Geology Reference
In-Depth Information
Site 2-97
Site 3-97
Site 1-97
0
0
5
5
10 cm
10
10 cm
10 cm
10
15
20 cm
15
20
25
20 cm
20
20 cm
30 cm
30
2 cm
2 cm
2 cm
25
0
246810
35
Salinity (‰)
0
246810 12 14 16 18
Salinity (‰)
Figure 4.34
Microstructure of ice representative of the three sites shown in Figure 4.33. The salinity profiles
in open and closed squires (attached to site 1‐97) are of site 1 and 3, respectively [
Johnston,
1998].
4.4.2.1. Air Entrapment in FY Ice
Since air is also rejected during the solidification pro-
cesses, it becomes entrapped together with the brine
pockets. These inclusions are trapped along the grain
and subgain boundaries along with the trapped brine
pockets. Since the air bubbles are mostly present in con-
junctions with the brine inclusions, often it is difficult to
identify them as separate entities. This was exemplified
earlier in Figure 2.36. As shown earlier in Figures 2.26
and 2.37 tiny, submillimeter size, air bubbles are often
trapped inside the brine pockets. Usually, the air bubbles,
isolated or inside brine pockets, are visible as black
objects with bright spots in thin sections observed
through optical microscopes under transmitted light. An
explanation for this appearance is given below.
Consider the example of brine and air entrapments pre-
sented here in Figure 4.35. Three types of elongated inclu-
sions are noticeable. They include clear brine pockets
outlined by dark lines, brine pockets with small dark
spherical air pockets, and dark individual large air pockets.
Note that these inclusions are mostly linked to subgrain
boundaries, and associated inclined surfaces, visible in
the form of diagonal bands. Note the inclusions along
the boundary in the shape of an arc at the bottom of the
picture. The long direction of the inclusions tends to
be parallel to the surfaces of the boundaries. All the small
0.5 mm
Figure 4.35
Photomicrograph of brine pockets and air entrap-
ment in FY ice (N. K. Sinha, unpublished).
air bubbles seem to be trapped inside the brine pockets as
dark spheres with bright dots in the center.
The dark appearance of the air bubbles can be explained
on the basis of transmission characteristics of light as
schematically shown Figure 4.36. The only light that can
go through the objective (lens) of a transmission‐type
optical microscope is the central beam incident at right
angle to the surface of the bubbles. This central beam,
