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Drill cores near bedrock often are challenging to analyze due to distorted flow patterns and composition likely to include materials from the underlying surface. Air in the atmosphere and firn are slowly exchanged by molecular diffusion through pore spaces, because gases move toward regions of lower concentration.
The gas age–ice age difference is as great as 7 kyr in glacial ice from Vostok.
Under increasing pressure, at some depth the firn is compressed into ice. Layers can be visually distinguished in firn and in ice to significant depths.
The fluid must also satisfy other criteria, for example those stemming from the analytical methods employed on the ice core.
A number of different fluids and fluid combinations have been tried in the past.
The length of the record depends on the depth of the ice core and varies from a few years up to 800 kyr (800,000 years) for the EPICA core. the shortest time period which can be accurately distinguished) depends on the amount of annual snowfall, and reduces with depth as the ice compacts under the weight of layers accumulating on top of it.
Upper layers of ice in a core correspond to a single year or sometimes a single season.
For material which is sufficiently soft, coring may be done with a hollow tube.
Deep core drilling into hard ice, and perhaps underlying bedrock, involves using a hollow drill which actively cuts a cylindrical pathway downward around the core.
The hole is filled with a fluid to keep the hole from closing.