ELSEVIER
Earth and Planetary Science Letters 170 (1999) 61–72
www.elsevier.com/locate/epsl
Sources of strontium and calcium in desert soil and calcrete
Rosemary C. Capo a,Ł , Oliver A. Chadwick b
a
Department of Geology and Planetary Science, 321 EH, University of Pittsburgh, Pittsburgh, PA 15260, USA
b Department of Geography, University of California, Santa Barbara, CA 93106, USA
Received 27 October 1998; accepted 8 April 1999
Abstract
The carbon-cycle significance of soil carbonate fluxes is subject to large uncertainties because it is not clear precisely
how much calcium is derived from atmospheric sources compared with that from the chemical weathering of silicate
minerals. In the petrocalcic horizon (calcrete) of a Pleistocene soil from the USDA–SCS Desert Project area near Las
Cruces, NM, approximately 1.5 g Ca=cm3 has been added, with an associated expansion of the profile of ¾200%.
Strontium isotope values for the labile cations and carbonate from the A, B and K soil horizons have 87 Sr=86 Sr values
that range from 0.7087 to 0.7093, similar to the values for easily soluble local dust and rain. The parent material,
non-calcareous Camp Rice alluvial sediment, has a 87 Sr=86 Sr ratio of ¾0.7165. Mixing calculations indicate a minimum
atmospheric contribution to soil carbonate calcium of ¾94%; the more likely scenarios indicate at least 98% of the Ca
originated from atmospheric input. The variations in 87 Sr=86 Sr ratios of soil silicate (0.7131 to 0.7173) are consistent
with weathering of volcanogenic sediments and neoformation of clay minerals in the petrocalcic horizon. Moreover, the
Sr isotope data suggest that 50–70% of silicate in the uppermost 25 cm of the profile could be atmosphere-derived. The
isotopic composition of labile strontium in the A horizon and the mass distribution of silicon and calcium indicate that
the uppermost portion of the profile is the present zone for the release of cations due to silicate weathering. Steady-state
models of the whole profile yield a Sr weathering flux ranging from ¾200 to 400 µg cm 2 Ma 1 . The results indicate that
both the present-day and long-term contribution of calcium from silicate weathering is less than 2% of that supplied from
the atmosphere, and confirm that desert soil formation is not a significant sink for atmospheric carbon.  1999 Elsevier
Science B.V. All rights reserved.
Keywords: carbonates; deserts; wind transport; geochemistry; soils; strontium; isotopes; weathering
1. Introduction
Arid regions comprise over a third of the global
terrestrial environment. The lack of intense leaching
by rainfall leads to the formation of laterally extensive, meters-thick deposits of pedogenic calcium
carbonate which can accumulate in cemented layers
Ł Corresponding
author. Tel.: C1 412 624 8873; Fax: C1 412
624 3914; E-mail: [email protected]
known as calcrete [1]. Globally, calcrete sequesters
about twice as much carbon as is held in the atmosphere and rivals soil organic carbon in total amount,
if not in rate of carbon turnover [2,3]. Knowledge of
the source of the calcium and carbon locked in these
vast terrestrial deposits is critical to understanding
linkages between their biogeochemical cycles [4].
Calcium provenance provides an important key for
the global significance of carbon sequestration in
calcrete. Each mole of calcium released by silicate
0012-821X/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 0 9 0 - 4
62
R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
weathering, when combined with carbonate, acts to
sequester a mole of carbon derived from photosynthetically fixed atmospheric carbon dioxide [5]. On
the other hand, calcium derived from dissolution
of calcium carbonate does not sequester any atmospheric carbon when it reprecipitates in calcrete.
Until now there has been little quantitative partitioning of the source of calcium in calcrete into that
derived from atmospheric addition from calcareous
sources and soil weathering of silicate minerals.
Atmospheric additions of calcium are thought to
control rates of calcrete accumulation in most arid
environments, because weathering rates are low and
the cycling of dust between the atmosphere and
soil is high [6–10]. Many deserts, including those
of the southwestern United States, were repeatedly
affected during the Pleistocene by pluvial conditions that roughly correlate with periods of enhanced
glacial activity [11]. These moister climate regimes
could have enhanced chemical weathering in Pleistocene-age soils, resulting in a greater proportion
of calcium derived from silicate minerals than that
indicated by present climatic conditions [12]. Here,
we present an intensive analysis of calcrete from an
early Pleistocene soil in southern New Mexico that
argues strongly for the dominance of the atmospheric
flux of labile cations to arid soils, regardless of the
climatic effect of glacial=interglacial cycles.
