An Estimate of the Dissolved oxygen Concentration in Subglacial Lake Vostok
Abstract
The upper section of 3.5 km thick glacier ice overlying Lake Vostok is characterized by abundance of air bubbles trapped during pore closure near the surface of the ice sheet. As the pressure increases with depth, the air occluded in ice gradually transforms to mixed air clathrate hydrate. In the region of interest the bubble-to-hydrate transition is complete at about 1300 mbs below which depth most of the air (about 99%) in the ice sheet is located within hydrate crystals. The basal-ice melting prevails in the north of Lake Vostok and the ice accretion occurs in the south. The concentration of air in the melting glacier ice is typically about 113 mg l-1 (86 mg N2 L-1 + 27 mg O2 L-1), whereas that in accreted ice is nearly zero. This suggests a net transfer of the atmospheric air (in the form of gas hydrate) through the ice-sheet thickness to Lake Vostok water. Available laboratory data and thermodynamic models indicate that, under conditions appropriate to Lake Vostok, the air-hydrate crystals released from the melting ice will persist within the water body provided there is enough air present in the system for a hydrate phase to coexist with dissolved N2 and O2. Neglecting biogeochemical inputs and losses of dissolved gases we have calculated the solubility of nitrogen (2.25 103 mg L-1) and oxygen (1.3 103 mg L-1) in equilibrium with air hydrate in lake water. Accordingly, the dissolved oxygen concentration is predicted to be between 27 and 1.3 103 mg L-1 (compare to 15 mg O2 L-1 for standard conditions). Assuming a steady state and taking 20 kyr for the residence time of the lake water, we have estimated that a 630 kyr period is needed to reach the upper bond of the dissolved O2 concentration, which is a prerequisite for air hydrate stability in the lake. Metabolic consumption of oxygen in the lake could only make this transition longer. We also demonstrate that strong hydrate-forming gases such as CO2 and CH4, if present in the lake together with N2 and O2, would lower the upper bound of the dissolved oxygen concentration. The presence of oxygen in glacier ice can cause a substantial degradation of DNA with time. As the age of glacier ice at the base of the ice sheet is about 1000 kyr, only few if any microorganisms reach the lake in viable state. Negative results of the molecular biology studies so far performed on the deep samples of glacier ice do support this conclusion. However, a few thermophilic bacteria containing DNA suitable for PCR analyses have been found in the accreted ice. Given the high oxygen tension in the open lake, this finding clearly indicates a presence of today's microbial life confined to sub-oxic lake sediments, most likely featured by a kind of hydrothermal activity.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2002
- Bibcode:
- 2002AGUSM.B21A..06L
- Keywords:
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- 0330 Geochemical cycles