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.