2. Geologic setting
The Desert Project research area covers 1040 km2
near Las Cruces, New Mexico in the southeastern
Basin and Range province [13,14]. Annual precipitation in the area currently ranges from <25 cm
in the lower elevation arid regions to 25–40 cm in
the semi-arid higher elevations (>1524 m). Summer
monsoon rains from the Gulf of Mexico and winter storms from the Pacific Ocean dominate Modern
climatic patterns in the region. Present vegetation
includes snakeweed, mesquite, creosote bush and
Yucca elata; mean annual temperature is 16ºC. In addition to massive carbonate, sulfates such as gypsum
are present in some of the soils in the area.
We analyzed archived soil samples from USDA
Soil Conservation Service Pedon S61NMex-7-7
(hereafter referred to as Pedon 7-7) [6]. The soil
profile, classified as a Petrocalcic Paleargid, is developed in unconsolidated, non-calcareous arkosic sand
and volcanogenic alluvial sediments in the Mesilla
basin. Deposition occurred from 3.4 to ¾0.73 Ma,
following the entrenchment of the ancestral Rio
Grande River into the Hatch–Rincon Basin during
extension of the southern Rio Grande Rift [15,16].
The soil formed on the Upper La Mesa geomorphic
surface that was isolated from fluvial deposition over
2 Ma ago [17]. Particle size analysis of the carbonate-free soil fraction and the existence of similar
profiles on both younger and buried landscape positions indicate that the soil formed in a vertically
continuous parent material [18]. Fault-controlled uplift and long-term downcutting of the Rio Grande
River preclude ground water involvement in calcrete
development.
3. Samples and analytical methods
Pedon 7-7 was sampled to a depth of 3.5 m, at
an elevation of 1353 m. Its reddish brown A (0–5
cm depth) and B (5–48 cm) horizons are non-calcareous, fine sandy loam to sandy clay loam with
few roots. The 2 m thick, white, petrocalcic K (Bkm )
horizon (48–236 cm) consists of >40% CaCO3 , with
up to 90% CaCO3 in the indurated laminar portions
of the horizon. The C horizon (below 236 cm) consists of unconsolidated sediments of the non-calcareous fluvial facies of the Camp Rice Formation. To
characterize atmospheric input, we analyzed archival
dust samples collected during the dry season from
February to June, 1970–1971 from Trap 3, in the
southern Robledo Mountains west of the Rio Grande
Valley, and Traps 5 and 7, in the eastern Doña Ana
Mountains [6] (Fig. 1). Rainwater was collected near
Las Cruces, in acid-cleaned polypropylene bottles,
during the summer of 1993.
Splits of 30–100 mg of powdered soil were taken
for isotopic and chemical analysis.
Major and trace elements were determined by
ICP–AES (Si, Ca, Ti) and XRF analysis (Zr). Samples for isotopic analysis were weighed and leached
in ultrapure 1 N acetic acid to remove labile Sr and
carbonate minerals. Post-leach silicate residues were
dissolved with concentrated ultrapure HF, HClO4
and HNO3 . Rainwater was evaporated and leached
R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
63
Fig. 1. Block diagram of the study area in the USDA=SCS Desert Project area near Las Cruces, New Mexico (after [38]). The soil profile
studied (square symbol) developed on the Plio–Pleistocene Upper La Mesa geomorphic surface. Dust samples analyzed are from traps
located near the Robledo and Doña Ana Mountains (triangles).
with HClO4 . Sr concentrations and Sr isotopic compositions were determined by thermal ionization
mass spectrometry on a VG Sector 54 multicollector and a Finnigan MAT 261. Total procedural
blanks were less than 500 pg. Measured ratios were
corrected for mass fractionation using 86 Sr=88 Sr D
0.1194. All samples were normalized to an NBS987
Sr standard value of 0.71024.
