Geology Reference
In-Depth Information
drainage areas were analogous to the catchment area
of a river and its tributaries. The frequency of occur-
rence of channels was roughly once every 33 cm 2 in the
case of thin ice (<100 mm thickness) and 180 cm 2 in
the case of thick ice (>100 mm). This gives an average
distance of 32.4 mm between brine channels for thin
ice and 134 mm for thick ice.
The brine channel formation and geometric charac-
teristics of these channels were addressed in several
studies [ Bennington , 1967; Martin , 1979; Weeks and
Ackley , 1982; Wakatsuchi and Saito , 1985; Wakatsuchi
and Kawamura , 1987]. In a laboratory experiments on the
growth of saline ice in a 0.5‐m‐deep freezing cell cooled
from above, Eide and Martin [1975] observed vertical brine
drainage channels with diameters of 1-3 mm and associ-
ated them with smaller feeder channels extended through-
out the ice thickness. They also observed that the diameter
of brine channels at the ice‐water interface is much nar-
rower than higher up in the bulk ice, so that the channel
has a bottle neck at the interface. They developed a quali-
tative theory based on the difference in pressure head
between the brine in the ice and the saline water under it
to explain the formation of the channels and the onset of
a convective instability. The latter explains the existence
of the “neck” at the ice‐water interface. Cole and Shapiro
[1998] examined ice sheet in Elson lagoon, northern
Alaska, during the winter of 1993-1994. They reported
networks extended through the entire thickness of 0.3 m
young ice (YI) sheet. However, they did not observe any
channel that extended completely through the ice sheet
when it was 1 m thick. The initial population of channels
terminated within the sheet, either randomly or at a spe-
cific depth. The typical length of the channel was found
to be in the range of 0.3-0.5 m.
Gravity drainage is enhanced under the following
conditions: (1) The channel's diameter is large. Larger
diameters reduce the viscous drag of brine flow hence
enhancing the drainage. (2) The local temperature gra-
dient within the ice that surrounds the channel is steep
enough to results in more expulsion of brine from the
pockets to feed into the channel. (3) The pressure asso-
ciated with channel formation is relatively low to facili-
tate the flow of brine into channels. (4) The intensity of
the channels and their tributaries that originate from the
pockets is high. The higher intensity increases permea-
bility, and therefore drainage.
Since all the major salts are precipitated at a tempera-
ture of −22.8 °C, there cannot be any measurable brine
drainage in the ice below this temperature. At tempera-
tures above −22.8 °C brine drainage is expected to increases
with the increase in ice temperature. Cox and Weeks [1975]
stated that the rate of gravity drainage depends on the
brine volume and temperature gradient of the ice. As either
factor increases, the rate of change of salinity due to
gravity drainage also increases. They also concluded that
the gravity drainage is more active when the brine volume
fraction is higher than 0.05, that is, the volume of pure ice
fraction is less than 0.95. This was also found by Notz and
Worster [2009] who measured the gravity drainage in labo-
ratory‐made saline ice made from a NaCl solution of salin-
ity 34‰, which was cooled from above with constant
temperature of −10 °C.
Wettlaufer et  al . [1997] employed the mush Rayleigh
number R a , which is defined as follows:
Rhz gS
k
(
)
l
brz
,
,max
(
)
(2.43)
a
where ( h z ) is the distance between the ice‐ocean inter-
face at depth h and the level z in the ice, g is the accel-
eration due to gravity, S br , z is the salinity of the brine
and ρ l βS br , z represents the difference between the den-
sity of seawater and that of brine at level z , k is ther-
mal diffusivity, m is the dynamic viscosity of the liquid,
ϕ υ , max is the maximum solid volume fraction and
∏ ( ϕ υ , max ) is the effective permeability of the ice as a
function of φ υ , max .
In laboratory experiments when saline ice was grown in
a tank using a cooling plate at the top of the tank
Wettlaufer et  al . [1997] found a delay of initial gravity
drainage from newly grown ice. The surface temperature
remained cold, and therefore ice had a low brine fraction.
This caused the Rayleigh number to be below its critical
value until the ice had reached a certain thickness. Notz
and Worster [2008] employed the same concept of mush
Rayleigh number to determine the susceptibility of sea
ice to gravity drainage. They concluded that gravity
drainage starts when the mush Rayleigh number reaches
10. This is more likely to happen when the ice is relatively
warm and hence permeable enough to allow for drainage.
It will be seen later that the permeability of warm ice is
linked to the grain as well as subgrain boundaries in sea
ice. At warm temperature, the interlinked grain and sub-
grain boundaries are impregnated with brine, forming a
network through which brine can move as described in
Sinha [1977a].
Going back to the field observations, it should be
emphasized that practically all the observations on natu-
ral sea ice were carried out by extracting blocks of ice
from ice sheets. Thick sections were then prepared for
examining the macroscopic characteristics and carrying
out investigations on the spatial distribution of brine chan-
nels. It is apparent that the previous investigators never
performed any grain‐ or subgrain‐scale microstructural
analysis of ice associated with the brine channels. An
example of cross‐sectional views of mature brine chan-
nels in the first‐year sea ice in Northstar Bay, Greenland
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