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| Titel |
Precipitation of solid phase calcium carbonates and their effect on application of seawater SA–T–P models |
| VerfasserIn |
G. M. Marion, F. J. Millero, R. Feistel |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1812-0784
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| Digitales Dokument |
URL |
| Erschienen |
In: Ocean Science ; 5, no. 3 ; Nr. 5, no. 3 (2009-07-21), S.285-291 |
| Datensatznummer |
250002578
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| Publikation (Nr.) |
 copernicus.org/os-5-285-2009.pdf |
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| Zusammenfassung |
| 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 CO2
increase. A geochemical model (FREZCHEM) was used to quantify the SA−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 SA=104 g kg−) and 40°C (at SA=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 SA 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. |
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