4. Results
4.1. Major and trace element geochemistry
Major and trace element data are shown in Table 1. Accurate determination of the enrichment or
depletion of cations in a soil relative to the parent
material (p) requires quantification of volume (V )
and bulk density (²) changes associated with soil
formation [19]. To compensate for the effects of volume change, a physiochemical strain parameter (")
is used in which the element of interest, j, in the
weathered material (w) is normalized to an immobile
element, i [20]:
²p Ci;p
1
(1)
"i;w D
²w Ci;w
To calculate absolute elemental mass gains and
losses of calcium per unit volume of parent material,
volume-corrected data in this study were normalized to Zr, which is thought to be immobile during
pedogenic processes:
m Ca;flux
∆Ca .g=cm3 / D
Vp
D
²w CCa;w ."Zr;w C 1/
100
² p CCa;p
(2)
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R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
Table 1
Strontium isotope and geochemistry data for Desert Project Pedon 86-INMex-7-7 soil (Pedon 7-7)
Sample
Horizon Pedon 7-7 Bulk Soil
Depth
(cm)
HOAc leachate
Residue
CaCO3 Ca
SiO2
Zr
Sr
Rb
Average
Sr
Average
Sr
(wt.%) (wt.%) (wt.%) (ppm) (ppm) (ppm) 87 86
(ppm) 87 86
(ppm)
Sr= Sr δ87 Sr
Sr= Sr δ87 Sr
DP14979 A
0–5
0.01
DP14980 B1t
5–25
0.01
DP14983 B22tea
36–48 16
DP14985 K22m
74–102 65
DP14987 K31
150–185 52
DP 14990 C2
272–353 2
CM-C1 (C1 Lower La Mesa)
0.65
0.64
4.81
27.42
14.39
1.25
0.79
79.3
78.2
68.9
24.5
50.6
81.1
83.4
281
235
217
55
122
138
102
254
251
349
407
488
320
277
102
91
93
16
41
91
98
0.709326
0.709133
0.708652
0.708732
0.708827
0.708989
0.708938
0.2 251
0.1
0.7
0.6 513
0.5
0.3
0.3 425
0.713919 6.7
0.713157 5.6
0.713100 5.6
0.717264 11.4
196
280
271
0.716361 10.1
0.716551 10.4
244
199
Sample numbers, horizon designations and CaCO3 content from Gile and Grossman [1]. All isotope data represent the average of 1–3
runs of 100–200 ratios each, and are expressed as 87 Sr=86 Sr and δ87 Sr (deviation from modern seawater ð103 ). 2¦ errors are: 0.04 δ
units. NBS 987 D 0.710243.
In the most indurated part of the petrocalcic K
horizon, approximately 1.5 g Ca cm 3 has been
added with an associated expansion of up to 200%
(Fig. 2). Nearly all of this calcium accumulated in
the soil as calcium carbonate [6]. Fig. 3a shows the
volume-change corrected depth distribution of Ca
and Si in Pedon 7-7 relative to the parent material.
Concomitant loss of Ca and Si, particularly in the
upper part of the profile, suggests some in situ
silicate weathering and leaching. If the observed
calcium accumulation and soil expansion were due
to breakdown of a Ca–silicate mineral, changes in
silica should be negatively correlated with calcium
with soil depth. In the upper K horizon, however,
Ca shows a significantly greater increase than Si.
Variations in Zr=Ti (< š15% below 100 cm) are
well within the limits suggested by Maynard [21]
for interpreting soil development in a single parent
material (Fig. 3b).
4.2. Strontium isotope results
Fig. 2. Plot of net change in calcium (g=cm3 ) relative to the
change in volume of the soil profile (percent). Note that the K
horizon points plot in the quadrant showing expansion of the
profile and addition of Ca. Zr was used as the immobile element,
although use of Ti does not substantially alter the results.
Strontium isotope data are shown in Table 2 and
Fig. 4. The Sr isotopic compositions of the atmospheric inputs to the Upper La Mesa soil and its
parent material are isotopically distinct. 87 Sr=86 Sr
values for the carbonate and easily exchangeable Sr
of local dust and rain are similar to, but slightly
lower than that of seawater, ranging from 0.7089
to 0.7092. The silicate fraction of the dust ranges
from 0.7109 to 0.7112. Our measurements are within
the range of seasonal rain and dust variations documented elsewhere in New Mexico [22,23].
