• A new model is presented for direct determination of [CO32−] in seawater.
• The model is appropriate for temperature 3 to 40 °C and salinity 20 to 40.
• Measured [CO32−] is best paired with CT and AT in CO2 system calculations.
• Measured [CO32−] alone can yield reliable estimates of CaCO3 saturation states.
• [CO32−] can be used in a variety of ways as a fifth measured CO2 system variable.
In ocean waters, the carbonate ion is of crucial importance to benthic and pelagic organisms that build their physical support structures out of calcium carbonate (CaCO3). Marine carbonate ion concentrations ([CO32−]) are measurable through spectrophotometric observations of the ultraviolet (UV) light absorbed by lead carbonate in Pb-enriched seawater, but previous characterizations of the Pb UV-absorption model have been applicable only at a fixed temperature of 25 °C. In this paper, the model is extended to a temperature range of 3 to 40 °C and a salinity range of 20 to 40. This advancement allows for determinations of [CO32−] with temperature measurement rather than temperature control, thus decreasing the required financial investment and instrumental complexity. The extended model also represents a significant step toward the development of automated inline or in situ [CO32−] sensors and promotes the utility of [CO32−] as a fifth measured variable for inclusion in studies of the marine carbon dioxide (CO2) system. Therefore, a quantitative evaluation of propagated uncertainties in CO2 system calculations based on [CO32−] as an input variable was also performed. The results show that total dissolved inorganic carbon (CT) and total alkalinity (AT) are the most suitable measured variables to pair with measured [CO32−] as input to such calculations. Pairing [CO32−] with the partial pressure of CO2 yields relatively low uncertainty in calculated pH — comparable to that resulting from conventional input pairs — but relatively high uncertainties in calculated AT and CT. Pairing [CO32−] with pH results in relatively high uncertainties in all calculated variables. CaCO3 saturation states (Ω) determined from measured [CO32−] (alone) can circumvent some sources of uncertainty inherent to conventional (two-variable) calculations. Simpler, more direct ways of measuring [CO32−] open up new opportunities for marine researchers and others interested in monitoring CaCO3 saturation states in seawater.
Sharp J. D. & Byrne R. H., in press. Carbonate ion concentrations in seawater: Spectrophotometric determination at ambient temperatures and evaluation of propagated calculation uncertainties. Marine Chemistry. Article (subscription required).