Acta geographica Slovenica, 60-2, 2020, 125–139
THE ISOTOPIC GEOCHEMISTRY
OF CaCO
3
ENCRUSTATIONS IN TAYLOR
VALLEY, ANTARCTICA: IMPLICATIONS
FOR THEIR ORIGIN
Berry Lyons, Kelly Foley, Anne Carey, Melisa Diaz, Gabriel Bowen, Thure Cerling
Ground image of the Lake Hoare basin (Lake Hoare on far left) looking west up Taylor
Valley, Antarctica.
M E l IS A D IA Z

1
The Ohio State University, School of Earth Sciences, Byrd Polar and Climate Research Center,
Columbus, Ohio, USA
lyons.142@osu.edu (https://orcid.org/0000-0002-3143-7251), carey.145@osu.edu
(https://orcid.org/0000-0002-0272-5254), diaz.237@osu.edu (https://orcid.org/0000-0001-6657-358X)
2
Ball Corporation, Denver, Colorado, USA
foleykk@gmail.com
3
University of Utah, Department of Geology and Geophysics, Salt Lake City, Utah, USA
gabe.bowen@utah.edu (https://orcid.org/0000-0002-6928-3104), thure.cerling@utah.edu
(https://orcid.org/0000-0002-3590-294X)
126
DOI: https://doi.org/10.3986/AGS.7233
UDC: 550.42:552.54(292.3)
COBISS: 1.01
Berry Lyons
1
, Kelly Foley
2
, Anne Carey
1
, Melisa Diaz
1
, Gabriel Bowen
3
, Thure Cerling
3
The isotopic geochemistry of CaCO
3
encrustations in Taylor Valley, Antarctica:
Implications for their origin
ABSTRACT: Calcium carbonate (CaCO
3
) encrustations occur in most desert soils, including polar ones,
and such encrustations preserve records of geochemical, hydrological, and atmosphere processes affect-
ing these soils. We have collected a series of CaCO
3
encrustations found underneath surface rocks in the
soils and tills of Taylor Valley, McMurdo Dry Valleys (~78°S lat.), Antarctica. These encrustations were
analyzed for 
87
Sr/
86
S and δ
18
O and δ
13
C to determine what relation they have with the underlying soils,
and the material in which they are in contact, and to identify the processes that control their formation.
In all but one case, the isotopic data indicate that the source of Sr to these encrustations is not from the rock
on which it is associated. The primary source of Sr (and by analogy Ca) is either from dust that has been
deposited through aeolian processes or from the aggregate of till material within the soils. The δ
13
C values
for Taylor V alley encrustations ranged from 5.7 to 11.0‰, and are consistent with a carbon source from atmos-
pheric CO
2
. The δ
18
O values range from –8.1 to –11.2‰ and are heavier than expected for equilibrium calcite
precipitation from Taylor Valley meteoric water. Taken together these results indicate that the CaCO
3
was
formed by rapid evaporation of films beneath clasts that had become supersaturated with respect to CaCO
3
. 
KEY WORDS: calcium carbonate, isotopic ratio, salt deposit, McMurdo Dry Valleys, Antarctica
Izotopska geokemija inkrustacij CaCO
3
v Taylorjevi dolini na Antarktiki: izsledki,
ki nakazujejo njihov izvor
IZVLEČEK: Inkrustacije kalcijevega karbonata (CaCO
3
) so značilne za večino puščavskih prsti, tudi polarnih,
v njih pa so ohranjeni podatki o geokemičnih, hidroloških in atmosferskih procesih, ki so vplivali na te
prsti. Zbrali smo niz inkrustacij CaCO
3
, odkritih pod površinskimi kamninami v prsteh in tilih v Taylorjevi
dolini – eni izmed McMurdovih suhih dolin na Antarktiki (~78° j. z. š.). Analizirali smo vsebnost 
87
Sr/
86
S
ter δ
18
O in δ
13
C v teh inkrustacijah, da bi ugotovili, kako so povezane s spodaj ležečimi prstmi in mate-
rialom, s katerim so v stiku, ter določili procese, ki uravnavajo njihov nastanek. Razen v enem primeru
izotopski podatki kažejo, da Sr v teh inkrustacijah ne izvira iz kamnine, s katero je inkrustacija povezana.
Glavni viri Sr (in po analogiji tudi Ca) so bodisi prah, ki ga je nanesel veter, bodisi agregati ledeniških sed-
imentov (tilov) v prsteh. V inkrustacijah iz Taylorjeve doline je bilo od 5,7 do 11 ‰ δ
13
C, kar se ujema
z vsebnostjo ogljika iz atmosferskega CO
2
. Vrednosti δ
18
O so od –8,1 do –11,2‰, to pa je več, kot bi pričakovali
za uravnoteženo izločanje kalcita iz meteornih vod v Taylorjevi dolini. Skupni rezultati kažejo, da je CaCO
3
nastal s hitrim izhlapevanjem slojev pod klasti, prenasičenimi s CaCO
3
.
