of the U.S. JGOFS Program and the NOAA Ocean-Atmosphere Carbon Exchange Study (DACES), measurements of C02 partial pressure were made in the atmosphere and in the surface waters of the central and eastern equatorial Pacific during the boreal spring and autumn of 1992, the spring of 1993, and the spring and autumn of 1994. Surface-water p C0 2 data indicate significant diurnal, seasonal, and interannual variations. The largest variations were associated with the 1991–1994 ENSO event, which reached maximum intensity in the spring of 1992. The lower values of surface-water Δ p C0 2 observed during the 1991–1994 ENSO period were the result of the combined effects of both remotely and locally forced physical processes. The warm pool, which reached a maximum eastward extent in January-February of 1992, began in September of 1991 as a series of westerly wind events lasting about 30 days. Each wind event initiated an eastward propagating Kelvin wave which caused a deepening of the thermocline. By the end of January 1992 the thermocline was at its maximum depth, so that the upwelled water was warm and C0 2 -depleted. In April of the same year, the local winds were weaker than normal, and the upwelling was from shallow depths. These changes resulted in a lower-than-normal C02 flux to the atmosphere. The results show that for the one-year period from the fall of 1991 until the fall of 1992, approximately 0.3 GtC were released to the atmosphere; 0.6 GtC were released in 1993, and 0.7 GtC in 1994, in good agreement with the model results of Ciais et al. [ Science , 269 ,1098–1102; J. Geophys. Res. , 100 , 5051–5070]. The net reduction of the ocean-atmosphere C02 flux during the 1991–1994 El Nifio was on the order of 0.8 – 1.2 GtC. Thus, the total amount of C02 sequestered in the equatorial oceans during the prolonged 1991–1994 El Nin˜o period was about 25% higher than the severe El Nin˜o of 1982–1983.

The international at-sea intercomparison of fCO2 systems during the R/V Meteor Cruise 36/1 in the North Atlantic Ocean

The ‘International Intercomparison Exercise of fCO2 Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO2) systems, one discrete fCO2 system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO2 measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation of final fCO2 were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO2 in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO2 measurements can be made in good agreement (±3 μatm) with underway fCO2 data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO2 system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO2 results. Calculation of fCO2 from high-quality total dissolved inorganic carbon (CT) and total alkalinity (AT) measurements does not yield results comparable in accuracy and precision to fCO2 measurements.

The pH of the North Atlantic Ocean: Improvements to the global model for sound absorption in seawater

At frequencies below 1 kHz, sound absorption coefficients in the ocean are a function of pH, and at higher frequencies they are dependent upon MgSO4. The pH dependent terms are attributable to relaxation of B(OH)3 and MgCO3 species, and the ensemble effect has been approximated (Mellen et al., 1987a) as α = α1(MgSO4) + α2(B(OH)3) + α3(MgCO3), where α is the absorption coefficient in decibels per kilometer, and αn = (S/35)An[ƒ2ƒn/(ƒ2 + ƒn2)], where S is the salinity, An is the amplitude of each component and depends on pH, ƒ is the frequency, and ƒn is the relaxation frequency. Overall accuracy of ±15% in a requires that pH be known to 0.05 pH units. The presently used oceanic pH field for sound absorption models is derived from a combination of Geochemical Ocean Sections Study (GEOSECS) data and Soviet data from the 1978 Gorshkov atlas for the North Atlantic where GEOSECS data are absent. We compare the North Atlantic fields with the well-constrained Transient Tracers in the Ocean North Atlantic CO2 data set and find large differences. We further show that sufficiently strong correlations exist between CO2 system variables and other more commonly available hydrographic properties and that improved renditions of the pH field in other regions of the ocean are possible once equivalent local correlations are established, thus leading to greatly improved estimates of the sound absorption field.

The carbonate system in the Black Sea

We have measured both alkalinity and total carbon dioxide on a selected set of Black Sea samples from cruise 134 of R.V. Knorr, using gas extraction/coulometry techniques, and improved titration procedures that permit more accurate data than those obtained in earlier expeditions. Earlier results had shown an apparent excess in alkalinity, by a factor of 1.6, from the stoichiometric ratio predicted from the sequential oxidation of Redfield ratio organic matter by the species O2, NO3− and SO42−. Thus both the nature of the organic substrate and our fundamental knowledge of reaction stoichiometry in anoxic systems were called into question. We show that the total CO2 balance is consistent, within narrower limits than found earlier, with oxidation of organic matter by sulfate: 2CH2O+SO42− → 2HCO3−+H2S and consistent with work on sediment interstitial waters in anoxic conditions (Berneret al., 1970, Limnology and Oceanography, 15. 544–549: Ben Yaakov, 1973, Limnology and Oceanography, 18, 86–94). The total CO2 results are lower by 300μmol kg−1 in surface waters, and 50μmol kg−1 in deep waters, than data reported from the 1969 Atlantis II expedition. While changes in Black Sea hydrography have been documented, these CO2 system changes are far too large to be accounted for by these processes and are more likely the result of improved technique, rather than geochemical evolution.

