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\begin{document}

\title{Episodic carbonate precipitation in perennially ice-covered Lake
Fryxell, Antarctica}



\author[1]{Jared Clance}%
\author[1]{Tyler Mackey}%
\author[1]{Marisol Juarez Rivera}%
\affil[1]{University of New Mexico}%


\vspace{-1em}



  
  \date{May 22, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Benthic carbonates in perennially ice-covered Lake Fryxell (Mc-Murdo Dry
Valleys, Antarctica) precipitated from pore waters in microbial mats as
calcite rhombs, acicular botryoids and interfering bundles. Carbonates
span the pronounced Lake Fryxell oxycline; variations in
carbonate-associated manganese and iron concentrations are consistent
with local oxycline conditions and seasonal fluctuations in pore water
oxygenation. Precipitation is most abundant in shallow oxic waters, but
extended through the oxycline during a discrete episode lasting multiple
years, as evidenced by patterns of cathodoluminescence consistent with
predicted seasonal changes in redox modulating dissolved manganese and
iron concentrations. Carbonates did not precipitate in isotopic
equilibrium with the water column, and are enriched in 18 O relative to
predicted equilibrium values. Carbonate layer 18 O values vary by
\textgreater{}20%0 at the mm-scale, suggesting precipitation was driven
by mixing of isotopically heterogeneous fluids in the mat pore waters.
Correlation of carbonate geochemistry and mat morphology with historical
observations indicates that precipitation postdates recent lake level
rise. Further investigation of the physical and geochemical carbonate
proxies from Lake Fryxell and other ice-covered lakes in the Dry Valleys
promises to provide a valuable framework for interpreting Antarctic
carbonates as records of modern and ancient climate, Antarctic
biogeochemical and hydrological systems, and the drivers of carbonate
precipitation at polar climate extremes.%
\end{abstract}%



\sloppy


\textbf{Episodic carbonate precipitation in perennially ice-covered Lake
Fryxell, Antarctica}

JARED J. CLANCE*, TYLER J. MACKEY*, MARISOL JUAREZ RIVERA*

\emph{*Department of Earth and Planetary Sciences, University of New
Mexico, 221 Yale Blvd NE, Albuquerque, NM 87131, USA (Email:
jclance@vols.utk.edu)}

\textbf{ABSTRACT}Benthic carbonates in perennially ice-covered Lake
Fryxell (McMurdo Dry Valleys, Antarctica) precipitated from pore waters
in microbial mats as calcite rhombs, acicular botryoids and interfering
bundles. Carbonates span the pronounced Lake Fryxell oxycline;
variations in carbonate-associated manganese and iron concentrations are
consistent with local oxycline conditions and seasonal fluctuations in
pore water oxygenation. Precipitation is most abundant in shallow oxic
waters, but extended through the oxycline during a discrete episode
lasting multiple years, as evidenced by patterns of cathodoluminescence
consistent with predicted seasonal changes in redox modulating dissolved
manganese and iron concentrations. Carbonates did not precipitate in
isotopic equilibrium with the water column, and are enriched
in\textsuperscript{18}O relative to predicted equilibrium values.
Carbonate layer \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values vary by \textgreater{}20%0
at the mm-scale, suggesting precipitation was driven by mixing of
isotopically heterogeneous fluids in the mat pore waters. Correlation of
carbonate geochemistry and mat morphology with historical observations
indicates that precipitation post-dates recent lake level rise. Further
investigation of the physical and geochemical carbonate proxies from
Lake Fryxell and other ice-covered lakes in the Dry Valleys promises to
provide a valuable framework for interpreting Antarctic carbonates as
records of modern and ancient climate, Antarctic biogeochemical and
hydrological systems, and the drivers of carbonate precipitation at
polar climate extremes.\textbf{Keywords} Antarctica, lacustrine
carbonates, cathodoluminescence, carbonate stable isotopes, microbial
mats

