FY 1996

The carbonate system in seawater: laboratory and field studies

Murphy, P.P.

Ph.D. dissertation, Chemistry Department, University of Washington, 288 pp (1996)

Comparison of CO concentrations in bubbles from ice cores with modern direct observations in the atmosphere suggests that the present atmospheric reservoir of CO and the rate of increase are larger than at any time over the past 160,000 years. Increases in atmospheric CO are attributed to fossil fuel burning and changing land use (deforestation and conversion to agricultural purposes). However, the atmospheric accumulation accounts for only half of the known CO sources. The ocean and the terrestrial biosphere can also act as sinks for CO, but the partitioning between these two reservoirs is uncertain. Oceanic uptake estimates of anthropogenic carbon have ranged from a third to nearly all of the 3.6 × 10 grams which do not remain in the atmosphere. Long-term, high quality measurements of carbonate system parameters, as well as an understanding of their geographical and seasonal variability, are required to assess and predict any increases in ocean carbon. The work presented here thus has two different, but related, aspects--a laboratory data component and a field data component. The laboratory work presented here indicates that a thermodynamic model for the carbonate system in seawater can account for the principal species and chemical reactions. A comparison of measured carbonate system parameters and model-predicted values suggests a new set of carbonate equilibrium constants from amongst those in common use. Improvements in existing methodologies for determination of the fugacity of CO are also proposed. A new set of measurements of air-sea CO disequilibrium made in the South Pacific Ocean during austral autumn is presented here which suggests that the South Pacific may be a smaller sink for carbon than previously thought. Data to calculate air-sea fluxes have been scarce in this region, and therefore uncertain. CO fluxes were calculated from the new pCO observations using four different wind fields and two different parameterizations for the gas exchange coefficient in order to examine the sensitivity of the basin-wide flux to different data choices. The sensitivity analysis used for the South Pacific measurements was also applied to measurements made in the subarctic North Pacific. Four sets of pCO observations made in the same region within the same 30-day period of the year were compared to examine the uncertainty of using one cruise track to determine a mean regional and seasonal value of CO disequilibrium. The results indicate that the high spatial and temporal variability observed here can translate into significant uncertainties in regional mean assessments of carbon uptake by the oceans.