Carbonate mineralogy and petrography
Rigid carbonate layers are present within mats at all depths surveyed.
Depth to carbonate (Table S2) from the mat-water interface ranges from
1-22 cm and is independent of depth and mat morphology. XRD spectra of
carbonate layers at 9.0 and 9.7 m show peaks characteristic of calcite
(Figure S1).
Lake Fryxell carbonates are closely associated with sediments and
organic matter; mm- to cm-scale clots of sediment and microbial biomass
are present surrounding and interspersed throughout carbonate cements
(Figure 3). Carbonates are present both as laminated layers and
irregularly-distributed void-filling cements in microbial mats.
The distribution of carbonate in benthic microbial mats varies with
depth. Samples from depths 9 m and shallower contain interconnected
rhombic carbonate crystals 100–200 µm in length in addition to multiple
layers of botryoidal cements (Figure 3A). At greater depths, carbonate
primarily consisted of a single layer of botryoidal cements hundreds of
µm to a few mm thick. At all depths, carbonates generally followed the
cm-scale macromorphology of benthic mats, but finer superimposed
pinnacles lacked this pervasive mineralization. Near the oxic-anoxic
transition depth, acicular crystals form interfering bundles rather than
botryoids (Figure 3B). Crystals most commonly grow outward from
dark-colored areas indicative of high sediment/organic matter content
(Figure 3). Crystal growth does not appear to occur preferentially in
any specific direction; growth direction is commonly oriented
perpendicular or downward relative to the mat-water interface.