Sea Ice
Part I. Bulk Salinity Versus Ice Floe Thickness
AUSTIN KOVACS
Brine entrapment originates in the root area
INTRODUCTION
(Fig. 1) of the 1- to 5-cm-thick mushy region called
When seawater freezes, small, flat platelets of
the SK or skeleton layer (Harrison and Tiller 1963)
fresh ice are first to form. As this ice mass grows,
or crystallization front (Cherepanov 1975). In this
the platelets commingle and bond one to another
growth zone the individual ice platelets extend
to form a compliant, highly saline slush called
into the seawater, like fingers on a hand, and are
grease ice. With sustained freezing temperatures,
typically less than 0.25 mm thick and more than 1
the platelet mass thickens and grows together,
cm wide. As each dendritic platelet grows thicker
forming an ever-stiffening ice cover. During this
at its root, it freezes to its neighbor. A high growth
ice growth process most of the seawater is dis-
rate at this stage (>2 cm/day) results in platelets
placed, but between the platelets of pure ice a
less than 0.5 mm thick, but at low growth rates
seawater concentrate called brine becomes en-
(<0.5 cm/day) the platelets can grow to be 1.6
trapped. This entrapment causes the ice to be
mm thick, with a related increase in crystal size
salty: the salinity of the surface layer of the new
(Weeks and Hamilton 1962a,b; Page 1966; Nakawo
ice can be greater than 40‰ (Martin 1979, Kasai
and Sinha 1984). During this crystal formation
and Ono 1984, Ono and Kasai 1985).
Many factors affect the thickness of the
platelet or frazil ice layer, including growth
PBrine
Fresh Ice
P
ockets
Brine
rate, sea state, ice pack motion, and under-ice
latelet
Gas
b
a
turbulence. However, under relatively quies-
cent conditions and after the frazil ice layer
has "stabilized," the stage is set for the growth
of congelation ice. At first, the growing ice
Columnar
I
ce
platelets, <0.5 mm thick, are randomly ori-
d
Root
Z
ented, but in time they become more and more
one
c
al
ordered, grow wider, and stack one against
s
Ba
ne
another. They freeze together in layered groups
~ SK Ice
Dc-axis
a
Pl
1 to 5 cm
irection
to form individual ice crystals. Due to thermo-
dynamic factors, preferred platelet growth in
seawater is vertical (Weeks and Ackley 1989).
Therefore, over a short growth span the ice
crystals become "vertically" aligned, with the
parallel platelet array having a predominantly
horizontal c-axes alignment. The ensuing
a≤b<
c
a ~ 0.1 to 0.3 mm; b ~ 1 to 5 a; c > 5 a
growth structure is called congelation ice. The
F
number and size of the vertical ice platelets in
d ~ 0.25 to 1.25 mm (avg 0.7)
any crystal vary with growth rate, as does the
rozen Interface
volume of brine trapped between the fresh-ice
Seawater Interface
platelets (Cox and Weeks 1988, Weeks and
Figure 1. Sea-ice crystal structure.
Ackley 1989).
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