Assessment of Ice Borehole Temperature Conditions at Interface with Subglacial Lake Vostok (Antarctica)

摘要

Borehole 5G drilled at Vostok Russian Research Station in Antarctica is currently the deepest ice hole ever drilled in the ice shields of Greenland and the Antarctic continent. This drilling project produced data that predetermined research trends for decades to come. The ice core recovered from Borehole 5G was used for palaeoclimatic reconstruction that covers over 420,000 years [7, 10, 13]. This made it possible to reconstruct the history of the Earth climate and atmosphere throughout the four recent glacial and five interglacial periods including the current one. It will provide an insight into the oncoming natural tendencies in climate change. The discovery of the largest sub-glacial lake on the Earth, Lake Vostok in Antarctica, was confirmed in 1996. Borehole 5G reached the interface between the atmospheric and congelation (lake) ice at the depth of 3538 m. This congelation ice was formed as the result of lake water freezing over the glacier subface. This gave a start to indirect investigations of Lake Vostok focusing on its properties and characteristics as well as ice formation patterns. Subsequently, the borehole reached a zone near the lake surface, where the ice temperature was close to the melting point and ice crystals were characterized with huge sizes of around 1.5 m. Our perception of Lake Vostok is based on the information obtained with remote sensing techniques (e.g. areal and ground radar soundings, seismic surveys, satellite radar altimetry) as well as on calculations using field data and available simulations. Further surveys will provide new information on the lake configuration and depths as well as the thickness and structure of its bottom sediments. This will help to carry out additional simulations of thermodynamic properties of the glacier-subglacial lake system. However, these investigations will neither be able to produce reliable information on the lake water composition nor confirm or dispel the assumption that some life forms exist in the lake. Therefore, sampling of the lake water is a key task to secure significant progress in research of this unique natural object [6]. Solution of this problem is a primary objective of further investigations in Borehole G5 that include water sampling from Lake Vostok and studies of the ice cores, which are expected to provide an insight into the origin and historical composition of lake water as well as possible existence of life in the lake. Unsealing Lake Vostok for a short time and collecting water samples from its surface layer helps to obtain source information about its current condition. Besides its intrinsic value, this data is also critical for developing technologies and technological tools for further direct investigations of the lake, collection of bottom samples as well as adjustment of remote investigation and simulation results. This paper presents results of time freezing calculations for the lake water with 'positive' temperature that entered the borehole once the subglacial lake was unsealed. These calculations were made using independent techniques. The paper studies temperature conditions in the borehole at the interface with the subglacial lake.