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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D16301, doi:10.1029/2005JD006669, 2006
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Quantifying sulfate components and their variations
in soils of the McMurdo Dry Valleys, Antarctica
Huiming Bao1 and David R. Marchant2
Received 14 September 2005; revised 9 February 2006; accepted 8 May 2006; published 16 August 2006.
[1] Many soils of the McMurdo Dry Valleys (MDV), Antarctica, being old, hyperarid,
and frigid, have accumulated abundant atmospheric salts over the last several million
years. This salt repository offers an opportunity to study atmospheric chemistry (past and
present), the origin and transport of ions in soils, the weathering activity of soils, and
postdepositional soil-leaching processes within this unique environment. In particular, soil
sulfate in the MDV is known to have multiple origins, but the precise proportions of
different sulfate components remain elusive. Here we test a hypothesis that soil sulfate in
the MDV is a predictable mixture of three major components: sea-salt sulfate, non-sea-salt
sulfate, and background sulfate (derived from weathering and volcanic sources) that to
a large extent, varies as a function of elevation and distance from the coast. By measuring
sulfate’s three stable isotope parameters, i.e., D17O, d18O, and d34S, plus estimating
independently the end-member stable isotope parameters for the three components on the
basis of published reports and our measured data, we solve explicitly the mixing
proportion for each component using a set of three simultaneous linear equations. The
results from four spatially representative soil profiles show that the sum of such calculated
proportions for the three components is very close to unity in most samples, suggesting
that a simple three-component mixing model is a good representation of the soil
sulfate budget. While significant uncertainties still exist in the isotope compositions of
different sulfate end-members, these data provide important initial constraints on sulfate
diversity as well as its spatial and vertical distribution in the MDV.
Citation: Bao, H., and D. R. Marchant (2006), Quantifying sulfate components and their variations in soils of the McMurdo Dry
Valleys, Antarctica, J. Geophys. Res., 111, D16301, doi:10.1029/2005JD006669.
1. Introduction
1.1. Overview
[2] Salts in Antarctica’s hyperarid McMurdo Dry Valleys
(MDV) are pervasive. In soils, they have been shown to
vary systematically in composition and concentration
[Bockheim, 1997, 2002; Campbell and Claridge, 1987].
Given the antiquity and stability of most soils in the
region [Denton et al., 1993; Marchant and Denton, 1996;
Marchant et al., 1996, 2002, 1993b; Sugden et al., 1995],
we postulate that salt accumulation over millions of years
has produced a unique, long-term record of atmospheric
aerosol deposition. We view soils in the MDV as extensive
and old ‘‘aerosol impactors’’ that have accumulated a
measurable quantity of atmospheric particles that can be
subjected to rigorous quantitative analyses. Here the composition and quantity of salts accumulated depend not only
on geographic location but also on soil age. The soil salt
data do not have the time resolution of ice core data.
1
Department of Geology and Geophysics, Louisiana State University,
Baton Rouge, Louisiana, USA.
2
Department of Earth Sciences, Boston University, Boston, Massachusetts, USA.
Copyright 2006 by the American Geophysical Union.
0148-0227/06/2005JD006669$09.00
Instead, most soil profiles have experienced atmospheric
deposition and vertical ion migration (leaching) over
millions of years. The soils (formed in till or otherwise)
do not vary in age with depth, are too dry to enable physical
reworking due to cryoturbation [Marchant et al., 2002] and
have not experienced significant postdepositional erosion
[Schafer et al., 1999; Summerfield et al., 1999]. Hence the
soils in the MDV offer information on the cumulative effects
of salt influx and subsequent ion migration measured over
million-year timescales. Given that the soils studied come
from different geographic locations, we can also test models
regarding the spatial distribution of salts in the MDV.
Ultimately, the MDV soils also offer a vast amount of
atmospheric salts valuable for studying atmospheric chemical processes in Antarctica.
[3] Pioneering studies on salt composition and concentration for soils in the MDV were conducted more than
thirty years ago [Bockheim, 1979, 1982, 1983, 2002;
Campbell and Claridge, 1968, 1975, 1978, 1981, 1982;
Claridge and Campbell, 1968a, 1968b, 1968c, 1974,
1977; Linkletter, 1972; Mueller, 1968; Nakai et al.,
1975; Nishiyama, 1979; Tedrow and Ugolini, 1966; Torii
et al., 1979; Wada et al., 1981]. These studies focused on
soil description, classification, ion concentration, and
migration, but lacked multiple stable isotope data for
major anions. In the 2002– 2003 Antarctic field season,
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BAO AND MARCHANT: SULFATE COMPONENTS IN ANTARCTIC SOILS
a team of us from Louisiana State University (LSU) and
Boston University collected !30 soil profiles across the
MDV. This campaign differs from the previous studies in
that we employ centimeter-scale, high-resolution vertical
sampling, measure multiple stable isotope compositions
for major anions, including newly discovered parameters
such as 17O anomaly (D17O) (see below), analyze welldated tills in the context of changes in local and global
climate, and highlight atmospheric chemistry and transport as the underlying mechanisms for the origin and
distribution of various salts in the MDV.
1.2. Soil Sulfate in the MDV
[4] The origin of sulfate in the MDV is debated; proposed
origins include wind-blown sea salt [Keys and Williams,
1981; Torii et al., 1979], chemical weathering [Linkletter,
1972], marine incursion [Nakai et al., 1975], hydrothermal
processes [Takamatsu et al., 1993], and sulfate from oxidation
of biogenic sulfur in the atmosphere [Campbell and Claridge,
1987]. Recently, simultaneous measurements of d18O and
d17O values of sulfate (defined as (Rsample/Rreference " 1) #
1000%, in which R = 18O/16O and 17O/16O, respectively)
from a variety of soils revealed that all sulfates in the MDV
have positive but variable 17O anomalies [Bao et al., 2000a].
The 17O anomaly, or the value of D17O, is defined as D17O =
d17O " 1000 # [(1 + d18O/1000) 0.52 " 1] [Bao et al., 2000b;
Farquhar et al., 1999]. It is found that sulfate formed from the
aqueous oxidation of sulfur gases by O3 or H2O2 possesses an
17
O anomaly. The anomaly has been shown to be transferred
ultimately from O3 [Savarino et al., 2000]. Atmospheric O3
has a D17O value ranging from +30% to +40% [Johnson et
al., 2000; Krankowsky et al., 2000; Lyons, 2001]. The precise
mechanism that contributes to the unique O3 isotopic
signatures has been elusive; and recently a treatment on
the basis of non-RRKM processes has been proposed
[Gao and Marcus, 2001]. Other oxidation pathways, such
as oxidation by air O2 at high temperature or at particle
surfaces via metal catalysts, or by $OH in gas phase
oxidation, do not produce an 17O anomaly in the product
[Savarino et al., 2000].