The isotopic composition of bulk parent material
was estimated by analyzing Camp Rice Formation
sediments from the C horizons of soils formed on
both the Upper and Lower La Mesa geomorphic
surfaces. The samples were leached prior to dissolution to remove the small amount (<1%) of calcium
carbonate added from overlying horizons. Total dissolution of the sediment yields 87 Sr=86 Sr values of
0.7164 (244 ppm Sr) and 0.7166 (199 ppm Sr) for
R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
65
Fig. 3. Variations in selected major and trace elements in bulk soil for Pedon 7-7. The silicate portion of the soil dominates the budget
of Zr, Ti and Si, whereas Ca is strongly affected by the carbonate. (a) Mass change in calcium and silica versus depth in the soil profile
relative to the parent material mass. These values are corrected for volume and bulk density changes associated with soil formation (using
the method of Brimhall et al. [44] and Chadwick et al. [43]; see text). Ca shows a significantly greater increase in the upper K horizon
than Si. (b) Plot of profile depth vs. deviation of Zr=Ti in Pedon 7-7 relative to the Camp Rice Formation parent material. Deviations of
<40% in the Zr=Ti ratio indicate that the soil in the profile represents in situ weathering [21].
the parent material of Upper and Lower La Mesa
soil profiles, respectively. The similar Sr concentration and isotopic composition of these two geographically disparate Camp Rice Formation fluvial
sediment samples, as well as the relatively small
variations in Zr=Ti, argue for the overall homogeneity of this parent material.
5. Discussion
5.1. Sources of soil silicate
If the low 87 Sr=86 Sr ratios in B horizon soil silicate are the result of differential weathering of
primary minerals in the soil (e.g., [24]), they must
reflect removal of a parent material component with
very radiogenic Sr. Rhyolitic volcanic rock fragments (VRF) comprise up to a third of the Camp
Rice alluvial sands, and decrease upward through
soil profiles found on the La Mesa surface [25].
These grains consist primarily of quartz and feldspar.
Analysis of VRF separated from the bulk C horizon material yielded 87 Sr=86 Sr values of 0.7075 and
0.7066, significantly lower than bulk parent material
values. Removal of Sr from the profile by selective
weathering of VRF of other low 87 Sr=86 Sr minerals
such as plagioclase would increase the 87 Sr=86 Sr of
the soil silicate remaining, and thus cannot explain
the silicate values in the A and B horizons. On the
other hand, the increase in soil silicate 87 Sr=86 Sr
from the C (0.7164) to the upper petrocalcic horizon (0.7173) could be the result of VRF weathering
(and consequent removal of low 87 Sr=86 Sr strontium)
to form palygorskite, as described by Monger and
Daugherty [26] and Wang et al. [27].
A more plausible explanation for the decrease in
silicate 87 Sr=86 Sr from the calcrete to the A horizon
(0.7139) is that the silicate in the uppermost part
of the soil profile is a mixture of parent material
and eolian input. The 87 Sr=86 Sr value for the silicate
portion of local dust is ¾0.7111. Dust collected from
the Desert Project area contains 63–78% silt and
sand, 20–40% clay, 3–7% organic carbon, and 1–6%
carbonate [28]. Assuming similar Sr concentrations
of the parent material and atmospheric silicate endmembers, 50–70% of A and B horizon silicate could
be atmosphere derived. Although Zr=Ti in Pedon 7-7
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R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
Table 2
Data for atmospheric inputs and parent material to Desert Project Pedon 86-1NMex-7-7 soil (Pedon 7-7)
Sample
Average
87 Sr=86 Sr
Atmospheric
Dust: HOAc leachates
S. Robledo Mtns Site 3a
S. Robledo Mtns Site 3
Doña Ana Mtns Site 5
E. Doña Ana Mtns Site 7
δ87 Sr
DP1200L
DP473L
DP476L
DP479L
Average
0.709012 š 24
0.709025 š 14
0.708886 š 09
0.708929 š 14
0.70896 š 07
DP473R
DP479R
Average
0.711244 š 09
0.710876 š 09
0.71106 š 37
DP4RSP93
0.709151 š 13
0.03 š 0.02
Parent material
Bulk sediment: non-calcareous Camp Rice Formation
C2 (272–353 cm)
DP14990R
C1 (Lower La Mesa)
CMC1-R
Average
0.716361 š 01
0.716551 š 09
0.71646 š 19
10.14 š 0.01
10.41 š 0.01
10.28 š 0.03
0.707467 š 18
0.706568 š 10
2.40 š 0.03
3.67 š 0.01
Dust: insoluble residue
S. Robledo Mtns Site 3
E. Doña Ana Mtns Site 7
Rain (acid soluble)
Las Cruces, summer 1993
0.22 š 0.03
0.20 š 0.02
0.40 š 0.01
0.34 š 0.02
0.29 š 0.09
2.93 š 0.01
2.41 š 0.01
2.67 š 0.52
Volcanic Rock Fragments (VRF)
DP14980V
DP14990V
All isotope data represent the average of 1–3 runs of 100–200 ratios each, and are expressed as 87 Sr=86 Sr. NBS 987 D 0.710243. To
facilitate interlaboratory comparison, we also express them in terms of δ87 Sr, where δ87 Sr D 103 [(87 Sr=86 Srsample =87 Sr=86 Srseawater ) 1 ].
