The role of seawater freezing in the formation of subsurface brines

Abstract

Abstract Several mechanisms (evaporation, water-rock interaction, ultra-filtration) have been suggested to explain the evolution of ubiquitous Ca-chloride subsurface brines. In the present paper, the freezing of seawater in polar regions, and in even wider areas during glacial periods, is proposed as an additional possible path of brine formation. Four detailed seawater freezing experiments to −14°C (resulting in a concentration factor of about 5) were carried out, and Na, K, Ca, Mg, Sr, Cl, SO4, and Br were analysed in the residual brines and in the ice. Br and Sr, whose behavior during the freezing of seawater is reported here for the first time, show a conservative behavior throughout the studied temperature range. Our data and earlier literature show that the high salinities, which are common in subsurface brines (>300 g/l), may be obtained by the removal of H2O as ice in the primary glacial environment. The decrease in the Na Cl ratio is caused by the crystallization of mirabilite (Na2SO4 · 10H2O), supplemented by hydrohalite (NaCl · 2H2O). Sulfate is removed both in mirabilite and by bacterial reduction. The brine then migrates to the subsurface, heats-up under the local geothermal gradient, and interacts with the adjacent rocks. At this stage, it may be diluted by meteoric waters, its Mg Ca ratio decreases (dolomitization and chloritization), the SO 4 Cl ratio varies according to the local gypsum-anhydrite equilibrium conditions, and the Ca (SO 4 + HCO 3 ) ratio increases as a result of dolomitization or chloritization. The interaction with rocks in the subsurface may affect both the original 87 Sr 86 Sr and the 18 O 16 O ratios of the brine. Although several of the processes which lead to the formation of Ca-chloride brines are common for both the evaporative and the freezing models, the Na-Br-Cl relationship in a given brine can be used to discriminate between the two modes of brine evolution. Several subsurface brines from the Canadian Shield and one brine from Finland are used as examples of the seawater freezing model, and an explanation is proposed for the necessary mass production of brines in glacial environments.