Brine Pockets Research Articles

Ion migration at the ice–water interface during the freezing process of salt solution – Experimental investigation

Within the scope of this work, systematic investigations of the ion migration process at the ice–water interface during the freezing of salt solutions are performed via experimental investigations (part I) and numerical investigations (part II). In this study, the ion migration trend under different freezing temperatures was revealed, and the formation process of the brine pockets during phase transition and the mechanism of ion release in ice were explored. The results show that the migration capabilities of Na+ and K+ are greater than those of Ca2+ and Mg2+, whereas the migration capability of Cl− is greater than those of HCO3− and SO42−. At −2 °C, the ion concentration in the surface ice decreased by 47.64 %, indicating the occurrence of ion release within the ice. When the moving speed of the phase transition interface is greater than the ion diffusion ability, the brine pockets form in the ice. Under a temperature gradient, the brine pockets in ice become thermodynamically unstable, resulting in the inevitable ion release toward the high-temperature side. The ion release depends on the temperature gradient and the brine pocket concentration. Furthermore, the unsynchronous migration of ions is related to the freezing temperature, diffusion coefficient, hydration radius, and hydration free energy.

Modeled variations in the inherent optical properties of summer Arctic ice and their effects on the radiation budget: a case based on ice cores from 2008 to 2016

Abstract. Variations in Arctic sea ice are apparent not only in its extent and thickness but also in its internal properties under global warming. The microstructure of summer Arctic sea ice changes due to varying external forces, ice age, and extended melting seasons, which affect its optical properties. Sea ice cores sampled in the Pacific sector of the Arctic obtained by the Chinese National Arctic Research Expedition (CHINARE) during the summers of 2008 to 2016 were used to estimate the variations in the microstructures and inherent optical properties (IOPs) of ice and determine the radiation budget of sea ice based on a radiative transfer model. The variations in the volume fraction of gas bubbles (Va) of the ice top layer were not significant, and the Va of the ice interior layer was significant. Compared with 2008, the mean Va of interior ice in 2016 decreased by 9.1 %. Meanwhile, the volume fraction of brine pockets increased clearly during 2008–2016. The changing microstructure resulted in the scattering coefficient of the interior ice decreasing by 38.4 % from 2008 to 2016, while no clear variations can be seen in the scattering coefficient of the ice top layer. These estimated ice IOPs fell within the range of other observations. Furthermore, we found that variations in interior ice were significantly related to the interannual changes in ice ages. At the Arctic basin scale, the changing IOPs of interior ice greatly changed the amount of solar radiation transmitted to the upper ocean even when a constant ice thickness is assumed, especially for thin ice in marginal zones, implying the presence of different sea ice bottom melt processes. These findings revealed the important role of the changing microstructure and IOPs of ice in affecting the radiation transfer of Arctic sea ice.

Water dynamics in eutectic solutions of sodium chloride and magnesium sulfate: implications for life in Europa's subsurface ocean and ice shell.

Liquid water is essential for life as we know it and the coupling between water and biomolecular dynamics is crucial for life processes. Jupiter's moon Europa is a good candidate for searching for extraterrestrial life in our outer solar system, mainly because a liquid water salty ocean in contact with a rocky seafloor underlies its ice shell. Little, however, is known about the chemical composition of the subglacial ocean of Europa or the brine pockets within its ice shell and their impacts on water dynamics. Here, we employ 1H, 17O, 23Na and 35Cl NMR spectroscopy, especially NMR spin relaxation and diffusion methods, and investigate the mobility of water molecules and ions in eutectic solutions of magnesium sulfate and sodium chloride, two salts ubiquitously present on the surface of Europa, over a range of temperatures and pressures pertinent to Europa's subglacial ocean. The NMR data demonstrate the more pronounced effect of magnesium sulfate compared with sodium chloride on the mobility of water molecules. Even at its much lower eutectic temperature, the sodium chloride solution retains a relatively large level of water mobility. Our results highlight the higher potential of a sodium chloride-rich than magnesium sulfate-rich Europa's ocean to accommodate life and support life origination within the eutectic melts of Europa's ice shell.

