Carbonate redox geochemistry
CL microscopy (Figure 4) shows that carbonate precipitation across the oxycline occurred through changes in pore water dissolved Mn and Fe concentration consistent with changes in local redox. Seasonal fluctuations in DO input and concentrations likely affect the depth of the oxic-anoxic transition within the density-stratified water column and the underlying benthic mats. During the austral summer, oxygenic photosynthesis would result in DO saturation to a greater depth. This DO increase would be greater within microbial mat pore space than in the surrounding waters due to local production within the mats, as evidenced by the emergence of seasonal oxygen oases in anoxic water below the oxycline (Sumner et al. 2015). Reciprocally, reducing conditions within mat pore spaces are expected to extend to shallower depths than the oxic-anoxic transition in the winter with local respiration in the mats. Seasonal shifts in mat oxygenation have not been directly observed in Lake Fryxell to date, but can be reasonably hypothesized from previous observation of seasonal changes in DO at the oxycline (Lawrence and Hendy 1985) and observed summer oxygen profiles (Jungblut et al. 2016, Sumner et al. 2015).
Because carbonates precipitated in the mat subsurface and experienced significant changes in redox during precipitation, they are interpreted to have precipitated within mm to cm of the contemporary mat-water interface, within or shallower than the depth extent of benthic mat oxygen oases (Sumner et al. 2015) at the time of their precipitation. Where depth to carbonate was greater than a few cm, the difference is interpreted to be the product of sand sedimentation through the perennial ice cover. This is most dramatically evidenced by the presence of a carbonate layer in two adjacent (~1.5 m apart) sand mounds on the lake bottom; the continuous carbonate layer lies on top of one mound and at the bottom of the other (Rivera-Hernandez et al. 2019).
Because these carbonates form in pore spaces within metabolically active microbial mats, pore water redox may differ significantly from that of the local water column (e.g. Sumner et al. 2015) and leave a corresponding signal on carbonate geochemistry. The presence of Mn-bearing oxides at the oxic-anoxic transition, as interpreted from dark brown to black spots displaying high Mn concentrations (Figure 5), indicates the importance of biological activity in this redox system; oxidized Mn is thermodynamically unstable in aqueous environments unless conditions are strongly oxidizing (Davison 1993), thus necessitating the presence of an oxidizing microenvironment in which these oxides can form. These environments are most likely facilitated by microbial activity, as photosynthetic oxygen production at these depths significantly increases pore water oxygenation (Sumner et al. 2015). The presence of these oxides in carbonates which precipitated from low-oxygen waters, as evidenced by CL banding, indicates early carbonate precipitation isolating the reactive oxides from reducing waters and thereby preventing re-reduction of the metals.
CL bands correspond well to fluctuations in carbonate Mn and Fe content in instances where bands can be accurately sampled by laser ablation (Figure S3). However, this relationship is not consistent in all carbonates throughout the oxycline; CL brightness is independent of metal concentrations at the majority of LA-ICP-OES sampling sites (Figure 5). This may be due in part to methodological limitations; LA-ICP-OES cannot differentiate between oxidation states, instead capturing all Mn and Fe present in the sample regardless of oxidation state. Since only reduced Mn and Fe affect calcite CL, ablation of oxidized phases may complicate interpretation of these data. Thus, careful interpretation of combined petrographic, CL, and LA-ICP-OES data sets is necessary to accurately assess the redox record contained in these carbonates.
From the combination of CL imaging and LA-ICP-OES data, it can be inferred that carbonate redox geochemistry in Lake Fryxell preserves a record of seasonal changes in pore water oxygenation and the oxic-anoxic transition depth. Both [Mn] and [Fe] show the broadest range at 9.3 m (Figure 5), providing a geochemical record of the frequent changes in redox evident from the abundant CL bands at the same depth (Figure 4). This implicates 9.3 m as a depth across which the oxic-anoxic boundary may regularly move over the course of an annual cycle. In contrast, carbonates at 9.0 m display fewer CL bands and lower [Mn] and [Fe] than at 9.3 m, indicating that pore waters at 9.0 m typically remain oxygenated perennially and are rarely reducing enough to allow Mn/Fe substitution into carbonates. Conversely, the scarcity of CL bands and high Mn/Fe content of carbonates at 9.7 m indicates that this depth is typically suboxic to anoxic, and rare episodes of oxygenation sequester Fe into insoluble oxides and prevent quenching of Mn-induced luminescence (Figure S5).
From petrographic and CL observations, it can be inferred that carbonate precipitation in the oxycline of Lake Fryxell is episodic, but that these episodes are not directly controlled by seasonal fluctuations in pore water redox. The precipitation event persisted through multiple annual light-dark cycles and associated changes in pore water redox. Although carbonate knob macromorphologies are reminiscent of liftoff features observed at these depths in Wharton et al. (1983), they are chemically consistent with precipitation under low oxygen conditions of the shallower oxycline as observed in 2006 and 2012. Thus, carbonates likely precipitated following lake level rise since 1981. Despite the lack of apparent metabolic control on carbonate precipitation, these observations highlight the potential for aspects of microbial ecosystem function to be preserved indirectly in extrinsically-driven carbonate precipitation as fluctuations in pore water geochemistry.