Geology Reference
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- Drainage network can be established
- Bubbles are formed from remains of
drainage network
- Drainage channels cannot be established
- Surface melt, snow, and rain accumulate
and later consolidate to form a new ice
layer
Figure 2.62 A graphical representation of multiyear ice surface during melt season. Hummock and depression
(melt pond) surfaces respond differently to surface melt and that leads to the generation of more air bubbles in
hummocks than in melt ponds [ Shokr and Sinha , 1994].
ponds is between 0.2 and 2.0‰. The density varies
between 0.7 and 0.85 kg.m −3 for hummock ice and 0.82
and 0.92 kg.m −3 for melt pond ice. Photographs of thin
sections that show the bubble contents of hummock and
pond ice are presented in section 4.4.3.
A scenario of bubble formation in MY ice is introduced
in Shokr and Sinha [1994, 1995]. It explains the abundance
of air bubbles in the subsurface of hummock ice and its
shortage in melt pond ice. As the ice temperature increases
during the melt season, the brine pockets increase in size
and group into larger pockets. When surface melt starts
the water percolates though these pockets, forming a more
connected and effective drainage network. In order for the
network to function as a conduit to drain the surface melt,
some conduits have to terminate at the ice surface as
shown in Figure 2.62. The surface melt continues to per-
colate through the ice volume during the summer until the
freezing starts in the fall. By that time, the surface melt
slows down, while the water in the drainage network con-
tinues to drain by gravity. As a result, channels will be par-
tially empty. At freezing, the empty parts become air
inclusions. In other words, bubbles at or near the surface
are formed from the remains of water drainage channels
established in the summer. This scenario explains also the
commonly observed interconnected air bubbles near the
surface of hummock ice. Perovich and Gow [1996] con-
firmed the origin of air bubbles in MY ice from the evolu-
tion of the brine pockets during the transition of FY ice
into MY ice. Moreover, they pointed out the importance
of understanding how the elongated brine pockets develop
into the rounder air bubbles. They recommended more
qualitative and quantitative studies of the evolution of
MY ice from FY ice.
The horizontal surface of melt pond ice, solidified
from pure or brackish melt water, does not form drainage
network in a significant manner (Figure  2.62). In this
case, surface melt along with possible snow and rainwa-
ter could accumulate on top of the ice. On subsequent
freezing, this layer consolidates to form a “newer” super-
imposed ice layer that could consist of snow‐ice.
Formation of air inclusions is possible within the granu-
lar texture of snow ice through two mechanisms: (1) air
entrapment between snow particles and (2) air originally
dissolved in water and later rejected during freezing. The
size and geometry of those air inclusions are quite smaller
than those found in the subsurface of hummock ice
(section 4.5.3).
Another significant difference between FY ice and
MY ice is manifested in their thickness. Old ice contin-
ues to grow by accretion at the ice/water interface.
Accordingly, there is always a layer of newly grown ice
under the older ice layers. This point is further explained
in section 5.1.4 in terms of polycrystalline ice structure.
Although ice thickness is one of the parameters that dis-
tinguish MY ice from FY ice, there is a thickness overlap
between the two types. MY ice thickness in the Arctic is
nominally 3 m or more, whereas the maximum thickness
of FY ice is around 2.5 m. Sea ice thickness in the Arctic
was measured during numerous studies and as part of
the operational programs conducted at Canadian
weather stations (e.g., Resolute Bay, Eureka, Mould Bay,
and Tuktoyaktuk). Johnston and Timco [2008] measured
MY ice thickness from several ice floes in the Arctic.
They confirmed the earlier‐mentioned ice thickness
ranges of the two ice types (although thickness of hum-
mock or old ridged ice may be significantly greater than
3 m). Figure  2.63 shows the freeboard and the draft of
two very thick MY ice floes in the Nares Strait (North of
Baffin Bay) and the Canadian Arctic Archipelago (CAA)
region. The CAA region is known to have the largest
accumulation of MY ice, blocking the North West
Passage for marine navigation. Melling [2002] found
thick MY ice floes (6-8 m thick) in this region with
hummocks reaching up to 20 m height. The location
was near‐shore areas where wind forces can thrust the
ice hard against the coast of several islands. Figure 2.63
shows that the top surface of the ice is smoother than
the underside of it.
Another aspect of ice aging is the increase of its
strength as it transforms from FY ice to MY ice. FY ice
loses its strength more than the MY ice during the sum-
mer. But Sinha [1987a] measured and compared, for the
first time, in-situ confined borehole indentation strength
of FY columnar-grained landfast ice and old ridged ice
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