JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C2, 3051, doi:10.1029/2001JC000887, 2003

2. Background

[6]   The physical properties of sea ice are highly variable in both time and space. In the Arctic, areas less than a square kilometer often include multiple ice types with varying ages and thicknesses. Variations in ice thickness, growth rate, age, and thermal forcing promote vertical gradients in salinity and temperature which, in turn, lead to vertical gradients in inclusion size and number distributions. Well-drained ice above freeboard level usually contains significant amounts of air, causing the ice to appear white due to high backscatter from these gas inclusions; interior samples from deeper in the ice generally have much lower gas volumes and hence produce less backscattering. Fractional brine, gas, and precipitated salt volumes have been parameterized for sea ice [Cox and Weeks, 1983] as a function of temperature, density, and salinity. Changes in these fractional volumes are dictated by freezing equilibrium chemistry. While such information is needed for understanding how other bulk properties of the ice respond to changes in temperature, it is not sufficient to characterize light scattering within the ice. For this purpose, it is also necessary to have at least a statistical description of how individual brine inclusions, gas bubbles, and salt crystals are distributed. In particular, we seek more detailed knowledge about the size, number density, and spatial arrangement of these inclusions.

2.1. Brine Inclusions

[7]   The underside of growing sea ice typically consists of an array of small ice platelets which grow vertically downwards from the interface, forming the so-called “skeletal layer”. Within a single crystal, these platelets are generally parallel with spacings in first-year ice of 0.4–1.0 mm, depending on the growth rate of the ice [Weeks and Hamilton, 1962; Lofgren and Weeks, 1969; Nakawo and Sinha, 1984]. As the platelets lengthen and grow wider, small ice bridges periodically form between adjacent platelets, trapping inclusions of brine. Since this mechanism is the source of most of the liquid found in sea ice, brine inclusions tend to be vertically oriented and to exhibit characteristic horizontal spacings. As the ice grows and cools, trapped brine must become more concentrated to maintain freezing equilibrium. This is accomplished by the removal of pure water from brine inclusions which freezes to the surrounding ice lattice, reducing the size of the inclusions and increasing their salinity. Conversely, when temperature increases, ice melts from inclusion walls increasing the brine volume and decreasing its salinity.

[8]   Measurements of brine inclusion size and number distributions have been made by imaging thin sections from both natural and laboratory-grown sea ice samples [e.g., Arcone et al., 1986; Eicken, 1993; Perovich and Gow, 1991, 1996; Light, 1995; Cole and Shapiro, 1998; Eicken et al., 2000]. Figure 1 shows a photograph of a vertical thick section cut from a depth of approximately 80 cm in naturally grown first-year ice. In this low resolution view, both brine and gas inclusions are visible. A majority of the visible features are elongated brine inclusions, and while there appears to be considerable small-scale spatial variability, the overall structure is much more uniform. Observations from higher resolution imagery on horizontal thin sections from first-year sea ice [Perovich and Gow, 1996] indicate that average brine inclusion number densities range from 1.0 to 4.5 per mm³ and cross-sectional areas of individual inclusions range from 0.0012 to 1.0 mm². Cole and Shapiro [1998], however, demonstrated that additional information about ice microstructure could be obtained by complementing the horizontal thin sections with high-resolution data from vertical thin sections. They found, for example, that brine inclusion shapes ranged from nearly spherical to elongated with vertical extent exceeding 15 times the diameter. While such studies have provided general information about how brine inclusions affect the microstructure, they do not address the roles played by gas bubbles and precipitated salts.

2.2. Gas Bubbles

[9]   The density of sea ice increases above that of pure ice (0.917 Mg m^-3 at -5°C) as its salinity is increased, and decreases as its gas content is increased. The bulk density of first-year sea ice typically ranges between 0.89–0.93 Mg m^-3 [Perovich et al., 1998a], where the lower end of this estimate corresponds to sea ice containing significant amounts of gas. Bubbles can be entrained into sea ice at the growth interface during the freezing process when gas dissolved in seawater comes out of solution. Air can also be included in ice above the freeboard level due to meltwater drainage during the summer. In addition, gas bubbles are expected to form in brine inclusions as the ice warms and lower density ice melts into higher density liquid, forming a void within the inclusion. This suggests that there should be two primary types of bubbles in sea ice: those that occur within the ice lattice itself, and those that form within brine inclusions. Data on the size and number distributions of bubbles in sea ice are sparse. Grenfell [1983] measured bubble number distributions in small samples cut from a freezing lead. Observed bubbles had radii ranging from 0.1 to 2 mm and number densities of 0.03 per mm³. These measurements were consistent with bubble data collected by Gavrilo and Gaitskhoki [1970] near the top of “bubbly”, first-year ice. These bubble densities are approximately two orders of magnitude smaller than the number of brine inclusions observed in first-year ice, suggesting that few of the brine inclusions contained visible bubbles. The apparent lack of smaller bubbles, however, may simply reflect the limited resolution available during these observations.

2.3. Precipitated Salts

[10]   As sea ice cools, brine in the liquid inclusions concentrates and becomes saturated with respect to certain salts. These salts begin to precipitate out of solution at various temperatures between -2° and -54°C [Nelson and Thompson, 1954; Richardson, 1976]. The mass of precipitated salt is a unique function of temperature and ice salinity. The two most abundant salts are mirabilite (Na₂SO₄ • 10H₂O) which begins to precipitate at -8.2°C, and hydrohalite (NaCl • 2H₂O) which begins to precipitate at -22.9°C. As shown in Figure 2, the precipitation of each salt occurs gradually over a range of temperatures; the total mass of hydrohalite precipitated from seawater is more than 4 times that of mirabilite. These two hydrated salts are known to form monoclinic crystals, but there are no direct observations of their sizes or precipitation patterns in sea ice. Roedder [1984] suggested that hydrohalite crystals in fluid inclusions are commonly small (~1 µm). Although crystal sizes are expected to range over some distribution, at this point we only have estimates of effective crystal sizes in sea ice that are based on comparison of observed and modeled optical properties. Light [1995] utilized this method to estimate an effective edge length of 8–9 µm for precipitated salt crystals in laboratory-grown sea ice.

Citation: Light, B., G. A. Maykut, and T. C. Grenfell, Effects of temperature on the microstructure of first-year Arctic sea ice, J. Geophys. Res., 108(C2), 3051, doi:10.1029/2001JC000887, 2003.

Copyright 2003 by the American Geophysical Union.