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GROUNDED ICEBERGS


Icebergs are large masses of floating ice calved from the fronts of glaciers or from permanent ice shelves. Pushed along by wind and mostly by the prevailing current, icebergs become grounded in shallow coastal water by plowing into the seafloor, just like a ship running aground.


When the ocean freezes over in winter, the grounded iceberg is locked in by the surrounding sea ice. This grounded iceberg was just south of Cape Evans on Ross Island. It is about two hundred feet long with about forty feet of iceberg showing above the sea ice.


The keel of the grounded iceberg causes gouges, scouring and plow marks on the sea floor. This natural physical disturbance of the seafloor, along with seafloor disturbance caused by anchor ice and drift ice, contributes to making shallow water undesireable for sessile (attached) organisms.


The iceberg makes a solid wall of ice all the way to the bottom which was ninety feet deep at the dive hole. This iceberg cut a large trench into the bottom when it drifted towards shore and ran aground; it is about 120 feet deep near the trench. How deep can icebergs scour the bottom? Tabular icebergs (flat-topped bergs that break away from ice shelves) can have a draft down to 330 meters depth; non-tabular icebergs (bergs from glaciers) and tilted tabular icebergs are the majority of Antarctic icebergs and can increase their draft 50% by rolling, so their draft may go down to 500 meters depth [1].


Icebergs may possess underwater spurs and ledges which extend a considerable distance from the visible topside portion of the iceberg. Here a diver explores the shallower reaches of the Cape Evans grounded iceberg which has a shelf of ice that sticks out at about the twenty foot level. The shelf is covered in ice algae and loaded with bright red amphipods grazing for food. These amphipods in turn attract fish that feed upon them.



Diving under the ice on a grounded iceberg is a visually stunning experience !

1: Antarctic Journal of the United States 30(5):130-131, 1987

Text ©Peter Brueggeman. Photographs ©Norbert Wu. Photographs may not be used in any form without the express written permission of Norbert Wu. Norbert Wu no longer grants permission for uncompensated use of his photos under any circumstances whatsoever; want more info?