LBM-PFM simulation of the effect of non-condensable gas on the directional crystallization of seawater

The present study employs the coupled Lattice Boltzmann Method-Phase Field Method (LBM-PFM) to investigate the impact of non-condensable gas bubbles on directional crystallization of seawater at the pore scale. Firstly, the LBM-PFM coupling model is proposed and experimentally validated. Subsequently, this model is applied to numerically simulate the two-phase (bubble -seawater) flow in directional crystallization. Additionally, the influence of factors such as bubble quantity, radius, and arrangement on ice crystal growth and the distribution, quantity, and area of internal brine pockets are discussed. The findings unequivocally demonstrate that the presence of non-condensable gas bubbles significantly enhances the rate of crystallization, resulting in a twofold increase in the area of freshwater ice. Bubbles with different radius, quantities, and spacing exert either promoting or inhibiting effects on the formation of brine pockets. Increasing bubble radius and quantity result in a larger area of freshwater ice crystals and a reduction in the quantity and area of brine pockets, ultimately resulting in an improved desalination efficiency. While changes in bubble spacing have a minimal impact on the generated ice crystal area, they significantly alter the distribution of brine pockets. Smaller spacing between bubbles leads to a lower number of brine pockets formed, thus inhibiting brine pocket generation.

Nanoscale insights on the freezing front propagation and ion behaviors during seawater freezing

Seawater freezing holds significant importance in scientific and industrial applications. However, there is a lack of in-depth understanding of the nanoscale mechanism of seawater freezing. In this study, the nanoscale characteristics and mechanisms of seawater freezing are investigated by the molecular dynamics method. Since the solid-liquid interfacial free energy decreases with increasing subcooling or decreasing NaCl solution concentration, the speed of freezing front propagation increases. The formation of nano brine pockets is observed, which is analyzed based on the solid-liquid interfacial free energy. The energy barriers of Na+ and Cl− diffusing from the freezing front into the NaCl solution are smaller than these of diffusing into the hexagonal ice. As a result, most Na+ and Cl− are rejected into the solution, whereas only a few dope into the hexagonal ice. Furthermore, the energy barrier of Cl− diffusion into hexagonal ice is smaller than that of Na+, which makes the amount of Cl− doping into hexagonal ice greater than that of Na+. This study contributes to a deeper understanding of nanoscale characteristics and mechanisms of seawater freezing, which shall promote related technologies such as seawater freeze desalination.

Salt Distribution from Freezing Intrusions in Ice Shells on Ocean Worlds: Application to Europa

Several icy moons and dwarf planets appear to have hosted subsurface liquid water. Liquid water intruding upwards into the icy outer shells of these worlds freezes, forming ice and (from ocean solutes) non-ice solids. Here, we model concentrated aqueous solutions below 273 K to simulate the compositional evolution of freezing spherical intrusions. Starting solutions are based on five previously reported compositional end members for Europa’s ocean. For moderate-pH end members dominated by chloride, sulfate, and/or carbonate, the solids formed include Ca-, Mg-, and Na-sulfates and -carbonates, as well as Na- and K-chlorides. For silica-rich, high-pH end members, abundant amorphous silica forms with, potentially, similarly abundant NaOH and KOH. We further develop a new numerical model to compute the spatial distribution of the formed solids and residual brine as freezing progresses. If non-ice solids settle to the bottom, their deposits tend to have stacked hourglass shapes, widening each time the crystallization temperature of a new solid is reached. We discuss the applicability of this model to vertical fractures and global freezing of a subsurface ocean. These results inform (i) how compositional heterogeneities may affect the thermophysical properties of ice shells, which in turn influence convective and cryovolcanic transport, (ii) the compatibility of brine pockets with physicochemical conditions suitable for microbial life, and (iii) possible measurements of compositional heterogeneities within ice shells by spacecraft such as NASA’s Europa Clipper and ESA’s JUICE missions. The methodology developed here is applicable to other ice-covered ocean worlds.

Investigation and application of microscopic mechanism of multiple bubble movement on directional frozen crystallization of seawater

In this study, we simulated the crystallization of seawater under multi bubble motion using the lattice Boltzmann method – phase field method (LBM - PFM) coupling method [1]. In addition, the crystallization mechanism with multiple bubbles movement is investigated, and the results indicate that the decisive factor for crystallization rate is boundary heat flux [1]. The change in bubble radius does not affect the formation of brine pockets (BP), while reducing bubble surface tension, increasing crystallization angle and bubble motion speed can effectively reduce BP area. Furthermore, we also propose an extremely low-cost, zero pollution application technology to improve desalination rate based on the influence of bubble velocity on crystallization.

Modeling sea ice albedo and transmittance measurements with a fully-coupled radiative transfer model.

