MICROBIAL CARBONATE RECORDS OF ENVIRONMENTAL CHANGE IN PERENNIALLY ICE-COVERED LAKE JOYCE, ANTARCTICA

Perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica form liquid water oases in a polar desert. Lake level, stratification, and ice cover are sensitive to changing climate, and these characteristics both influence and are partially recorded by benthic microbial mats. In this study, we assess the response of mats to lake level rise in Lake Joyce. Mats are variably calcified and their morphology differs with depth. Shallower than 8 m, microbial mats experience buoyant deformation with bubble nucleation from supersaturated gasses. Mats grade from cm-scale pinnacle and flat to bulbous-conical morphologies around 15 m depth. Within the lake’s main salinity gradient from 21-23 m depth, mats form dm-scale columnar stromatolites interpreted as calcified liftoff mats from formerly lower lake levels.

Inner layers of bulbous-conical mats and columnar stromatolites were characterized from micro X-ray computed tomography scans and then subsampled. Sampled bulbous-conical mats contain calcified pinnacles morphologically identical to those of shallower mats. Microfossils within the interior calcified pinnacles preserved as filament molds are consistent with the cyanobacteria morphotypes identified in actively growing shallower pinnacles. The inner layer of deeper columnar stromatolites are also morphologically similar to shallow water liftoff mats, but differ in their mineralization: modern liftoff mats do not contain sufficient carbonate to preserve delaminated morphology, whereas the inner layers are marked by 0.5–1.5 mm thick calcite crusts. Recurring morphologies like pinnacles and liftoff mat indicate that microbial communities have grown similarly with mat expansion through lake level rise. The restriction of mineralized columnar morphologies to a specific depth range is likely a product of different mineralization in the salinity gradient, and thus preservation, rather than different community composition or liftoff processes at lower lake levels.