Abstract
The sea ice microstructure is permeated by brine channels and pockets that contain concentrated seawater-derived brine. Cooling the sea ice results in further formation of pure ice within these pockets as thermal equilibrium is attained, resulting in a smaller volume of increasingly concentrated residual brine. The coupled changes in temperature and ionic composition result in supersaturation of the brine with respect to mirabilite (Na2SO4·10H2O) at temperatures below −6.38°C, which consequently precipitates within the sea ice microstructure. Here, mirabilite solubility in natural and synthetic seawater derived brines, representative of sea ice at thermal equilibrium, has been measured in laboratory experiments between 0.2 and −20.6°C, and hence we present a detailed examination of mirabilite dynamics within the sea ice system. Below −6.38°C mirabilite displays particularly large changes in solubility as the temperature decreases, and by −20.6°C its precipitation results in 12.90% and 91.97% reductions in the total dissolved Na+ and SO42− concentrations respectively, compared to that of conservative seawater concentration. Such large non-conservative changes in brine composition could potentially impact upon the measurement of sea ice brine salinity and pH, whilst the altered osmotic conditions may create additional challenges for the sympagic organisms that inhabit the sea ice system. At temperatures above −6.38°C, mirabilite again displays large changes in solubility that likely aid in impeding its identification in field samples of sea ice. Our solubility measurements display excellent agreement with that of the FREZCHEM model, which was therefore used to supplement our measurements to colder temperatures. Measured and modelled solubility data were incorporated into a 1D model for the growth of first-year Arctic sea ice. Model results ultimately suggest that mirabilite has a near ubiquitous presence in much of the sea ice on Earth, and illustrate the spatial and temporal evolution of mirabilite within sea ice as it grows throughout an Arctic winter, reaching maximum concentrations of 2.3gkg−1.
Highlights
Sea ice is a porous medium comprised of a pure ice framework dotted by gas pockets and permeated by channels of concentrated seawater-derived brine (Light et al, 2003; Golden et al, 2007)
Mirabilite solubility in natural and synthetic seawater derived brines, representative of sea ice at thermal equilibrium, has been measured in laboratory experiments between 0.2 and À20.6 °C, and we present a detailed examination of mirabilite dynamics within the sea ice system
By idealising the system to an equilibrium environment, it is possible to accurately model the conditions of sea ice brine pockets at equilibrium on a larger, laboratory scale. Using this approach we investigated the solubility of mirabilite between 0.2 and À20.6 °C at 1 atm to elucidate its dynamics in an equilibrium sea ice system with changing temperature
Summary
Sea ice is a porous medium comprised of a pure ice framework dotted by gas pockets and permeated by channels of concentrated seawater-derived brine (Light et al, 2003; Golden et al, 2007). The lower the temperature of sea ice, the more concentrated the brine becomes as more pure water freezes to maintain thermal equilibrium. The precipitation of minerals in this setting is unusual in that, rather than sinking to the ocean floor, they become encapsulated in the pores of the ice (Light et al, 2003) and are retained close to the ice–atmosphere interface.
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