Carbonate stable isotopes
Enrichment of carbonates in 13C provides a potential signature of net photosynthetic activity in these benthic mats. Slight (up to ~3‰) enrichment in 13C relative to values expected for precipitation in isotopic equilibrium with the water column (Knoepfle et al. 2009, Romanek et al. 1992) (Figure 6) is consistent with preferential uptake of 12C by photosynthesis, leaving a more enriched pool from which isotopically heavier carbonates precipitate. However, this δ13C signature does not covary with the changes in redox inferred from CL microscopy, which are interpreted as effects of seasonal fluctuation in microbial metabolic activity. Additionally, δ13C does not covary with depth. Therefore, carbonate δ13C may be interpreted as a general signature of microbial metabolic activity, but cannot be used to quantitatively assess seasonal or spatial changes at the resolution of this study.
The broad range of ~23‰ in δ18O of Lake Fryxell carbonates is unexpected. The water column of Lake Fryxell is isotopically light in oxygen (average δ1818O; across 10 meters of depth, δ18Owatervaries over a range of <1‰ (Dowling and Lyons 2014). Equilibrium fractionation calculations predict carbonate δ18O values of -27.5‰ to -25.6‰ VPDB at all depths in the lower oxycline (after Coplen 2007, Kim and O’Neil 1997, Wostbrock et al. 2020), whereas measured δ18O values range from -26.4 to -3.1‰ VPDB. Carbonate δ18O often exhibits large differences over small (sub-mm to mm) distances; for example, two subsampling sites ~4 mm apart in carbonate from 9.3 m exhibit a difference in δ18O of >15‰ (Figure S2). This isotopic variability contrasts δ18O data from carbonates in perennially ice-covered Lake Joyce (Pearse Valley, MDV), in which δ18O spans a range of only 5.5‰ and does not display such broad variability on the μm-to-mm scale (Mackey et al. 2018). Thus, the oxygen isotope composition of Lake Fryxell carbonates must be influenced by some process which does not occur in Lake Joyce.