KLJUČNE BESEDE: kalcijev karbonat, izotopsko razmerje, nanos soli, McMurdove suhe doline, Antarktika
Berry Lyons, Kelly Foley, Anne Carey, Melisa Diaz, Gabriel Bowen, Thure Cerling, The isotopic geochemistry of CaCO
3
…

1 Introduction
Calcium carbonate layers within soils and as encrustations on the undersides of rocks and pebbles occur
in many landscape types. It has been postulated that these layers and crusts, when moisture is drawn upwards
evaporates, precipitating carbonate minerals from supersaturated films. Although pedogenic carbonate
has been observed and analyzed from a variety of soils from different latitudes, most of the work has been
done in warm desert and semi-arid environments. Several of investigations, however, have described pedo-
genic carbonates in polar locales where desert conditions exist. Authigenic carbonates in cold climates can
take many forms and are formed by several different processes (Lacelle 2007). Pedogenic carbonates in
cold environments, such as polar regions, have been defined as those that form on the inside of soil clasts
(Lacelle 2007). This particular type of carbonate deposition has been observed in the Canadian High Arctic,
the Antarctic Maritime and Svalbard (Bunting and Christensen 1978; Vogt and Corte 1996; Forman and
Miller 1984; Courty et al. 1994). In all these various types of such carbonates, isotopic analysis has been
used to elucidate the source of C and O and to discern the processes involved in the formation of these
minerals (e.g., Lacelle 2007). In addition, 
87
Sr/
86
Sr analyses have served as a useful tool to define the source
of Ca in several past investigations of pedogenic carbonates (e.g., Quade, Chivas and McCulloch 1995; Capo
and Chadwick 1999; Naiman, Quade and Patchett 2000; Van der Hoven and Quade 2002).
In this paper we present δ
13
C, δ
18
O, and 
87
Sr/
86
Sr data from pedogenic carbonate encrustations from
soils of Taylor Valley, Antarctica, one of the McMurdo Dry Valleys. Taylor Valley (TV) is a polar desert
with low precipitation rates (~3 cm a-
1
) and a mean annual temperature of approximately –20
o
C (Doran
et al. 2002; Fountain et al. 2010). In Taylor Valley low relative humidities generate rapid sublimation of
snow and produce very low soil moisture concentrations thus limiting chemical weathering and dissolu-
tion/precipitation reactions. 
The objectives of this work were to assess the sources of water, carbon and calcium in the formation
of calcium carbonate and to ascertain the processes that produced these minerals. In addition, we com-
pare the formation mechanisms of these Antarctic encrustations to encrustations formed in other desert
environments.
2 Materials and methods
2.1 Study area
The McMurdo Dry Valleys (MCM) (77–78°S) are the most extensive ice-free region in Antarctica (Figure 1).
The valleys are a mosaic of glaciers, soils, exposed bedrock, ephemeral streams and perennially ice-cov-
ered lakes. Since 1993 Taylor Valley (TV) has been the primary operational location of the McMurdo Dry
V alleys Long-T erm Ecological Research (MCM-LTER) site where meteorological, glaciological, hydrological,
geochemical, and ecological research has been conducted regularly (Fountain et al. 1999).
The soils and tills in TV are derived from a number of rock types within the region. These include the
Precambrian to Ordovician granitoid and metamorphic basement rocks, the Devonian aged Beacon
Supergroup (sandstones, shales, conglomerates), and the Ferrar Dolerite (Jurassic age). The Ferrar intrudes
the Beacon and basement rocks in the form of sills and dikes in numerous locations in the McMurdo Dry
Valleys. In addition, the more recent McMurdo Volcanic rocks are present both in-situ as erupted cones,
and also as debris transported within the tills. In Taylor Valley, the volcanics range in age from 1.50 to 3.89
Ma (Wilch et al. 1993). The TV soils are composed of unconsolidated material ranging from primarily
sand to boulder size, although small grain sizes exist. Relatively flat areas are often covered by desert pave-
ment and the active layer is only ~50 cm in depth.
Much work has been done in determining the glacial chronology and soil/till age of the MCM region.
Several eastward advances and retreats of the Taylor Glacier (from the East Antarctic Ice Sheet) deposit-
ed till in the western parts of TV and northward advances of the West Antarctic Ice Sheet has deposited
tills in eastern to central TV during glacial times (Denton et al. 1989; Hall et al. 2000). The lakes in TV
have fluctuated dramatically in size, with valley-wide glacial Lake Washburn existing from approximate-
ly 23,000 to 8500 years ago (Hall, Denton and Hendy 2000) to low stands at ~1kyr ago (Lyons et al. 1998).