A rigorous treatment of the sea ice medium has been incorporated in the advanced Coupled Ocean-Atmosphere Radiative Transfer (COART) model. The inherent optical properties (IOPs) of brine pockets and air bubbles over the 0.25-4.0 µm spectral region are parameterized as a function of the sea ice physical properties (temperature, salinity and density). We then test the performance of the upgraded COART model using three physically-based modeling approaches to simulate the spectral albedo and transmittance of sea ice, and compare them with measurements collected during the Impacts of Climate on the Ecosystems and Chemistry of the Arctic Pacific Environment (ICESCAPE) and the Surface Heat Budget of the Arctic Ocean (SHEBA) field campaigns. The observations are adequately simulated when at least three layers are used to represent bare ice, including a thin surface scattering layer (SSL), and two layers to represent ponded ice. Treating the SSL as a low-density ice layer yields better model-observation agreement than treating it as a snow-like layer. Sensitivity results indicate that air volume (which determines the ice density) has the largest impact on the simulated fluxes. The vertical profile of density drives the optical properties but available measurements are scarce. The approach where the scattering coefficient for the bubbles is inferred in lieu of density leads to essentially equivalent modeling results. For ponded ice, the albedo and transmittance in the visible are mainly determined by the optical properties of the ice underlying the water layer. Possible contamination from light-absorbing impurities, such as black carbon or ice algae, is also implemented in the model and is able to effectively reduce the albedo and transmittance in the visible spectrum to further improve the model-observation agreement.

Brine Volume Fraction as a Habitability Metric for Europa's Ice Shell

AbstractBrine systems in Europa's ice shell have been hypothesized as potential habitats that could be more accessible than the sub‐ice ocean. We model the distribution of sub‐millimeter‐scale brine pockets in Europa's ice shell. Through examination of three habitability metrics (water activity, ionic strength, and salinity), we determine that brine pockets are likely not geochemically prohibitive to life as we know it for the chloride and sulfate‐dominated ocean compositions considered here. Brine volume fraction is introduced as a novel habitability metric to serve as a proxy for nutrient transport and recycling—because of its role in governing permeability—and used to define regions where nutrient‐open, nutrient‐closed, and relict habitats are stable. Whereas nutrient‐closed habitats could exist wherever brine is stable, nutrient‐open habitats are confined to meter‐scale regions near the ice‐ocean interface where freezing is occurring. This classification scheme can help guide future life‐detection missions to ocean worlds.

Snowball Earth Initiation and the Thermodynamics of Sea Ice

AbstractSnowball Earth is a hypothesized state in the deep past of Earth in which the ocean was completely or nearly completely covered by sea ice, resulting from a runaway ice‐albedo feedback. Here, we address how the treatment of sea‐ice thermodynamics affects the initiation of a Snowball Earth in the global climate model ICON‐A run in an idealized slab‐ocean aquaplanet setup. Specifically, we study the impact of vertical resolution and brine pockets of ice by comparing the 3‐layer Winton and a 0‐layer Semtner scheme, and we investigate the impact of limiting ice thickness to 5 m. The internal heat storage of ice is increased by higher vertical resolution and brine pockets, which weakens surface melting and increases global albedo by allowing snow and ice to persist longer into the summer season. The internal heat storage weakens the melt‐ratchet effect, as is confirmed with offline simulations with the two ice schemes. The result is a substantially easier Snowball Earth initiation and an increase in the critical CO2 for Snowball initiation by 50%. Limiting ice thickness impedes Snowball initiation as the removal of excess ice leads to an artificial heat source. Yet, the impact is minor and critical CO2 is decreased by 5% only. The results show that while the sea‐ice thickness limit plays only a minor role, the internal heat storage of ice represents an important factor for Snowball initiation and needs to be taken into account when modeling Snowball Earth initiation.

Impact of Microstructure on Solar Radiation Transfer Within Sea Ice During Summer in the Arctic: A Model Sensitivity Study

The recent rapid changes in Arctic sea ice have occurred not only in ice thickness and extent, but also in the microstructure of ice. To understand the role of microstructure on partitioning of incident solar shortwave radiation within the ice and upper ocean, this study investigated the sensitivity of the optical properties of summer sea ice on ice microstructures such as the volume fraction, size, and vertical distribution of gas bubbles, brine pockets, and particulate matter (PM). The results show that gas bubbles are the predominant scatterers within sea ice. Their effects on the scattering coefficient and ice albedo are 5 and 20 times stronger respectively than the effect of brine pockets. Albedo and transmittance of ice decrease with higher concentration and larger size of PM particles. A 4-cm top layer of ice with high PM concentration (50 g/m3) results in a 10% increase in radiation absorption. The role of ice microstructure in the partitioning of radiation transfer is more important for seasonal than for multiyear ice, and more important for ponded than for snow-covered ice. Varying ice microstructure can obviously alter solar radiation transfer in the ice-ocean system, even with a constant ice thickness. Our results suggest that numerical models should take the variable microstructure of sea ice into account to improve model accuracy and to understand the interaction between internal variations in Arctic sea ice and the ocean, especially in summer.

