Total individual ion activity coefficients of calcium and carbonate in seawater at 25°C and 35%. salinity, and implications to the agreement between apparent and thermodynamic constants of calcite and aragonite

Abstract

We have calculated the total individual ion activity coefficients of carbonate and calcium, γ TCO 3 2− and γ TCa 2+ , in seawater. Using the ratios of stoichiometric and thermodynamic constants of carbonic acid dissociation and total mean activity coefficient data measured in seawater, we have obtained values which differ significantly from those widely accepted in the literature. In seawater at 25°C and 35%. salinity the (molal) values of γ TCO 2 3− and γ TCa 2+ are 0.038 ± 0.002 and 0.173 ± 0.010, respectively. These values of γ TCO 3 2− and γ TCa 2+ are independent of liquid junction errors and internally consistent with the value γ TCl − = 0.651 . By defining γ TCa 2+ and γ TCO 3 2− on a common scale ( γ TCl − ), the product γ TCa 2+ γ TCO 3 2− is independent of the assigned value of γ Cl − and may be determined directly from thermodynamic measurements in seawater. Using the value γ TCa 2+ γ TCO 3 2− = 0.0067 and new thermodynamic equilibrium constants for calcite and aragonite, we show that the apparent constants of calcite and aragonite are consistent with the thermodynamic equilibrium constants at 25°C and 35%. salinity. The demonstrated consistency between thermodynamic and apparent constants of calcite and aragonite does not support a hypothesis of stable Mg-calcite coatings on calcite or aragonite surfaces in seawater, and suggests that the calcite critical carbonate ion curve of Broecker and Takahashi (1978, Deep-Sea Research 25, 65–95) defines the calcite equilibrium boundary in the oceans, within the uncertainty of the data.