Geology Reference
In-Depth Information
(a)
(b)
50
μ
m
50
μ
m
Figure 2.38
Scanning electron micrographs (SEMs) showing precipitated sodium sulfate crystals (Na
2
SO
4
.10H
2
O)
in brine pocket, taken at −10 °C. (a) indicates crystals observed at the bottom of a brine tube, while (b) indicates
clusters strewn throughout the side of the tube [
Sinha
, 1977a].
pockets at −10 °C in first‐year ice obtained from
Strathcona Sound, Nunavut, Canada, in 1975. Replicas
of thin sections of ice be prepared and examined
according to the technique described in
Sinha
[1977a].
The details of the procedures are described in sec-
tion 6.4.4. These SEM images are quite different from
the optical images seen at −5 °C in Figure 2.37. The
liquid brine in the pockets at −10 °C resulted in the
smooth walls of the replicas as the replicating solu-
tion penetrated the brine tubes. Only sodium sulfate
(Na
2
SO
4
.10H
2
O) crystals are expected to be present in
the pockets in noticeable quantities at this tempera-
ture. It is conceivable that the salt crystals are pushed
either to the side (Figure 2.38b) or near the bottom
(Figure 2.38a) of the cavities by the replicating solu-
tion. This indicates that the salt crystals remain sus-
pended in the liquid brine, in which case no reinforcement
in strength due to the inclusion of these crystals in the
ice matrix around the brine pockets is possible, contra-
dicting a theory originally proposed by
Peyton
[1966].
At temperatures of −30 °C, most of the dissolved salts
in sea‐ice brine are expected to be precipitated. Optical
micrographs shown earlier in Figure 2.23 illustrated this
in a very clear manner. The brine pockets were revealed
to be fully packed with various crystals. The shallow
depth of field of the optical microscope and the pres-
ence of the pockets below the surface of the thin sec-
tions, however, were responsible for lack of clarity of
the shape of crystals in these illustrations. This apparent
difficulty was removed by the great depth of field of the
SEM, which clearly showed details inside cylindrical
cavities (Figure 2.38), albeit, at −10 °C. Figure 2.39
shows two images at different magnifications of a brine
pocket at −30 °C in the same first‐year ice from
Strathcona Sound illustrated in Figures 2.21, 2.23, and
2.38. These are, of course, SEM images of a second rep-
lica made at −30 °C. The first replica of sea ice almost
always contains debris from the microtoming procedure
used in preparing the surfaces of the thin sections
[
Sinha
, 1977a]. Precipitated salts within the pocket are
clearly seen in these images. Based on the phase diagram
and the temperature at which the replica was made, it is
believed that the majority of the salt crystals were
NaCl.2H
2
O, although individual crystals were not posi-
tively identified. It can also be seen that the salt crystals
were loosely packed in the cavity and could be removed
by making successive replicas. This provided another
confirmation of the observations made at −10 °C that
the precipitated crystals remained suspended in the
brine pockets until the closer of the cavities when most
of the predominant slats were precipitated.
The rate of salt precipitation after reaching the eutec-
tic temperature has not been accurately estimated in the
literature. However,
Weeks
[2010] communicated an
impression based on early studies on ice grown from a
Na
2
SO
4
-NaCl.H
2
O solution that precipitation occurred
within a couple of hours from reaching the precipita-
tion temperature. In a recent study,
Light et al
. [2003]
presented a photomicrograph of a brine pocket in a
thin section of first‐year ice taken at −15 °C. At this
temperature, the main salt that should have precipi-
tated is Na
2
SO
4
.10H
2
O (sodium sulfate or mirabilite).
The figure shows that, unlike the NaCl.2H
2
O crystals,
mirabilite has rounded edges and an irregular shape.
The largest and smallest crystals have diameters of 140
and 15
µ
m, respectively. The authors used a CCD video
camera of high resolution that allows recording inclu-
sions as small as 10
µ
m.
