This suggests that fracture processes are suppressed or inhibited in the deepest section of the accreted ice (to be drilled).

The latter conclusion is fully supported by the high crystallographic structure of the ice crystals, as revealed by X-ray diffraction measurements (Montagnat et al. 2001), which is possible only if the lake ice does not plastically deform under the in situ conditions. Hence, the formation of open crevasses often observed in grounded ice can hardly be expected in the case of basal ice at Vostok (Russian Federation 2006).

The permeability of the accreted ice to the drilling fluid is expected to be low, because the accreted ice crystals show a lack of distortion, which essentially rules out a significant diffusion of the drilling fluid through the ice lattice (Montagnat et al. 2001). However the presence of liquid water along the grain boundaries provides a route for liquid transport. The veins form a continuous network of microscopic channels that remain liquid at subfreezing temperatures. Hence, contamination from the drilling fluid may be associated with both the downward advection of intercrystalline water and the diffusion of the drilling fluid through the vein system. Estimates of the total advection of the drilling fluid from the borehole toward the ice-water interface will be about 1 m over millennial time scales (Russian Federation 2006). In addition, helium data (Jean-Baptiste et al. 2001) support the presence of an upward component of velocity in the accretion ice layer of about 6 mm per year, which can be the result of hydrostatic compensation in this region (Souchez et al. 2004). This upward movement of ice, if present, would overcompensate a very slow downward advection of the fluid (Russian Federation 2006).

NEEDS FOR TECHNOLOGY DEVELOPMENTS

Some technologies currently exist that can be used immediately in the exploration of subglacial aquatic environments. These include airborne radar, magnetic, andgravity surveys, land-based seismic surveys; and certain operational sensors that could be deployed within lakes exist for some of the more fundamental properties. Other technologies however, will require development. For example, experimental sensor arrays need to be developed for the detection of dissolved gases such as H2S, CH4, N2O, N2, and argon, as well as major anions and cations and bioreactive redox couples such as ammonium and dissolved manganese. Currently available sensors will also need to be field-tested for compatibility with the expected temperature and pressure conditions of subglacial lake environments and environmental restrictions.

The participants of SCAR SALE (2006) stressed that the size of the sensor packages may not be suitable for the size of lake access holes and that limitations on the borehole size may require the miniaturization of existing technologies. It is conceivable that at least some of the subglacial aquatic environments may be substantially overpressure. Further technological developments, such as introducing pressure-tight locks in the bottom parts of a borehole may eventually be advisable to deal with this possibility as well as to sample subglacial lake bottom sediments.

Sample recovery poses its own set of challenges. Many of the standard oceanographic techniques for remote collection using water sampling bottles (e.g., Rosette Samplers with Niskin bottles) and sediment retrieval by coring devices (piston corers, gravity corers, box corers, grab samplers, etc.) use very large devices that are too large to fit through a drill hole; specialized sampling devices and techniques may have to be developed (SCAR SALE 2006). Sediment trap technologies and other water particu-



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