Modeling Virus and Bacteria Populations in Europa's Subsurface Ocean.

The search for life in the universe is often informed by the study of “extreme” environments on Earth, which provide analogs for habitable locations in the Solar System, and whose microbial inhabitants may therefore also serve as analogs for potential life forms in extraterrestrial milieus. Recent work has highlighted the ubiquity and importance of viral entities in terrestrial ecosystems, which calls for a greater understanding of the roles that viruses might play in hypothetical extraterrestrial biomes. While some studies have modeled the dynamics of viral and bacterial populations in icy ocean environments on Earth, previous work has yet to apply these findings to icy ocean worlds such as Jupiter’s moon Europa. It is commonly theorized that hydrothermal vents on Europa could produce the necessary reductants for chemosynthesis to take place on the ocean bottom. In the case that Europa’s ocean is a reductant-limited environment, how might reductants and organic matter reach the sub-ice region to power a more easily accessible ecosystem? Here, we propose a ‘viral elevator,’ a mechanism that functions similarly to the ‘viral shunt’ in Earth’s oceans, which could create and shuttle dissolved organic matter (DOM) to a hypothetical sub-ice biosphere through viral carriers. Current models of Europa’s ocean currents and stratification support the movement of DOM to the sub-ice biosphere. We adapt an existing model for bacterial and viral population dynamics in Earth’s Arctic sea ice to Europa and use parameters from various Arctic-based studies as proxies for Europa’s environment. We find that viral burst size has the most significant effect on the virus-to-bacteria ratio (VBR) and system longevity in closed systems (such as brine pockets within Europa’s icy crust), with higher burst sizes clearly increasing both. When applying our model to an open system with an influx of DOM from the viral elevator, we found that a steady-state system is attainable, with resulting sub-ice biofilms on the order of 0.1 mm thick (global equivalent layer). This has implications for future searches for life on Europa, given that life directly under the ice will be easier to detect and observe than life near the ocean bottom.

Importance of rigidity of ice-binding protein (FfIBP) for hyperthermal hysteresis activity and microbial survival

Ice-binding proteins (IBPs) are well-characterized proteins responsible for the cold-adaptation mechanisms. Despite extensive structural and biological investigation of IBPs and antifreeze proteins, only a few studies have considered the relationship between protein stabilization and thermal hysteresis (TH) activity as well as the implication of hyperactivity. Here, we investigated the important role of the head capping region in stabilization and the hyper-TH activity of FfIBP using molecular dynamics simulation. Data comparison revealed that residues on the ice-binding site of the hyperactive FfIBP are immobilized, which could be correlated with TH activity. Further comparison analysis indicated the disulfide bond in the head region is mainly involved in protein stabilization and is crucial for hyper-TH activity. This finding could also be generalized to known hyperactive IBPs. Furthermore, in mimicking the physiological conditions, bacteria with membrane-anchored FfIBP formed brine pockets in a TH activity-dependent manner. Cells with a higher number of TH-active IBPs showed an increased number of brine pockets, which may be beneficial for short- and long-term survival in cold environments by reducing the salt concentration. The newly identified conditions for hyper-TH activity and their implications on bacterial survival provide insights into novel mechanistic aspects of cold adaptation in polar microorganisms.

Inhibition effect of adsorption on brine pockets formation during seawater freeze desalination

High concentration of brine pockets formed during seawater freeze crystallisation reduces the freeze desalination efficiency. In this study, the inhibitory effect of salt-adsorption on brine pockets formation was investigated by simulation and experimental methods. A phase-field model coupled with the adsorption kinetic equation was established. The experiments on oriented growth of sea ice assisted with adsorption were conducted. In addition, the effects of adsorption on the morphology of ice crystals and brine pockets were studied. The results show that under the effect of adsorption, the planar crystal layer became thicker, the columnar crystals had a lower aspect ratio, the growth of brine pockets was inhibited, and the brine channels between adjacent ice crystals were reduced significantly. Stronger adsorption effect leads to less brine pockets. The area of ice crystals is increased by 6.08% in simulation and 11.79% in experiments, respectively. Moreover, the ice crystal growth rate and Peclet number were consistent between the simulation and experiment. The tip solute migration rate under adsorption increased by 17.69% compared with that without adsorption. This indicates that adsorption inhibits brine pockets formation in seawater freeze desalination and is favourable for improving the desalination rate.

