Icebergs start as ice sheets attached to the land or a glacier. They are large monsters of solid ice but they do break off the ice sheet before they float out to sea. How do they break up afterwards at sea? An international team of scientists has discovered a previously unknown mechanism by which large tabular icebergs break up out at sea as part of a study carried out on the Peterman Iceberg in Baffin Bay over the summer. Scientists observed that the gradual creation of a huge underwater ice foot produced so much buoyancy that it broke large chunks off the main iceberg thus causing the iceberg to slowly disintegrate. This discovery was captured on camera as a film crew followed the expedition for Operation Iceberg.
An iceberg is a large piece of freshwater ice that has broken off a glacier or an ice shelf and is floating freely in open water. It may subsequently become frozen into pack ice (one form of sea ice). As it drifts into shallower waters, it may come into contact with the seabed, a process referred to as seabed gouging by ice.
Scientists have long assumed the decay of icebergs was either caused by wave action physically splitting them apart, or by warmer sea water gradually melting them. The truth now appears to be an interesting combination of factors.
During the arctic expedition, scientists found that warm surface water, aided by wave action, erodes a deep notch around the edge of the iceberg. This notch is at water level, and the unsupported ice above the notch quickly sloughs away. Over time this creates a large underwater ram or ice foot sticking out which is protected by colder, deeper water. This underwater ram creates so much buoyant upwards pressure that it finds a weak point within the iceberg which then snaps off. This can be a sizable chunk of ice. During filming for Operation Iceberg, camera crews and scientists were aboard the iceberg when a chunk of ice, 200m by 50m, broke off in this way.
Dr Keith Nicholls of the British Antarctic Survey argues that the notch is the key start to this process. "The reason this iceberg is breaking up really fast in front of us is all about what’s happening at the wave cut in the warm water layer right at the surface."
Also on the expedition was one of the world leading experts on ice science, Prof Peter Wadhams of Cambridge University, who believes their observation of the process is unique: "We have discovered a new mechanism. People don’t normally sit beside melting icebergs in a ship. Normally when they come across an iceberg they give it a wide berth. So in a way, we’re the first people to be doing it."
Alon Stern of New York University was responsible for taking the temperature measurements of the sea water around the iceberg, including key data gathered by the BBC’s dive team. These readings confirmed that the deeper water measured -1.5 degrees Celsius, and was therefore able to protect the ice foot from melting.
Understanding the mechanism by which icebergs break up is important for shipping in the North Atlantic. It is also becoming clear that because of climate change, the number of tabular icebergs in Arctic seas is increasing.
Before April 1912 there was no system in place to track icebergs to guard ships against collisions. The sinking of the RMS Titanic, which caused the deaths of 1,514 of its 2,223 passengers, created the demand for a system to observe icebergs. For the remainder of the ice season of that year, the United States Navy patrolled the waters and monitored ice flow. In November 1913, the International Conference on the Safety of Life at Sea met in London to devise a more permanent system of observing icebergs. Within three months the participating maritime nations had formed the International Ice Patrol (IIP). The goal of the IIP was to collect data on meteorology and oceanography in order to measure currents, ice-flow, ocean temperature, and salinity levels.
Keith Nicholls added: "In recent years we’ve been seeing a lot more big tabular icebergs come off the Greenland ice sheet and they’re now ending up in Baffin Bay. That’s a change and the only reason that can change is because the climate around Greenland is changing."
For further information see Berg Break Up.
Iceberg image via Wikipedia.