Large Chemical Variations in ice Formed above Lake Vostok, Antarctica

Lake Vostok is the largest subglacial lake identified in Antarctica. It is 260 km long, 80 km wide, over 500 m deep and lies beneath $\sim$4 km of ice. It has received great scientific interest since it may provide clues to the survival of life on Earth during global glaciations, and provides an analogue for environments that may harbour life on icy planets and moons, such as Europa. Our current understanding of physical, chemical and biological conditions in the lake comes from samples of water which have refrozen to the underside of the ice sheet. Analysis of this `accreted ice' reveals large concurrent variations in the concentrations of major ions. For example, Mg^{2+}$ and SO4^{2-}$ vary by a factor of $\sim$400, Na^{+}$ and Cl^{-}$ by a factor of $\sim$40, and K^{+}$ and Ca^{2+}$ by a factor of $\sim$10. The inferred ionic content of the lake water, from which this accreted ice formed, ranges from $\sim$1 to 40 mM in terms of total dissolved anions. Clearly, these recorded ionic extremes have implications for the lake's environment, which in turn may be important to microbial life in the lake. We seek explanations for this variation by considering plausible perturbations to the lake system, possible effects of the varying environmental origin of accreted ice, and sample treatment. Specifically, these are: i) changes in lake volume in response to changes in ice sheet thickness and flow direction; ii) the sporadic contribution of saline water from a deeper-rock reservoir; iii) periodic upwelling events, bringing more concentrated waters from depth; iv) variations in the chemistry of icemelt feeding the lake; v) formation of ice near the grounding lines of a shallow embayment to the west of the main lake; vi) the effect of sampling ice crystal boundaries; and vii) post-sampling reactions with mineral particulates. The likelihood of each of these possibilities will be discussed.