There have been numerous fluctuations in lake levels between these times (Hall et al. 2010). As the lake
127
Acta geographica Slovenica, 60-2, 2020

Figure 1: Map of Taylor Valley, McMurdo Dry Valleys, Antarctica with locations of samples. Samples included 1) NE shore of Lake Bonney, basement
dike at elevation 82 m asl; 2) Defile, dolerite, gneiss, granite and marble at elevation 193 m; 3) North shore of Lake Freyxell, kenyite at elevation 37 m;
4) NE shore of Lake Fryxell, dolerite, and granite at elevation 46 m; 5) SW shore of Lake Fryxell, basement dike and dolerite at elevation 57 m; 6) Lake
Fryxell to Explorer’s Cove, dolerite and granite at elevation 83 m; 7) N shore of explorer’s Cove, dolerite at elevation 26 m. Not show on this map is 8)
Marble Point, basement rock at elevation 108 m located approximately 19 km north of location 7.p
level rises, soils are inundated and become lake sediments, and as the lakes decrease in size, the lacustrine
sediments become part of the terrestrial environment. A portion of the organic C currently observed in
the soils is ancient lacustrine organic matter, or what has been termed »legacy« organic C that had been
deposited during earlier aged high stands of the lakes (Burkins et al. 2000). CaCO
3
that forms in the lakes
and in ephemeral streams today is usually associated with algal mats, and it is clear that much of the pre-
sent day CaCO
3
observed in the soils of MCM was actually produced within a lacustrine environment when
lake levels were higher. In fact, lacustrine-derived CaCO
3
in soils has been observed throughout the TV
(Doran et al. 1999; Hendy 2000). The stable isotope composition of current lacustrine carbonate has been
previously measured (Lawrence and Hendy 1989) and can be compared to pedogenic forms of carbonate
in order to ascertain the origin of carbonate minerals in these soils.
2.2 Sample collection, preparation, and protocol
Thirteen groupings of surface »rocks« sitting at the soil surface were collected in locations throughout TV
(Figure 1). Each rock was chosen for the noticeable amounts of carbonate encrustation on its underside
(Figure 2).
Each group of rocks was collected within a one-meter radius area and the individual specimens ranged
from about 1 cm to about 6 cm in diameter. GPS coordinates were taken at each of the 13 sampling loca-
tions and local geological and surrounding landscape features were also recorded. The rocks in each group
were of different lithologies, including the major bedrock types in the area: marble, basement dike, gran-
ite, gneiss (all from the basement complex), dolerite and kenyite (McMurdo Volcanics). On return to the
USA, these rocks were visually identified according to type. Rocks were kept separated throughout the
sampling and analytical process. In the lab, pre-cleaned stainless steel dental tools were used to scrape off
much of the encrusted calcite onto a clean piece of paper. Vinyl gloves were worn during this procedure,
and each time a new rock was scraped, the dental tools and gloves were cleaned with distilled-deionized
water and wiped dry between samples. About 10–100 mg of scraped material was collected and transferred
into new plastic vials.
A portion of this material was analyzed for its strontium isotopic composition (
87
Sr/
86
Sr) and Sr con-
centration at the Radiogenic Isotopes Laboratory at The Ohio State University. Prior to dissolution, the
aliquots were pre-washed using ultrapure ammonium acetate as described by Montañez et al. (1996). Roughly
10–100 mg of the carbonate scrapings were dissolved using high-purity acetic acid, and a spike of 
84
Sr was
added to analyze Sr by isotope dilution. The Sr in the samples was purified using cation exchange chro-
matography following procedures described by Foland and Allen (1991). The Sr isotopic measurements
were made using a multi-collector MAT-261A thermal ionization mass spectrometer using dynamic mul-
tiple ion collection as described in Foland and Allen (1991). The accuracy of these measurements was
determined by the analysis of reference standard, SRM987, for which the value has been determined to
be 0.710242 ± 0.000010 (one sigma external reproducibility). The analytical precision of the total Sr analy-
sis was ± 1%.
Another fraction of the carbonate from each sample was analyzed for its δ
13
C and δ
18
O compositions.
Scrapings were first washed with deionized water using a vacuum filtration assembly and at least 100 µ g
was packaged in clear plastic vials in Columbus and sent to University of Utah, where they were analyzed
as CO
2
on a Finnigan MAT 252 mass spectrometer following reaction with orthophosphoric acid and cryo-
genic purification using an automated Isocarb system (see Swart, Burns and Leder 1991). All data (including
values from the literature) are reported using delta notation relative to the Pee Dee belemnite (PDB) stan-
dard for carbonates and the standard mean ocean water (SMOW) standard for waters. Analytical precision
for both the δ
13
C and δ
18
O was ~0.1‰.
