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