The formation of natural cryogenic brines

Geochimica et Cosmochimica Acta, 1990

Several mechanisms (evaporation, water-rock interaction, ultra-filtration) have been suggested to explain the evolution of ubiquito~ C&chloride subsurface brines.

Chemical Geology, 1988

Several mechanisms (evaporation, water-rock interaction, ultra-filtration) have been suggested to explain the evolution of ubiquito~ C&chloride subsurface brines.

Geochimica et Cosmochimica Acta, 2013

Two 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 (Na 2 SO 4 Á10H 2 O) 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.

Chemical Geology, 2010

Chlorine and bromine are two major anionic components of most brines, and typically behave conservatively in groundwater systems. Chlorine isotopes have been utilized to determine brine evolution during water rock evolution, very few investigations have analyzed for bromine isotopes. In this paper, brines and fluids from the Canadian and Fennoscandian Shields are characterized through a survey of chlorine and bromine stable isotopes. Stable chlorine and bromine isotopic values in Fennoscandian Shield fluids were more positive, and a greater range of values than was observed for Canadian Shield fluids. For the Fennoscandian Shield, isotopic values for δ37Cl varied between − 0.54‰ and + 1.52‰ SMOC, while δ81Br values ranged between + 0.26‰ and + 2.04‰ SMOB, while values in the Canadian Shield varied between − 0.78‰ and + 0.98‰ SMOC and + 0.01‰ and +1.29‰ SMOB, respectively. A weak positive correlation between chlorine and bromine isotopes was also observed. At one site with serpentinite rocks, a large variation in δ37Cl isotopic values compared with minimal variation in δ81Br values is attributed to ion filtration through serpentinite, which affected the Cl but not Br ions. Comparisons with other isotopic systems, such as 87Sr/86Sr, indicate water–rock interactions at some sites are likely to influence halogen isotopic composition (δ37Cl, δ81Br). The δ37Cl and δ81Br values of the investigated samples do not support a marine origin for these brines. However, if a seawater origin were to be considered for the fluids, a process or combination of processes significantly altered chlorine and bromine isotopic signatures. A positive correlation between the fluid halide isotopic composition (δ37Cl, δ81Br) and methane gas isotopic composition (δ2H, δ13C) may be due to changes in redox, pH, temperature and pressure conditions, as well as diffusion over geologic time. Although overlap occurs, the differences between the chlorine and bromine stable isotope ranges and behaviors for crystalline shields and sedimentary basins presented in this paper are significant, which indicates either different sources or different evolutionary processes in the two different environments. This could have implications to several shield evolutionary pathways published in the present literature.

Geochimica et Cosmochimica Acta, 2008

Dissolved noble gas concentrations were measured in high salinity (270 g/L) Ca(Na)-Cl groundwaters from the Con Mine, Yellowknife, Canada in an effort to discriminate between two possible origins, as either a brine generated by evaporative enrichment in a Paleozoic inland sea, or marine water concentrated by freezing during glacial times. Major ion and isotope geochemistry indicate that brines from the deepest level remain relatively undisturbed by mixing with modern water introduced by mining. Mixing calculations are used to quantify fractions of brine, glacial meltwater and modern water. From this, noble gas concentrations were corrected for excess air with Ne and normalized to 100% brine solution. Over-pressuring of helium and argon in the brine provide age constraints based on the accumulation of geogenic 4 He and 40 Ar. Radiogenic age calculations together with the local geological history suggest brine emplacement during early Palaeozoic time, likely during the Devonian when evaporitic inland seas existed in this region. The concentrations of the atmospherically derived noble gases in the brine fraction (Kr = 1.4E-8, Xe = 8.5E-10 cc STP =cc H 2 O ) are close to atmospheric equilibrium for brine at 25°C (Kr = 7.3E-9, Xe = 8.0E-10 cc STP =cc H 2 O ), but are far lower than would be expected for closed-system concentration of seawater by freezing (Kr = 2.8E-6, Xe = 4.2E-7 cc STP =cc H 2 O ). Thus, despite the complicated mixing history of the brine, the atmospheric and geogenic noble gases provide strong evidence for an origin as air-equilibrated brine from evaporated Paleozoic seawater, which infiltrated via density displacement through existing fractures and faults into the Canadian Shield.

Geochimica et Cosmochimica Acta, 1994

Deep groundwaters in crystalline rocks typically are very saline and are characterized by a rather unique Ca-Na-Cl-dominated chemistry. Sulfate is present in variable amounts and may be linked to both the geochemical evolution of these fluids as well as to recent processes initiated through mining activities. It is possible to distinguish on the basis of isotopic compositions between brine sulfate and secondary sulfate formed by oxidation of sulfides: The latter is characterized by 634S values which reflect the local mineral sulfide precursor and 6'sO close to or below 0% SMOW. The isotopic composition of the brine sulfate is characterized by 6 "0 and 6 34S values which resemble marine isotopic compositions at some localities, at others they could be explained as being of magmatic/hydrothermal origin. It is likely that the sulfate participated in the geochemical evolution of these brines. Thus, its isotopic composition reflects geochemical processes rather than a primary origin. No evidence for the influence of bacterial reduction was found. VAN EVERDINGEN R. 0. and KROUSE H. R. ( 1985) Isotopic composition of sulfates generated by bacterial and biological oxidation. Nature 315, 395-396. YANAGL~AWA F. and SAKAI A. ( 1983) Thermal decomposition of preparation of sulfur dioxide in sulfur isotope ratio measurements. Anal. Chem. 55,985-987.