128
Berry Lyons, Kelly Foley, Anne Carey, Melisa Diaz, Gabriel Bowen, Thure Cerling, The isotopic geochemistry of CaCO
3
…

162°
77°45’
77°35’
163°
Glaciers
Lakes
Boundary
Study area
Ross Island
Ross Ice Shelf
170°
168° 166° 164° 162° 160°
McMurdo Sound
0 1 2 3 4 5 6 7 km
180°
85°
80°
75°
70°
0°
90° 270° 77°
78°
Ross Sea
Scale: 1:24.000
Content by: Anne Carey
Map by: Christopher Gardner, Kathleen Welch
Source: McMurdo Dry Valleys Long Term Ecological Research Project
© 2019 School of Earth Science, "e Ohio State University
T a y l o r   G l a c i e r
R h o n e  G l a c i e r
East
Lake Bonney
Lake Hoare
Lake Fryxell
C a n a d a  G l
C o m m o n w e a l t h  G l
West
Lake Bonney
Suess Gl
1
2
3
4
5
6
7
129
Acta geographica Slovenica, 60-2, 2020

Berry Lyons, Kelly Foley, Anne Carey, Melisa Diaz, Gabriel Bowen, Thure Cerling, The isotopic geochemistry of CaCO
3
…
130
Figure 2: Example of rocks with carbonate encrustation at Mount Speed
along the Shackleton Glacier at -84.46567, -177.1357167. M E l IS A D IA Z 3 Results
87
Sr/
86
Sr data for CaCO
3
encrustations (calcite) from TV are presented in Table 1. Previously published
lake and stream 
87
Sr/
86
Sr values within each of the three lake basins are similar in their geographic dis-
tributions as the encrustation data (Lyons et al. 2002; Dowling, Lyons and Welch 2013). In general, the
87
Sr/
86
Sr values for all TV soils and waters increase (i.e., become more radiogenic) with increasing distance
from the Ross Sea coast (Jones and Faure 1968; Lyons et al. 2002; Dowling, Lyons and Welch 2013). There
also is an increase in 
87
Sr/
86
Sr with increasing distance from the coast for all rock types except for the mar-
ble (Lyons et al. 2002). The range of 
87
Sr/
86
Sr values for lakes and streams is on the lower end of 
87
Sr/
86
Sr
values of the various rock sources within the entire MCM regime, perhaps suggesting the relative impor-
tance of both seawater and McMurdo Volcanic sources of Sr to the lakes (Lyons et al. 2002; Dowling, Lyons
and Welch 2013). The range of 
87
Sr/
86
Sr ratios for carbonate encrustations in TV is smaller and is slightly
less radiogenic than the lakes and streams (Table 1). There is no relation between the 
87
Sr/
86
Sr ratios in
carbonate encrustations and the rock type from which they were taken nor between the 
87
Sr/
86
Sr ratios of
the encrustations and elevation.
The δ
13
C and δ
18
O data from the CaCO
3
encrustations and other materials in TV are presented in T able 2.
The δ
13
C and δ
18
O values range from –5.70 to –11.02, and –8.13 to –11.18 (PDB), respectively. In general,
the δ
13
C values are much more enriched than the soil organic matter (Burkins et al. 2000) and the δ
18
O
values are more enriched than the lacustrine carbonate in TV (Hendy et al. 1977; 1979).
4 Discussion
4.1 Source of Ca to the CaCO
3
encrustations based on Sr isotopic analysis
Naiman, Quade and Patchett (2000) and Van der Hoven and Quade (2002) have demonstrated in Arizona
soil, in the desert southwest of the USA, that there are two primary sources of Ca that form pedogenic

Acta geographica Slovenica, 60-2, 2020
131
Table 1: 
87
Sr/
86
Sr ratios for calcite encrustations and soils in TV (*Jones and Faure 1968).
Rock type coated with the CaCO
3
crust
87
Sr/
86
Sr Mean Standard Deviation Sr (ppm)
Marble Point (basement dike) 0.7081 890
Northern shore of Explorers Cove (dolerite) 0.7081 584
Between coast and Lake Fryxell 0.7083 0.00000
Granite 0.7083 431
Basement dolerite 0.7083 827
North east of Lake Fryxell 0.7089 0.00000
Dolerite 0.7089 504
Granite 0.7089 443
North central of Lake Fryxell (kenyite) 0.7090 202
South east of Lake Fryxell 0.7087 0.00010
Basement dike 0.7088 268
Basement dike 0.7087 470
Dolerite 0.7086 429
Defile 0.7101 0.00148
Gneiss 0.7109 1030
Granite 0.7109 497
Marble 0.7079 1740
Dolerite 0.7108 625
Northeast of Lake Bonney (basement dike) 0.7118 327
Soils
Basin of Lake Bonney* 0.7136
Near Lake Fryxell* 0.7089
Near LaCroix Glacier* 0.7125
Near Canada Glacier* 0.7101
Table 2: δ
13
C and δ
18
O for Taylor Valley soil CaCO
3
encrustations. The far-right column reflects the δ
18
O of water in equilibrium with the δ
18
O of the
carbonate. Values are in ‰.
Pee Dee Belemnite δ
18
O Standard Mean Ocean Water
Sample no. δ
13
Cδ
18
0 Coplen et al. 1983 Water
12BD 7.47 –8.56 22.09 –12.3
25A* 5.70 –10.30 20.29 –14.1
25B 8.39 –8.23 22.43 –12.0
25D 9.14 –9.33 21.29 –13.1
46D 8.49 –10.82 19.76 –14.6
46G 8.18 –11.18 19.38 –15.0
59D 7.95 –8.13 22.53 –11.9
59Gn 8.42 –9.37 21.25 –13.2
59Gr 11.02 –10.37 20.22 –14.2
59M 7.24 –9.64 20.97 –13.4
69DB1 7.15 –10.50 20.09 –14.3
69G 7.33 –10.33 20.26 –14.1
75D*1 7.04 –8.54 22.11 –12.3
88B1 6.73 –10.39 20.20 –14.2
99K 7.20 –10.74 19.84 –14.6

132
carbonates: the local parent geologic material and aeolian dust. Much of the Ca in CaCO
3
in TV has been
hypothesized to have originated from either the in-situ weathering of marble, mafic volcanic rock debris
within the tills, and/or dust containing carbonates (Campbell and Claridge 1977; Keys and Williams 1981;
Green, Angle and Chave 1988). The McMurdo Volcanics source of dissolved Sr is important to Lake Fryxell
as it is found only in tills in the Lake Fryxell basin (Lyons et al. 2002). Measurements from the surface of
nearby Canada Glacier yielded a value of 0.70991 (Dowling, Lyons and Welch 2013).