\textbf{INTRODUCTION}

Ice core records provide a rich record of climate change across much of
Antarctica, but a similar record has not been thoroughly constructed for
the ice-free regions of the continent. In ice-free areas like the
McMurdo Dry Valleys (MDV) in West Antarctica, modern and ancient
lacustrine carbonate deposits provide a means to develop such a record.
Lacustrine carbonate deposits are useful sedimentary archives, often
preserving rich multi-proxy records of changes in local climate,
hydrology, and biogeochemistry. The physical and geochemical information
contained in lacustrine carbonates can aid in detailed reconstruction of
past local environmental conditions. The rich lacustrine carbonate
record has been most thoroughly recognized and explored in low- and
mid-latitude lakes, including both open- and closed-basin lakes (e.g.
DeMott et al. 2019, Hudson et al. 2017, Petryshyn et al. 2015, Talbot
1990). In Antarctic closed-basin lakes, where processes of sedimentation
differ considerably from those at lower latitudes, the carbonate record
has not yet been thoroughly characterized. Sedimentation in Antarctic
lakes is impacted by factors such as transport of sediment through
permanent ice covers on lakes, restricted liquid water input due to
sub-freezing temperatures, low weathering rates, prevalence of freeze
concentration over evaporative concentration, and limitation of primary
productivity by the seasonal light-dark cycle and low nutrient
availability (Craig et al. 1992, Green et al. 1988, Rivera-Hernandez et
al. 2019, Vincent et al. 2008 and references therein). Thus, it is
necessary to explore modern carbonate deposition in order to more
accurately interpret drivers of carbonate precipitation in paleolake
deposits (e.g. Doran et al. 1994) and their implications for terrestrial
Antarctic climate records.

At present, the MDV contain a number of perennially ice-covered lakes
which contain liquid water year-round. Many of these lakes contain
abundant benthic carbonate deposits (e.g. Lawrence and Hendy 1985,
Mackey et al. 2018); the fraction of the total sediment represented by
these carbonates varies among the lakes (Lawrence and Hendy 1985, Mackey
et al. 2015, Sutherland and Hawes 2008). Modern benthic carbonates in
Lake Fryxell (Taylor Valley, MDV) comprise a record of modern climate,
hydrology, and biogeochemistry in the lake and its surroundings. Lake
Fryxell carbonates are found in close spatial association with
metabolically active microbial mats on the lake floor (Wharton et al.
1982), and sediment cores indicate that carbonate precipitation occurred
through changes in regional climate and hydrology over tens of thousands
of years (Lawrence and Hendy 1985; 1989). This study assesses the
distribution, texture, and composition of carbonates in modern microbial
mats of Lake Fryxell to better interpret the roles of climate,
hydrology, and biology in carbonate precipitation in this perennially
ice-covered lake. Such terrestrial carbonates present a promising
sedimentary archive that can enhance scientific understanding of
terrestrial Antarctic environmental change.

\textbf{BACKGROUND}

\textbf{History of Taylor Valley}

Taylor Valley has been the site of extensive glaciation and deglaciation
during the past 20,000 years, which has strongly affected lake water
levels in the valley over time (Hall et al. 2000, Lawrence and Hendy
1989, Myers et al. 2021). Ice from the Ross Ice Shelf advanced into the
valley during the Last Glacial Maximum (LGM) (28.5-12.8 years BP) when
local air temperatures were 4-9 \selectlanguage{ngerman}°C lower than modern (Steig et al.
2000). Ice sheet advance resulted in an ice dam at the mouth of Taylor
Valley which reached elevations up to 300-350 meters above sea level
(m.a.s.l.) and allowed the formation of a large paleolake, Glacial Lake
Washburn (GLW), which filled all of Taylor Valley (Hall and Denton 2000,
Lawrence and Hendy 1989, Myers et al. 2021). Lake levels in Taylor
Valley have undergone drastic changes since the LGM; post-LGM ice dam
retreat caused drainage of GLW and lowered lake levels to approximately
81 m.a.s.l. (Levy et al. 2017, Myers et al. 2021). Lake levels fell
further as drainage into the Ross Sea continued, and high rates of
evaporation likely resulted in the formation of a shallow brine pond in
the Fryxell basin around 1000 years BP (Lawrence and Hendy 1989, Levy et
al. 2017). Lake basins subsequently refilled with glacial melt waters,
and lake levels have been rising since observations began during Robert
Scott's first expedition in 1903 (Doran and Gooseff 2022, Myers et al.
2021, Scott 1905).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1.} (A) Location of the McMurdo Dry Valleys in
Antarctica. (B) USGS Landsat image of Taylor Valley, MDV. (C) Depth
profile of photosynthetically active radiation (PAR), dissolved oxygen,
and conductivity in the Lake Fryxell water column (Doran 2020, Priscu
2022).

