Accreted Ice Under Vostok Station

It was mentioned above that on agreement with SCAR, a new round of drilling, with the purpose of collecting an ice core, was started as a collaboration program between Russia, France, and the U.S.A. This stopped at a depth of 3,623 m, about 130 m above the lake. New and interesting information developed as a result. A very sharp and significant variation in stable isotope and dust concentrations around the 3,320 m depth were found, which could not be of climatic origin (Figure 7.6). The ice stratum from 3,538-3607 m displays visible inclusions of millimeter size, which may originate from bedrock (it is difficult to date this depth). However, in general this layer surprised investigators - it had very large ice crystals, meters in size (the size was measured in the vertical direction, along the length of the core). Electrical conductivity of this layer was two orders of magnitude lower than in previous layers, but the stable isotope content was only slightly different. It was clear that a new area in the ice had been reached, but what did it represent?

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Age, 1,000 years B.P.

Figure 7.4. A change in temperature of the Antarctic Ice Sheet surface over the course of 420,000 years (adapted from Kotlyakov and Lorius, 2000). Upper curve was reconstructed on the basis of "S180%o" changes (deuterium profile) measured in ice cores from deep borehole 5G at Vostok Station, taken from different depths (upper scale) or ages of cores (lower scale). Lower curve shows variations of the total ice volume on the Earth, reconstructed on the basis of the profile of "S180%o" interpreted from ice cores from the 5G borehole core. Four total 100,000 year long climatic circles of the Earth's history are clearly visible.

Figure 7.4. A change in temperature of the Antarctic Ice Sheet surface over the course of 420,000 years (adapted from Kotlyakov and Lorius, 2000). Upper curve was reconstructed on the basis of "S180%o" changes (deuterium profile) measured in ice cores from deep borehole 5G at Vostok Station, taken from different depths (upper scale) or ages of cores (lower scale). Lower curve shows variations of the total ice volume on the Earth, reconstructed on the basis of the profile of "S180%o" interpreted from ice cores from the 5G borehole core. Four total 100,000 year long climatic circles of the Earth's history are clearly visible.

Figure 7.5. Change in temperature and content of particulates in ice cores noted as a result of the study of the 3,330 m deep core from borehole 5G at Vostok Station (adapted from Kotlyakov and Lorius, 2000). (A) Deuterium profile. (B) Sodium profile obtained from samples every 3-4 m on average. (C) Dust profile for every 4 m on average - the dust concentration is expressed in parts per million on the assumption that Antarctic dust has a density of 2.5 x103 kgm

Figure 7.6. Internal structure of the bottom part of the Antarctic Ice Sheet at Vostok Station as revealed by deep-core drilling (adapted from Lipenkov and Barkov, 1998; Souchez et al., 2000). Ice above the 3,310m horizon is glacier ice with a meteoritic signature. Ice below 3,310 m (down to 3,539 m) is deformed glacier ice. Ice below the 3,539 m horizon to the 3,609 m horizon is accreted frazil ice with visible dirt inclusions. Ice below 3,609 m down to the bottom end of the core (3,623 m) is accreted frazil ice without visible dirt inclusions (Lipenkov and Barkov, 1998; Souchez et al., 2000).

Figure 7.6. Internal structure of the bottom part of the Antarctic Ice Sheet at Vostok Station as revealed by deep-core drilling (adapted from Lipenkov and Barkov, 1998; Souchez et al., 2000). Ice above the 3,310m horizon is glacier ice with a meteoritic signature. Ice below 3,310 m (down to 3,539 m) is deformed glacier ice. Ice below the 3,539 m horizon to the 3,609 m horizon is accreted frazil ice with visible dirt inclusions. Ice below 3,609 m down to the bottom end of the core (3,623 m) is accreted frazil ice without visible dirt inclusions (Lipenkov and Barkov, 1998; Souchez et al., 2000).

It was a surprise when from the isotope studies, ECM, and gas content measurements it became clear (Jouzel et al., 1999) that the lowest (below the 3,538-3,539-m horizon) part of the ice core was composed of accreted ice (i.e., ice possibly formed from the frozen water of Lake Vostok). This part of the layer is highlighted using the letter "E" in Figure 7.6.

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