- Email: [email protected]
Sources of strontium and calcium in desert soil and calcrete
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 Don˜a 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 Don˜a 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)
64
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
66
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 Don˜a Ana Mtns Site 5 E. Don˜a 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. Don˜a 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.
68
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.
70
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-
References [1] L.H. Gile, F.F. Peterson, R.B. Grossman, The K horizon: a master soil horizon of carbonate accumulation, Soil Sci. 99 (1965) 74–82. [2] H. Eswaran, E. Ven Den Berg, P. Reich, Organic carbon in soils of the world, Soil Sci. Soc. Am. J. 57 (1993) 192–194. [3] H. Eswaran, E. Van Den Berg, P. Reich, J. Kimble, Global soil carbon resources, in: Lal, R., Kimble, J., Levine, E., Stewart, B.A. (Eds.), Soils and Global Change, CRC Lewis, Boca Raton, FL, 1995, pp. 27–43. [4] O.A. Chadwick, E.F. Kelly, D.M. Merritts, R.G. Amundson, Carbon dioxide consumption during soil development, Biogeochemistry 24 (1994) 115–127. [5] R.A. Berner, A.C. Lasaga, R.M. Garrels, The carbonate– silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years, Am. J. Sci. 283 (1983) 641–683. [6] L.H. Gile, R.B. Grossman, The Desert Project soil monograph, US Government Printing Office, Washington, DC, 1979, 984 pp. [7] L.D. McFadden, J.C. Tinsley, Rate and depth of pedogenic carbonate accumulation in soils: formulation and testing of a compartment model, in: Weide, D.L. (Ed.), Spec. Pap. Geol. Soc. Am. 203 (1985) 23–42. [8] O.A. Chadwick, J.O. Davis, Soil-forming intervals caused by eolian sediment pulses in the Lahontan basin, northwestern Nevada, Geology 18 (1990) 243–246. [9] M.C. Reheis, J.C. Goodmacher, J.W. Harden, L.D. McFadden, T.K. Rockwell, R.R. Shroba, J.M. Sowers, E.M. Taylor, Quaternary soils and dust deposition in southern Nevada and California, Geol. Soc. Am. Bull. 107 (1995) 1003–1022. [10] R.W. Simonson, Airborne dust and its significance to soils, Geoderma 65 (1995) 1–43.
R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72 [11] W.B. Bull, Geomorphic responses to climatic change, Oxford University Press, New York, 1991. [12] O.A. Chadwick, W.D. Nettleton, G.J. Staidl, Soil polygenesis as a function of climate change, northern Great Basin, USA, Geoderma 68 (1995) 1–26. [13] R.V. Ruhe, Geomorphic surfaces and surficial deposits in southern New Mexico, New Mexico State Bur. Mines Min. Res. Mem. 18, 1967, 65 pp. [14] J.W. Hawley, F.E. Kottlowski, Quaternary geology of the south-central New Mexico border region, New Mexico Bur. Mines Min. Res. Circ. 104 (1969) 89–115. [15] G.O. Bachman, H.H. Mehnert, New K–Ar dates and the late Pliocene to Holocene geomorphic history of the central Rio Grande region, New Mexico, Geol. Soc. Am. Bull. 89 (1978) 283–292. [16] G.H. Mack, W.C. James, Calcic paleosols of the Plio– Pleistocene Camp Rice and Palomas Formations, southern Rio Grande Rift, USA, Sediment. Geol. 77 (1992) 89–109. [17] G.H. Mack, S.L. Salyards, C. James, Magnetostratigraphy of the Plio–Pleistocene Camp Rice and Palomas Formations in the Rio Grande rift of southern New Mexico, Am. J. Sci. 293 (1993) 49–77. [18] H.C. Monger, L.A. Daugherty, Pressure solution: possible mechanism for silicate grain dissolution in a petrocalcic horizon, Soil Sci. Soc. Am. J. 55 (1991) 1625–1629. [19] O.A. Chadwick, G.H. Brimhall, D.M. Hendricks, From a black to a gray box — a mass balance interpretation of pedogenesis, Geomorphology 3 (1990) 369–390. [20] G.H. Brimhall, O.A. Chadwick, C.J. Lewis, W. Compston, I.S. Williams, K.J. Danti, W.E. Dietrich, M.E. Power, D. Hendricks, J. Bratt, Deformational mass transport and invasive processes in soil evolution, Science 255 (1991) 695– 702. [21] J.B. Maynard, Chemistry of modern soils as a guide to interpreting Precambrian paleosols, J. Geol. 100 (1992) 279–289. [22] J.R. Gosz, D.I. Moore, Strontium isotope studies of atmospheric inputs to forested watersheds in New Mexico, Biogeochemistry 8 (1989) 115–134. [23] W.C. Graustein, 87 Sr=86 Sr ratios measure the source and flow of strontium in terrestrial ecosystems, in: Rundel, P.W., Ehleringer, J.R., Nagy, K.A. (Eds.), Stable Isotopes in Ecological Research 68 (1989) 491–512, Springer-Verlag, New York. [24] J.D. Blum, Y. Erel, K. Brown, 87 Sr=86 Sr ratios of Sierra Nevada stream waters: implications for relative mineral weathering rates, Geochem. Cosmochim. Acta 58 (1994) 5019–5025. [25] H.C. Monger, Soil mineral transformations in a southern New Mexico Aridisol: pedogenic palygorskite, mineral dissolution, and microbial-related calcite, Ph.D. dissertation, New Mexico State University, 1990. [26] H.C. Monger, L.A. Daugherty, Neoformation of palygorskite in a southern New Mexico Aridisol, Soil Sci. Soc. Am. J. 55 (1991) 1646–1650. [27] Y. Wang, D. Nahon, E. Merino, Dynamic model of the
[28]
[29]
[30] [31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
71
genesis of calcretes replacing silicate rocks in semi-arid regions, Geochim. Cosmochim. Acta 58 (1994) 5131–5145. L.H. Gile, J.W. Hawley, R.B. Grossman, Soils and geomorphology in the Basin and Range area of southern New Mexico, Guidebook to the Desert Project, New Mexico Bureau of Mines and Mineral Resources, Socorro, 1981, 222 pp. L.H. Gile, F.F. Peterson, R.B. Grossman, Morphological and genetic sequences of carbonate accumulation in desert soils, Soil Sci. 101 (1966) 347–360. M.N. Machette, Calcic soils of the American Southwest, Spec. Pap. Geol. Soc. Amer. 203 (1985) 1–22. S.G. Wells, L.D. McFadden, J.C. Dohrenwend, Influence of Late Quaternary climate changes and pedogenic processes on a desert piedmont, eastern Mojave Desert, California, Quaternary Res. 27 (1987) 130–146. L.D. McFadden, R.G. Amundson, O.A. Chadwick, Numerical modeling, chemical, and isotopic studies of carbonate accumulation in soils of arid regions, in: Nettleton, W.D. (Ed.), Occurrence, Characteristics and Genesis of Carbonate, Gypsum, and Silica Accumulations in Soils, SSSA Spec. Publ. 26 (1991) 17–35, Soil Science Society of America, Madison. M.C. Reheis, R. Kihl, Dust deposition in southern Nevada and California, 1984–1989: relations to climate, source area, and source lithology, J. Geophys. Res. 100 (1995) 8893–8918. M.C. Rabenhorst, L.P. Wilding, C.L. Girdner, Airborne dusts in the Edwards Plateau region of Texas, Soil Sci. Soc. Am. J. 48 (1984) 621–627. J.M. Prospero, E. Bonatti, C. Schubert, T.N. Carlson, Dust in the Caribbean atmosphere traced to an African dust storm, Earth Planet. Sci. Lett. 9 (1970) 287–293. D.R. Muhs, C.A. Bush, T.R. Rowland, C.C. Stewart, Geochemical evidence of Saharan dust parent material for soils developed on Quaternary limestones of Caribbean and western Atlantic islands, Quaternary Res. 33 (1990) 157–177. G.H. Brimhall, J. Donovan, B. Singh, Provenance and discordance mechanism of zircons in the Australian regolith using SHRIMP and electron microprobe mapping, Abstr. 8th Int. Conf. Geochron. Cosmochron. Isotope Geol., US Geol. Surv. Circ. 1107 (1994) 39. L. Mayer, L.D. McFadden, J.W. Harden, Distribution of calcium carbonate in desert soils: a model, Geology 16 (1988) 303–306. C.E. Junge, R.T. Werby, The concentration of chloride, sodium, calcium and sulfate in rain water over the United States, J. Meteorol. 15 (1958) 417–425. W.H. Schlesinger, J.T. Gray, F.S. Gilliam, Atmospheric deposition processes and their importance as sources of nutrients in a chaparral ecosystem of southern California, Water Resources Res. 18 (1982) 623–629. ˚ berg, G. Jacks, T. Wickman, P.J. Hamilton, Strontium G. A isotopes in trees as an indicator for calcium availability, Catena 17 (1990) 1–11. E.K. Miller, J.D. Blum, A.J. Friedland, Determination of
