Complexities in Estimating the Contribution of Minor, Unidentified Bases to Total Alkalinity in Seawater

Accurately measuring CO₂ system variables in the ocean is necessary not only to estimate carbon fluxes, but also to understand changes in seawater chemistry and its effect on marine organisms. Total alkalinity (A_T) is one of the most commonly used parameters to characterize the marine CO₂ system, but compared to the other measureable CO₂ system variables, the measurement of A_T necessarily titrates all bases present in seawater, and not just those of interest (carbonate and bicarbonate). While the majority of non-CO₂ bases in seawater are identified and easily accounted for, there is extensive evidence for unidentified bases (excess alkalinity, A_X) at levels exceeding the uncertainty of the A_T measurement. Failure to account for these unidentified bases in a quantitatively meaningful way will lead to errors in interpreting A_T as a CO₂ system variable. Few methods estimating A_X are presented with uncertainty estimates, and none have so far dealt with the uncertainty in the total boron content (B_T), the second largest acid-base system in the ocean following the CO₂ system. I will describe an open-cell titration approach that can, together with equilibrium modelling, quantify A_X as a combination of one or several acid-base systems with apparent total concentration(s) (X_T) and acid dissociation constant(s) (K_X). The method includes titration of a seawater sample with a strong acid to estimate A_T, followed by quantitative removal of CO₂. The sample is then titrated with a strong base, and data from this “back-titration” can be used to quantify A_X. An uncertainty assessment is provided that includes systematic errors and effects of CO₂ contamination, supplemented by colorimetric measurements of B_T. This work provides realistic uncertainties of A_T measurements made in areas suspected of having a high presence of unidentified acid-base systems, thus contributing to a more accurate assessment of changes in marine CO₂ chemistry and speciation in many environments.