Lake Fryxell
Lake Fryxell is located near the eastern end of Taylor Valley and is the closest lake in the valley to the ocean, ~5 km from the McMurdo Sound (Figure 1B) (Chinn 1993). The lake is approximately 5 by 1.5 km at the surface (Lyons and Priscu 2014). Lake Fryxell is endorheic, with its level determined by the balance between glacial melt water influx and ablation of the ice cover (Dugan et al. 2013, Lawrence and Hendy 1985). About 1000 years BP, high evaporation rates resulted in a shallow sodium chloride-rich brine in the Fryxell basin (Lawrence and Hendy 1989), which was then refilled with glacial melt waters. The maximum lake level has increased gradually in recent decades, from 16 m.a.s.l. in 1980 to its present level of 18.3 m.a.s.l (Doran and Gooseff 2022, Wharton et al. 1982).
The water column of Lake Fryxell is chemically stratified based on density with conductivity increasing steadily with depth below a thin mixed layer immediately beneath the ice cover (Jungblut et al. 2016, Lawrence and Hendy 1985, Spigel and Priscu 1998). A steep oxycline exists between 8-10 m deep; the dissolved oxygen (DO) maximum is present around 8 m, and DO is absent from the water column below 10 m (Figure 1C) (Jungblut et al. 2016, Sumner et al. 2015). DO input to the lake is predicted to fluctuate seasonally due to the influence of the polar day-night cycle on glacial melting and microbial photosynthesis; sunlight is available 24 hours a day for about four months during summer, and no sunlight is available for about four months during winter (Doran 2020).
Lake bottom sedimentation is influenced by extensive benthic microbial mats that coat the lakebed where sufficient photosynthetically active radiation penetrates. The microbial mat community composition and metabolic potential varies across the oxycline (Dillon et al. 2020, Jungblut et al. 2016, Sumner et al. 2015). Microbial mat morphologies transition from pinnacle to ridge-pit, prostrate, and flocculent mat with increasing depth (Jungblut et al. 2016). Calcium carbonates are present in close spatial association with the microbial mats (Jungblut et al. 2016, Lawrence and Hendy 1985, Wharton et al. 1982). The depth range in which mat-associated carbonates occur spans the oxycline as well as density-stratified gradients of solutes including major ions, nutrients, and transition metals (Harnish et al. 1991, Jungblut et al. 2016, Lawrence and Hendy 1985). As a result, the mat-water-carbonate system in Lake Fryxell provides a useful “natural laboratory” in which to investigate the processes of carbonate precipitation in a polar lacustrine setting.