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.