72
R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
soil exchangeable-cation loss and weathering rates using Sr isotopes, Nature 362 (1993) 438–441. [43] J. Quade, A.R. Chivas, M.T. McCulloch, Strontium and carbon isotope tracers and the origins of soil carbonate in South Australia and Victoria, Palaeogeogr. Palaeoclimatol. Palaeoecol. 113 (1995) 103–117. [44] S.W. Bailey, J.W. Hornbeck, C.T. Driscoll, H.E. Gaudette, Calcium inputs and transport in a base-poor forest ecosystem as interpreted by Sr isotopes, Water Resources Res. 32 (1996) 707–719.
[45] R.C. Capo, B.W. Stewart, O.A. Chadwick, Strontium isotopes as tracers of ecosystem processes: theory and methods, Geoderma 83 (1998) 515–524. [46] B.W. Stewart, R.C. Capo, O.A. Chadwick, Quantitative strontium isotope models for weathering, pedogenesis and biogeochemical cycling, Geoderma 83 (1998) 525–537. [47] L.D. McFadden, The impacts of temporal and spatial climatic changes on alluvial soils genesis in southern California, Ph.D. thesis, University of Arizona, 1982.
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 Don˜a 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 Don˜a 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)
64
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
66
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 Don˜a Ana Mtns Site 5 E. Don˜a 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. Don˜a 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.
68
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.
70
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-
References [1] L.H. Gile, F.F. Peterson, R.B. Grossman, The K horizon: a master soil horizon of carbonate accumulation, Soil Sci. 99 (1965) 74–82. [2] H. Eswaran, E. Ven Den Berg, P. Reich, Organic carbon in soils of the world, Soil Sci. Soc. Am. J. 57 (1993) 192–194. [3] H. Eswaran, E. Van Den Berg, P. Reich, J. Kimble, Global soil carbon resources, in: Lal, R., Kimble, J., Levine, E., Stewart, B.A. (Eds.), Soils and Global Change, CRC Lewis, Boca Raton, FL, 1995, pp. 27–43. [4] O.A. Chadwick, E.F. Kelly, D.M. Merritts, R.G. Amundson, Carbon dioxide consumption during soil development, Biogeochemistry 24 (1994) 115–127. [5] R.A. Berner, A.C. Lasaga, R.M. Garrels, The carbonate– silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years, Am. J. Sci. 283 (1983) 641–683. [6] L.H. Gile, R.B. Grossman, The Desert Project soil monograph, US Government Printing Office, Washington, DC, 1979, 984 pp. [7] L.D. McFadden, J.C. Tinsley, Rate and depth of pedogenic carbonate accumulation in soils: formulation and testing of a compartment model, in: Weide, D.L. (Ed.), Spec. Pap. Geol. Soc. Am. 203 (1985) 23–42. [8] O.A. Chadwick, J.O. Davis, Soil-forming intervals caused by eolian sediment pulses in the Lahontan basin, northwestern Nevada, Geology 18 (1990) 243–246. [9] M.C. Reheis, J.C. Goodmacher, J.W. Harden, L.D. McFadden, T.K. Rockwell, R.R. Shroba, J.M. Sowers, E.M. Taylor, Quaternary soils and dust deposition in southern Nevada and California, Geol. Soc. Am. Bull. 107 (1995) 1003–1022. [10] R.W. Simonson, Airborne dust and its significance to soils, Geoderma 65 (1995) 1–43.