As noted above, there is little relationship between the 
87
Sr/
86
Sr of the rock types and the encrusta-
tions taken from them. The 
87
Sr/
86
Sr values from the CaCO
3
encrustations on the dike samples were less
radiogenic than the dikes themselves. The CaCO
3
encrustations from the dolerites have much lower 
87
Sr/
86
Sr
values than the Ferrar Encrustations on the granites and the dolerite samples were much less radiogenic
than the granites (e.g., Lyons et al. 2002). Similarly, the one 
87
Sr/
86
Sr value from an encrustation on a gneiss
sample was 0.7109, which is also much less radiogenic than the Olympus Granite-Gneiss, which ranges
between 0.7150 and 0.7210. The authigenic carbonate from the kenyite (McMurdo Volcanics) sample had
an 
87
Sr/
86
Sr value more radiogenic than the McMurdo Volcanics themselves (~0.7030 to 0.7045).
Clearly Sr within the encrustations has different Sr isotopic compositions than their »carrier« rocks.
These data strongly suggest that the source of Sr, and by inference Ca, is not from the direct chemical weath-
ering of the in-situ parent rocks but is also at least in part from another source. There is one encrustation
that did not fit this pattern – the one in association with a piece of marble from ~22 km inland (Figure 1)
had an 
87
Sr/
86
Sr ratio of 0.7079. The only reported 
87
Sr/
86
Sr value from the Asgard Marble is 0.7088, but
that sample came from Wright Valley just to the northwest of Marble Point, far from our sample location
(Faure, Jones and Owen 1973). Since the 
87
Sr/
86
Sr value for the marble encrustation (0.7079) is only slight-
ly less radiogenic than the Asgard Marble, it is probable that the Ca in the CaCO
3
encrustation was derived
from the marble itself, thereby overwhelming any contribution from the aerosol.
Extensive chemical weathering occurs within the TV environment, but that weathering has only been
documented in the austral summer where there is abundant liquid water in the fixed stream channels that
drain glacier melt water (Nezat, Lyons and Welch 2001; Gooseff et al. 2002). Because there is no overland
flow and only a few locations where subsurface melt affects the surficial soils (Levy et al. 2012), most of
the soil in TV has little contact with liquid water during the year (Fountain et al. 1999). Snow provides
this water, but snow is rapidly sublimated instead of going through prolonged melting and any accumu-
lated snow remaining from the previous winter disappears by the end of the austral summer (Fountain et
al. 1999).
4.2 Dust flux to MCM and its chemical signature
Aeolian transport significantly redistributes particulate material in the MCM and it has been suggested
that the geometry of TV dictates the direction of net dust flux with transport from higher elevations to
lower elevations down-valley (Lancaster 2002; Šabacká et al. 2012; MacDonell, Fitzsimons, and Moelg 2013).
Mean daily wind speeds are higher up-valley to the west nearer to the Taylor Glacier and East Antarctic
Ice Sheet and decrease near the coast (Fountain et al. 1999; Doran et al. 2002). This results in higher aeo-
lian sediment flux up-valley (Lancaster 2002). Strong katabatic winds (especially in the austral winter) can
transport dust from higher elevations to the lower elevations down valley (i.e., to the east). The aeolian
dust flux in the MCM region has been measured ~1 m above the landscape surface and the values gener-
ally decrease from west to east as described above, ranging from 1.10 to 0.24gm
–2
yr
–1
for Lake Bonney
and Explorers Cove/New Harbor (eastern most portion of TV), respectively (Lancaster 2002). The dust
fluxes in the Lakes Fryxell and Hoare basins are dominated by fine grained (<50 μm) material, while Lake
Bonney basin is primarily sand size particles (>90% by weight). The general up-valley increase in particle
flux and grain size may suggest that the Lake Bonney basin and the Lakes Fryxell-Hoare basin dust inputs
have different chemical compositions. Recent work from collectors placed close to the ground indicates
that the aeolian material being blown in TV is of local origin, different size fractions have different major
element geochemistries, and the finer grained fraction has higher Ca concentrations (Deuerling et al. 2014).
More recent work on aeolian samples collected at higher elevations off the ground (1 m) suggest a more
homogeneous geochemistry (Diaz et al. 2018).
