资源环境科技发展态势分析平台

We analyzed helium and neon in 24 samples from between 3,607 and 3,767m (i.e., down to 2m above the lake-ice interface) of the accreted ice frozen to the ceiling of Lake Vostok. Within uncertainties, the neon budget of the lake is balanced, the neon supplied to the lake by the melting of glacier ice being compensated by the neon exported by lake ice. The helium concentration in the lake is about 12 times more than in the glacier ice, with a measured He-3/He-4 ratio of 0.120.01R(a). This shows that Lake Vostok's waters are enriched by a terrigenic helium source. The He-3/He-4 isotope ratio of this helium source was determined. Its radiogenic value (0.057xR(a)) is typical of an old continental province, ruling out any magmatic activity associated with the tectonic structure of the lake. It corresponds to a low geothermal heat flow estimated at 51mW/m(2).

Plain Language Summary Extending over 15,000km(2) in a deep trough north of Vostok station, Lake Vostok is the largest and the deepest among the many subglacial lakes to have been discovered in Antarctica. Its ice ceiling is tilted, with an ice thickness of 3,750m in the south and 4,300m in the north. As the melting point is pressure dependent, the base of the glacier melts on the thick side (northern region) whereas lake water refreezes in the south, where the Vostok station is located. Unlike most gases, helium and neon can be incorporated into the crystal structure of ice during freezing. This property makes helium and neon isotopes in the accreted ice a valuable source of information on the concentration and isotope composition of both gases in the lake water itself. Between 2006 and 2012, we collected 24 samples from between 3,607 and 3,767m (i.e., down to 2m above the lake-ice interface) of the accreted ice frozen to the ceiling of Lake Vostok (lake ice) for analyzing helium and neon. Within uncertainties, the neon concentration measured in the lake ice is equal to that in the glacier ice. This indicates that the neon budget of the lake is balanced, the neon supply to the lake by the melting of glacier ice being compensated by the Ne export by lake ice. This confirms earlier suggestions from radar data and GPS measurements of surface ice velocity that the water added to the lake by the melting of glacier ice is balanced by the lake ice, which is exported by the glacier's movement out of the lake. Helium isotopes (He-3 and He-4) are sensitive indicators of tectonic-magmatic activity. In continental areas of recent tectonic-magmatic activity such as geothermal areas, the ratio He-3/He-4 is at its highest, accompanied by excess heat flow. On the contrary, in stable continental areas, low He-3/He-4 ratios are found. The low He-3/He-4 ratio in Lake Vostok clearly demonstrates the absence of volcanic and/or magmatic activity associated with the tectonic structure of the lake, in agreement with the absence of magnetic anomaly. The helium concentration in the lake is about 12 times the concentration measured in the glacier ice. This shows that the Lake Vostok's waters are enriched from beneath by a flux of helium typical of an old stable continental province. Helium isotopes points to a low geothermal heat flow beneath the lake. We estimate this heat flow at 51mW/m(2). This value is fully consistent with the heat flow map for Antarctica inferred from satellite magnetic data and corresponds to the baseline heat flow in Antarctica. Our data do not allow us to determine the helium flux and the residence time of the lake waters independently, but only as the product of these two quantities. If we adopt the previously determined residence time of 13,300years based on geophysical inferences, we note that the helium flux value is close to the maximum of the lognormal distribution of continental helium fluxes, corresponding to the maximum likelihood of the continental helium flux.