Geology Reference
In-Depth Information
2.5. Ice Decay anD aGInG
snow cover. Secondary factors include tides, albedo,
mechanical disruption (e.g., ice breakup due to wind),
and water temperature (warm ocean current washing
against the bottom of the ice in late spring causing ero-
sion). Ice decay starts at the surface in the form of melt-
ing initiated by two heat sources: (1) the absorbed solar
radiation and (2) the conductive heat from the surround-
ing air. The amount of absorbed solar radiation is deter-
mined by the surface albedo, which varies with the type
of surface; namely snow‐covered or snow‐free as well as
the degree of snow wetness or ice surface dryness. Albedo
values of these surfaces are presented in section 8.3.
Water absorbs 90% or more of the ISR, while melting
snow absorbs 40-60% and dry snow absorbs 10-20%.
Higher absorption of solar radiation leads to greater
increase of local temperature of the surface.
Because of the variation of the absorption of the ISR
at the ice surface, areas that contain high concentration
of dust usually act as centers for melting. Moreover, areas
covered with snow (especially when the snow is thin) with
brine-rich bottom become susceptible for melting rapidly.
This leads to the formation of a few small ponds at the
locations of these spots (Figure 2.56). Once this radia-
tion‐driven process starts, the water in the pond absorbs
even more radiation and the pond starts to expand and
deepen slowly. The other force that comes into action is
heat conduction. When warm air comes in contact with
the initial pond, heat is transferred from the air to the
water by conduction.
The ponds tend to form in minor depressions on the
ice surface, or simply being retained within surviving
snow pack as areas of slush. While the hydrostatic head
of the surface melt provides the driving force for the
water to penetrate through pores and remnants of brine
channels in the ice fabric, an interconnected network of
pores is also formed to complete this process.
Billelo
[1977] described the appearance of an ice sheet while
being in the state of decay as if it is covered with “giant
spiders” with the “body” being the thaw hole and the
“legs” being channels of melt water draining laterally
toward the hole. The ponding stage of decaying FY
level‐ice in Allen Bay (74.72°N and 95.25°W) near
Resolute is shown in Figure 2.57. The scene is covered
by melt ponds interspersed by raised areas of snow‐cov-
ered ice. The figure also includes another scene of melt
pond on a MY ice surface depression in the same
geographic site. The ice was 1.27 m thick under a hum-
mock surface (the white areas) and 0.9 m thick under
depression [
Johnston et al
., 2003]. The drainage of water
during ice decay washes away brine (see brine flushing
in section 2.3.3.2.). Therefore, the decayed ice (also
called rotten ice) has a much lower salinity than the
original ice. As the ice underneath the ponds becomes
thinner and more solar radiation is absorbed, melting
is enhanced producing “thaw holes” (Figure 2.56). A
It would probably have been more appropriate to start
this section with discussions on ice aging followed by ice
decay (or melting). However, the term “ice aging” is used
in this section to refer to the transformation of FY ice to
MY ice. On the other hand, the term “ice decay” is often
used to refer to the decay or partial melting of FY ice
before it turns into SY ice. Therefore, the ice decay is
discussed before moving to ice aging. It should be men-
tioned here that decay and aging of ice are presented in
Chapter 5 in a more focused viewpoint.
Similar to ice formation and accretion, the onset of ice
decay depends on the latitude (a proxy of air tempera-
ture). For example, ice decay is observed during June-
July around Eureka in Ellesmere Island, Canada (80°N);
2 months later than ice decay in Cartwright in Labrador,
Canada (54°N) (the rate of ice accretion in Eureka is
greater than twice that in Cartwright). In the peripherals
of the Arctic region, the overlying snow layer on FY ice
starts to melt in mid‐June. By the end of July or August,
most of the FY ice would have melted. In the Labrador
Sea (latitude between 50°N and 60°N), the ice shows its
first signs of disintegration in March and melts com-
pletely by the end of April.
2.5.1. Ice Decay
The onset of FY ice decay is marked by systematic
melting of the ice surface which then proceeds slowly
through the subsurface layer before it accelerates as it
infiltrates the ice sheet. This process takes between two to
several weeks, depending on the climatic conditions and
air temperature of the region and average thickness of
the ice. Melting may also start at the ice bottom albeit at
a much smaller rate. The decay process may accelerate in
the presence of microalgae, which absorb light and con-
vert it into heat [
Zeebe et al
., 1996]. In this case, the mag-
nitude of melting is negligible from the aspects of energy
or mass balance of sea ice but may be considerable from
the biological viewpoint.
Ice decay causes both ice thickness and its mechanical
strength to decrease [
Hibler III
, 1979]. For that reason
operational ice centers have incorporated information
on ice decay within their products of ice strength charts. In
May 2002, the Canadian Ice Service (CIS) began the pro-
duction of a prototype of ice strength charts based on an
algorithm developed by the Canadian Hydraulics Centre
of the National Research Council (NRC) of Canada. The
algorithm estimates the ice strength using modeled surface
air temperature. The output charts display the mechanical
strength of undeformed FY ice during its decay relative
to its mid‐winter strength [
Langlois et al
., 2003].
The prime factors that trigger ice decay are air tem-
perature, incoming solar radiation (ISR) and melting of
