Precipitation of solid phase calcium carbonates and their effect on application of seawater iSsubA/sub/i–iT/i–iP/i models

摘要

At the present time, little is known about how broad salinity andtemperature ranges are for seawater thermodynamic models that are functionsof absolute salinity (S_A), temperature (T) and pressure (P). Suchmodels 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 eventuallyprecipitate. This will change the compositional ratios, and these salinitymodels will no longer be applicable. A future complicating factor is thelowering of seawater pH as the atmospheric partial pressures of CO₂increase. A geochemical model (FREZCHEM) was used to quantify the S_A−Tboundaries at P=0.1 MPa and the range of these boundaries for futureatmospheric CO₂ increases. An omega supersaturation model forCaCO₃ minerals based on pseudo-homogeneous nucleation was extended from25–40°C to 3°C. CaCO₃ minerals were the boundary definingminerals (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 thedominant boundary defining mineral for colder temperatures, which culminatedin a low temperature boundary of −4.93°C. Increasing atmosphericCO₂ 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 as11 g kg−1 and 0.66°C, respectively. The model-calculatedcalcite-ikaite transition temperature of 2.82°C is in excellentagreement with ikaite formation in natural environments that occurs attemperatures of 3°C or lower. Furthermore, these results provide aquantitative theoretical explanation (FREZCHEM model calculation) for whyikaite is the solid phase CaCO₃ mineral that precipitates duringseawater freezing.