Geology Reference
In-Depth Information
the aging processes involving partial thawing, percola-
tion of meltwater to the lower levels, and subsequent
refreezing. The only inclusion found in hummock ice is
air bubbles. Air exists with larger volume fraction within
the upper layer of hummock ice (see rationale in sec-
tion 2.5 and Figure 2.62). In this case, bubbles exist in
relatively large size (order of a few millimeters) and at
much closer distances. When hummock surface melts as
a result of exposure to solar radiation, meltwater accu-
mulates in neighboring melt ponds. Albedo of the pond
then decreases and the absorption of the incident radia-
tion increases. This positive feedback scenario accelerates
melting of ice in the melt pond. About 1 m of ice was
observed to melt on a MY floe in Sabine Bay, Melville
Island, during the summer of 1992 by the authors. The
melt is usually brackish water with salinity less than 2‰.
Melt pond ice, on the other hand, features relatively large
crystals at the top, which originate from freezing of
brackish water at the melted surface. Unlike hummock
ice, MP ice usually contains fewer bubbles and more
brine pockets, though the salinity near the surface is gen-
erally low (1‰−3‰), depending on the flooded water
salinity from which ice was formed. In the case of sec-
ond‐year ice, hummocks are usually formed at locations
of ridges while MP ice is formed at leveled surface areas.
The scenario of air bubble formation at the surface of
old hummocks, schematically presented in Figure 2.62,
depicts three commonly observed geometrical features
of the bubble‐rich layer in hummock ice. The first is the
interconnectivity of the drainage network and conse-
quently the bubbles, particularly at locations near the
hummock surface. The second is the decrease in the depth
of the bubbly layer with the increase in distance from the
top of the hummock. The third is the sharp interface that
marks the end of the bubble‐rich layer.
The concave surface of melt pond ice, on the other hand,
does not allow the formation of a drainage network in a
significant manner. In this case, water from the surface
melt along with snowmelt and/or rain, could accumulate
on top of the ice surface. When this water freezes, it
forms a new ice layer on top of the older ice. The interface
between the two layers can sometimes be abrupt and well
defined. The crystalline structure of the new layer depends
on the water quality and the atmospheric conditions. S1
type of ice with very large crystals develops for calm con-
ditions during freezing. However, if snow falls before the
initiation of freezing, then a granular layer followed by S2
type of ice develop depending on the amount of snow
deposition. Formation of air inclusions in this case is pos-
sible through two mechanisms. The first is air entrapment
between snow particles. The second originates from the air
dissolved in the water. In both cases, the air inclusions in
MP ice are much smaller than air bubble in hummock ice.
Examples of structural features of MY hummock and MP
Ice
Air
Brine
Figure 4.36
Schematic to illustrate the appearance of a dark air
bubble inside a clear brine pocket when viewed through trans-
mission‐type optical microscope. Only a narrow beam of light
propagating normal to the surface of the bubble can go through
the lens of the microscope, making a visible white spot in the
middle of the bubble (sketch by N. K. Sinha, unpublished).
therefore, produces the bright spot in the middle of the
dark air bubbles. The dark areas of the air bubbles are
caused by the scattering of the transmitted light from
the source before reaching the lenses of the microscope.
4.4.3. Multiyear Ice, MY (
Qavvaq
)
For Inuit communities living in the Low Arctic, usually
the annual ice melts by the end of the winter. However,
there are two Inuit communities in the High Arctic, namely
Resolute and Grise Fjord in Canada and Qaanaaq in
Greenland, where they do encounter old sea ice they call
qavvaq
in Inuktitut. Occasionally, large masses of pack ice
from further north move inside the ice‐free fjords and
sounds during the summers. This does not happen every
year in the Low Arctic. However, often grounded ridges or
rubbles piled up on the land near the shorelines survive
through the summer melt season and become virtually
free from trapped brine. The local people are, naturally,
aware of that and call the white or partially clear, virtually
salt‐free ice
Qavvaq
. According to the WMO terms, it is
known as multiyear (MY) ice. MY ice floes are the com-
mon features in the western High Arctic in the Canada
Basin and the Arctic Ocean. Large‐scale characteristics of
MY ice floes have been described in section 2.5.
The multiyear ice surfaces are characterized by hummocks,
depressions, or melt ponds (MP) and rounded ridges.
Hummock and melt pond ice are characteristically different
in terms of surface and internal microstructure. Both types,
however, are different from the top layer of the FY ice from
which they originate due to the aging and melting/refreezing.
Hummock ice features a variety of crystal shapes and
sizes with convoluted boundaries. This is mainly due to
