Figure 4. CL (A, D, G), plane-polarized light (B, E, H), and
cross-polarized light (C, F, I) photomicrographs of Lake Fryxell
carbonates from depths of 9.0 (A-C), 9.3 (D-F), and 9.7 (G-I) m.
Acicular calcite crystals form botryoids and interfering bundles that
fill voids in sediment- and mat biomass-rich zones. Carbonates
incorporate varying amounts of reduced Mn and Fe through the oxycline,
resulting in different abundances and thicknesses of CL bands. Arrows
indicate direction to mat-water interface.
Where carbonates lack incorporated sediment or microbial biomass,
precipitation is interpreted to have occurred in mm-scale voids within
microbial mats. Voids may form due to mat growth processes, such as when
microbial films overgrow open pits in ridge-pit mats (Jungblut et al.
2016), or when gas bubbles build up in mats as is the case of shallow
water liftoff mats in Lake Fryxell (Wharton et al. 1982, Wharton 1994).
Liftoff influence on mineralized mats is further supported by cm-scale
knob macromophologies (see Doran et al. 1994). Knob morphologies across
the oxycline (Figure 2C) are more consistent with liftoff mats than with
the typical prostrate or pinnacle mat morphologies observed growing at
these depths (Jungblut et al. 2016, Sumner et al. 2015). Therefore,
knobs are interpreted to be remnant structures formed during periods of
lower lake level.