Groundwater infiltration as a potential trigger for carbonate
precipitation
Oxygen isotope variability may be indicative of isotopic heterogeneity
in the fluid from which these carbonates precipitated, potentially
caused by mixing of multiple water sources with significantly different
oxygen isotope compositions. Stream waters in the Fryxell basin flow
into Lake Fryxell during the summer and have slightly heavier (0.8-1.2‰)
δ18O values than the water column (Gooseff et al.
2006, Harris et al. 2007). This relative enrichment is not of a
sufficient magnitude to fully explain the oxygen isotope offset observed
in Lake Fryxell carbonates, nor are other documented water sources in
the basin. Lake and glacier ices, surface waters, and regional
evaporatively concentrated Ca-chloride brines derived from meteoric
waters cannot account for the heaviest carbonate δ18O
values (Figure S4) (Fountain 2014, Matsubaya et al. 1979). Carbonate
δ18O values are consistent with predicted
precipitation from marine fluids at ambient temperatures (Figure S4),
but despite the proximity of McMurdo Sound ~5 km down
valley from Lake Fryxell, there is no evidence for surface inflow to the
lake.
More diverse waters may interact with lake sediments through groundwater
infiltration. Lake Fryxell is connected via a talik (an unfrozen region
of ground bounded by permafrost) to a regional subsurface calcium
chloride-rich brine aquifer (Foley et al. 2019, Mikucki et al. 2015,
Toner and Sletten 2013). The aquifer is most likely a remnant of GLW
which infiltrated the ground after extensive freezing and/or evaporation
created a concentrated brine (Mikucki et al. 2015, Myers et al. 2021).
Although no isotopic data exist for this brine at present, the brine
would likely have higher δ18O values than modern
meteoric-sourced waters due to incorporation of marine fluids; seawater
has previously been suggested as an explanation for carbonate18O enrichment in sediments deposited from GLW
(Lawrence and Hendy 1989). Any evaporative concentration with brine
formation would further enrich these values as observed in other surface
waters of the McMurdo Dry Valleys (Horita 2009). The isotopic data
presented here indicate that this brine could have infiltrated through
the lake sediments and mixed with mat pore waters, disrupting the
geochemical equilibrium and inducing short-lived precipitation of
isotopically heterogeneous carbonates documented in this study.
The lack of covariation between δ18O and
δ13C is also consistent with precipitation from a
low-DIC calcium chloride brine, such as those which have been observed
in Taylor and Wright Valleys (Dickson et al. 2013, Toner and Sletten
2013). In this case, the induction of carbonate precipitation could be
the product of locally increasing calcium concentrations in the pore
waters of benthic mats, counteracting the otherwise low calcium levels
within the pycnocline (Lawrence et al. 1985).
Geochemical constraints on Lake Fryxell carbonates suggest that an
episode of enhanced connection between existing lake water and
evaporatively-modified marine or marine-influenced ground water occurred
in recent decades following lake level rise and shallowing of the lake
oxycline. Future work on oxygen isotope composition of Antarctic
carbonates and their source waters promises new insights into the
hydrology and climate history of this region, as well as the processes
which induce carbonate precipitation in Antarctic lakes. Additionally,
the high variability of δ18O across μm to mm in Lake
Fryxell carbonates highlights the importance of high-resolution sampling
in environmental reconstructions in these polar deposits; sample
homogenization during bulk analysis may obscure variability important to
interpret processes surrounding carbonate precipitation. Applying
similar combined geochemical and petrographic characterization to
Antarctic paleolake deposits is necessary to reconstruct the sources of
carbonates and their significance for interpretation of terrestrial
Antarctic environments.