Figure 5. Log-log plot of [Mn] and [Fe] as determined by LA-ICP-OES. The lowest concentrations of both metals are found at 9.0 m, while the highest concentrations are found at 9.3 and 9.7 m. The highest concentrations of both metals are found in dark brown to black spots surrounded by calcite (black symbols), here interpreted as Mn/Fe-bearing oxides entombed in the carbonate. Neither [Mn] nor [Fe] displays covariance with CL brightness.
Both lake level and the observed depth distribution of liftoff mats at Lake Fryxell have varied through time. In 1980-1981, Wharton et al. (1983) recorded liftoff mat down to 8 m depth (10.1 m depth in 2012). Observations from 2006 show liftoff mats extended to 6.4 m depth (7.0 m depth in 2012). Liftoff depth variation is interpreted as a result of shifts in both freeze-concentration of gasses from meltwater streams entering the lake and the relative rates of net respiration versus net photosynthesis following lake level rise (Andersen et al. 1998, Craig et al. 1992, Wharton et al. 1986, Wharton et al. 1987). The precipitation of carbonates on these shallow water mats (Wharton et al. 1982, Wharton 1994) creates a sedimentary record of liftoff mat (Parker et al. 1981) and, by extension, environmental change. Relic knob liftoff structures were overgrown with subsequent lake level rise and shoaling of the oxycline as observed in the 2006 transect. Similar juxtaposition of shallow- and deep-water microbial mat morphologies have been described at Lake Joyce (e.g. Hawes et al. 2011, Mackey et al. 2015) and are tied to changing lake levels.