\textbf{Lake Fryxell}

Lake Fryxell is located near the eastern end of Taylor Valley and is the
closest lake in the valley to the ocean, \textasciitilde{}5 km from the
McMurdo Sound (Figure 1B) (Chinn 1993). The lake is approximately 5 by
1.5 km at the surface (Lyons and Priscu 2014). Lake Fryxell is
endorheic, with its level determined by the balance between glacial melt
water influx and ablation of the ice cover (Dugan et al. 2013, Lawrence
and Hendy 1985). About 1000 years BP, high evaporation rates resulted in
a shallow sodium chloride-rich brine in the Fryxell basin (Lawrence and
Hendy 1989), which was then refilled with glacial melt waters. The
maximum lake level has increased gradually in recent decades, from 16
m.a.s.l. in 1980 to its present level of 18.3 m.a.s.l (Doran and Gooseff
2022, Wharton et al. 1982).

The water column of Lake Fryxell is chemically stratified based on
density with conductivity increasing steadily with depth below a thin
mixed layer immediately beneath the ice cover (Jungblut et al. 2016,
Lawrence and Hendy 1985, Spigel and Priscu 1998). A steep oxycline
exists between 8-10 m deep; the dissolved oxygen (DO) maximum is present
around 8 m, and DO is absent from the water column below 10 m (Figure
1C) (Jungblut et al. 2016, Sumner et al. 2015). DO input to the lake is
predicted to fluctuate seasonally due to the influence of the polar
day-night cycle on glacial melting and microbial photosynthesis;
sunlight is available 24 hours a day for about four months during
summer, and no sunlight is available for about four months during winter
(Doran 2020).

Lake bottom sedimentation is influenced by extensive benthic microbial
mats that coat the lakebed where sufficient photosynthetically active
radiation penetrates. The microbial mat community composition and
metabolic potential varies across the oxycline (Dillon et al. 2020,
Jungblut et al. 2016, Sumner et al. 2015). Microbial mat morphologies
transition from pinnacle to ridge-pit, prostrate, and flocculent mat
with increasing depth (Jungblut et al\emph{.} 2016). Calcium carbonates
are present in close spatial association with the microbial mats
(Jungblut et al. 2016, Lawrence and Hendy 1985, Wharton et al. 1982).
The depth range in which mat-associated carbonates occur spans the
oxycline as well as density-stratified gradients of solutes including
major ions, nutrients, and transition metals (Harnish et al. 1991,
Jungblut et al. 2016, Lawrence and Hendy 1985). As a result, the
mat-water-carbonate system in Lake Fryxell provides a useful ``natural
laboratory'' in which to investigate the processes of carbonate
precipitation in a polar lacustrine setting.

\textbf{Geochemical proxies in Lake Fryxell}

Manganese and iron are useful biogeochemical proxies in carbonates,
largely due to their redox sensitivity; reduced (divalent) cations of
both Mn and Fe are able to occupy Ca sites in calcite, while their more
oxidized (trivalent and higher) cations cannot (e.g. Barnaby and
Rimstidt 1989, Braithwaite 2016). Additionally, the ratio of Mn to Fe in
carbonates allows for more detailed investigation, as Fe oxidizes more
readily and is more thermodynamically stable in oxidized phases than Mn
in aqueous environments (Davison 1993). Lake Fryxell contains both
manganese and iron throughout the water column, with very low
(\textless{}50 \selectlanguage{greek}μ\selectlanguage{english}g/l) concentrations above the oxycline increasing to
hundreds of \selectlanguage{greek}μ\selectlanguage{english}g/l around the oxic-anoxic transition (Harnish et al.
1991).

Water column Mn and Fe concentrations may not directly correspond to
carbonate concentrations in Lake Fryxell, as carbonates are present
within metabolically active microbial mats. Benthic mat metabolisms
modulate mat pore water oxygenation; photosynthesis further
supersaturates pore water oxygen within the hyperoxic DO maximum (Sumner
et al. 2015), contributing to expected perennial oxygenation of surface
mats that persists through the polar winter based on extended season
observations of nearby Lake Hoare (Wharton et al. 1994). The depth limit
for photosynthesis extends below the oxycline in Lake Fryxell, leading
to mm-scale seasonal oxygen oases within mats performing oxygenic
photosynthesis (Sumner et al. 2015). Thus, seasonal changes to pore
water redox vary with depth due to both the water column oxycline and
local metabolic effects in the benthic microbial mats. In this study, Mn
and Fe concentrations in carbonate are examined as a redox-sensitive
proxy for seasonal changes to local pore water redox conditions at the
time of carbonate precipitation.

