Precipitation of solid phase calcium carbonates and their effect on application of seawater S_A–T–P models

Abstract. At the present time, little is known about how broad salinity and temperature ranges are for seawater thermodynamic models that are functions of absolute salinity (S_A), temperature (T) and pressure (P). Such models rely on fixed compositional ratios of the major components (e.g., Na/Cl, Mg/Cl, Ca/Cl, SO₄/Cl, etc.). As seawater evaporates or freezes, solid phases [e.g., CaCO₃(s) or CaSO₄2H₂O(s)] will eventually precipitate. This will change the compositional ratios, and these salinity models will no longer be applicable. A future complicating factor is the lowering of seawater pH as the atmospheric partial pressures of CO₂ increase. A geochemical model (FREZCHEM) was used to quantify the S_A−T boundaries at P=0.1 MPa and the range of these boundaries for future atmospheric CO₂ increases. An omega supersaturation model for CaCO₃ minerals based on pseudo-homogeneous nucleation was extended from 25–40°C to 3°C. CaCO₃ minerals were the boundary defining minerals (first to precipitate) between 3°C (at S_A=104 g kg⁻) and 40°C (at S_A=66 g kg⁻). At 2.82°C, calcite(CaCO₃) transitioned to ikaite(CaCO₃6H₂O) as the dominant boundary defining mineral for colder temperatures, which culminated in a low temperature boundary of −4.93°C. Increasing atmospheric CO₂ from 385 μatm (390 MPa) (in Year 2008) to 550 μatm (557 MPa) (in Year 2100) would increase the S_A and t boundaries as much as 11 g kg⁻¹ and 0.66°C, respectively. The model-calculated calcite-ikaite transition temperature of 2.82°C is in excellent agreement with ikaite formation in natural environments that occurs at temperatures of 3°C or lower. Furthermore, these results provide a quantitative theoretical explanation (FREZCHEM model calculation) for why ikaite is the solid phase CaCO₃ mineral that precipitates during seawater freezing.