THE EVOLUTION OF ACE-LAKE, ANTARCTICA, DETERMINED FROM SEDIMENTARY DIATOM ASSEMBLAGES

The sediments in Ace Lake in the Vestfold Hills, East Antarctica, cont ain a continuous sequence of diatom frustules that record the lake's h istory since its formation during the retreat of continental ice more than 9200 years ago. Frustules from six indicator groups; Pinnularia m icrostauron, Nitzschia cylindrus, Nitzschia curta, Eucampia antarctica , a freshwater Stauroneis species and several centric species in a sed iment core were used to determine the sequence of the lake's evolution . The history of the lake falls into five stages. Ace Lake began as a marine inlet influenced by dynamic mixing of ocean and meltwater input s. As the ice sheet retreated, isostatic uplift isolated the lake allo wing it to be flushed by meltwater input from the retreating ice sheet . Over the course of 800 years (similar to 9200-8400 C-14 yr B.P.) the lake became meromictic supporting a freshwater diatom assemblage. App roximately 6700 years ago, coinciding with Antarctic sea level maxima, diatom assemblages indicate that seawater flooded over the sill into Ace Lake disturbing the freshwater meromixis. The sediments in this pe riod were laminated and contained elemental sulphur suggesting that th e marine input was limited in extent and energy. Approximately 5500 ye ars ago this marine input ceased and the lake again became a meromicti c basin which stabilised over 1700 years to become the lake that it is today and has been with little change for about the past 4000 years.