Geology Reference
In-Depth Information
In natural states, the temperature of the top of the ice
sheet decreases with the advent of the winter. When the
ice temperature decreases, different salts start to precipi-
tate at different temperatures (corresponding to their
eutectic temperature according to Table 2.1). This leads
to a somewhat reversible mechanism of changes in brine
volume. It takes place inside brine pockets as well as in
the channels. The size of the brine inclusions decreases
as more of the water solidifies with the concurrent
precipitation of salts. Consequently, the brine salinity
increases and the freezing temperature of the higher
salinity brine decreases. This process continues until the
freezing temperature becomes too low to allow further
freezing, that is, reaches a state of thermal equilibrium.
In this process, brine volume decreases but gives rise to
an increase in the volume of solid salt. The reverse is
also true, namely, when ice temperature increases brine
volume also increases because more solid salt will dis-
solve into the brine solute in order to bring the freezing
temperature back to the point of thermal equilibrium.
Calculations of these volume fractions are presented in
section 3.4.
In agreement with the phase diagram of sea ice,
Figures 2.36 and 2.37 exhibit the brine pockets in the
form of long tubes filled by liquid brine and entrapped
air bubbles at −5 °C. No solid crystals can be seen inside
these tubes. Calcium carbonate (CaCO
3
.6H
2
O), which
begins to precipitate at −2.2 °C, is present in seawater
as trace quantities. Figure 2.38 illustrates scanning
electron micrographs (SEMs) of the replica of brine
0.5mm
Figure 2.36
Optical micrograph of brine tubes mostly with air bubbles (black) in a vertical thin section of
first‐year sea ice at −5 °C (micrograph by N. K. Sinha, unpublished).
(b)
(a)
200
μ
m
200
μ
m
Figure 2.37
Air bubble forming a neck in a brine tube (a) and bubbles inside a long brine tube (b) in a vertical
section at −5 °C (micrograph by N. K. Sinha, unpublished).