SO2"
4
[5] The D17O parameter distinguishes a sulfate component that is derived from the oxidation of reduced
sulfur gases in the atmosphere. In coastal Antarctica, this
17
O-anomalous sulfate component is predominantly of
dimethylsulfide (DMS) origin. Monitoring studies of atmospheric chemical composition in coastal Antarctica demonstrate that the non-sea-salt (NSS) sulfate aerosol body is
dominated by photochemically generated sulfate, mostly
derived from the oxidation of DMS emitted biologically
from the adjacent oceans [Davis et al., 1998; Gras, 1993a;
Minikin et al., 1998; Savoie et al., 1992]. Sulfate derived
from the atmospheric oxidation of volcanic gases or sulfate
transported from midlatitudes/the stratosphere can also
bear 17O anomalies. These additional sources of sulfate,
however, are believed to be of minor volumetric importance
in Antarctica [Chuan et al., 1986; Gras, 1993b; Hogan et
al., 1982; Minikin et al., 1998; Rose et al., 1985]. Therefore,
in this paper, NSS sulfate refers only to the sulfate component that has a positive D17O value.
[6] Among the sulfates that do not possess an 17O
anomaly (i.e., D17O ! 0%) in MDV soils, there exist at
least two major components. One likely widespread com-
D16301
ponent is sea-salt (SS) sulfate, being transported and deposited as wind-blown sea-salt particles. Another probably
less ubiquitous component is the sulfate that is derived from
weathering processes or volcanic sources (i.e., ash leachates); we label this third component as background (BG)
sulfate. It should be noted that BG sulfate does not include
those derived from the oxidation of volcanic SO2 in the
atmosphere. The overall contribution of sulfate from the
atmospheric oxidation of local volcanic SO2 should be low
relative to DMS source, since the measured emission rates
for sulfur from Mt. Erebus are not very high compared to
other volcanoes [Zreda-Gostynska et al., 1997]. The BG
sulfate likely covaries with soil development. It could be a
significant component in soils of relatively warm coastal
sites, or in soils with a fine-grained, dark volcanic matrix,
but may be minimal in the little weathered and hyperarid
soils of interior regions in the MDV [Bockheim, 2002;
Campbell and Claridge, 1987].
1.3. Goal and Strategy of This Study
[7] The goal of this study is to characterize, delineate, and
quantify the three major sulfate components in terms of
their spatial distribution within the MDV and their vertical
distribution within a given soil profile. The results have
broad implications. First, the delineation of individual
sulfate components and the characterization of their stable
isotope compositions can help to understand their origins
and underlying physical, chemical, and biological mechanisms; second, the patterns of atmospheric transport and
deposition in the MDV can be uncovered for the two airborne sulfate components: NSS and SS sulfates; third, the
proportion of the BG sulfate component can be used to
quantify and compare the degree of weathering in soils in
the hyperarid cold desert; and fourth, a high-resolution
vertical data set can reveal detailed leaching dynamics in
soil profiles in the Mars-like environment.
[8] To achieve the goal, we first assume that a soil sulfate
sample from a given location or depth is a simple mixture of
three sulfate components: NSS, SS, and BG sulfates. As for
any three-component mixing system, the proportion of each
component in the mixture can be explicitly calculated if
the system is satisfied with the following two conditions:
(1) three isotope parameters shared by each sulfate component can be identified; and (2) the end-member parameters
for each sulfate component are sufficiently different to
provide independent constraints on their mixing ratios.
[9] The first condition is satisfied because in addition to
the parameter D17O, we measure the d18O and d34S, i.e., two
additional isotope compositions for the same sulfate component. The d34S is defined as (Rsample/Rreference " 1) #
1000%, in which R = 34S/32S. For the second condition,
end-member stable isotope compositions for SS and BG
sulfates will be defined using published data with variable
uncertainties. The end-member parameters for NSS sulfate
is not available in literature, but can be deduced from the
measured data in the MDV. The error for the calculated
proportions depends on the differences among end-member
isotope parameters of the three sulfate components. If
the differences are large, accurate proportioning among
the three sulfate components can be achieved. Finally, the
sum of the three proportions should be close to unity if the
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BAO AND MARCHANT: SULFATE COMPONENTS IN ANTARCTIC SOILS
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Table 1. Glacial Geology Context of the Four Soil Profiles Reported in This Paper
Soil
MB07
MB13
MB53
MB11
Unit
a
Loop Moraine
b
Taylor glacier moraine
Sessrumnir tillc
Asgard tillc
Age
Depositional Setting
%3.7 Ma
maximum Pliocene (?) expansion of grounded glacier ice
from west Antarctica into central Dry Valleys region
lateral moraine from expanded East Antarctic Ice Sheet
basal till from alpine glaciers
basal till from expanded East Antarctic Ice Sheet
!2.7 Ma
>15 Ma
>14.8 Ma
a
Stuiver et al. [1981].
Wilch et al. [1993].
Marchant et al. [1993a].
b
c
simple mixing model is a good representative of the reality.
If not, our initial assumption needs to be modified.
2. Field Sampling
[10] A set of diverse soil profiles with a high vertical
sampling resolution is required to determine the origin of
sulfate in the MDV (Table 1). In our field campaign we
excavated soil pits to a depth of !1.0 m, or to the top of icecemented soil if such was encountered. In terms of texture,
the soil profiles contained a mixture of unsorted pebbles,
fine sands, and silt-sized grains. We sampled each soil
profile from the bottom up in order to minimize sample
contamination. This explains the seemingly reversed sample
order in that the first sample in a vertical sequence (e.g.,
MB07-1) is always from the deepest section of the soil
excavation (Table 2). Samples were taken every 2 to 10 cm,
with each sample covering a depth of !1 to 3 cm. Given
that these dry soils are noncohesive, this is the highest
vertical sampling resolution physically attainable.
[11] The four soil profiles reported in this study, MB07,
MB13, MB53, and MB11, are all developed on exposed
(i.e., not buried) basal tills (Figure 1). Their descriptions in
the context of glacial geology are provided in Table 1.
MB53 and MB11 are located furthest from the coast and are
much older in age than MB07 and MB13 (Table 1). Each of
the four profiles represents type soils for their respective
geographic region.