Stable isotopes of carbon and oxygen in carbonates record a wealth of
biogeochemical, hydrological, and climatic information, such as
metabolic processes and changes in fluid sources. In the Lake Fryxell
water column, \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{DIC} follows a
similar trend to the DO profile;
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{DIC} increases from
\textasciitilde{}0%0 VPDB below the ice cover to a maximum of 3-4%0 around
8 m, then decreases to approximately -4%0 near the lake bottom (Knoepfle
et al. 2009, Neumann et al. 2004). This is interpreted as a result of
preferential uptake of \textsuperscript{12}C by photosynthesizing
microbes and a lack of water column mixing, resulting in the
largest\textsuperscript{13}C\textsubscript{DIC} enrichment at the same
depth where maximum primary productivity occurs (Neumann et al. 1998,
Neumann et al. 2004). \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O\textsubscript{water} is
nearly constant throughout the water column, varying over a narrow range
of -31.9 to -31.3%0 VSMOW (Dowling and Lyons 2014). Existing isotopic
data for the benthic carbonates are limited to bulk values of 10-20 mg
samples from sediment cores (Lawrence 1982, Lawrence and Hendy 1989).

The stable stratification of the Lake Fryxell water column and the
lake's depth profiles of oxygenation, redox-sensitive metal
concentrations, and stable isotope compositions offer a robust suite of
geochemical proxies which can be used to construct a facies model of
carbonate precipitation in this perennially ice-covered Antarctic lake.
The petrography and geochemistry of Lake Fryxell carbonates presented in
this study help to describe the timing of carbonate precipitation and
the changes in redox and isotope geochemistry during precipitation.

\textbf{METHODS}

\textbf{Sample collection and preparation}

Benthic observations and sampling were carried out by divers in November
2012, following a transect established in 2006 (see Dillon et al. 2020,
Jungblut et al. 2016). Video imagery was collected from 8.5 to 10.5 m
depth in 2012 and from below the ice cover to 7.1 m depth in 2006. All
water depths referenced in this study are based on the lake level in
2012. Divers measured the depth to carbonate from the mat-water
interface by probing mats with an incrementally-marked metal wire until
a rigid surface was encountered. Selected push cores and dredge samples
were dissected in the field, dried, and shipped to the University of
California, Davis. Remaining samples were frozen during the field season
for shipment and subsequently stored below -20 \selectlanguage{ngerman}°C.

Carbonates in sediment cores were located by halving the frozen cores.
Acrylic core tubes were cut lengthwise using a hand-held rotary cutting
tool with a circular diamond-edged blade, then cores were halved along
the same line using a water-cooled Hi-Tech Diamond 6'' trim saw
(Westmont, IL, USA) to expose the interior of the core. Carbonate pieces
and surrounding sediments were excised from cores using the trim saw,
then lyophilized in order to separate carbonates from sediments. Dried
carbonates were shipped to Spectrum Petrographics (Vancouver, WA, USA)
for preparation as thin sections; carbonate pieces which appeared
particularly fragile were first embedded in Epo-Tek 301-2 clear epoxy
(Billerica, MA, USA) according to manufacturer instructions to prevent
damage during shipping. Samples were prepared as thin sections 30 or 80
\selectlanguage{greek}μ\selectlanguage{english}m thick.

\textbf{Analyses}

X-ray diffraction (XRD) was used to determine the mineralogy of Lake
Fryxell carbonates. Four pieces of freeze-dried carbonate, two each from
depths of 9.0 and 9.7 m (Table S1), were crushed into powders using a
ceramic mortar and pestle, and 230-340 \selectlanguage{greek}μ\selectlanguage{english}g of each powder was weighed out
for analysis. XRD analysis was performed using a Rigaku SmartLab
diffractometer (Tokyo, Japan) with Cu K\selectlanguage{greek}α \selectlanguage{english}radiation (wavelength = 1.54059
\selectlanguage{ngerman}Å) and a DeteX Ultra detector. Spectra were collected in the 3-150° 2\selectlanguage{greek}θ
\selectlanguage{english}range with a step size of 0.02\selectlanguage{ngerman}° and a count time of 6°/minute.

Transmitted- and reflected-light microscopy were performed on thin
sections and billets using Nikon SMZ25 stereo and Optiphot-Pol
petrographic microscopes (Tokyo, Japan) to assess petrographic
characteristics of these carbonates. Cathodoluminescence (CL) microscopy
was conducted using a Nuclide Corporation Luminoscope ELM-2A
cold-cathode microscope. Polished thin sections and billets were placed
in the Luminoscope chamber under low vacuum (30-50 mtorr) and CL images
were captured using a voltage of 5.5-6.0 kV to obtain a beam current of
0.7-0.8 mA.

