Ice coring
The length of the drill barrel determines the maximum length of a core sample (6 m at GISP2 and Vostok). Collection of a long core record thus requires many cycles of lowering a drill/barrel assembly, cutting a core 4-6m in length, raising the assembly to the surface, emptying the core barrel, and preparing a drill/barrel for drilling. The length of the record depends on the depth of the ice core and varies from a few years up to 800 kyr for the EPICA core. The time resolution (i.e. the shortest time period which can be accurately distinguished) depends on the amount of annual snowfall, and dminishes with depth as the ice compacts under the weight of layers accumulating on top of it. Upper layers of ice in a core corresponds to a single year or sometimes a single season. Deeper into the ice the layers thin and annual layers become indistinguishable.
