Carbonate distribution and precipitation timing
Prior to this study, the presence of carbonates in Lake Fryxell’s benthic microbial mats had been attributed to microbial photosynthesis inducing precipitation of calcite in the water column and within benthic mats to form laminated stromatolitic structures (Doran et al. 1994, Lawrence and Hendy 1985, Wharton et al. 1982). This study presents an updated facies model describing in greater detail the spatial distribution, precipitation timing, and geochemistry of carbonates in this Antarctic lacustrine system in order to assess their potential to preserve evidence of local microbial activity and lake hydrology. The petrographic images presented here show that Lake Fryxell carbonates are closely associated with microbial mat biomass, typically surrounded by and/or interspersed with large amounts of sediment and biomass (Figure 3, Figure 4), indicating these carbonates precipitated from pore waters in voids distributed throughout sediments and microbial mat biomass and fill available pore spaces. Carbonates form botryoids and interfering bundles with varied growth orientations, both across the depth transect and within individual samples on the scale of tens of microns to millimeters (Figure 3). This style of precipitation contrasts that of microbial mat-associated carbonate precipitation in two other perennially ice-covered MDV lakes: Lake Joyce (Pearse Valley), where carbonates precipitate as crusts near the mat-water interface and grow outward from the surfaces of microbial mat structures, and voids are not filled in by later generations of cements (Mackey et al. 2015), and Lake Hoare (Taylor Valley), where calcite rhombs are disseminated in annual laminae and correlate to predicted changes in net photosynthetic activity (Hawes et al. 2016, Vopel and Hawes 2006).