3. Analytical Methods
[12] Two aspects of the analytical approaches are new in
this study: (1) a wet chemistry protocol was tested and
implemented to ensure that >99% of the water-soluble salts
were extracted from the salt-rich soils for the measurements
of ion concentration and isotope compositions, and (2) a
newly developed cleanup procedure [Bao, 2006] was used
to remove occluded nitrate from precipitated barite (BaSO4)
for multiple oxygen isotope analyses.
3.1. Wet Chemistry
3.1.1. Measurement of Water-Soluble Ion
Concentration Using IC
[13] Approximately 300 to 500 mg samples (<2 mm
fraction) were mixed with double-deionized water in an
!100:1 water to soil ratio. The mixtures were shaken
rigorously to induce the dissolution of water soluble salts.
Supernatants were collected from three repeated extractions
per sample. We noticed that gypsum is often not completely
extracted when repeated 5:1 water-to-soil mixtures were
applied. The problem of incomplete salt extraction for
extremely salt-rich soils was also noted by previous authors
[Berger and Cooke, 1997]. We established that after the third
100:1 water-to-soil extraction, >99% of the sulfate was
extracted into solution from various soils in the MDV. It
should be noted that previous water-soluble ion concentration
data for soils in the MDV [e.g., Bockheim, 1997; Gibson et al.,
1983] adopted a conventional 5:1 water-to-soil extraction
method. It is likely therefore that their concentration data may
underestimate the total water-soluble ion concentrations in
salt-rich horizons, especially those rich in gypsum. After
filtration (through 0.22 mm) of the collected supernatant, total
water-soluble anion and cation concentrations were measured
on an Ion Chromatograph (IC) system (Dionex ICS-90). The
following ion concentrations are measured for each soil
"
+
2+
2+
sample: [Cl"], [SO2"
4 ], [NO 3 ], [Na ], [Ca ], [Mg ],
[K+], and [NH+4 ]. The IC measurement error is !5% of the
reported values, although significant larger differences may
be encountered for replicate analyses (whole process replicates). The reported concentration data are the average
values of two or three whole process replicate analyses for
+
a single sample. Here we report only [SO2"
4 ] and [Na ] in
weight percentage (wt %), the two ions that are pertinent to
the theme of this paper.
3.1.2. Sulfate Extraction From Soils
[14] After ion concentrations are determined by IC using
a small aliquot of a soil sample, a larger aliquot (0.2 to
120 g depending on sulfate concentration) of the same soil
(unsieved) is soaked in double-deionized water. Because of
the large sample size, the extraction procedure is slightly
different from that for IC measurement. We checked a
variety of soils in the MDV and found that in order to
extract >99% of the water-soluble sulfate from bulk soil
samples we had to repeat soil-water extractions (in this case
5:1 water-to-soil mixture) six to seven times. Solutions from
all the repeated extractions are collected and evaporated
down to a small volume (!50 ml). After filtration (0.22 mm)
and acidification, droplets of saturated BaCl2 are added to
the solution to precipitate sulfate as barite.
[15] The precipitated barite can have a high content (up to
10% weight) of occluded nitrate due to high nitrate content
in some soils in the MDV. Barite was therefore further
purified by a DTPA (a chelating agent) – dissolution – and–
reprecipitation (DDARP) method in which barite was dissolved and reprecipitated twice [Bao, 2006]. The DDARP
treatment is essential in that nitrate in the MDV has
extremely high D17O and d18O values [Bao and Marchant,
2004; Michalski et al., 2005], and a trace amount of
occluded nitrate may impair the true sulfate values. This
report also constitutes an effort to reexamine previously
3 of 13
BAO AND MARCHANT: SULFATE COMPONENTS IN ANTARCTIC SOILS
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Table 2. Measured Ion Concentrations ([Na] and [SO4]) and Multiple Oxygen and Sulfur Isotope Compositions (D17O, d18O, and d34S)
as Well as Calculated Mole Fraction of Three Sulfate Components (mNSS, mSS, and mBG) and Total Mole Fraction (Mtotal) for WaterSoluble Sulfate in Individual Soil Horizons for Four Different Soil Profiles in the MDVa
Sample
Depth, cm
[SO4], wt %
[Na], wt %
D17O
d18O
d34S
mNSS
mSS
mBG
Mtotal
18.1
18.1
18.4
17.9
18.0
17.7
17.4
17.6
17.4
17.2
17.0
16.7
15.9
15.9
16.4
15.9
15.9
0.39
0.26
0.31
0.33
0.30
0.41
0.48
0.39
0.42
0.42
0.40
0.38
0.29
0.28
0.23
0.31
0.26
0.61
0.66
0.65
0.62
0.64
0.58
0.53
0.58
0.56
0.55
0.55
0.52
0.52
0.52
0.57
0.52
0.52
"0.01
0.07
0.06
0.05
0.06
0.02
0.01
0.04
0.02
0.02
0.05
0.12
0.19
0.21
0.23
0.14
0.26
0.99
1.00
1.02
1.00
1.00
1.01
1.02
1.01
1.00
0.99
0.99
1.03
1.00
1.01
1.03
0.97
1.04
1.01 ± 0.02
MB02-07-17
MB02-07-16
MB02-07-15
MB02-07-14
MB02-07-13
MB02-07-12
MB02-07-11
MB02-07-10
MB02-07-9
MB02-07-8
MB02-07-7
MB02-07-6
MB02-07-5
MB02-07-4
MB02-07-3
MB02-07-2
MB02-07-1
Average
0
4
7
9
12
18
23
27
30
34
38
48
57
64
71
80
90
0.584%
1.026%
0.484%
0.798%
0.476%
0.161%
0.110%
0.069%
0.065%
0.057%
0.045%
0.027%
0.025%
0.038%
0.019%
0.025%
0.034%
0.276%
0.483%
0.479%
1.533%
3.193%
2.139%
0.823%
0.791%
0.660%
0.606%
0.534%
0.338%
0.204%
0.166%
0.180%
0.123%
0.174%
Soil Profile MB07
0.98
0.0
0.65
"1.7
0.77
0.4
0.83
0.0
0.76
0.3
1.03
"1.2
1.21
"2.6
0.98
"1.3
1.04
"1.5
1.04
"1.6
0.99
"1.8
0.96
"3.4
0.72
"3.1
0.70
"3.4
0.58
"2.6
0.77
"2.4
0.64
"4.0
MB02-13-22
MB02-13-21
MB02-13-20
MB02-13-19
MB02-13-18
MB02-13-17
MB02-13-16
MB02-13-15
MB02-13-14
MB02-13-13
MB02-13-12
MB02-13-11
MB02-13-10
MB02-13-9
MB02-13-8
MB02-13-7
MB02-13-6
MB02-13-5
MB02-13-4
MB02-13-3
MB02-13-2
MB02-13-1
Average
0
3
5
8
11
16
25
30
34
40
46
55
58
64
72
77
80
88
94
104
118
126
1.409%
2.277%
1.930%
0.187%
0.334%
0.336%
0.023%
0.011%
0.017%
0.083%
0.064%
0.033%
0.043%
0.024%
0.027%
0.011%
0.010%
0.007%
0.011%
0.010%
0.005%
0.005%
0.398%
0.388%
0.466%
0.462%
0.395%
0.342%
0.198%
0.226%
0.279%
0.185%
0.270%
0.194%
0.196%
0.167%
0.138%
0.129%
0.142%
0.149%
0.134%
0.151%
0.120%
0.102%
Soil Profile MB13
0.49
"2.4
0.57
"3.1
0.53
"3.7
0.76
"1.4
0.63
"2.8
0.69
"1.6
0.68
"2.0
0.51
"4.7
0.50
"4.0
0.60
"4.8
0.63
"2.7
0.65
"3.9
0.63
"3.0
0.65
"3.8
0.70
"2.8
n. a.
