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TitleCryogenic formation of brine and sedimentary mirabilite in submergent coastal lake basins, Canadian Arctic
AuthorGrasby, S E; Smith, I R; Bell, T; Forbes, D L
SourceGeochimica et Cosmochimica Acta vol. 110, 2013 p. 13-28,
Alt SeriesEarth Sciences Sector, Contribution Series 20120440
Mediapaper; on-line; digital
File formatpdf
AreaBanks Island; Sachs Harbour
Lat/Long WENS-126.0000 -125.0000 72.0000 71.7500
Subjectshydrogeology; brine; lake sediment geochemistry; sedimentary basins; basins; salinity; lake water geochemistry; lake water; Mirabilite Basin 1; Mirabilite Basin 2; mirabilite
Illustrationslocation maps; tables; photomicrographs; plots; ternary diagrams
ProgramCoastal Infrastructure, Climate Change Geoscience
AbstractTwo informally named basins (Mirabilite Basins 1 and 2) along a submergent coastline on Banks Island, Canadian Arctic Archipelago, host up to 1 m-thick accumulations of mirabilite (Na2SO4\'0210H2O) underlying stratified water bodies with basal anoxic brines. Unlike isostatically uplifting coastlines that trap seawater in coastal basins, these basins formed from freshwater lakes that were transgressed by seawater. The depth of the sill that separates the basins from the sea is shallow (1.15 m), such that seasonal sea ice formation down to 1.6 m isolates the basins from open water exchange through the winter. Freezing of seawater excludes salts, generating dense brines that sink to the basin bottom. Progressive freezing increases salinity of residual brines to the point of mirabilite saturation, and as a result sedimentary deposits of mirabilite accumulate on the basin floors. Brine formation also leads to density stratification and bottom water anoxia. We propose a model whereby summer melt of the ice cover forms a temporary freshwater lens, and rather than mixing with the underlying brines, it is exchanged with seawater once the ice plug that separates the basins from the open sea melts. This permits progressive brine development and density stratification within the basins.
Summary(Plain Language Summary, not published)
A discovery of unusual deposits of the rare mineral mirabilite, found in the sediments of two lakes on Banks Island, was made. A new model explains this occurrence. Sea water moves into coastal lakes as a result of relative sea level change and then freezes in winter. During freezing only water go into ice, leaving the remaining seawater more concentrated (called cryogenic brine formation). Progressive concentration through this manner leads to precipitation of the mineral mirabilite (similar to how salt precipitates when seawater is concentrated through heating). This work provides the first direct evidence that cryogenic brine formation occurs, important for understanding the origin of deep brines in the Canadian Shield. The study also shows that the process creates a thermal anomaly that could potentially enhance decay of coastal permafrost. Whether this is a unique occurrence or prevalent is uncertain and requires more research.