Samples for \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C/\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O analysis were
extracted from billets using a hand-held Proxxon Micromot 60/E drill
(Hickory, NC, USA) with Kodiak Cutting Tools carbide end mill bits
(Syracuse, NY, USA) of diameters 1/64'' and 1/32''. Carbonate powders
were washed in 30\% hydrogen peroxide overnight to remove organic
carbon, then dried at 50-70 \selectlanguage{ngerman}°C for several hours. Carbon and oxygen
isotope ratios were measured at the Center for Stable Isotopes,
University of New Mexico using the method described by Spotl and
Vennemann (2003). Samples were loaded in 12 mL borosilicate exetainers,
then the exetainers were flushed with He and the samples were reacted
with H\textsubscript{3}PO\textsubscript{4} for 12 hours at 50º C. The
evolved CO\textsubscript{2} gas was measured by continuous flow isotope
ratio mass spectrometry using a Thermo Fisher Scientific Gasbench
coupled to a Thermo Fisher Scientific Delta V Plus Isotope Ratio Mass
Spectrometer (Waltham, MA, USA). Isotope ratios are reported in delta
notation on the VPDB scale.

Elemental composition data were acquired by laser ablation and
inductively coupled plasma optical emission spectrometry (LA-ICP-OES).
Elemental analysis focused on concentrations of Mn and Fe ({[}Mn{]} and
{[}Fe{]}, respectively). In-house standards were prepared by diluting
BCS-CRM 393 limestone and NBS 2710 Montana soil with BCS-CRM 313
high-purity silica to create a set of references encompassing a wide
range of {[}Mn{]} and {[}Fe{]}. LA-ICP-OES was performed using a New
Wave Research UP-213 laser ablation system (Fremont, CA, USA) connected
to a PerkinElmer Optima 4300 DV optical emission spectrometer (Waltham,
MA, USA) with Ar plasma and Ar as a carrier gas at a flow rate of 600
mL/min. {[}Mn{]} and {[}Fe{]} were measured at selected locations on
four billets corresponding to thin sections of Lake Fryxell carbonates
(see Table S1). Ablation was conducted with the laser set to 80\% power
and 10 Hz firing speed. Linear transects were scanned with a spot size
of 55 or 80 \selectlanguage{greek}μ\selectlanguage{english}m and a scanning speed of 4-8 \selectlanguage{greek}μ\selectlanguage{english}m/s; single-spot analyses
were performed with a spot size of 55-100 \selectlanguage{greek}μ\selectlanguage{english}m and a dwell time of 45-75
seconds.

\textbf{RESULTS}

\textbf{Benthic mat observations}

The morphology of shallow water microbial mats at Lake Fryxell is
influenced by the nucleation of gas bubbles on the mats (Wharton et al.
1982, Wharton et al. 1994). Gas bubbles build up in mats and induce
buoyant delamination of mat layers, creating microbial mat liftoff
structures. Liftoff mat was present to 6.4 m depth in 2006 (7.0 m depth
in 2012; Figure 2A-B). Liftoff was not present below 7.0 meters in 2006
and was also absent from the deeper transect starting at
\textasciitilde{}8.5 m in 2012. Video footage throughout the 2012
oxycline shows ridge-pit and prostrate mat overlying discrete cm-scale,
mineralized topographic highs or knobs (Figure 2C).

\textbf{Carbonate mineralogy and petrography}

Rigid carbonate layers are present within mats at all depths surveyed.
Depth to carbonate (Table S2) from the mat-water interface ranges from
1-22 cm and is independent of depth and mat morphology. XRD spectra of
carbonate layers at 9.0 and 9.7 m show peaks characteristic of calcite
(Figure S1).

Lake Fryxell carbonates are closely associated with sediments and
organic matter; mm- to cm-scale clots of sediment and microbial biomass
are present surrounding and interspersed throughout carbonate cements
(Figure 3). Carbonates are present both as laminated layers and
irregularly-distributed void-filling cements in microbial mats.