"4.0
0.61
"3.2
0.65
"4.0
0.70
"2.0
0.64
"4.4
0.65
"5.5
0.55
"6.2
17.6
17.3
17.0
17.1
17.1
17.6
17.6
15.8
15.7
16.6
16.7
16.5
16.7
16.3
16.8
16.0
16.6
16.3
17.4
16.5
16.4
16.3
0.20
0.23
0.21
0.30
0.25
0.28
0.27
0.20
0.20
0.24
0.25
0.26
0.25
0.26
0.28
n. a.
0.24
0.26
0.28
0.26
0.26
0.22
0.63
0.60
0.59
0.58
0.59
0.61
0.61
0.53
0.53
0.55
0.57
0.55
0.57
0.54
0.57
n. a.
0.57
0.54
0.60
0.54
0.53
0.53
0.28
0.27
0.31
0.12
0.23
0.17
0.19
0.34
0.31
0.32
0.22
0.26
0.23
0.25
0.20
n. a.
0.25
0.26
0.18
0.29
0.33
0.40
1.10
1.10
1.11
1.01
1.07
1.05
1.07
1.07
1.04
1.11
1.04
1.07
1.05
1.05
1.04
n. a.
1.06
1.06
1.05
1.09
1.12
1.15
1.07 ± 0.03
MB02-53-14
MB02-53-13
MB02-53-12
MB02-53-11
MB02-53-10
MB02-53-9
MB02-53-8
MB02-53-7
MB02-53-6
MB02-53-5
MB02-53-4
MB02-53-3
MB02-53-2
Average
0
2
4
6
8
10
12
14
16
18
23
35
40
0.065%
11.606%
8.454%
10.960%
5.383%
2.326%
1.081%
1.000%
1.704%
0.637%
0.355%
0.027%
0.027%
0.007%
0.013%
0.010%
0.052%
0.093%
0.115%
0.110%
0.099%
0.090%
0.366%
0.279%
0.225%
0.179%
Soil Profile MB53
1.59
"8.5
1.47
"7.8
1.63
"10.1
1.63
"10.7
1.65
"10.7
1.82
"10.7
1.86
"9.6
1.84
"10.1
1.89
"12.0
2.21
"13.1
1.88
"12.5
1.81
"11.4
2.01
"10.5
16.4
16.8
16.5
15.9
15.8
15.9
16.3
16.4
15.9
15.4
15.7
15.2
16.1
0.64
0.59
0.65
0.65
0.66
0.73
0.74
0.74
0.76
0.88
0.75
0.72
0.80
0.38
0.41
0.36
0.33
0.32
0.30
0.32
0.33
0.28
0.21
0.27
0.27
0.29
0.10
0.13
0.16
0.18
0.17
0.10
0.05
0.08
0.14
0.05
0.16
0.12
0.03
1.11
1.13
1.17
1.16
1.15
1.13
1.12
1.14
1.17
1.14
1.18
1.12
1.12
1.14 ± 0.02
MB02-11-21
MB02-11-20
MB02-11-19
MB02-11-18
MB02-11-17
MB02-11-16
MB02-11-15
MB02-11-14
MB02-11-13
0
3
5
8
12
16
20
26
30
0.052%
1.078%
0.336%
0.973%
0.108%
0.183%
0.047%
0.071%
0.010%
0.088%
0.113%
0.126%
0.466%
0.188%
0.175%
0.081%
0.064%
0.053%
Soil Profile MB11
1.90
"8.5
1.59
"7.5
1.59
"7.4
1.54
"9.6
1.78
"10.2
1.89
"11.1
1.93
"10.6
1.95
"10.8
1.69
"10.2
15.5
15.9
15.6
14.3
13.8
14.4
14.3
14.0
14.5
0.76
0.64
0.64
0.62
0.71
0.76
0.77
0.78
0.68
0.30
0.36
0.35
0.28
0.23
0.23
0.22
0.21
0.27
"0.03
0.05
0.04
0.13
0.05
0.06
0.02
0.02
0.10
4 of 13
1.02
1.05
1.03
1.03
0.99
1.05
1.02
1.01
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BAO AND MARCHANT: SULFATE COMPONENTS IN ANTARCTIC SOILS
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Table 2. (continued)
Sample
Depth, cm
[SO4], wt %
[Na], wt %
D17O
d18O
d34S
mNSS
mSS
mBG
Mtotal
MB02-11-12
MB02-11-11
MB02-11-10
MB02-11-9
MB02-11-8
MB02-11-7
MB02-11-6
MB02-11-5
MB02-11-4
MB02-11-3
MB02-11-2
MB02-11-1
Average
36
42
45
50
54
62
70
76
84
93
104
105
0.009%
0.016%
0.020%
0.098%
0.006%
0.011%
0.006%
0.005%
0.021%
0.004%
0.006%
0.021%
0.046%
0.054%
0.055%
0.059%
0.057%
0.042%
0.056%
0.044%
0.051%
0.043%
0.048%
0.050%
2.12
2.10
2.05
1.96
2.08
2.04
1.96
1.87
1.41
1.54
n. a.
1.54
"12.1
"11.3
"12.1
"10.4
"10.9
"13.8
"12.9
"13.0
"12.6
"12.4
n. a.
"14.7
13.6
13.8
13.4
14.0
12.3
13.0
11.9
12.5
11.8
11.5
11.9
11.5
0.85
0.84
0.82
0.78
0.83
0.82
0.78
0.75
0.56
0.62
n. a.
