Precipitation of solid phase calcium carbonates and their effect on application of seawater SA-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 (SA), 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 CO 2 increase. A geochemical model (FREZCHEM) was used to quantify the SA-T boundaries at P=0.1MPa 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 SA=104 g kg-1) and 40°C (at SA=66 g kg-1). At 2.82°C, calcite(CaCO3) transitioned to ikaite(CaCO 36H2O) as the dominant boundary defining mineral for colder temperatures, which culminated in a low temperature boundary of -4.93°C. Increasing atmospheric CO2 from 385μatm (390MPa) (in Year 2008) to 550μatm (557MPa) (in Year 2100) would increase the S A and t boundaries as much Abstract: 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. © 2009 Author(s).