Geology Reference
In-Depth Information
unconformities are sometimes inferred from the study of soil microfabrics and structures
(Derbyshire et al., 1985; van Vliet-Lanoë, 1988).
7.4.4. Ice Crystallography
The petrofabric analysis of ground ice is not only useful for descriptive purposes but, like
the study of cryostructures, helps to infer growth processes and conditions. This is
because the crystal size, shape, boundary characteristics, and
c
-axis orientations are
directly related to the direction and speed of the freezing process. Ice crystals normally
grow at right angle to the direction of freezing, and crystal size varies inversely with the
rate of freezing. In recent years, petrofabric analyses of ground ice bodies have become
standard procedures (French and Pollard, 1986; Pollard, 1990; Solomatin, 1986). The
techniques and procedures follow closely those developed for glacier ice (Ostrem,
1963).
Although different ice types may display a characteristic range of fabric and texture
patterns, the reality is that a wide range of fabrics exist. Without good cryostratigraphic
control, ice fabrics do not permit unambiguous identifi cation of ice types. However, seg-
regated ice tends to be composed of large equigranular anhydral crystals whose
c
-axes
form a loose girdle oriented normal to the plane of the ice layer. By contrast, buried snow-
bank ice is composed of small enhedral equigranular crystals with a high concentration
of vertically-oriented inter-crystalline bubble trains and tubular bubbles. The petrography
of intrusive ice refl ects the groundwater transfer mechanism and freezing conditions. For
example, in seasonal-frost mounds, the ice mass is composed of large tabular crystals ori-
ented normal to the freezing direction with
c
-axes forming a horizontal girdle normal to
the long axes of the crystals (Pollard and French, 1985).
7.4.5. Ice Geochemistry
Standard chemical analyses, including conductivity and cations (Ca, Na, Mg, and K), can
characterize ground ice (water). Although such determinations usually refl ect local geo-
logic and/or hydrologic conditions, they are useful for comparison purposes and for dif-
ferentiating between ice bodies.
More inferential from the viewpoint of cryostratigraphy is the use of isotopic data
(e.g.
O
18
is preferentially
incorporated into the ice, which becomes isotopically heavier. Usually isotopic values are
compared to standard mean ocean water (SMOW) values and expressed in ‰. There is
also a positive linear relationship between temperature and
O
18
, deuterium, and tritium). For example, when water freezes,
δ
δ
O
18
, as demonstrated from
ice cores from Greenland. Finally, there is also a relationship between
δ
O
18
and
O
16
: the
δ
δ
colder the climate, the lower the
O
16
ratio becomes. It follows that the isotopic
analysis of ground ice is not only a useful descriptive tool but also allows inferences to be
made concerning the approximate temperature of the water prior to freezing and, by
comparing the isotopic signatures with those from adjacent groundwater, the water
source.
A simple example of the effects of freezing upon the oxygen-isotope composition of
groundwater is provided by data in Table 7.5. Two sites are compared; one a large drained
lake where aggrading permafrost is 20-35 m in thickness, the other a more-recently
drained lake where permafrost is currently only 15-20 m thick. It is assumed that initial
groundwater conditions were similar. The ice (water) samples from within newly-formed
permafrost is
δ
O
18
:
δ
−
16‰ while the subpermafrost waters range from
−
27‰ to
−
29‰, indicating
