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Frozen seawater is a composite material with a sponge-like structure. The framework of the structure is composed of pure ice, and within the pores exists a concentrated seawater brine. When the temperature is reduced, the volume of this residual brine decreases, while its salinity increases. As a result of the paired changes to temperature and salinity, the brine eventually becomes supersaturated with respect to a mineral, resulting in the precipitation of microscopic crystals throughout the ice structure. Due to experimental constraints, the current understanding about the formation of these minerals relies on the analysis of the residual brine, rather than the mineral phase. Here synchrotron X-ray powder diffraction was used to assess the dynamics that occur between ice, brine, and mineral phases within frozen seawater brines that were subjected to cooling and warming at subzero temperatures. The method was able to detect crystalline phases of ice, mirabilite (Na2SO4·10H2O), and hydrohalite (NaCl·2H2O). Results illustrate a highly dynamic geochemical environment where ice-brine-mineral interactions tend toward an equilibrium crystallization process, which supports the process of seawater freezing that is described by the Gitterman Pathway and FREZCHEM model. This study highlights the power of synchrotron techniques in observing the mineralogical dynamics of inaccessible environmental systems.

We report studies on the thermal and radiolytic stability of the hydrated salt minerals epsomite (MgSO4·7H2O), mirabilite (Na2SO4·10H2O), and natron (Na2CO3·10H2O) under the low-temperature and ultrahigh vacuum conditions characteristic of the surface of the Galilean satellite Europa. We prepared samples, ran temperature-programmed dehydration (TPD) profiles and irradiated the samples with electrons. The TPD profiles are fit using Arrhenius-type first-order desorption kinetics. This analysis yields activation energies of 0.90±0.10, 0.70±0.07, and 0.45±0.05 eV for removal of the hydration water for epsomite, natron, and mirabilite, respectively. A simple extrapolation indicates that at Europa surface temperatures (

Frost flowers collected from the surface of new sea ice near the Brunt Ice Shelf, Antarctica, show depletion in sulphate and sodium relative to other sea water ions. This is consistent with loss of mirabilite (Na2SO4) during formation of the brine from which the frost flowers grow. Aerosol generated from frost flowers would have higher sodium:sulphate ratios than aerosol generated from sea water. This would explain low values of non-sea-salt sulphate encountered in winter aerosol, and winter layers in ice cores, at coastal Antarctic sites. Calculations confirm that the frost flower source should be significant compared to an open water source for coastal regions.

[1] Aerosol sampling was conducted at Syowa Station, Antarctica (coastal station) in 2004–2006. SO42−depletion by mirabilite precipitation was identified from April through November. The fractionated sea-salt particles were distributed in ultrafine– coarse modes. Molar ratios of Mg2+/Na+ and K+/Na+ were higher than in bulk seawater ratio during winter–spring. The Mg2+/Na+ ratio in aerosols greatly exceeded the upper limit in the case only with mirabilite precipitation. The temperature dependence of Mg2+/Na+ ratio strongly suggested that higher ratios of Mg2+/Na+ and K+/Na+were associated with sea-salt fractionation by precipitation of mirabilite at −9°C, hydrohalite at ca. −23°C and other salts such as ikaite at ca. −5°C and gypsum at ca. −22°C during winter–spring. Mg-salts with lower deliquescence relative humidity can be enriched gradually in the fractionated sea-salt particles. Results suggests that sea-salt fractionation can alter aerosol hygroscopicity and atmospheric chemistry in polar regions.

[1] Linear mixture modeling has been applied to Galileo Near Infrared Mapping Spectrometer (NIMS) spectra of Europa's dark, non-water-ice terrain using cryogenic laboratory spectra of hydrated compounds. Water ice alone could not account for the distorted and asymmetric spectral features attributed to water of hydration in the Europa spectrum. The modeling was conducted using cryogenic reference spectra of hexahydrite, epsomite, bloedite, sodium sulfide nonahydrate, mirabilite, magnesium sulfate dodecahydrate, sulfuric acid hydrate, and saturated brines of magnesium sulfate, sodium carbonate, and sodium sulfate. All were measured between 77 and 130K. The best model fit was attained using abundances of 14% hexahydrite, 11% bloedite, 12% mirabilite, and 62% sulfuric acid hydrate. Results indicate that sulfuric acid hydrate and hydrated sulfate salts, taken together, produce a better spectral match than either class of compound considered alone.

