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Mirabilite solubility in equilibrium sea ice brines

DOI: 10.1016/j.gca.2016.03.008 DOI Help

Authors: Ben Butler (Bangor University) , Stathys Papadimitriou (Bangor University) , Anna Santoro (Bangor University) , Hilary Kennedy (University of Bangor)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Geochimica Et Cosmochimica Acta

State: Published (Approved)
Published: March 2016
Diamond Proposal Number(s): 3897

Open Access Open Access

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 (Na2SO4·10SO4·10H2O) at temperatures below View the MathML source-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 View the MathML source-20.6°C, and hence we present a detailed examination of mirabilite dynamics within the sea ice system. Below View the MathML source-6.38°C mirabilite displays particularly large changes in solubility as the temperature decreases, and by View the MathML source-20.6°C its precipitation results in View the MathML source12.90% and View the MathML source91.97% reductions in the total dissolved Na+ and View the MathML sourceSO42- 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 p H, whilst the altered osmotic conditions may create additional challenges for the sympagic organisms that inhabit the sea ice system. At temperatures above View the MathML source-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 2.3 g kg−1

Journal Keywords: Mirabilite; Sea ice; FREZCHEM

Subject Areas: Earth Science

Instruments: I11-High Resolution Powder Diffraction

Added On: 15/03/2016 12:40

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