R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72 [11] W.B. Bull, Geomorphic responses to climatic change, Oxford University Press, New York, 1991. [12] O.A. Chadwick, W.D. Nettleton, G.J. Staidl, Soil polygenesis as a function of climate change, northern Great Basin, USA, Geoderma 68 (1995) 1–26. [13] R.V. Ruhe, Geomorphic surfaces and surficial deposits in southern New Mexico, New Mexico State Bur. Mines Min. Res. Mem. 18, 1967, 65 pp. [14] J.W. Hawley, F.E. Kottlowski, Quaternary geology of the south-central New Mexico border region, New Mexico Bur. Mines Min. Res. Circ. 104 (1969) 89–115. [15] G.O. Bachman, H.H. Mehnert, New K–Ar dates and the late Pliocene to Holocene geomorphic history of the central Rio Grande region, New Mexico, Geol. Soc. Am. Bull. 89 (1978) 283–292. [16] G.H. Mack, W.C. James, Calcic paleosols of the Plio– Pleistocene Camp Rice and Palomas Formations, southern Rio Grande Rift, USA, Sediment. Geol. 77 (1992) 89–109. [17] G.H. Mack, S.L. Salyards, C. James, Magnetostratigraphy of the Plio–Pleistocene Camp Rice and Palomas Formations in the Rio Grande rift of southern New Mexico, Am. J. Sci. 293 (1993) 49–77. [18] H.C. Monger, L.A. Daugherty, Pressure solution: possible mechanism for silicate grain dissolution in a petrocalcic horizon, Soil Sci. Soc. Am. J. 55 (1991) 1625–1629. [19] O.A. Chadwick, G.H. Brimhall, D.M. Hendricks, From a black to a gray box — a mass balance interpretation of pedogenesis, Geomorphology 3 (1990) 369–390. [20] G.H. Brimhall, O.A. Chadwick, C.J. Lewis, W. Compston, I.S. Williams, K.J. Danti, W.E. Dietrich, M.E. Power, D. Hendricks, J. Bratt, Deformational mass transport and invasive processes in soil evolution, Science 255 (1991) 695– 702. [21] J.B. Maynard, Chemistry of modern soils as a guide to interpreting Precambrian paleosols, J. Geol. 100 (1992) 279–289. [22] J.R. Gosz, D.I. Moore, Strontium isotope studies of atmospheric inputs to forested watersheds in New Mexico, Biogeochemistry 8 (1989) 115–134. [23] W.C. Graustein, 87 Sr=86 Sr ratios measure the source and flow of strontium in terrestrial ecosystems, in: Rundel, P.W., Ehleringer, J.R., Nagy, K.A. (Eds.), Stable Isotopes in Ecological Research 68 (1989) 491–512, Springer-Verlag, New York. [24] J.D. Blum, Y. Erel, K. Brown, 87 Sr=86 Sr ratios of Sierra Nevada stream waters: implications for relative mineral weathering rates, Geochem. Cosmochim. Acta 58 (1994) 5019–5025. [25] H.C. Monger, Soil mineral transformations in a southern New Mexico Aridisol: pedogenic palygorskite, mineral dissolution, and microbial-related calcite, Ph.D. dissertation, New Mexico State University, 1990. [26] H.C. Monger, L.A. Daugherty, Neoformation of palygorskite in a southern New Mexico Aridisol, Soil Sci. Soc. Am. J. 55 (1991) 1646–1650. [27] Y. Wang, D. Nahon, E. Merino, Dynamic model of the
[28]
[29]
[30] [31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
71