We use an 87 Sr=86 Sr value of 0.70917 for Modern seawater; 2¦ errors of individual runs are 0.04 δ units.
is relatively constant, there is a slight shift in the
upper 50 cm that is related to mineral aerosol input
with a lower Zr=Ti than the Camp Rice parent material (Fig. 3b). Coppice dunes formed around shrubs
indicate eolian reworking of Camp Rice derived soil
materials. Based on the range of measured silicate
dust fluxes from 0.0009 to 0.012 g cm 2 yr 1 [28],
it would take a maximum of only 55,000 years to
accumulate the fraction of dust-derived silicate calculated from the Sr isotope data. This suggests that
the silicate portion of the dust has a short residence
time at the surface of the soil.
5.2. Sources of calcium to soil carbonate
Mass balance considerations indicate that the bulk
of the calcium addition to the Upper La Mesa calcrete originated from a source other than the chemical weathering of silicate minerals. However, bulk
elemental analysis cannot identify the sources of this
external calcium. We use strontium isotopes to distinguish between the sources of alkaline earth ions
and to quantify their relative contributions to the
carbonate.
The strontium isotope data show that weathering
of the silicate parent material was not an important
source of ions for carbonate formation. Selective
weathering of Camp Rice Formation components
to produce the observed mass of carbonate would
shift the 87 Sr=86 Sr ratio of the silicate residue in
the profile to >0.72. The observed shift of silicate
87
Sr=86 Sr values in the A and B horizons is, in fact,
in the opposite direction.
The calcium necessary to fuel carbonate formation in many continental soils comes primarily from
atmospheric dry- and wet-fall that originates from
limestone and evaporites [13,29–33]. The source of
this input can be locally or regionally derived (e.g.,
R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
67
Fig. 4. Sr isotope data from atmospheric components and soil developed on the Pleistocene Upper La Mesa geomorphic surface in the
Desert Project area, New Mexico. Sr isotopic data are expressed as 87 Sr=86 Sr and in terms of δ87 Sr (deviation from seawater in parts per
103 ; see Table 1). The upper region shows the atmospheric (rain and dust) samples, and the lower portion shows the soil samples plotted
against depth in the profile. Open symbols are the acetic acid soluble leaches of dust and soil, which represent both carbonate and labile
Sr. The solid symbols are the HF C nitric C perchloric acid-soluble residues that represent the silicate fraction of dust and soil. Error
bars are smaller than the size of the symbol.
[34]), but dust can also travel great distances (e.g.,
[35–37]). The present carbonate dust flux in the
deserts of the southwestern U.S. is high (¾0.5 mg
cm 2 yr 1 ) [9,38]. Rain and snow also carry a significant amount of Ca and Sr; precipitation in the
Desert Project area contains an average of 3 mg
Ca=l [39,40]. The labile cations and carbonate from
the A, B and K soil horizons have 87 Sr=86 Sr values that range from 0.7087 to 0.7093. These ratios
are similar to the values for the acetic acid-soluble
local dust and rain, and confirm that the Sr in the
pedogenic carbonate is derived almost entirely from
atmospheric sources.
5.3. Quantification of atmospheric contribution to
soil carbonate
Sr can be used as a geochemical proxy to estimate
Ca budgets in precipitation, throughfall, runoff, soilwater, soil, and trees [22,23,41–45]. The fraction of
Ca contributed by one component in a two-component mixture can be estimated from the isotopic composition of the endmembers and that of the mixture
[45]. Fig. 5 shows the relationship between Sr=Ca
of the silicate parent material, Sr=Ca of atmospheric
inputs, and the fraction of calcium derived from atmospheric sources. Model mixing curves show the
contributions to pedogenic carbonate of atmospheric
Ca (dissolved in rain and as a labile component
in dust) vs. Ca released by weathering of parent
material, as a function of 87 Sr=86 Sr of the pedogenic carbonate. The curvature of the mixing line
depends on the Sr=Ca of the endmembers. Even for
the unrealistic cases represented by curves b and d
(extremely low atmospheric 87 Sr=86 Sr, or a parent
material with an order of magnitude more Ca than
measured today), the maximum weathering contribution is less than 6%. The more likely scenarios
(curves a and c) indicate that 98–100% of the Ca in
the soil carbonate originated from atmospheric input.
