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.