Ice Core Drilling
In the 1970s the Soviet Union drilled a set of cores 500–952 m deep. These have been used to study the oxygen isotope composition of the ice, which showed that ice of the last glacial period was present below about 400 m depth. Then three more holes were drilled: in 1984, Hole 3G reached a final depth of 2202 m; in 1990, Hole 4G reached a final depth of 2546 m; and in 1993 Hole 5G reached a depth of 2755 m; after a brief closure, drilling continued during the winter of 1995. In 1996 it was stopped at depth 3623 m, by the request of the Scientific Committee on Antarctic Research that expressed worries about possible contamination of Lake Vostok. This ice core, drilled collaboratively with the French, produced a record of past environmental conditions stretching back 420,000 years and covering four previous glacial periods. For a long time it was the only core to cover several glacial cycles; but in 2004 it was exceeded by the EPICA core, which whilst shallower, covers a longer time span. In 2003 drilling was permitted to continue, but was halted at the estimated distance to the lake of only 130 m.
The ancient lake was finally breached on 5 February 2012 when scientists stopped drilling at the depth of 3770 meters and reached the surface of the sub-glacial lake.
The brittle zone is approximately between 250 and 750 m and corresponds to the Last Glacial Maximum, with the end of the Holocene climatic optimum at or near the 250 m depth.
Although the Vostok core reached a depth of 3623 m the usable climatic information does not extend down this far. The very bottom of the core is ice refrozen from the waters of Lake Vostok and contains no climate information. The usual data sources give proxy information down to a depth of 3310 m or 414,000 years. Below this there is evidence of ice deformation. It has been suggested that the Vostok record may be extended down to 3345 m or 436,000 years, to include more of the interesting MIS11 period, by inverting a section of the record. This then produces a record in agreement with the newer, longer EPICA record, although it provides no new information.
Read more about this topic: Vostok Station
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