Origin and occurrence of Antarctic lacustrine carbonates, with special reference to Lake Fryxell, Taylor Valley

Abstract

Lake Fryxell is a permanently ice-covered 19 m deep lake located 15m a.s.l. in the lower Taylor Valley, Antarctica (75° 35'S, 163° 35'E). The lake occupies the lowest part of the Fryxell drainage basin and is surrounded by glacial tills with associated lacustrine carbonate and algal material of about Ross Sea I age. Lake Fryxell is a mesotherrnal, stratified, amictic lake with a euphotic (aerobic) zone above 10 m depth and an anaerobic (anoxic) zone below. The lake waters were derived from glacial meltwater into which upward diffusion of salts deposited in the basin or remnant brines occurred creating a gradient in density, which results in a physically stable lake. The so formed diffusion cell gave an estimated age of about 1000 years. Analysis of both lake water and lake bottom sediment cores show precipitation of CaCO₃ to have occurred both in the past and the present. Current carbonate precipitation is a biologically induced process occurring in the euphotic zone lake waters. over deep waters, precipitated carbonates, in this case calcite, fall from suspension through the water column and form flakes on the lake bed. Where the lake bed is within the euphotic zone stromatolites occur. Dissolution processes predominate in Lake Fryxell bottom waters and account for the discontinuous carbonate record in the uppermost lithologic unit (unit E) of Lake Fryxell sediments. At the time of the Ross Sea I advance a calcareous mud (unit B) was deposited on top of fluvialglacial sediments. The mud, a mixed calcite-aragonite deposit of up to 20 wt% CaCO₃ is C ¹⁴ dated at about 20,000 yrs B.P., was deposited in a deep pro-glacial Lake Washburn. At this time the Ross Sea Ice occupied part of the Fryxell basin damming the lower Taylor Valley and supplying the lake with meltwater. In ~1is ancient lake precipitation occurred in a similar manner to present Lake Fryxell except that aragonite was precipitated. Post-depositional change in the form of the aragonite/calcite transformation caused the observed dual mineralogy. Retreat of the ice sheet caused the deposition of transitional unit C and an evaporite deposit (unit D). Unit D is a varve-like aragonite ~10,000 years old, deposited in a lake of much reduced volume. The iv ice retreat from the drainage basin meant that the lake had an increased ablation surface enabling evaporation to exceed lake inputs resulting in brine concentration and subsequent precipitation of aragonite. Comparison of the Lake Fryxell carbonates with those of the Marshall Valley indicates that the Lake Fryxell depositional model, particularly evaporitic unit D, can adequately explain other Dry Valley carbonate deposits.