The distribution of carbonate in benthic microbial mats varies with
depth. Samples from depths 9 m and shallower contain interconnected
rhombic carbonate crystals 100--200 \selectlanguage{greek}µ\selectlanguage{english}m in length in addition to multiple
layers of botryoidal cements (Figure 3A). At greater depths, carbonate
primarily consisted of a single layer of botryoidal cements hundreds of
\selectlanguage{greek}µ\selectlanguage{english}m to a few mm thick. At all depths, carbonates generally followed the
cm-scale macromorphology of benthic mats, but finer superimposed
pinnacles lacked this pervasive mineralization. Near the oxic-anoxic
transition depth, acicular crystals form interfering bundles rather than
botryoids (Figure 3B). Crystals most commonly grow outward from
dark-colored areas indicative of high sediment/organic matter content
(Figure 3). Crystal growth does not appear to occur preferentially in
any specific direction; growth direction is commonly oriented
perpendicular or downward relative to the mat-water interface.

\textbf{Cathodoluminescence}

Luminescence in Lake Fryxell carbonates is predominantly red and ranges
from non-luminescent to very bright (Figure 4A, D, G). Red CL occurs in
both ``clean'' (optically clear low sediment/organic matter content) and
``dirty'' (opaque high sediment/organic matter content) carbonates
(Figure 4B, E, H). Shallower samples (9.0-9.3 m) commonly display CL
``bands'': visually distinct, concentric or roughly parallel zones
alternating between luminescent and non-luminescent calcite oriented
approximately perpendicular to the crystal growth direction (Figure 4A,
D, G). Most luminescent bands are several microns wide and bright,
although band width and intensity are variable; luminescent bands can be
dim to bright and occasionally over 100 \selectlanguage{greek}μ\selectlanguage{english}m wide (Figure 4D).
Non-luminescent zones are typically longer in the crystal growth
direction than luminescent zones (Figure 4A, D, G). CL bands occur
rarely in deeper (9.4-9.7 m) carbonates; approaching the oxic-anoxic
transition, carbonate CL is predominantly dim and diffuse or
non-luminescent with rare, very narrow (\textless{}10 \selectlanguage{greek}μ\selectlanguage{english}m wide) bright
bands (Figure 4G). Bright bands in deeper samples are flanked by much
wider zones of dim red to purple luminescence or non-luminescent
carbonate (Figure 4G).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2.} Comparison of Lake Fryxell microbial mat morphology
at shallow depths and the oxycline. Bubbles nucleate on microbial mats
at shallow waters creating buoyant liftoff mat structures. (A) Liftoff
mats below the ice-cover creating tented structures. (B) Liftoff mats at
the deepest end of the liftoff zone with variable morphologies. (C)
Prostrate mat in the oxycline overlying calcified liftoff structures.

\textbf{Elemental analysis}

Mn and Fe concentrations in Lake Fryxell carbonates range from tens to
thousands of ppm; in samples from four depth points within the oxycline,
{[}Mn{]} ranges from 44--8972 ppm, and {[}Fe{]} from 36--2182 ppm. The
ratio of {[}Mn{]} to {[}Fe{]} also displays a broad range from
0.24--20.31. {[}Mn{]} and {[}Fe{]} display a rough covariance; the
shallowest carbonates contain the lowest concentrations of both metals,
while in deeper samples, the ablation sites yielding the highest
{[}Mn{]} also yield the highest {[}Fe{]} (Figure 5). The minimum values
of both {[}Mn{]} and {[}Fe{]} increase with depth. Carbonate {[}Mn{]}
and {[}Fe{]} vary widely at 9.3 and 9.7 m, with {[}Mn{]} ranging from
52--4173 ppm and {[}Fe{]} from 178--2418 ppm at these depths. The
highest values of both {[}Mn{]} and {[}Fe{]} represent dark brown to
black spots in carbonates interpreted as oxide minerals (Figure 5).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3.} Transmitted-light photomicrographs of Lake Fryxell
carbonates showing acicular calcite crystals forming botryoids (A) and
interfering bundles (B) at depths of 9.0 and 9.7 meters, respectively.
Adjacent zones of crystals often grow in different directions. Sediment
and organic matter (dark-colored zones) are commonly found surrounding
and interspersed throughout carbonates. Scale bars are 100 \selectlanguage{greek}μ\selectlanguage{english}m. Arrows
indicate direction to mat-water interface.