Coulometric total carbon dioxide analysis for marine studies : assessment of the quality of total inorganic carbon measurements made during the US Indian Ocean CO2 Survey 1994-1996

Two single-operator multiparameter metabolic analyzers (SOMMA)-coulometry systems (I and II) for total carbon dioxide (TCO2) were placed on board the R/V Knorr for the US component of the Indian Ocean CO2 Survey in conjunction with the World Ocean Circulation Experiment-WOCE Hydrographic Program (WHP). The systems were used by six different measurement groups on 10 WHP Cruises beginning in December 1994 and ending in January 1996. A total of 18,828 individual samples were analyzed for TCO2 during the survey. This paper assesses the analytical quality of these data and the effect of several key factors on instrument performance. Data quality is assessed from the accuracy and precision of certified reference material (CRM) analyses from three different CRM batches. The precision of the method was 1.2 μmol/kg. The mean and standard deviation of the differences between the known TCO2 for the CRM (certified value) and the CRM TCO2 determined by SOMMA-coulometry were −0.91±0.58 (n=470) and −1.01±0.44 (n=513) μmol/kg for systems I and II, respectively, representing an accuracy of 0.05% for both systems. Measurements of TCO2 made on 12 crossover stations during the survey agreed to within 3 μmol/kg with an overall mean and standard deviation of the differences of −0.78±1.74 μmol/kg (n=600). The crossover results are therefore consistent with the precision of the CRM analyses. After 14 months of nearly continuous use, the accurate and the virtually identical performance statistics for the two systems indicate that the cooperative survey effort was extraordinarily successful and will yield a high quality data set capable of fulfilling the objectives of the survey.

The carbon dioxide system in the Arabian Sea

In 1995 the WHOI (C. Goyet) and MIAMI (F.J. Millero) groups participated on a number of research cruises in the Arabian Sea as part of the U.S. Joint Global Ocean Flux Study (JGOFS) sponsored by the National Science Foundation (NSF). This paper gives the results of our total inorganic carbon dioxide (TCO2), total alkalinity (TA) and potentiometric pH measurements made on Arabian Sea water samples during these cruises. Measurements made on Certified Reference Material (CRM) indicate that the reproducibility of the measurements was ±0.007 in pH, ±3.2 μmol kg-1 in TA, and ±1.2 μmol kg-1 in TCO2 (N=180). The surface measurements (0–30 m) of pH and normalized TCO2 and TA were quite uniform throughout the year (pH=8.1±0.05, NTCO2=1950±20 μmol kg-1 and NTA=2290±5 μmol kg-1). The larger variations in NTCO2 in the surface waters are related to changes in primary production and upwelling in the coastal waters. The depth profiles of pH, pCO2, TA, and TCO2 were similar to those in the Equatorial Pacific Ocean. The components of the carbonate system (CO2, HCO-3, CO2-3) and the saturation state (Ω) for calcite and aragonite were determined from the measurements of TA and TCO2. The waters below 600 and 3400 m in the Arabian Sea were undersaturated (Ω<1.0) for aragonite and calcite, respectively. The CO2 measurements have been combined with the nutrient data to examine the stoichiometric ratios of C/N, C/P, C/O2, and C/SiO2 of the waters. Marked differences were found for the waters above and below the oxygen minimum zone. The surface water results have been used to develop the following stoichiometry for phytoplankton in the Arabian Sea (CH2O)125(NH3)14(H3PO4)(SiO2)13. The oxidation of this material is due to reactions with O2 (77%) and NO3 (23%) with the resultant formation of N2 and N2O. The maximum amount of organic carbon oxidized has been estimated to be 3.1 μmol kg-1 in the deep waters with as much as 0.9 μmol kg-1 in the oxygen minimum zone with NO3. The maximum amount of CaCO3 dissolved in the deep waters is 116 μmol kg-1. These results, together with the organic material collected from the sediment traps, should be useful in characterizing the formation and degradation of plant material in the Arabian Sea.