[1] We selected two Europan-ice-shell candidate highly hydrated sulfate salts for a laboratory survey of ductile flow properties: MgSO4·7H2O (epsomite) and Na2SO4·10H2O (mirabilite), called MS7 and NS10, respectively. Polycrystalline samples in pure form and in mixtures with water ice I were tested using our cryogenic high-pressure creep apparatus at temperatures 232 ≤ T ≤ 294 K, confining pressures P = 50 and 100 MPa, and strain rates 4 × 10−8 ≤ ≤ 7 × 10−5 s−1. Grain size of NS10 samples was >100 μm. The flow strength σ of pure MS7 was over 100 times that of polycrystalline ice I at comparable conditions; that of pure NS10 over 20 times that of ice. In terms of the creep law = Aσn e−Q/RT, where R is the gas constant, we determine parameter values of A = 1012.1 MPa−ns−1, n = 5.4, and Q = 128 kJ/mol for pure NS10. Composites of ice I and NS10 of volume fraction ϕ NS10 have flow strength σc = [ϕNS10σNS10J + (1 − ϕNS10)σiceIJ]1/J where J ≈ −0.5, making the effect on the flow of ice with low volume fractions of NS10 much like that of virtually undeformable hard rock inclusions. Being much stronger and denser than ice, massive sulfate inclusions in the warmer, ductile layer of the Europan ice shell are less likely to be entrained in convective ice flow and more likely to be drawn to the base of the ice shell by gravitational forces and eventually expelled. With only smaller, dispersed sulfate inclusions, at probable sulfate ϕ < 0.2, the shell may be treated rheologically as pure, polycrystalline ice, with boundary conditions perhaps influenced by the high density and low thermal conductivity of the hydrated salts.

[1] Observations of aerosol constituents and acidic gases in the Antarctic area were carried out at Syowa (39.58°E, 69.00°S) in 1997 and 1998 and Dome Fuji stations (39.62°E, 77.37°S) in 1997. Sea-salt concentrations decreased to background levels in the summer at both Syowa (Na+, ≤4 nmol m−3) and Dome Fuji (Na+, ∼0.44 nmol m−3 on average). During the winter, blizzard and strong wind may cause an increase of sea-salt particles at Syowa, whereas long-range transport from the boundary layer at midlatitudes and coastal Antarctic regions may contribute significantly to the increase in sea-salt particles observed at Dome Fuji. Particulate Cl− and Br− are liberated preferentially from sea-salt particles at Syowa and Dome Fuji in the summer. The molar ratio of Cl−/Na+ and Br−/Na+ at Syowa decreased to ∼0.5 and ≈0, respectively, in summer. At Dome Fuji more Cl− tend to be liberated from sea-salt particles thorough heterogeneous NO3− formation. The concentrations of gaseous chlorine species (mostly HCl) and bromine species ranged from 0.2 to 5.3 nmol m−3 and below detection limit (BDL) to 1.5 nmol m−3, respectively, corresponding to sea-salt modification. In the present study, SO42− depletion due to mirabilite formation was observed not only at Syowa but also at Dome Fuji. This evidence suggests that SO42− depletion might occur through sublimation on snow surfaces in addition to seawater freezing. At Syowa, sea-salt fractionation relating to Mg2+, K+, and Ca2+ was also observed mostly under strong wind conditions.

[1] Year-round composition of bulk and size-segregated aerosol was examined at a coastal Antarctic site (Dumont d'Urville). Sea-salt particles display a summer depletion of chloride relative to sodium, which reaches ∼10%. The mass chloride loss is maximum on 1- to 3-μm-diameter particles, nitrate being often the anion causing the chloride loss. The summer SO42−/Na+ ratio exceeds the seawater value on submicron particles due to biogenic sulfate and on coarse particles due to ornithogenic (guano-enriched soils) sulfate and to heterogeneous uptake of SO2 (or H2SO4). HCl levels range from 47 ± 28 ng m−3 in the winter to 130 ± 110 ng m−3 in the summer, being close to the mass chloride loss of sea-salt aerosols. In the winter, sea-salt particles exhibit Cl−/Na+ and SO42−/Na+ mass ratios of 1.9 ± 0.1 and 0.13 ± 0.04, respectively. Resulting from precipitation of mirabilite during freezing of seawater, this sulfate-depletion-relative sodium takes place from May to October. From March to April, warmer temperatures and/or smaller sea ice extent offshore the site limit the phenomenon. A range of 14–50 ng m−3 of submicron sulfate is found, confirming the existence of nssSO42− in the winter at a coastal Antarctic site, highest values being found in the winters of 1992–1994 due to the Pinatubo volcanic input. Apart from these three winters, nssSO42− levels range between 15 and 30 ng m−3, but its origin is still unclear (quasi-continuous SO2 emissions from the Mount Erebus volcano or local wintertime dimethyl sulfide [DMS] oxidation, in addition to long-range transported by-product of DMS oxidation).