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).