Berry Lyons, Kelly Foley, Anne Carey, Melisa Diaz, Gabriel Bowen, Thure Cerling, The isotopic geochemistry of CaCO
3
…

133
Acta geographica Slovenica, 60-2, 2020
The geologic materials up-wind (i.e., west) of TV are primarily Ferrar Dolerite and the Beacon Supergroup
rocks. The dolerite has 
87
Sr/
86
Sr values of 0.712–0.714, while to our knowledge only the carbonates in the
Beacon rocks have been analyzed, and they are very radiogenic (i.e. >0.725) (see data and references in
Lyons et al. 2002). Samples collected in aeolian collectors on Taylor Glacier in the Bonney basin (west)
and in Explorers Cove (east) yielded 
87
Sr/
86
Sr ratios of 0.71575 and 0.70864, supporting the notion that
the dust is from very local (i.e., individual basin) sources (Diaz 2017) and are generally within the range
of the values for our carbonate encrustations (Table 1).
4.3 Local soil as a source of Sr
As noted above, the Sr isotope ratios of the encrustations are geographically similar to what has been pre-
viously reported for TV stream waters (Lyons et al. 2002; Dowling, Lyons and Welch 2013). The most recent
values for the Fryxell and Hoare basins range 0.70837–0.71084 and 0.70837–0.71091, respectively, while
the Bonney Basin samples are more radiogenic, ranging between 0.71255 and 0.71418 (Dowling, Lyons
and Welch 2013). As previously noted, the few measurements on TV dust yield similar values. The stream
values have a narrower range than the encrustations, but the geographic trends are similar. Dowling, Lyons
and Welch (2013) concluded that in the Fryxell and Hoare basins the dissolution of CaCO
3
must play an
important role in contributing dissolved Sr and Ca to these streams, and that the primary source may be
from pieces of Asgard Marble within the stream beds. They also suggested that the initial Sr
2+
that occurs
in the authigenic carbonate minerals within the soils probably originates from the weathering of a wide
range of sources, including previously existing secondary carbonates and Ca-rich dust.
Much research has clearly demonstrated that calcite within crystalline rocks, even at very low abun-
dances, is the major contributor of dissolved Ca to aquatic systems (e.g., Lyons et al. 2005; Andrews and
Jacobson 2017). As a polar example, it has been shown that the dissolution of fracture-filling calcite from
moraines on Baffin Island is a major source of Sr, and hence Ca, to pedogenic crusts (Lacelle, Lauriol and
Clark 2007). The 
87
Sr/
86
Sr values available for the lithologies in TV are whole rock values. Unfortunately,
we are unable to evaluate whether calcite in the crystalline rocks in the soils and tills of TV is a potential
source of Sr. It should be noted that calcite fillings in volcaniclastic rocks from Minna Bluff, ~150 km south
of our study area, had 
87
Sr/
86
Sr values of 0.70327 (Antibus et al. 2014), suggesting that calcite within at
least the McMurdo Volcanics could be a source of Ca and Sr to these soils.
The MCM soils are primarily glacier deposited tills and lacustrine sediments and, as mentioned above,
consist of materials from all the regional bedrock groupings; they are not derived from one lithology. There
are differences in the amounts of various types of material (e.g. the Ross tills having abundant kenyite)
and their ages, so the soils are a heterogeneous mix of geologic materials (Foley et al. 2006). These dif-
ferences in both lithology and age lead to important differences in the geochemistry of the soils from basin
to basin in TV . These geographical differences superimpose important ecological and biogeochemical dif-
ferences such as landscape-scale differences in N:P ratios (Barrett et al. 2007). Soils also demonstrate an
increase in radiogenic Sr moving inland, with a value of 0.7089 in the Fryxell Basin, increasing to 0.7101
along the transition from the Fryxell to the Hoare Basin, 0.7125 as the eastern reaches of the Bonney Basin,
to 0.7136 within the more western portions of the Bonney Basin (Jones and Faure 1968) which is reflect-
ed in both stream waters and local dusts. 
The general pattern of increasing radiogenic ratios of these carbonate encrustations from east to west
in TV fits with all the available stream, lake, dust and soil data. In general, the carbonate samples are slight-
ly less-radiogenic than their corresponding soils, suggesting that in the Fryxell and Hoare basins the McMurdo
Volcanics may be preferentially weathered within the soils. This idea of selective weathering of the till mate-
rials is not a new one as it has been mentioned as a mechanism to explain the variation of major ion stream
chemistry and the Sr isotopic compositions for the stream waters of the Fryxell Basin (Lyons et al. 2002;
Dowling, Lyons and Welch 2013). In the Bonney basin, the less radiogenic (than the soil) carbonate val-
ues might reflect input from marine aerosol or more intensive weathering of the Ferrar Dolerite. The process
of dissolution and reprecipitation of CaCO
3
within the soil profile may play an important role in maintaining
the Sr isotopic variation constrained to some overall local basin-wide value. All these data support the idea
that this trend of less to more radiogenic values from east to west in TV reflects local, rather than far-afield,
input of Sr.