genesis of calcretes replacing silicate rocks in semi-arid regions, Geochim. Cosmochim. Acta 58 (1994) 5131–5145. L.H. Gile, J.W. Hawley, R.B. Grossman, Soils and geomorphology in the Basin and Range area of southern New Mexico, Guidebook to the Desert Project, New Mexico Bureau of Mines and Mineral Resources, Socorro, 1981, 222 pp. L.H. Gile, F.F. Peterson, R.B. Grossman, Morphological and genetic sequences of carbonate accumulation in desert soils, Soil Sci. 101 (1966) 347–360. M.N. Machette, Calcic soils of the American Southwest, Spec. Pap. Geol. Soc. Amer. 203 (1985) 1–22. S.G. Wells, L.D. McFadden, J.C. Dohrenwend, Influence of Late Quaternary climate changes and pedogenic processes on a desert piedmont, eastern Mojave Desert, California, Quaternary Res. 27 (1987) 130–146. L.D. McFadden, R.G. Amundson, O.A. Chadwick, Numerical modeling, chemical, and isotopic studies of carbonate accumulation in soils of arid regions, in: Nettleton, W.D. (Ed.), Occurrence, Characteristics and Genesis of Carbonate, Gypsum, and Silica Accumulations in Soils, SSSA Spec. Publ. 26 (1991) 17–35, Soil Science Society of America, Madison. M.C. Reheis, R. Kihl, Dust deposition in southern Nevada and California, 1984–1989: relations to climate, source area, and source lithology, J. Geophys. Res. 100 (1995) 8893–8918. M.C. Rabenhorst, L.P. Wilding, C.L. Girdner, Airborne dusts in the Edwards Plateau region of Texas, Soil Sci. Soc. Am. J. 48 (1984) 621–627. J.M. Prospero, E. Bonatti, C. Schubert, T.N. Carlson, Dust in the Caribbean atmosphere traced to an African dust storm, Earth Planet. Sci. Lett. 9 (1970) 287–293. D.R. Muhs, C.A. Bush, T.R. Rowland, C.C. Stewart, Geochemical evidence of Saharan dust parent material for soils developed on Quaternary limestones of Caribbean and western Atlantic islands, Quaternary Res. 33 (1990) 157–177. G.H. Brimhall, J. Donovan, B. Singh, Provenance and discordance mechanism of zircons in the Australian regolith using SHRIMP and electron microprobe mapping, Abstr. 8th Int. Conf. Geochron. Cosmochron. Isotope Geol., US Geol. Surv. Circ. 1107 (1994) 39. L. Mayer, L.D. McFadden, J.W. Harden, Distribution of calcium carbonate in desert soils: a model, Geology 16 (1988) 303–306. C.E. Junge, R.T. Werby, The concentration of chloride, sodium, calcium and sulfate in rain water over the United States, J. Meteorol. 15 (1958) 417–425. W.H. Schlesinger, J.T. Gray, F.S. Gilliam, Atmospheric deposition processes and their importance as sources of nutrients in a chaparral ecosystem of southern California, Water Resources Res. 18 (1982) 623–629. ˚ berg, G. Jacks, T. Wickman, P.J. Hamilton, Strontium G. A isotopes in trees as an indicator for calcium availability, Catena 17 (1990) 1–11. E.K. Miller, J.D. Blum, A.J. Friedland, Determination of
72
R.C. Capo, O.A. Chadwick / Earth and Planetary Science Letters 170 (1999) 61–72
soil exchangeable-cation loss and weathering rates using Sr isotopes, Nature 362 (1993) 438–441. [43] J. Quade, A.R. Chivas, M.T. McCulloch, Strontium and carbon isotope tracers and the origins of soil carbonate in South Australia and Victoria, Palaeogeogr. Palaeoclimatol. Palaeoecol. 113 (1995) 103–117. [44] S.W. Bailey, J.W. Hornbeck, C.T. Driscoll, H.E. Gaudette, Calcium inputs and transport in a base-poor forest ecosystem as interpreted by Sr isotopes, Water Resources Res. 32 (1996) 707–719.
[45] R.C. Capo, B.W. Stewart, O.A. Chadwick, Strontium isotopes as tracers of ecosystem processes: theory and methods, Geoderma 83 (1998) 515–524. [46] B.W. Stewart, R.C. Capo, O.A. Chadwick, Quantitative strontium isotope models for weathering, pedogenesis and biogeochemical cycling, Geoderma 83 (1998) 525–537. [47] L.D. McFadden, The impacts of temporal and spatial climatic changes on alluvial soils genesis in southern California, Ph.D. thesis, University of Arizona, 1982.