A Random Pore Model of sea ice for predicting its mechanical properties

Sea ice contains gas and brine pockets arising from its special growth environments.The mechanical properties are strongly dependent on the microstructure of sea ice, the most prominent of which is described by its porosity. In this paper a Random Pore Model (RPM) is proposed, the basic assumption of which is that sea ice is made of pure ice sheet embedded with a lot of randomly distributed pores of random sizes. Monte Carlo method is then employed to generate random pore field. Simple numerical tests show that there exists upper limit value for porosity, which is close to 0.61. Simple numerical tests are then performed to find the dependence of tensile and compressive strengths of sea ice on porosity. The results turn out to be in good agreement with experimental results. This is followed by further studies on the effect of porosity on mechanical properties of sea ice, such as elastic modulus, Poisson's ratio, failure strains. The mechanical properties of sea ice at low porosity and scale effect are also discussed.

First Insights into the Microbiology of Three Antarctic Briny Systems of the Northern Victoria Land

Different polar environments (lakes and glaciers), also in Antarctica, encapsulate brine pools characterized by a unique combination of extreme conditions, mainly in terms of high salinity and low temperature. Since 2014, we have been focusing our attention on the microbiology of brine pockets from three lakes in the Northern Victoria Land (NVL), lying in the Tarn Flat (TF) and Boulder Clay (BC) areas. The microbial communities have been analyzed for community structure by next generation sequencing, extracellular enzyme activities, metabolic potentials, and microbial abundances. In this study, we aim at reconsidering all available data to analyze the influence exerted by environmental parameters on the community composition and activities. Additionally, the prediction of metabolic functions was attempted by the phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt2) tool, highlighting that prokaryotic communities were presumably involved in methane metabolism, aromatic compound biodegradation, and organic compound (proteins, polysaccharides, and phosphates) decomposition. The analyzed cryoenvironments were different in terms of prokaryotic diversity, abundance, and retrieved metabolic pathways. By the analysis of DNA sequences, common operational taxonomic units ranged from 2.2% to 22.0%. The bacterial community was dominated by Bacteroidetes. In both BC and TF brines, sequences of the most thermally tolerant and methanogenic Archaea were detected, some of them related to hyperthermophiles.

Extracellular enzyme activity under subzero hypersaline conditions by model cold-adapted bacteria and Arctic sea-ice microbial communities

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 87:99-111 (2021) - DOI: https://doi.org/10.3354/ame01974 Extracellular enzyme activity of model cold-adapted bacteria and Arctic sea-ice microbial communities under subzero hypersaline conditions G. M. Showalter*, J. W. Deming School of Oceanography and Astrobiology Program, University of Washington, Seattle, WA 98195, USA *Corresponding author: gmshowalter@gmail.com ABSTRACT: Bacterially produced extracellular enzymes (EEs) play an important role in the cycling of organic matter in the marine environment, breaking down large compounds to those small enough to be transported across the cell membrane. EEs may play an especially important role within the brines of sea ice, as freezing concentrates both bacteria and organic materials into brine pockets, leading to higher encounter rates between EEs and their substrates. However, whether EEs are able to perform under the extreme conditions of sea-ice brines, particularly during winter, is unknown. Here, we characterized EE activity (EEA) of leucine aminopeptidase produced by the psychrophilic bacterium Colwellia psychrerythraea strain 34H and the cold-tolerant Psychrobacter strain 7E, under analogue sea-ice conditions using a standard fluorescence-based activity assay. EEs produced by the psychrophile were active at the most extreme conditions tested, i.e. temperature of -8°C and salt concentration of 120 ppt, with activity enhanced if the EEs concerned were produced under subzero hypersaline conditions. EEs produced by the Psychrobacter strain were less cold- and salt-active. When high-latitude Arctic samples of sea-ice brine, under-ice water, and the sea-surface microlayer were analyzed using the same assay after a freeze-thaw cycle, EEA was highest in the sea-ice samples, with activity at -10°C and salinity of 142 ppt. Overall, these results indicate that EEA can contribute to the degradation of organic material in sea ice through winter, likely sustaining microbial communities in brine pores in the process and altering the nature of organic material released at spring melt. KEY WORDS: Arctic marine environment · Carbon cycle · Colwellia psychrerythraea strain 34H · Enzymatic activity · Psychrophiles · Sea ice · Psychrobacter strain 7E Full text in pdf format PreviousNextCite this article as: Showalter GM, Deming JW (2021) Extracellular enzyme activity of model cold-adapted bacteria and Arctic sea-ice microbial communities under subzero hypersaline conditions. Aquat Microb Ecol 87:99-111. https://doi.org/10.3354/ame01974 Export citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in AME Vol. 87. Online publication date: August 26, 2021 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2021 Inter-Research.