134
4.4 Sources of Carbon and Oxygen to the CaCO
3
encrustations
The stable isotopic geochemistry of C and O in modern soil carbonate is primarily determined by the local
climate. Other work has clearly demonstrated that δ
18
O values are well correlated with the isotopic com-
position of the local meteoric water and δ
13
C values are related to vegetation type and soil respiration rates
(Cerling 1984; Lipar et al. 2017). At low biological respiration rates in the soil, the influence of atmospheric
CO
2
on the δ
13
C signal is more pronounced. In cold climates several types of carbonate precipitates exist
that are produced through different processes which lead to different isotopic compositions (Lacelle 2007).
The evaporation and transpiration of water and sublimation of snow and ice can lead to loss of pCO
2
that
can also greatly affect both the δ
13
C and δ
18
O signatures of pedogenic carbonates (Marion, Introne, Van
Cleve 1991; Clark and Lauriol 1992). The freezing of liquid water causes supersaturation with respect to
CaCO
3
and the non-equilibrium conditions produced by this process can affect the stable isotopic com-
position of the CaCO
3
produced (Clark and Lauriol 1992; Courty et al. 1994; Lacelle, Lauriol and Clark
2007). Such kinetic fractionation usually leads to enrichment of the isotopes in the carbonate minerals.
The difference in thermal conductivity between the larger clasts, where precipitation occurs, and the finer
grained soil influences this process, as the freezing front moves more rapidly in the larger material. This
type of pedogenic carbonate, formed in the active layer of high latitude soil, is common (Lacelle 2007).
The δ
13
C signature of soil respiratory CO
2
depends in part on the nature and amount of organic mat-
ter present in the soil. In modeling the δ
13
C values of pedogenic carbonates, Cerling (1984) used values of
–13‰ and –27‰ for pure C
4
and pure C
3
plant biomass, but CO
2
from this organic matter source can mix
with atmospheric CO
2
(δ
13
C=–6.5‰) to produce an intermediate value of δ
13
C of soil CO
2
. Measurements
of materials in the sub-Arctic boreal region of Saskatchewan, Canada show a positive correlation between
δ
13
C values of pedogenic carbonates and organic carbon in the soils (Landi, Mermut and Anderson 2003).
Even pedogenic carbonates from interior Alaskan floodplain sediments fall in the range of those of Cerling
(1984), having values as depleted as –7.9‰, clearly demonstrating a biologically influenced signal (Marion
Introne, Van Cleve 1991).
Soil organic matter in TV is extremely low, resulting in great part from past climate and glacial histo-
ries, and has accumulated in soils in a distinct low elevation pattern (<250m above sea level) corresponding
to the spatial distribution of ancient lacustrine systems. The mean δ
13
C values for various organic matter
sources in MCM soils range between –20.8 and –24.3‰ (Burkins, Virginia and Chamberlain 2000). Because
the organic matter concentrations in soil are so low, the CO
2
flux from Antarctic soils is also extremely
low. The CO
2
present in Antarctic soils may originate from atmospheric sources or via the in-situ respi-
ration of organic C (Parsons et al. 2004). CO
2
fluxes in TV soils fluctuate throughout the day and show
very low concentrations of CO
2
escaping from the soil. In the Lake Fryxell and Hoare basins, CO
2
is actu-
ally taken up by soils during rapid decreases in soil temperature (Parsons et al. 2004). Recent work in hot
desert soils clearly shows that similar diel variations driven by inorganic processes play a significant role
in CO
2
dynamics (Ma et al. 2013). It is unknown if these diel variations have influence on CaCO
3
dynam-
ics within the active layer of Antarctic soils.
The δ
13
C values for the TV carbonate encrustations are very enriched with values ranging from +5.7
to +11.0‰ (Table 2). These values contrast greatly with the modern lacustrine carbonates in Lake Fryxell
which range from +3 to –18% depending on water depth (Lawrence and Hendy 1989). Using a CO
2
-CaCO
3
fractionation factor of –14.4 (10
3
lnα) at 0°C (Friedman and O’Neil 1977), the δ
13
C values for a TV pedo-
genic carbonate with a 100% atmospheric CO
2
source would be +7.4‰. Ten out of fifteen of the TV
encrustations are within ±1‰ of this value, suggesting that the primary source of C to these carbonates
is atmospheric CO
2
. Only one sample is more than 1‰ greater than this value. That one sample (from a base-
ment dike specimen from SW Lake Fryxell) may have some carbon derived from CO
2
respired organic
matter or, perhaps, it was formed at a warmer temperature. Ancient lacustrine carbonates obtained in what
are now TV soils show a more depleted, but still positive, set of δ
13
C values (2.4–6.8‰), suggesting a larg-
er percentage of biologically respired CO
2
(Hendy et al. 1979). The lack of a biological source or influence
on the C in these TV CaCO
3
soil encrustations is not a surprise given the low organic carbon present in
the soils and the very low CO
2
fluxes from the soils, as noted above (Burkins et al. 2000; Parsons et al. 2004).
The δ
13
C of the TV encrustations are very similar to what has been previously reported for pedogenic car-
bonates (1.5–9.0‰) in Wright Valley just north of TV (Nakai et al. 1975).