0.62
0.15
0.17
0.16
0.21
0.12
0.12
0.10
0.13
0.17
0.14
n. a.
0.12
0.00
"0.02
0.03
0.00
"0.06
0.10
0.07
0.12
0.26
0.20
n. a.
0.30
1.01
0.99
1.00
0.99
0.89
1.04
0.95
1.00
1.00
0.96
n. a.
1.04
1.00 ± 0.04
a
The calculation assumes a simple three-component mixing model; n.a. indicates not available due to small sample sizes.
published sulfate D17O data [Bao et al., 2000a] that were
potentially nitrate contaminated.
3.2. Stable Isotope Analyses
3.2.1. Sulfate #17O Measurement Using a CO2 Laser
Fluorination System
[16] Molecular O2 is generated nonquantitatively (!30%
O 2 yield), together with SOx F y gaseous compounds,
through a CO2 laser heating on a powdery barite sample
in a BrF5 atmosphere [Bao and Thiemens, 2000]. After the
gas mixture goes through several liquid N2 traps, pure O2 is
collected in a sieved sample tube submerged in liquid N2
and is analyzed on a Finnigan MAT 253 in a dual-inlet
mode. The d18O value can have a large error due to
nonquantitative yield. The D17O for barite samples is in
general, however, within 0.08% among replicate analyses,
due to covariation between the d18O and the d17O. The
analytical error (1s) for the D17O for our reference barites
(LSU-BaSO4 and LSU-BaSO4-17) is less than 0.05%. Our
O2 reference gas LSU-O2 was independently calibrated
against UWG-2, a garnet sample from John Valley’s laboratory at University of Wisconsin through laser fluorination.
Subsequent measurement of LSU-O2 at Mark Thiemens’
laboratory at University of California San Diego against its
O2 reference gas yielded the same d18O and d17O values
within analytical errors.
3.2.2. Sulfate D18O Measurement Using TCEA
[17] Approximately 200 mg of BaSO4 were loaded into a
silver capsule. The capsule is dropped through a zero-blank
autosampler (Costech) into a graphite oven heated to
1450!C. Carbon monoxide (CO) and other gases are produced and are carried by Helium gas at a flow rate of
85 ml/min. and subsequently separated by a gas chromatographer (GC) at 85!C. The d18O of sulfate or nitrate is
measured against reference gas through a conflo-III interface on a Finnigan MAT253. The d18O of samples is
calibrated against NSB127 with an assigned d18O value of
+9.3%. This high-temperature conversion plus elemental
analyzer (TCEA) approach achieves ±0.5% in standard
deviation for d18O value among replicate analyses of
reference barite. The reported d18O values are the average
of two or three replicate analyses for a single sample.
3.2.3. Sulfate D34S Measurement Using
Combustion-Reduction EA
[18] Approximately 0.2 mg BaSO4 is packed with !2 mg
V2O5 in a tin capsule and is combusted (reduction/oxida-
tion) at !1050!C in a quartz tube filled with oxygen buffer
(Cu + CuO or quartz chips). SO2 together with other gases
are generated and are carried by Helium gas through a GC.
The d34S value is measured against a reference SO2 through
a continuous flow inlet system and is calibrated against
NBS127 with an assigned d34S value of +21.3%. The
reported d34S values are the average of two replicate
analyses for a single sample, with an analytical error of
±0.3%. The d34S measurement was conducted at A. J.
Kaufman’s laboratory at the University of Maryland. All
other analytical works were conducted at LSU Oxy-Anion
Stable Isotope Center.
4. Results
[19] Table 2 shows tabulated data measured for the four
soil profiles.
Figure 1. Location and feature of four soil profiles in the
McMurdo Dry Valley, Antarctica.
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Table 3. Weighted Average Sulfate Stable Isotope Compositions for Four Individual Soil Profiles in the MDVa
Soil Profile
Distance to the coast, km
Estimated elevation (±20 m)
D17O-weighted (±0.08%)
d18O-weighted (±0.5%)
d34S-weighted (±0.3%)
MB07
MB13
MB53
MB11
32.5
290
0.83 (N = 17)
"0.9 (N = 17)
17.8 (N = 17)
35.0
880
0.60 (N = 21)
"2.8 (N = 22)
17.2 (N = 22)
63.8
1520
1.66 (N = 14)
"10.0 (N = 14)
16.2 (N = 14)
70.0
1490
1.71 (N = 20)
"9.3 (N = 20)
14.7 (N = 21)
a
N is the number of samples measured in a profile; ‘‘weighted’’ values are normalized by the water-soluble sulfate content in
individual horizons.
4.1. Spatial Patterns Among the Four Soil Profiles
[20] Sulfate’s stable isotope parameters from the four soil
profiles exhibit spatial trends along a transect from soils at
low-elevation sites near the coast to upland valley sites far
away from the coast: (1) weighted sulfate D17O value
increases from !0.60 to 1.71%; (2) weighted sulfate d18O
value decreases from "1.1 to "10.0%; and (3) weighted
sulfate d34S value also decreases from 17.8 to 14.7%
(Table 3 and Figure 2). Nonweighted average values of
surface samples parallel the weighted mean values of an
individual soil profile in the spatial patterns (e.g., the
surface samples’ d34S in Figure 2).
[21] Although the correlations among the three isotope
parameters points to a dominantly two-component mixing
scenario (Figure 3), there are sizable scatterings in the
overall data distribution. Among them, the d34S versus
d18O are the best correlated, whereas the D17O versus
d18O is the most clustered, and the D17O versus d34S the
most scattered. One salient feature is that profile MB13 has
persistently lower D17O values than predicted from the
corresponding d18O or d34S values in the general D17O –
d18O or the D17O– d34S plot (Figure 3). Also, the d34S– d18O
plot reveals a characteristic difference between profile
MB11 and MB53.
4.2. Vertical Patterns for Individual Soil Profiles
[22] The most prominent and consistent vertical pattern is
the decrease of both sulfate’s d18O and d34S values with
increasing depth in individual soil profiles (Figure 4). In
profile MB07, the d18O value decreases from !0% near the
surface to "3% or "4% at depth (D ! 1%). Similarly, the
d18O value in profile MB13 decreases from !"2% to
!"5% with increasing depth (D ! 3%). Finally, at upland
valley sites, d18O values in both MB11 and MB53 decrease
from !"8% to !"13% (D ! 5%), a change larger in
magnitude than those in the low-elevation sites. Meanwhile,
the d34S also decreases with depth for the low-elevation
profiles MB07 and MB13, changing from !+18% to
!+16% and from !+17% to !+16%, respectively, but
the two upland valley sites are not quite the same: MB11’s
d34S value decreases from !+15.5% to !+11.5% (D =
4%) whereas MB53’s only from +16.9% to +15.9% (D =
1%). In the deeper part of the profiles, however, the datum
points often diverge from the main trend and become less
negative than would otherwise be the case (Figure 4).
