Stormy and icy seas - Antarctica: Global Science from a Frozen Continent - page 145

Figure 5.7 Pancake ice forms at the end of summer in March as the ocean surface begins
to freeze and form winter sea ice. The exclusion of salt from the seawater during freezing
generates a very strong brine which ows down to the sea floor to form Antarctic Bottom
Water. (Credit: Peter Bucktrout, BAS)

freezing again. Ice platelets are formed which either ascend to the underside of
the ice shelf, increasing its vertical extent, or are swept out of the cavities by
ocean currents. The refrozen part of the ice shelf is called marine ice.
Icebergs are formed from glaciers reaching the coast or from the ice shelves
which break off and float northwards towards the open ocean. By melting icebergs

add fresh water to the ocean further offshore. With 90% of their mass underwater
icebergs are at the mercy of the ocean currents. But if the sea ice cover becomes
suf ciently compacted, it takes over the control of the drift, which then becomes

wind driven. As long as the icebergs are in relatively cold water the melt rate is
moderate but as soon as they break into smaller pieces the melt rate is signi cantly

enhanced due to the increased surface area. Icebergs formed from marine ice attract
attention after capsizing since they are of greenish colour. Icebergs can persist
for several years, especially if they run aground, and during that period the ocean
and the weather can produce some startling shapes and patterns.