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R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
Fig. 5. Model mixing curves showing the relative contributions to pedogenic carbonate of atmospheric Ca (dissolved in rain and labile
in dust) vs. Ca released by silicate weathering, as a function of 87 Sr=86 Sr of the pedogenic carbonate. Full mixing curves are shown
in the inset. The shaded area represents the range of soil carbonate 87 Sr=86 Sr values, and the horizontal dashed line the labile Sr in
the uppermost soil. The vertical dotted lines show the intercept between the highest measured pedogenic carbonate 87 Sr=86 Sr value and
each model curve, and represent the maximum possible contribution from parent material. For curves a, b and c, atmospheric Sr=Ca D
0.006 (upper limit for precipitation [23]), and parent material Sr=Ca ratio D 0.034 (Table 1). For curve a, atmospheric 87 Sr=86 Sr (ATM)
D 0.7087, and the parent material (PM) D 0.7165; these values reflect the measured endmembers. Curve b assumes that atmospheric
87 Sr=86 Sr was considerably lower in the past; ATM D 0.7067 (lowest value for Phanerozoic limestone) and PM D 0.7165. Curve c
addresses preferential weathering of a low 87 Sr=86 Sr component (such as VRF) in the parent material: ATM D 0.7087 and PM D 0.7127.
Curve d assumes equal Sr=Ca ratios for both endmembers; ATM D 0.7087 and PM D 0.7165.
5.4. Present-day weathering fluxes
Labile (HOAc extractable) strontium in the A
horizon appears to have a slightly greater (by ¾1%)
weathering component, possibly reflecting greater
present-day weathering in the upper portions of the
soil profile. Sr=Ca ratios and the absence of Ca-carbonate in this part of the profile demonstrate that this
trend is not linked to the presence of soil carbonate.
These data, and the mass distribution of Si and Ca
suggest that the uppermost portion of the profile is
the present zone for the release of Sr and Ca due to
silicate weathering.
A simplified steady-state model can be used to
quantify present-day weathering fluxes [46]. We assume that in the arid region of the La Mesa site,
nutrient cycling in the soil profile by vegetation is
minimal, and that most of the precipitation and dryfall reaches the upper part of the profile, with the
amount of wetting from rainfall decreasing exponentially with depth. Atmospheric strontium (dissolved
Sr in rainwater and water-soluble Sr in dryfall) is
delivered to the profile via downward movement of
pore water, accompanied by limited exchange with
Sr held in more tightly bound soil solution and exchangeable sites (‘labile’ Sr) [45]. In this model, the
rate of weathering at any depth is directly proportional to the amount of water passing through that
depth; thus the flux of Sr from weathering decreases
exponentially with depth. The solute concentration
increases with depth as the amount of water decreases; this eventually leads to precipitation of cal-
R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
cium carbonate and formation of a K horizon. The
isotopic composition of labile strontium (δL ) in the
profile within a given layer from i D 1 at the top to
i D n at the base is determined by the sum of the Sr
fluxes entering that layer:
δLi D
Sr
Sr
C δM JMi
δWi J.W!L/i
Sr
Sr
J.W!L/i
C JMi
(3)
Sr
is the flux of strontium from the
Here J.W!L/i
Sr
is the
mobile water to labile sites in layer i, and JMi
Sr flux from mineral weathering in layer i. δWi and
δM are the respective 87 Sr=86 Sr ratios of mobile soil
water and soil minerals in layer i (δM is taken to be
uniform throughout the profile). A similar expression
can be written for δWi :
δWi D
δW.i
Sr
Sr
1/ JW.i 1/ C δLi J.L!W/i
Sr
Sr
JW.i
1/ C J.L!W/i
(4)
Sr
JW.i
1/ and δW.i 1/ represent the flux and isotopic
composition of Sr from mobile water flowing down
to layer i from the layer above. Because of the
69
mutual interdependence of δL and δW , model curves
for these parameters are generated by iteratively
solving Eqs. 3 and 4 while maintaining a flux balance
in each layer.