Berry Lyons, Kelly Foley, Anne Carey, Melisa Diaz, Gabriel Bowen, Thure Cerling, The isotopic geochemistry of CaCO
3
…

135
Acta geographica Slovenica, 60-2, 2020
Previous work on authigenic carbonates in Antarctica reflects the influence of the δ
18
O of the water
present. Aragonite that was deposited subglacially on gneissic bedrock in the Vestfold Hills, Antarctica
(68°30’ S) has very depleted δ
18
O values ranging from –17.3 to –14.1‰ (Aharon 1988). Lacustrine car-
bonates deposited at higher lake stands in TV are also depleted with δ
18
O values of –32 to –46‰ (Hendy
et al. 1977; 1979). In both cases, these δ
18
O values reflect the extremely depleted values associated with
the local meteoric waters derived from glacier melt (Gooseff et al. 2006).
These TV encrustations have δ
18
O values (Table 2) more similar to the Type 3 evaporite calcite crusts
for other polar regions reviewed by Lacelle (2007) than the subglacial and lacustrine-derived carbonates
from the Antarctic. There is no reflection in the TV carbonates of the very depleted δ
18
O values for the
snow/ice melt waters observed in the TV region, which range from –27 to –40.4‰ (Gooseff et al. 2006).
Our δ
18
O values from the encrustations when converted to water at 0°C in equilibrium with the carbon-
ate produce values of –11.9 to –15.0‰ (Table 2). Given this enrichment with respect to the TV meteoric
waters, the data suggest strong evaporative loss of 
16
O prior to carbonate precipitate. Evaporation of thin
films and water droplets has also been used to explain enriched δ
18
O in carbonate encrustations in warm
deserts (Knauth, Brilli and Klonowski 2002; Quade et al. 2007). These high δ
18
O values in the TV encrus-
tations are undoubtedly due to precipitation during extensive evaporation and light isotope loss.
4.5 Formation of Taylor Valley pedogenic carbonate
We envision the following scenario for carbonate mineral encrustation formation in polar desert systems
like the McMurdo Dry Valleys. During snowfall events small amounts of Ca are solubilized from the Ca-
containing minerals and rocks (probably CaCO
3
) in the soils, and/or mineral dusts and/or desiccated marine
aerosols. Given that snowfall deposition is minimal (≤ 3 cm a-
1
), and relative humidity is extremely low,
sublimation and evaporation rates are high. During the evaporative process calcite becomes supersatu-
rated in the remaining water film and is precipitated with CO
2
as the primary carbonate source as indicated
by the δ
13
C values. These Taylor V alley carbonate encrustations form in an extremely arid setting and demon-
strate that carbonate mineral formation in soils can occur with little to no biological processes being involved.
This lack of biological process affecting δ
13
C compositions of ΣCO
2
is also observed in TV streams (Lyons
et al. 2013). These Antarctic carbonate encrustations form an extreme position in δ
13
C and δ
18
O when
compared to other cold environment authigenic carbonates and a rapid rate of precipitation leading to non-
equilibrium conditions due to a rapid loss of CO
2
as outlined in Lacelle (2007).
5 Conclusions
Except for the carbonate crust on a marble sample, which obtained its Sr (Ca) from weathering of the par-
ent marble, it is hypothesized that the carbonate encrustations on all the other TV rock samples receive
their Sr (and by inference their Ca) from surface soil sources including potential locally derived aeolian
debris. The encrustations are less radiogenic than the soils from their respective lake basins indicating that
selective dissolution of volcanic materials and/or marine aerosol may play an important role in their for-
mation. The δ
13
C values of the rock encrustations indicate they are produced in place with atmospheric
CO
2
as the primary carbon source. Our results suggest that the δ
13
C signature can be used to ascertain
the difference between pedogenic and lacustrine carbonate in MCM soils. The δ
18
O values of the CaCO
3
encrustations do not directly reflect the very depleted values of the local melt water source. The δ
18
O val-
ues for the CaCO
3
encrustations suggest extensive evaporation has led to their formation, similar to deposits
from warm deserts, as well as other active layer and polar soil environments. The δ
18
O data indicate that
evaporation/sublimation of water, perhaps in thin films, plays a major role in the production of these encrus-
tations.
ACKNOWLEDGEMENT: The fieldwork was supported through NSF Grants OPP-9813061 and ANT-
0423595. We thank Kate Harris, Nevada Hanners, and A. J. (Rabbit) Kaltenback for their help in sample
collection. We are extremely grateful to Ken Foland and Jeff Linder for the Sr isotope analyses. We thank
Kathy Welch, Chris Gardner, Sarah Fortner and Michele Cook for their editing of the initial draft of the
manuscript. We also thank Chris Gardner for creating Figure 1 for this paper. The first draft of this paper

136
was completed while the senior author was supported by a Royal Society Travel Grant; he thanks Prof. J.
Laybourn-Parry and the School of Physical Sciences and Geography at Keele University for their hospitality
during that time. Finally, the senior author greatly appreciates being asked to contribute to and to co-edit
this special addition of Acta geographica Slovenica. Thanks to Matija Zorn and Blaž Komac!
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