\textbf{Stable isotopes}

Carbonate \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O in this depth range is highly variable
and does not follow a clear trend with depth. Carbonate
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values span \textgreater{}20%0, and carbonate
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O does not match predicted values for precipitation
in isotopic equilibrium with the Lake Fryxell water column (Figure 6).
Carbonate \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O does not co-vary with
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C. At the shallowest depths (8.9-9.0 m), average
carbonate \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C values are similar to calculated values
for precipitation in isotopic equilibrium with the DIC pool. From
9.3-9.7 m, carbonate \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C at nearly all sampling sites
is approximately 0-3%0 heavier than expected from precipitation in
isotopic equilibrium with DIC (Figure 6).

\textbf{DISCUSSION}

\textbf{Carbonate distribution and precipitation timing}

Prior to this study, the presence of carbonates in Lake Fryxell's
benthic microbial mats had been attributed to microbial photosynthesis
inducing precipitation of calcite in the water column and within benthic
mats to form laminated stromatolitic structures (Doran et al. 1994,
Lawrence and Hendy 1985, Wharton et al. 1982). This study presents an
updated facies model describing in greater detail the spatial
distribution, precipitation timing, and geochemistry of carbonates in
this Antarctic lacustrine system in order to assess their potential to
preserve evidence of local microbial activity and lake hydrology. The
petrographic images presented here show that Lake Fryxell carbonates are
closely associated with microbial mat biomass, typically surrounded by
and/or interspersed with large amounts of sediment and biomass (Figure
3, Figure 4), indicating these carbonates precipitated from pore waters
in voids distributed throughout sediments and microbial mat biomass and
fill available pore spaces. Carbonates form botryoids and interfering
bundles with varied growth orientations, both across the depth transect
and within individual samples on the scale of tens of microns to
millimeters (Figure 3). This style of precipitation contrasts that of
microbial mat-associated carbonate precipitation in two other
perennially ice-covered MDV lakes: Lake Joyce (Pearse Valley), where
carbonates precipitate as crusts near the mat-water interface and grow
outward from the surfaces of microbial mat structures, and voids are not
filled in by later generations of cements (Mackey et al. 2015), and Lake
Hoare (Taylor Valley), where calcite rhombs are disseminated in annual
laminae and correlate to predicted changes in net photosynthetic
activity (Hawes et al. 2016, Vopel and Hawes 2006).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4.} CL (A, D, G), plane-polarized light (B, E, H), and
cross-polarized light (C, F, I) photomicrographs of Lake Fryxell
carbonates from depths of 9.0 (A-C), 9.3 (D-F), and 9.7 (G-I) m.
Acicular calcite crystals form botryoids and interfering bundles that
fill voids in sediment- and mat biomass-rich zones. Carbonates
incorporate varying amounts of reduced Mn and Fe through the oxycline,
resulting in different abundances and thicknesses of CL bands. Arrows
indicate direction to mat-water interface.

Where carbonates lack incorporated sediment or microbial biomass,
precipitation is interpreted to have occurred in mm-scale voids within
microbial mats. Voids may form due to mat growth processes, such as when
microbial films overgrow open pits in ridge-pit mats (Jungblut et al.
2016), or when gas bubbles build up in mats as is the case of shallow
water liftoff mats in Lake Fryxell (Wharton et al. 1982, Wharton 1994).
Liftoff influence on mineralized mats is further supported by cm-scale
knob macromophologies (see Doran et al. 1994). Knob morphologies across
the oxycline (Figure 2C) are more consistent with liftoff mats than with
the typical prostrate or pinnacle mat morphologies observed growing at
these depths (Jungblut et al. 2016, Sumner et al. 2015). Therefore,
knobs are interpreted to be remnant structures formed during periods of
lower lake level.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5.} Log-log plot of {[}Mn{]} and {[}Fe{]} as determined
by LA-ICP-OES. The lowest concentrations of both metals are found at 9.0
m, while the highest concentrations are found at 9.3 and 9.7 m. The
highest concentrations of both metals are found in dark brown to black
spots surrounded by calcite (black symbols), here interpreted as
Mn/Fe-bearing oxides entombed in the carbonate. Neither {[}Mn{]} nor
{[}Fe{]} displays covariance with CL brightness.

