Structure and Properties of Lake Ice - Freezing of Lakes and the Evolution of their Ice Cover - page 71

3.3 Impurities in Lake Ice Cover

Lake ice sheet contains impurities, which originate from the water body, bottom sedi-
ments, or atmospheric deposition (Fig. 3.10 ). These impurities consist of gas bubbles,
liquid inclusions, and particles, which are called ice sediments, and they in

uence on the
physical properties of ice and biological processes inside the ice. The concentration of
impurities is low, compared with river ice or sea ice. Freezing of lake water releases
impurities from the growing ice to the liquid water body. Atmospheric deposition is stored
in the ice sheet for the period of the ice cover. Flooding of a snow-covered ice by lake
water forms slush, and when the slush freezes the impurities are kept in the ice sheet.
The size, shape and concentration of gas bubbles can be examined from thick sections
using a microscope (e.g., Li et al. 2011). In congelation ice, gas bubbles appear sometimes
as layers, and the bubbles may extend vertically into cylinders if there is continuous
supply of gas during an ice growth episode. Lake ice liquid inclusions and sediments are
examined from the meltwater of ice samples as usually water samples are treated (Lep-
p

fl

ranta et al. 2003b). Ice cores are sectioned according to the stratigraphy, for a good
resolution the slices are 5

ä

10 cm thick. Gas bubbles are the most important impurities in
freshwater lakes, while in saline ice liquid brine inclusions possess this role. Brine pockets
can serve as habitats of biota, and they have been examined widely for sea-ice ecology.
The gas content ( m a ) of congelation ice is usually small, magnitude 1 %, and in snow-
ice it is 3

-

6 % (Palosuo 1965). Therefore the properties of freshwater congelation ice are
close to the properties of pure ice, but in snow-ice the gas content is more signi

-

cant. The

size of the gas bubbles is in the range 0.1 10 mm (Fig. 3.11 ). In particular, the gas bubbles

-

have a major in fl uence on the backscattering of electromagnetic waves, and minor

in uences, e.g., on the thermal conductivity and strength of ice. In congelation ice the gas

is derived from the water column or the lake bottom sediments, while snow-ice contains
air inclusions from the parent snow. Gas exchange between lake water bodies and the
atmosphere may take place through leads and fractures during the ice season.

fl

Fig. 3.10 The sources of
impurities and their catchment
processes in lake ice. Usually
atmospheric fallout and
flooding of lake water are the
main sources, but where frazil
ice and anchor ice forms the
amount of impurities is at
largest