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

Abstract

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, SO 4 /Cl, etc.). As seawater evaporates or freezes, solid phases [e.g., CaCO 3 (s) or CaSO 4 2H 2 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 2 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 CO 2 increases. An omega supersaturation model for CaCO 3 minerals based on pseudo-homogeneous nucleation was extended from 25-40 • C to 3 • C. CaCO 3 minerals were the boundary defining minerals (first to precipitate) between 3 • C (at S A =104 g kg −1 ) and 40 • C (at S A =66 g kg −1 ). At 2.82 • C, calcite(CaCO 3 ) transitioned to ikaite(CaCO 3 6H 2 O) as the dominant boundary defining mineral for colder temperatures, which culminated in a low temperature boundary of −4.93 • C. Increasing atmospheric CO 2 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 Correspondence to: G. M. Marion (giles.marion@dri.edu) (FREZCHEM model calculation) for why ikaite is the solid phase CaCO 3 mineral that precipitates during seawater freezing.

Environmental controls on coccolithophore calcification

“…Increased salinity increases Ω cal , as does increased temperature (Green et al 1998, Marion et al 2009). These changes in Ω cal are complicated, in terms of CaCO 3 precipitation, in the ocean by the general correlation of salinity with carbonate alkalinity, and the temperature and salinity effects on the speciation of inorganic carbon and the solubility of CO 2 .…”

Introducing the bromide/alkalinity ratio for a follow-up discussion on "Precipitation of salts in freezing seawater and ozone depletion events: a status report", by Morin et al., published in Atmos. Chem. Phys., 8, 7317–7324, 2008

“…It should be noted though, that all results and discussions presented here assume thermodynamic equilibrium. For future studies, the kinetics of precipitation and dissolution should also be taken into account, a topic which is mentioned in Marion et al (2009).…”

Thermodynamic properties of seawater, ice and humid air: TEOS-10, before and beyond

“…The new reference salinity scale supported this extension beyond the validity range of the previous practical salinity (see Sect. 3.4) and at the same time raised the question of the solubilities of the dissolved salts (Marion et al, 2009) On the basis of mainly these two documents, TEOS-10 was endorsed by IOC/UNESCO in 2009 at Paris as a new international seawater standard "to replace EOS-80 and thus updating this valuable, but no longer state-of-the-art, 30-yearold UNESCO standard" (IOC, 2009, p. 5 therein;Wright et al, 2010b;IUGG, 2011;Valladares et al, 2011a, b). In Berlin, in addition to its four permanent working groups, the IAPWS established a new Subcommittee on Seawater that (2009); "G": data of Gleitz et al (1995).…”

“…A workable practical approach to this problem may be the use of so-called Pitzer equations. They approximate singleion activities as series expansions with respect to the ion concentrations and adjust the unknown empirical coefficients to other measurable properties (Nesbitt, 1980;Marion and Grant, 1994;Prausnitz et al, 1999;Marion and Kargel, 2008;Marion et al, 2011). Pitzer equations for seawater ions successfully describe colligative properties, while other thermodynamic properties such as sound speed may not yet be represented as accurately as by TEOS-10 (Feistel and Marion, 2007;Feistel, 2008a;Sharp et al, 2015).…”