Both lake level and the observed depth distribution of liftoff mats at
Lake Fryxell have varied through time. In 1980-1981, Wharton et al.
(1983) recorded liftoff mat down to 8 m depth (10.1 m depth in 2012).
Observations from 2006 show liftoff mats extended to 6.4 m depth (7.0 m
depth in 2012). Liftoff depth variation is interpreted as a result of
shifts in both freeze-concentration of gasses from meltwater streams
entering the lake and the relative rates of net respiration versus net
photosynthesis following lake level rise (Andersen et al. 1998, Craig et
al. 1992, Wharton et al. 1986, Wharton et al. 1987). The precipitation
of carbonates on these shallow water mats (Wharton et al. 1982, Wharton
1994) creates a sedimentary record of liftoff mat (Parker et al. 1981)
and, by extension, environmental change. Relic knob liftoff structures
were overgrown with subsequent lake level rise and shoaling of the
oxycline as observed in the 2006 transect. Similar juxtaposition of
shallow- and deep-water microbial mat morphologies have been described
at Lake Joyce (e.g. Hawes et al. 2011, Mackey et al. 2015) and are tied
to changing lake levels.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6.} \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O of
Lake Fryxell carbonates. Carbonate \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C is, on
average, enriched relative to expected values for precipitation in
isotopic equilibrium with water column DIC (closed symbols). Carbonate
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O is also enriched relative to expected equilibrium
values and varies over a range of \textgreater{}20%0.
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C follows an overall increasing trend with depth,
while \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O does not covary with depth or with
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C.

\textbf{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 (\textasciitilde{}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.

\textbf{Carbonate stable isotopes}

Enrichment of carbonates in \textsuperscript{13}C provides a potential
signature of net photosynthetic activity in these benthic mats. Slight
(up to \textasciitilde{}3%0) enrichment in \textsuperscript{13}C relative
to values expected for precipitation in isotopic equilibrium with the
water column (Knoepfle et al. 2009, Romanek et al. 1992) (Figure 6) is
consistent with preferential uptake of \textsuperscript{12}C by
photosynthesis, leaving a more enriched pool from which isotopically
heavier carbonates precipitate. However, this \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C
signature does not covary with the changes in redox inferred from CL
microscopy, which are interpreted as effects of seasonal fluctuation in
microbial metabolic activity. Additionally, \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C does
not covary with depth. Therefore, carbonate \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C may
be interpreted as a general signature of microbial metabolic activity,
but cannot be used to quantitatively assess seasonal or spatial changes
at the resolution of this study.

The broad range of \textasciitilde{}23%0 in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O of
Lake Fryxell carbonates is unexpected. The water column of Lake Fryxell
is isotopically light in oxygen (average \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{1818}O;
across 10 meters of depth,
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O\textsubscript{water}varies over a range of
\textless{}1%0 (Dowling and Lyons 2014). Equilibrium fractionation
calculations predict carbonate \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values of -27.5%0
to -25.6%0 VPDB at all depths in the lower oxycline (after Coplen 2007,
Kim and O'Neil 1997, Wostbrock et al. 2020), whereas measured
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values range from -26.4 to -3.1%0 VPDB. Carbonate
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O often exhibits large differences over small
(sub-mm to mm) distances; for example, two subsampling sites
\textasciitilde{}4 mm apart in carbonate from 9.3 m exhibit a difference
in \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O of \textgreater{}15%0 (Figure S2). This
isotopic variability contrasts \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O data from
carbonates in perennially ice-covered Lake Joyce (Pearse Valley, MDV),
in which \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O spans a range of only 5.5%0 and does not
display such broad variability on the \selectlanguage{greek}μ\selectlanguage{english}m-to-mm scale (Mackey et al.
2018). Thus, the oxygen isotope composition of Lake Fryxell carbonates
must be influenced by some process which does not occur in Lake Joyce.

\textbf{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%0)
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O 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 \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O
values (Figure S4) (Fountain 2014, Matsubaya et al. 1979). Carbonate
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values are consistent with predicted
precipitation from marine fluids at ambient temperatures (Figure S4),
but despite the proximity of McMurdo Sound \textasciitilde{}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 \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O values than modern
meteoric-sourced waters due to incorporation of marine fluids; seawater
has previously been suggested as an explanation for
carbonate\textsuperscript{18}O 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 \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C 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 \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O across \selectlanguage{greek}μ\selectlanguage{english}m 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.

\textbf{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 \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C shows overall enrichment
relative to predicted equilibrium values, consistent with preferential
uptake of \textsuperscript{12}C by metabolic processes, but does not
covary with precipitation timing or redox at this sampling resolution.
Carbonate \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O 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 \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{18}O 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.

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\textbf{Hosted file}

\verb`Clance_SupportingInfo.docx`  available at \url{https://authorea.com/users/563941/articles/610994-episodic-carbonate-precipitation-in-perennially-ice-covered-lake-fryxell-antarctica}

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