[23] Another important observation is the change in
sulfate’s D17O value with depth. The change is not uniform.
The variability increases in soils with increasing distance
from the coast. At low-elevation sites, profile MB13’s D17O
value varies only from +0.49% to +0.76% (D = 0.27%).
MB07, also near the coast, has slightly larger variability,
ranging from +0.58 % to +1.21% (D = 0.63%). At upland
valley sites, the D17O value varies from +1.41% to +2.12%
(D = 0.71%) for MB11, and from +1.47% to +2.21% (D =
0.74%) for MB53. A closer inspection also reveals that,
except for profile MB13, there is a general ‘‘S’’ form in the
vertical patterns in which the D17O value decreases initially
at the top 5 to 10 cm and is followed by a gradual increase
with depth but decreases again at the much deeper part of
the soil profiles (Figure 4). The depths of these transitions,
however, vary among different profiles. Note that the
analytical error for D17O is <0.1%, so the vertical variations
are significant. Similarly, mirrored (opposite) ‘‘S’’ forms in
the profiles can be identified near the top 5 to 10 cm for the
corresponding d18O and d34S values. These ‘S’ forms for the
d18O and d34S vertical trends are, however, less distinctive
than that for the D17O.
+
4.3. Water-Soluble Ion Concentration: SO2"
4 and Na
+
[24] Both [SO2"
4 ] and [Na ] vary tremendously among
individual soil profiles; this includes the depth where ions
have the highest concentration and/or the ion’s total content
within sampled depths. Profile MB53 has a much higher salt
content than the other three profiles. Sulfate concentration
peaks at depths between 5 and 10 cm and drops to
background levels (!50 ppm) below 40-cm depth for all
four soils. Sodium generally shows a similar decrease in
concentration with depth, but it exceeds the concentration of
sulfate by a slight margin in the deep parts of the profiles
(Figure 4).
5. Discussion
[25] As the data show, the degrees of correlations between
any two of the isotope parameters are variables, signaling
that sulfate in the MDV soils is not the result of a simple
two-component mixing. Before testing a simple threecomponent mixing model, we must provide independent
constraints on the isotope compositions of each sulfate endmembers.
5.1. End-Member Isotope Parameters for Individual
Sulfate Components
5.1.1. SS Sulfate
[26] SS sulfate in soils of the MDV comes from the
accumulative sea-salt depositions of the last several million
years. Its D17O value is close to 0 [Bao, 2005; Bao et al.,
2000b]. Data from disseminated marine barite show that
seawater sulfate’s d34S value has changed little during the
6 of 13
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BAO AND MARCHANT: SULFATE COMPONENTS IN ANTARCTIC SOILS
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[Zachos et al., 2001], the input of SS sulfate to the MDV
has had a constant d34S value of !+22.0 ± 0.5%. On the
other hand, seawater sulfate’s d18O value appears to have a
larger variability than its d34 S value during the last
Figure 2. Plots of the weighted average sulfate stable
isotope compositions, D17O, d18O, and d34S, in a soil profile
versus the shortest distance to the coast in the MDV. The
larger squares are weighted average value normalized by
sulfate concentration and soil depth. The d34S for samples at
the ground surface (0 to 2 cm in depth) at the four soil
profiles are also plotted for comparison.
last 35 million years; its value has been almost constant at
+22.0 ± 0.3%, with only a slight decrease since two million
years ago to its modern value at 21.3% [Paytan et al.,
1998]. We therefore conclude that since the onset of
Antarctic glaciation back in the late Eocene or Oligocene
Figure 3. Correlations among the D17O, d18O, and d34S
for all datum points from four soil profiles in the MDV. R2
value for d18O-D17O, d34S-D17O, and d18O-d34S correlations
are 0.76, 0.42, and 0.68, respectively.
7 of 13
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BAO AND MARCHANT: SULFATE COMPONENTS IN ANTARCTIC SOILS
Figure 4. Vertical profiles of water-soluble sulfate and sodium concentration and D17O, d18O, and d34S
values for four soil profiles (MB07, MB13, MB53, and MB11) from the MDV. Vertical axis is sample
depth in cm. Concentration is in wt %, and isotope compositions are with respect to VSMOW and VPDB
for oxygen and sulfur, respectively.
8 of 13
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BAO AND MARCHANT: SULFATE COMPONENTS IN ANTARCTIC SOILS
10 million years [Turchyn and Schrag, 2004], with an
average value at !+10 ± 2%.
5.1.2. BG Sulfate
[27] BG sulfate’s D17O value is also close to 0 [Bao,
2005; Bao et al., 2000b, 2003]. Their d34S and d18O values,
however, can be quite variable among individual soils. It is
known that the d34S of sulfate derived from sulfide oxidation is largely determined by its source. Unfortunately, soil
materials in the MDV have not been measured for their d34S
value. Nevertheless, since volcanic rocks (e.g., ashes, scoria, and dolerite) contain much more sulfur and are much
darker in color than other materials in the MDV (and
therefore are more susceptible to albedo-driven surface
snowmelt), BG sulfate is likely dominated by magmatic
sources. We estimate therefore that BG sulfate has a d34S
value of 5 ± 5%, an average value for magmatic sulfur
materials [Sakai et al., 1982]. This end-member value
would not change appreciably even if the BG sulfate
component consisted of a large portion of sulfate from ash
leachates, since measured ash leachates from other sites in
the world show similar d34S values (!+5 ± 5%) [Bao et al.,
2003].
[28] The d18O value for BG sulfate is determined by its
oxidation pathway that is often pH-dependent and can be
biologically mediated or abiotic [van Stempvoort and
Krouse, 1994]. Nevertheless, it is known that sulfate derived
from the oxidation of sulfide minerals has more than half of
its oxygen isotope composition determined by that of
ambient water, regardless of oxidants (Fe3+ or O2) or pathways. Data from modern snowfall and secondary ice in soils
[Marchant et al., 2002] suggest that the d18O of the soil
moisture in the MDV ranges from "45% to "10%.
Therefore it is estimated that BG sulfate component has a
d18O value of "20 ± 5%, on the basis of complied natural
and experimental data [van Stempvoort and Krouse, 1994].
