*Department of Earth and Planetary Sciences, University of New Mexico, 221 Yale Blvd NE, Albuquerque, NM 87131, USA (Email: jclance@vols.utk.edu)
ABSTRACTBenthic carbonates in perennially ice-covered Lake Fryxell (McMurdo Dry Valleys, Antarctica) precipitated from pore waters in microbial mats as calcite rhombs, acicular botryoids and interfering bundles. Carbonates span the pronounced Lake Fryxell oxycline; variations in carbonate-associated manganese and iron concentrations are consistent with local oxycline conditions and seasonal fluctuations in pore water oxygenation. Precipitation is most abundant in shallow oxic waters, but extended through the oxycline during a discrete episode lasting multiple years, as evidenced by patterns of cathodoluminescence consistent with predicted seasonal changes in redox modulating dissolved manganese and iron concentrations. Carbonates did not precipitate in isotopic equilibrium with the water column, and are enriched in18O relative to predicted equilibrium values. Carbonate layer δ18O values vary by >20‰ at the mm-scale, suggesting precipitation was driven by mixing of isotopically heterogeneous fluids in the mat pore waters. Correlation of carbonate geochemistry and mat morphology with historical observations indicates that precipitation post-dates recent lake level rise. Further investigation of the physical and geochemical carbonate proxies from Lake Fryxell and other ice-covered lakes in the Dry Valleys promises to provide a valuable framework for interpreting Antarctic carbonates as records of modern and ancient climate, Antarctic biogeochemical and hydrological systems, and the drivers of carbonate precipitation at polar climate extremes.Keywords Antarctica, lacustrine carbonates, cathodoluminescence, carbonate stable isotopes, microbial mats