Journal of Hydrology, 1992

Typical Ca-C1 brines occur in crystalline and metamorphic rocks below freshwater horizons at various localities in Sweden and Finland. Total dissolved solids (TDS) range in concentration between 2 and 120gl t and have long been thought to derive from water-rock interactions. The relationships between Na, C1 and Br in these brines suggest, however, that they were derived from freezing of seawater during glacial periods. The brines were subsequently diluted by meteoric waters and their Ca/Mg ratio was increased through water-rock interactions in the subsurface. The hydrogeological model for both the formation of freeze-derived marine brines and their lateral intrusion involves restricted inland marine basins in recent and subrecent polar climatic belts. Seawater in such basins gradually freezes in response to glaciation. The solutes which concentrate in the remaining water body are residual after precipitation of a sequence of minerals which include carbonates, mirabilite and hydrohalite. Hydraulic pressure of the growing ice sheet over the frozen seas is gradually added to the ambient hydrostatic pressure exerted on the brines. This, together with their increased density, increases the intrusional potential of the brines. As the land ice cannot exert hydraulic pressure on continental groundwater in the aquifers, the balance of pressure favours deep landward intrusion of brines. Post-glacial processes cause the subsurface dilution and replacement of the brines both by seawater and fresh waters. The presence of such brines also far from present-day coastal settings reflects the shifting of coastlines as a result of isostatic movements and eustalic sea-level changes associated with glaciation and deglaciation.

Marine Chemistry, 2016

The sea ice cover of high latitude oceans contains concentrated brines which are the site of in-situ chemical and biological reactions. The brines become supersaturated with respect to mirabilite (Na 2 SO 4 • 10H 2 O) below −6.4 • C, and the associated removal of Na + and SO 2− 4 from the brine results in considerable non-conservative changes to its composition. The changes are reflected in the brine salinity, which is a fundamental physico-chemical parameter in the sea ice brine system. Here, measurements of electrical conductivity and brine composition in synthetic sea ice brines between −1.8 and −20.6 • C, obtained during a comprehensive investigation of the brine-mirabilite equilibrium at below-zero temperatures reported elsewhere, are combined with modelled estimates to assess the behaviour of the absolute (S A) and practical (S P) salinities of sea ice brines. Results display substantial divergence of S P from S A below −6.4 • C, reaching a 7.2 % difference at −22.8 • C. This

Geochimica et Cosmochimica Acta, 2018

The isotopic analyses (δ 13 C, δ 18 O, and ∆ 47) of carbonate phases recovered from a core in McMurdo Sound by ANtarctic geologic DRILLing (ANDRILL-2A) indicate that the majority of secondary carbonate mineral formation occurred at cooler temperatures than the modern burial temperature, and in the presence of fluids with δ 18 O water values ranging between-11 and-6‰ VSMOW. These fluids are interpreted as being derived from a cryogenic brine formed during the freezing of seawater. The ∆ 47 values were converted to temperature using an in-house calibration presented in this paper. Measurements of the  47 values in the cements indicate increasingly warmer crystallization temperatures with depth and, while roughly parallel to the observed geothermal gradient, consistently translate to temperatures that are cooler than the current burial 2 temperature. The difference in temperature suggests that cements formed when they were~ 260±100 m shallower than at the present day. This depth range corresponds to a period of minimal sediment accumulation from 3-11 Myr; it is therefore interpreted that the majority of cements formed during this time. This behavior is also predicted by time-integrated modeling of cementation at this site. If this cementation had occurred in the presence of these fluids, then the cryogenic brines have been a longstanding feature in the Victoria Land Basin. Brines such as those found at this site have been described in numerous modern high-latitude settings, and analogous fluids could have played a role in the diagenetic history of other ice-proximal sediments and basins during glacial intervals throughout geologic history. The agreement between the calculated δ 18 O water value and the measured values in the pore fluids shows how the ∆ 47 proxy can be used to identify the origin of negative δ 18 O values in carbonate rocks and that extremely negative values do not necessarily need to be a result of the influence of meteoric fluids or reaction at high temperature.

Geochimica et Cosmochimica Acta, 2012

Results from cryogenic column experiments are compared with the geochemical data collected in the Canadian and Fennoscandian Shields over the past 25 years to investigate the relative influence of the glacial-interglacial cycle; specifically, the impact of continental glaciers, permafrost, and methane hydrate, on the evolution of groundwater from crystalline shield environments. Several different geochemical indicators of freezing processes (either glacial or permafrost-related) were utilized: comparisons of Na/Cl and Br/Cl ratios, d 18 O and d 2 H values, and d 18 O values and Cl À concentration. During freezing, fluids with different dominant cations follow distinctly different linear trends when Na/Cl and Br/Cl ratios are compared. Significantly, none of the freezing trends follows the trend hypothesized by for the evolution of seawater chemistry during freezing. Intrusion of glacial meltwater and in situ freezing (i.e., permafrost formation) result in a similar end-member when comparing d 18 O values and Cl À concentration. The geochemical influence of a freezing process on fresh, brackish, and some saline fluids was identified at some, but not all Canadian Shield sites, regardless of site location with respect to modern-day permafrost. Appreciably, physical and geochemical data do not support the formation of brines through any freezing process in the Canadian and Fennoscandian Shields, as hypothesized by . Rather, on all diagnostic freezing plots, brines are an end-member, indicating a different evolutionary pathway. Significant depletions in 18 O with respect to modern precipitation, an indication of either glacial meltwater or a freezing process, were identified at depths of up to 1 km at some sites in the Canadian Shield, and to shallower depths in the Fennoscandian Shield. The potential of this fluid to reach such depths could be attributable to artificial gradients and mixing, glacial recharge, permafrost or paleo-permafrost formation, or methane hydrate or paleo-methane hydrate formation. At most locations it was not possible to distinguish between the different scenarios using the current geochemical database.