5.1.3. NSS Sulfate
[29] Differing from those of SS and BG sulfates, NSS
sulfate’s end-member isotope parameters have to be deduced from the measured data from the four different soil
profiles. The total stable isotope compositions for NSS
sulfate in the Antarctic atmosphere have not been characterized by direct measurement. This is largely due to an
inevitable mixing of multiple sulfate sources in sampling
matrices, such as sample filter, soil, snow, or ice. The most
positive sulfate D17O value that we have measured so far in
soils is !+2.21%, which is, however, a minimum long-term
average value for NSS sulfate in the MDV. The reason is
that we do not know what the proportions for SS and BG
sulfates are in these high-D17O samples.
[30] One way to improve our estimation is to examine the
[Na+] in soil profiles. The quantity of SS sulfate in soils can
be estimated using the accumulative quantity of water+
soluble [Na+] (seawater [SO2"
4 ]/[Na ] = 0.252) [Calhoun
+
et al., 1991], assuming that all Na originated from sea salts.
There are three complications here. (1) An important winter
sea-salt aerosol source in coastal Antarctica is probably the
sulfate-depleted brine on sea ice surface due to the precipitation of mirabilite (Na2SO4.10H2O) from original seawater [Wagenbach et al., 1998]. (2) BG sulfate is likely to be
minimal in the upland valleys, but not necessarily absent.
(3) On the basis of our high-resolution ion concentration
data, we found that while in many cases the sulfate has been
D16301
Table 4. Estimated End-Member Stable Isotope Values for LongTerm Average Sulfate in the MDV
End-Member
NSS sulfate
SS sulfate
BG sulfate
D17O
d18O
d34S
+2.5 ± 0.2%
0%
0%
"16 ± 2%
+10 ± 1%
"20 ± 5%
+12 ± 2%
+22 ± 1%
+5 ± 5%
retained in the top 1-m soil profiles over the time, the Na+,
Cl", and NO"
3 may have not (Table 2 and Figure 4).
Regarding the above complications, factor 1 tends to give
a maximum estimate of SS sulfate proportion, whereas the
factors 2 and 3 tend to provide minimum estimates. These
factors are hard to quantify at this time. However, considering the fact that most soil horizons that have a D17O value
> 2.0% still bear a significant amount of [Na+], e.g., an
equivalent of >10% SS sulfate for profile MB11, a minimum NSS sulfate’s D17O value is calculated to be +2.30%.
We assume, at this time, a value of +2.50 ± 0.2% for the
long-term average D17O value of NSS sulfate in the MDV.
Note that NSS sulfate may have lower or higher D17O value
than the +2.50% at a shorter timescale, e.g., seasonal or
glacial versus interglacial periods.
[31] Similarly, the end-member d18O and d34S values for
long-term average NSS sulfate are estimated to be !"16 ±
2% and !+12 ± 2%, respectively. Slight modification of
these end-member values is anticipated with the addition of
soil sulfate data from different geographic locations or with
the deletion of soil data that are significant in BK sulfate
component. Recently, a complete sulfate stable isotope ratio
measurement has been undertaken for the Vostok and Dome
C ice cores that cover the last glacial cycle [Alexander et al.,
2002, 2003]. The trace amount of sulfate extracted from
these ice cores has an average D17O, d18O and d34S values
of +2.69 ± 0.95% (N = 17), "4.3 ± 2.4% (N = 17), and
+12.4 ± 1.5% (N = 15), respectively, with a range from
+1.0% to +4.6%, "7.8% to "0.1%, and +9.5% to
+15.4%, respectively (calculated from Alexander et al.
[2002, 2003] and from B. Alexander (personal communication, 2005)). If corrected for SS sulfate contributions, the
corresponding NSS sulfate’s D17O, d18O and d34S values are
+2.91 ± 0.96%, "5.4 ± 2.9%, and +11.1 ± 1.9%, respectively. While our estimated D17O and d34S values for the
long-term average NSS sulfate in the MDV are close to
those from recent ice cores, the d18O value is much lower. It
should be noted that our NSS sulfate’s d34S value is also
!6% to 7% smaller than the value (+18.6 ± 0.9%)
obtained for total sulfur (including both sulfate and methane
sulfonic acid) from shallow ice cores near the South Pole by
[Patris et al., 2000]. Variation in contributions from the
stratospheric and local volcanic sources have been suggested for the observed variances in sulfate d34S value
among different sites in the Antarctica [Pruett et al.,
2004]. It is also intriguing to note that NSS sulfate’s D17O
value in the MDV is much higher than those obtained from
many midlatitude sites where the average NSS sulfate’s
D17O value is !+0.7% [Jinkens and Bao, 2006].
[32] Given these independent estimates of the endmember isotope parameters and their corresponding uncertainties for the three sulfate components (Table 4), we now
9 of 13
BAO AND MARCHANT: SULFATE COMPONENTS IN ANTARCTIC SOILS
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proceed to examine the simple three-component mixing
model for soil sulfate in the MDV.
5.2. Evaluating the Simple Three-Component Mixing
Model
[33] Each of the three isotope parameters D17O, d18O, and
34
d S is linearly additive in the sense that a particular
parameter of a mixture is the sum of that parameter from
each individual components normalized by its mole fraction
in the mixture. This can be derived from the definition of
the d notation. Strictly speaking, however, the D17O of a
mixture is not a simple linear addition of its component
values, as its definition is in a logarithmic form [Ono et al.,
2006]. The D17O value of a two-component mixture is
slightly lower than the value of a simple linear mixing,
with the difference the largest at a half-half mixing. However, for the ranges of sulfate D17O and d18O values in the
MDV, the difference is <0.02%, much less than the analytical error for the D17O. Therefore a simple linear addition
can be applied to the parameter D17O with good precision.
Meanwhile, since the three components’ three end-member
isotope parameters have been independently estimated
(Table 4), the proportions of the three sulfate components
can be solved explicitly for any given soil sample (Table 2).
The system of linear algebraic equation is:
2:5 * mNSS þ 0 * mSS þ 0 * mBG ¼ D17 O
ð1Þ
12 * mNSS þ 22 * mSS þ 5 * mBG ¼ d34 S
ð2Þ
"16 * mNSS þ 10 * mSS þ "20 * mBG ¼ d18 O
ð3Þ
Or, in its nonhomogeneous 3 # 3 matrix form:
2
2:5
4 12
"16
32
3 2 17 3
0
0
mNSS
D O
22
5 5:4 mSS 5 ¼ 4 d34 S 5
10 "20
mBG
d18 O
ð4Þ
where mNSS is the proportion or mole fraction of NSS
sulfate in a sample, mSS is the proportion or mole fraction of
SS sulfate in a sample, mBG is the proportion or mole
fraction of BG sulfate in a sample, and the D17O, d34S, and
d18O are the measured stable isotope parameters for a given
soil sulfate. It should be noted that in equation (1), the D17O
for mSS and mBG are not exactly zero, but slightly negative
at !"0.05% [Bao, 2005]. Errors associated with this
approximation are on the order of 10"3 for calculated ms.