Model curves for δW are shown in Fig. 6a. The
weathering component was modeled using the present-day 87 Sr=86 Sr profile for the soil silicate. Two
atmospheric endmember 87 Sr=86 Sr values were chosen: the lowest soil carbonate value (0.7087) as in the
previous section (Model I), and the average value for
the labile component of present-day dust (0.7090;
Model II). In the modeling presented here for Pedon 7-7, no carbonate precipitation occurs until the
concentration of Sr in the mobile soil water builds
up to a certain critical level, forcing precipitation.
The sharp discontinuities in the curves around 35 cm
represent the point where calcium carbonate begins
precipitating. The best-fit weathering rate curves for
each model are shown in Fig. 6b. Reasonable fits
to the observed present-day 87 Sr=86 Sr profile are
obtained with relatively modest weathering rates,
reaching maximum values of 2–5% Ma 1 at the sur-
Fig. 6. Results of steady-state modeling for the Desert Project soil profile. (a) Model curves showing labile and carbonate 87 Sr=86 Sr with
depth, compared with measured values. The 87 Sr=86 Sr ratio of the atmospheric endmember is 0.7087 for Model I, and 0.7090 for Model
II. The discontinuity in the curve around 35 cm results from precipitation of pedogenic carbonate, which cycles strontium rapidly from
the mobile soil water into carbonate. (b) The modeled pattern of weathering rate with depth for the two atmospheric endmember models.
The pattern reflects the distribution of soil water from rain in the profile, which decreases exponentially with depth.
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R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
face and decreasing exponentially with depth. These
values are in good agreement with the small amount
of total weathering inferred for the profile [6,28].
The integrated flux of Sr derived from silicate weathering over the whole profile ranges from ¾0.0002 to
0.0004 µg cm 2 yr 1 with this model, which corresponds to alteration of <2% of the parent material
over the last 2 million years. Compared to the estimated flux of Sr from the atmosphere of ¾0.07 µg
cm 2 yr 1 (35 cm rain per year with ¾2 ppb Sr), the
total present-day contribution of Sr from weathering
is 0.2–0.6%. This corresponds to >98% contribution
of calcium from atmospheric sources, in accordance
with the calculations in the previous section.
plicable, they provide quantitative confirmation that
pedogenic carbonate is not a significant sink in the
global carbon cycle.
Acknowledgements
We thank Bob Grossman and Lee Gile for assistance in choosing and obtaining soil and dust
samples, and H. Curtis Monger for samples of Camp
Rice sediments, the collection of Las Cruces rain
water and numerous discussions. R.A. Berner, Brian
Stewart and two anonymous reviewers improved earlier versions of this manuscript. This research was
supported by JPL–Caltech on contract to NASA
(OAC) and NSF–EAR 9614875 (RCC). [MK]
6. Conclusions
Soil carbonate from the B and K soil horizons
of a calcrete that has been developing in southern
New Mexico for most of the Quaternary period have
87
Sr=86 Sr values similar to those for easily soluble
dust and rain. The Sr in the pedogenic carbonate
is derived almost entirely from atmospheric sources.
Mixing calculations and steady-state models of the
measured isotope data using reasonable atmospheric
input parameters indicate that parent material calcium released by silicate weathering contributes less
than 2% to the calcrete, consistent with earlier estimates [47]. The increase in soil silicate 87 Sr=86 Sr
ratios from 0.7164 to 0.7173, observed in the transition from the C to the K horizon, is consistent with
neoformation of secondary clays associated with calcrete formation. The decrease in 87 Sr=86 Sr from the
K to the A horizon indicates addition of eolian silicate material to the top of the profile. Sr=Ca ratios
and the absence of Ca-carbonate support Sr isotope
data that suggest that the uppermost portion of the
soil is the present zone for the release of Sr and
Ca due to silicate weathering. Steady-state modeling
shows that chemical weathering of <1% of parent
material from the top 50 cm of the profile is capable of releasing enough strontium to produce the
observed present-day trend of labile and carbonate
87
Sr=86 Sr with depth. The data also suggest a low
weathering rate in the profile as a whole, with >98%
of the parent material Sr retained over 2 million
years of weathering. If these results are widely ap-
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