Antarctic Carbonate Records of Ice-Covered Lake Ecosystems in Transition

Perennially ice-covered Antarctic lakes can contain abundant carbonates associated with benthic microbial mats. In this study, we present depositional models for microbial carbonates of perennially ice-covered lakes Fryxell and Joyce of the McMurdo Dry Valleys, Antarctica. Carbonate texture and chemistry are influenced by benthic microbial mats that cover the lake bottom, but in association with different porewater redox conditions in each lake. In Lake Joyce, carbonates form layered stromatolitic deposits, where carbonate texture and geochemistry constrains precipitation to the oxic surface of photosynthetically active microbial mats. Changes in carbonates from older to younger stromatolite layers record decreasing rates of primary productivity alongside shifts in microbial community composition with lake level rise. In Lake Fryxell, carbonates precipitated episodically in benthic mats across a redox-stratified water body. Variations in pore water redox at the time of carbonate precipitation are preserved through the incorporation of redox-sensitive (Fe and Mn) elements into these microbial carbonates. The distribution of Fe and Mn indicate that carbonates precipitated through changes to pore water redox, here interpreted as seasonal shifts in rates of oxygenic photosynthesis in this polar setting. Carbonate precipitation was episodic within microbial mats across the persistently stratified Lake Fryxell water column, and this episodic nature indicates that precipitation was primarily driven by an extrinsic perturbation to local geochemistry rather than microbial activity. In both lakes Joyce and Fryxell, lacustrine carbonate sediments provide a record of benthic microbial ecosystems through time and their response to recent environmental change. Models for carbonate precipitation in these lakes may further inform interpretations of paleolake carbonates from previous glacial-interglacial transitions.