Precipitation of solid phase calcium carbonates and their effect on application of seawater <i>S<sub>A</sub></i>–<i>T</i>–<i>P</i> models
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 (<i>S<sub>A</sub></i>), temperature (<i>T</i>) and pressure (<i>P</i>). Such models rely...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2009-07-01
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| Series: | Ocean Science |
| Online Access: | http://www.ocean-sci.net/5/285/2009/os-5-285-2009.pdf |
| Summary: | 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 (<i>S<sub>A</sub></i>), temperature (<i>T</i>) and pressure (<i>P</i>). Such models rely on fixed compositional ratios of the major components (e.g., Na/Cl, Mg/Cl, Ca/Cl, SO<sub>4</sub>/Cl, etc.). As seawater evaporates or freezes, solid phases [e.g., CaCO<sub>3</sub>(s) or CaSO<sub>4</sub>2H<sub>2</sub>O(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<sub>2</sub> increase. A geochemical model (FREZCHEM) was used to quantify the <i>S<sub>A</sub></i>−<i>T</i> boundaries at <i>P</i>=0.1 MPa and the range of these boundaries for future atmospheric CO<sub>2</sub> increases. An omega supersaturation model for CaCO<sub>3</sub> minerals based on pseudo-homogeneous nucleation was extended from 25–40°C to 3°C. CaCO<sub>3</sub> minerals were the boundary defining minerals (first to precipitate) between 3°C (at <i>S<sub>A</sub></i>=104 g kg<sup>−</sup>) and 40°C (at <i>S<sub>A</sub></i>=66 g kg<sup>−</sup>). At 2.82°C, calcite(CaCO<sub>3</sub>) transitioned to ikaite(CaCO<sub>3</sub>6H<sub>2</sub>O) as the dominant boundary defining mineral for colder temperatures, which culminated in a low temperature boundary of −4.93°C. Increasing atmospheric CO<sub>2</sub> from 385 μatm (390 MPa) (in Year 2008) to 550 μatm (557 MPa) (in Year 2100) would increase the <i>S<sub>A</sub></i> and t boundaries as much as 11 g kg<sup>−1</sup> 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 CaCO<sub>3</sub> mineral that precipitates during seawater freezing. |
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| ISSN: | 1812-0784 1812-0792 |