Cathodoluminescence
Luminescence in Lake Fryxell carbonates is predominantly red and ranges from non-luminescent to very bright (Figure 4A, D, G). Red CL occurs in both “clean” (optically clear low sediment/organic matter content) and “dirty” (opaque high sediment/organic matter content) carbonates (Figure 4B, E, H). Shallower samples (9.0-9.3 m) commonly display CL “bands”: visually distinct, concentric or roughly parallel zones alternating between luminescent and non-luminescent calcite oriented approximately perpendicular to the crystal growth direction (Figure 4A, D, G). Most luminescent bands are several microns wide and bright, although band width and intensity are variable; luminescent bands can be dim to bright and occasionally over 100 μm wide (Figure 4D). Non-luminescent zones are typically longer in the crystal growth direction than luminescent zones (Figure 4A, D, G). CL bands occur rarely in deeper (9.4-9.7 m) carbonates; approaching the oxic-anoxic transition, carbonate CL is predominantly dim and diffuse or non-luminescent with rare, very narrow (<10 μm wide) bright bands (Figure 4G). Bright bands in deeper samples are flanked by much wider zones of dim red to purple luminescence or non-luminescent carbonate (Figure 4G).