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.