Brine Migration and Impact‐Induced Cryovolcanism on Europa

AbstractDespite evidence for plumes on Jupiter's moon Europa, no surface features have been definitively identified as the source of the plumes to date. Furthermore, it remains unknown whether the activity originates from near‐surface water reservoirs within the ice shell or if it is sourced from the underlying global ocean. Here we investigate brine pocket migration, studied previously in the context of sea ice on Earth, as a process for transporting brine along thermal gradients. We show that the fracture system located in the center of Europa's Manannán crater is consistent with the formation of a subsurface brine reservoir. After the initial impact, residual aqueous melt concentrated via brine pocket migration as the target material cooled. Freezing and overpressurization then resulted in a cryovolcanic eruption. The volume of the emptied reservoir and the critical composition at the end of migration provide further constraints on the average salinity of Europa's ice shell.

Microbial Assemblages in Pressurized Antarctic Brine Pockets (Tarn Flat, Northern Victoria Land): A Hotspot of Biodiversity and Activity.

Two distinct pressurized hypersaline brine pockets (named TF4 and TF5), separated by a thin ice layer, were detected below an ice-sealed Antarctic lake. Prokaryotic (bacterial and archaeal) diversity, abundances (including virus-like particles) and metabolic profiles were investigated by an integrated approach, including traditional and new-generation methods. Although similar diversity indices were computed for both Bacteria and Archaea, distinct bacterial and archaeal assemblages were observed. Bacteroidetes and Gammaproteobacteria were more abundant in the shallowest brine pocket, TF4, and Deltaproteobacteria, mainly represented by versatile sulphate-reducing bacteria, dominated in the deepest, TF5. The detection of sulphate-reducing bacteria and methanogenic Archaea likely reflects the presence of a distinct synthrophic consortium in TF5. Surprisingly, members assigned to hyperthermophilic Crenarchaeota and Euryarchaeota were common to both brines, indicating that these cold habitats host the most thermally tolerant Archaea. The patterns of microbial communities were different, coherently with the observed microbiological diversity between TF4 and TF5 brines. Both the influence exerted by upward movement of saline brines from a sub-surface anoxic system and the possible occurrence of an ancient ice remnant from the Ross Ice Shelf were the likely main factors shaping the microbial communities.

Evidence of Freezing Pressure in Sea Ice Discrete Brine Inclusions and Its Impact on Aqueous‐Gaseous Equilibrium

AbstractSea ice in part controls surface water properties and the ocean‐atmosphere exchange of greenhouse gases at high latitudes. In sea ice, gas exists dissolved in brine and as air bubbles contained in liquid brine inclusions or as bubbles trapped directly within the ice matrix. Current research on gas dynamics within the ocean‐sea ice‐atmosphere interface has been based on the premise that brine with dissolved air becomes supersaturated with respect to the atmosphere during ice growth. Based on Henry's law, gas bubbles within brine should grow when brine reaches saturation during cooling, given that the total partial pressure of atmospheric gases is above the implicit pressure in brine of 1 atm. Using high‐resolution light microscopy time series imagery of gas bubble evolution inside discrete brine pockets, we observed bubbles shrinking during cooling events in response to the development of freezing pressure above 3 atm. During warming of discrete brine pockets, existing bubbles expand and new bubbles nucleate in response to depressurization. Pressure variation within these inclusions has direct impacts on aqueous‐gaseous equilibrium, indicating that Henry's law at a constant pressure of 1 atm is inadequate to assess the partitioning between dissolved and gaseous fractions of gas in sea ice. This new evidence of pressure build‐up in discrete brine inclusions controlling the solubility of gas and nucleation of bubbles in these inclusions has the potential to affect the transport pathways of air bubbles and dissolved gases within sea ice‐ocean‐atmosphere interface and modifies brine biochemical properties.