[34] Not considering the uncertainties in end-member
parameters, the error introduced by analytical precision is
estimated to be at a maximum of !5% for each of the
calculated proportions. The distinct differences among the
end-member parameters (Table 4) help reducing the errors
associated with the calculated proportions. If the simple
three-component mixing model is a good account of the
sulfate budget in soils, we should expect that the sum of
mNSS, mSS, and mBG be very close to unity:
mNSS þ mSS þ mBG ¼ 1
ð5Þ
D16301
[35] The results reveal four important conclusions. First,
as shown in Table 2, the sum of the three components is
very consistent (i.e., small standard deviations) within
individual soil profiles and fairly close to unity for all
soil profiles, suggesting that the simple mixing model is a
valid representation of the sulfate budget in the MDV.
Profile MB13 and MB53 have a sum of 1.07 and 1.14,
respectively, higher than 1.00. A preliminary sensitivity
test suggests that increasing the d34S value (from 5% to
8%) and/or decreasing the d18O value (from "20% to
"25%) for BG sulfate help the sum of the three
components approaching 1.00. This is consistent with
the fact that BG sulfate has the poorest defined endmember isotopic parameters among the three sulfate
components, and its parameters are expected to be the
most variable among different soil profiles in the valleys.
For example, the !2.7 million-years-old profile MB13 is
unique among the four in that it occurs !200 m east of a
lava flow emplaced at !2.7 Ma [Wilch et al., 1993]. It
has much more volcanic influence than the other profiles,
as evident from the calculated !25% BG sulfate component. Also, profile MB53 has undergone the most prolonged weathering among the four profiles (Table 1).
[36] Second, the fact that a simple three-component mixing model produced a remarkably good description of the
sulfate budget in the soils leads to another important
conclusion: other than the process that may change the
mixing ratio of the three sulfate components at different soil
horizons (e.g., selective leaching), postdepositional processes
that alter stable sulfur or oxygen isotope composition of
sulfate are not significant in soils of the MDV. In other
words, microbial sulfur redox reactions or superimposed
oxygen and sulfur isotope fractionations during dissolution
and reprecipitation [Lloyd, 1968; Thode and Monster, 1965]
along the vertical profiles is negligible, if any.
[37] Third, the soil location (Figure 1) and the calculated proportions (Table 2) indicate that NSS sulfate
dominates the upland valleys, while SS sulfate dominates
the coastal or low-elevation sites. Likewise, BG sulfate is
more abundant near the coast than in the upland valleys.
The significant data scattering in Figure 3 where two
isotope parameters are correlated for all the data is,
however, the result of a simple three-component rather
than a simple two-component mixing.
[38] Finally, it is shown that the overall vertical patterns
for each stable isotope values in Figure 4 are determined by
the relative proportions of the three sulfate end-members at
different depths of the soils. Although all three sulfate
components have their maximum ion concentrations in the
top 15 cm, the vertical profiles of their relative proportions
are, however, not necessarily in line with their absolute ion
concentration profiles (Figure 5). It is these relative proportions that determine the seemingly intricate vertical patterns
for D17O, d34S, and d18O values as described in section 4.2.
For example, the ‘‘S’’ and its mirror forms are the result of
different peak concentration depths for different sulfate endmembers. In the case of profile MB13, the mNSS initially
increases with depth, a trend opposite to that of all other
profiles measured, explaining the opposite ‘‘S’’ form of the
D17O trend in MB13 (Figure 4). The reason for the vertical
differentiation of the three sulfate components may have to
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Figure 5. Calculated proportions of the three sulfate components along vertical soil profiles in the
MDV, assuming a simple three-component mixing model; mNSS: mole fraction (MF) of non-sea-salt
sulfate; mSS: MF of sea-salt sulfate; mBG: MF of background sulfate. An ! 5% error for each datum
point on the horizontal axis is not plotted.
do with differential leaching, a topic beyond the scope of
this study.
6. Conclusions and Implications
[39] Every soil profile in the MDV is unique in its salt
budget and vertical distribution, even when considering the
concentration and isotopic characteristics of a single ion
such as sulfate. This uniqueness originates from a combination of factors including, among others, age, distance to
the ocean, elevation, prevailing wind direction, surface
albedo, and parent material. We have demonstrated that
there is an intricate variability in space and in depth in soil
sulfate’s three stable isotope compositions, the D17O, d18O,
and d34S; and the variability is best explained by a simple
mixing of three isotopically distinct sulfate components:
NSS, SS, and BG sulfates. The independently estimated
long-term average D17O, d18O, and d34S values for SS, NSS,
and BG sulfate components are (0%, !10%, +22%),
(+2.5%, "16%, +12%), and (0%, 5%, "20%) with
variable uncertainties, respectively. These values themselves
have placed important constraints on their origins, transport
patterns, and chemical reaction models.
[40] This study established an approach to quantify different sulfate components in soils in the MDV. This approach is
extremely valuable in that factors contributed to the observed
data can be delineated and models explaining these data can
be tested. For example, the degree of weathering can be
quantified by comparing the proportion of BG sulfate among
different soil profiles, and the nature and dynamics of ion
migration or leaching can be revealed using a high-resolution
vertical data set for these unique soils.
[41] Acknowledgments. We thank Greg Michalski, Jim Head, Sara
Burns, Helen Margerison, Stephanie Thomas, and other team members for
fieldwork assistance and helpful discussions. We are especially indebted to
Margarita Khachaturyan, Kathryn Jenkins, Chee-Haur Siew, and Joanie
Wisekal at LSU and A. J. Kaufman at the University of Maryland for their
countless hours of analytical assistance. Financial and logistic support is
provided by grants from the National Science Foundation to H.B. and
D.R.M. (EAR-0129793 and OPP-0125842).
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""""""""""""""""""""""
H. Bao, Department of Geology and Geophysics, E235 Howe-Russell
Complex, Louisiana State University, Baton Rouge, LA 70803, USA.
(bao@lsu.edu)
D. R. Marchant, Department of Earth Sciences, Boston University, 685
Commonwealth Avenue, Boston, MA 02215, USA. (marchant@bu.edu)
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