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Ice is lost at the edges of the glacier to icebergs, or to summer melting, and the overall shape of the glacier does not change much with time.
Impurities in the ice provide information on the environment from when they were deposited.
These data can be combined to find the climate model that best fits all the available data. Coastal areas are more likely to include material of marine origin, such as sea salt ions.
Greenland ice cores contain layers of wind-blown dust that correlate with cold, dry periods in the past, when cold deserts were scoured by wind.
A design for ice core augers was patented in 1932 and they have changed little since.
An auger is essentially a cylinder with helical metal ribs (known as flights) wrapped around the outside, at the lower end of which are cutting blades.
Below this depth, electromechanical or thermal drills are used.
The cutting apparatus of a drill is on the bottom end of a drill barrel, the tube that surrounds the core as the drill cuts downward.
Since heat flow in a large ice sheet is very slow, the borehole temperature is another indicator of temperature in the past.The cuttings (chips of ice cut away by the drill) must be drawn up the hole and disposed of or they will reduce the cutting efficiency of the drill.The fluid must have a low kinematic viscosity to reduce tripping time (the time taken to pull the drilling equipment out of the hole and return it to the bottom of the hole).Ice cores are collected by cutting around a cylinder of ice in a way that enables it to be brought to the surface.Early cores were often collected with hand augers and they are still used for short holes.
Hand augers can be rotated by a T handle or a brace handle, and some can be attached to handheld electric drills to power the rotation.