CONCLUSIONS
Microbial mat-associated carbonates in Lake Fryxell provide a sedimentary record of episodic carbonate precipitation across biogeochemical gradients of the chemocline. CL imaging demonstrates that the episode of carbonate precipitation persisted for multiple years, with precipitation continuing through changes in pore water redox interpreted as the result of annual polar light-dark cycles. Carbonate Mn/Fe content further indicates variability of local redox in microbial mat pore spaces, both with depth as the lake transitions from oxygen supersaturation to anoxia and seasonally with varying rates of oxygenic photosynthesis.
Carbonate isotope compositions are inconsistent with predictions for precipitation in isotopic equilibrium with the Lake Fryxell water column. Carbonate δ13C shows overall enrichment relative to predicted equilibrium values, consistent with preferential uptake of 12C by metabolic processes, but does not covary with precipitation timing or redox at this sampling resolution. Carbonate δ18O is offset from expected values for precipitation in equilibrium with the water column, and is highly variable on small spatial scales, likely indicating mixing of multiple water sources during carbonate precipitation. As indicated by previous water δ18O results from Taylor Valley, the carbonate source waters were a mix between lake and another non-meteoric source. Carbonate composition is most consistent with precipitation from a mix of lake waters and a subsurface brine in the porewaters of benthic mats. Formation of a brine aquifer via evaporative concentration of GLW waters provides a possible source for foreign waters to Lake Fryxell.
Petrographic and geochemical observations indicate that although these carbonates precipitate in close proximity to metabolically active microbes, and that microbial metabolism influences the geochemistry of these carbonates, biological activity is not a controlling factor in precipitation timing. Rather, precipitation is interpreted as a result of short-term groundwater influx, possibly driven by changes to the local climate in recent decades, disrupting the carbonate equilibrium in pore waters and inducing this episode of carbonate precipitation. Continuing investigation of the redox and isotope geochemistry of both modern lacustrine carbonates and paleolake deposits in the MDV will facilitate more detailed reconstruction of local environmental response to past and present climate change.