Isotopic evidence for eolian recycling of pedogenic carbonate and variations in carbonate dust sources throughout the southwest United States

Geochimica et Cosmochimica Acta, Vol. 64, No. 18, pp. 3099 –3109, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 ⫹ .00

Pergamon

PII S0016-7037(00)00410-5

Isotopic evidence for eolian recycling of pedogenic carbonate and variations in carbonate dust sources throughout the southwest United States ZACHARY NAIMAN, JAY QUADE, and P. JONATHAN PATCHETT* Department of Geosciences, University of Arizona, Tucson AZ 85721, USA (Received January 11, 1999; accepted in revised form March 30, 2000)

Abstract—Using isotopic ratios of Sr, C and O, we trace calcium carbonate through surface systems across a wide region of semi-arid terrain in Arizona, southwestern USA, in order to evaluate the contribution of cations from silicate weathering to soil carbonate. We present 87Sr/86Sr ratios of soil carbonate, parent rock, dry river course silts, floodplain, playa, dust and rain samples, as well as ␦18O and ␦13C values of selected samples. Results show that both parent rock and dust are important sources of cations for soil carbonate in this inland setting where bedrock is dominated by silicate lithologies. Dust in southeast Arizona has higher 87 Sr/86Sr ratios (0.7100 – 0.7123) than Phanerozoic sea water (0.7070 – 0.7096). These high ratios derive ultimately from silicate rocks. Our ␦18O and ␦13C data show clearly that the dominant source of carbonate dust is eroded older soil carbonate, not bedrock limestone. Because dust contributes significantly to newly-forming soil carbonate, some products of silicate weathering may reside in soil carbonate two or more times before being removed from the region, and this recycling retards the rate at which the products of silicate weathering enter the sedimentary system. Comparison of the 87Sr/86Sr ratios of carbonate dust from southeast Arizona with those from surrounding regions shows that dust 87Sr/86Sr ratios, and by inference carbonate dust sources, vary on a scale of 200 –300 km in the southwest United States. Copyright © 2000 Elsevier Science Ltd carbonate (West et al., 1988; Rabenhorst et al., 1984; Eghbal and Southard, 1993). Other studies argue that dust is the dominant source of calcium (Chiquet et al., 1999; Capo and Chadwick, 1999; Van der Hoven and Quade, 1994; Stewart et al., 1998). These studies have used detailed soil profiles, soil particle size and density analysis, petrographic observations, and trace element and isotope data to support models which preclude a significant contribution of calcium from parent rock at the sites studied. We sampled soils and parent rocks for Sr isotopic analysis on a regional scale, encompassing Arizona and one site in eastern California, in order to evaluate whether these conclusions hold on a large geographic scale. Many studies demonstrate that dust is an important source of Ca in soils (e.g., McFadden and Tinsley, 1985; Whipkey et al., 1999; Reheis et al., 1995; Reheis and Kihl, 1995). Researchers have identified alluvial, playa, bedrock, and agricultural sources of dust (Reheis and Kihl, 1995; Pe´we´ et al., 1981), but have not utilized isotopic tracers to constrain the ultimate source of the calcium carbonate fraction. Reheis and Kihl (1995) showed that fluxes of carbonate dust are greatest downwind of carbonate bedrock, suggesting that limestone and dolostone may be the dominant source for carbonate dust. Significant amounts of carbonate are observed in dust from regions with dominantly silicate bedrock (Reheis and Kihl, 1995; Pe´we´ et al., 1981). In such cases, playas, floodplains, or calcareous soils may be the dominant sources. To evaluate these potential sources, we analyzed the 87Sr/86Sr ratios, ␦13C, and ␦18O values of soil carbonate, carbonate bedrock, dust, wash silts, flood-plain silts, playa silts, and rain samples from the Tucson, Arizona area.

1. INTRODUCTION

Numerous authors have linked global climate change to continental weathering rates because the chemical weathering of silicate minerals consumes atmospheric CO2, which is the most significant greenhouse gas (e.g., Raymo and Ruddiman, 1992; Berner, 1991). Calcium is a common constituent of silicate rocks and is the dominant cation in continental waters; knowledge of the sources and sinks of calcium is essential for constraining the global weathering budget. Pedogenic carbonate is an important continental calcium reservoir because thick petrocalcic horizons characterize many soils in arid and semi-arid regions. The origin of soil carbonate, especially petrocalcic horizons developed on silicate rocks, has been debated for more than sixty years. Isotopic data provide a powerful means to trace the movement of elements through surface systems (Graustein and Armstrong, 1983; Starinsky et al., 1987; Miller et al., 1993; Clow et al., 1997). The 87Sr/86Sr ratio and the ␦13C and ␦18O values have been used to trace the origin of soil calcium, carbon, and oxygen in soil carbonate (e.g., Nordt et al. [editors], 1998). The ␦18O value of soil carbonate mainly derives from soil waters, whereas plant carbon, through respiration and decay, dominates the carbon component (Cerling, 1984; Quade et al., 1989; Cerling and Quade, 1993; Amundson et al., 1998; Hsieh et al., 1998). Calcium is derived from chemical weathering of the local parent rock and from atmospheric contributions to soils, including dust, precipitation, and sea-spray (Quade et al., 1995; Van der Hoven and Quade, 1994; Chiquet et al., 1999; Capo et al., 1998; Stewart et al., 1998). Studies of soil carbonate from inland continental settings which are removed from the influence of sea-spray indicate that parent rock is the most significant source of calcium in soil

2. GEOLOGIC SETTING

Southern Arizona is part of the Basin and Range physiographic province, and is characterized by mountain ranges

*Author to whom correspondence should be addressed (patchett@ geo.arizona.edu). 3099

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Z. Naiman, J. Quade, and P. J. Patchett

Fig. 1. a) Generalized geologic map of the Tucson area (modified from Reynolds, 1988) showing sample locations (numbers correspond to sample descriptions in Table 1). b) Regional map of Arizona (and surrounding states) showing sample locations and the location of the Tucson area map.

separated by extensive arid to semi-arid basins. Igneous lithologies dominate rock exposures in southern Arizona; carbonate bedrock is not very common. In contrast, there is an abundance of Mesozoic limestone exposed on the Colorado Plateau in northern Arizona. Southwest Arizona is near the Gulf of California; it is likely that sea spray is an important source of Ca in soils from this region. Most samples for this study come from in and around the Tucson Basin (Fig. 1). Mountain ranges surrounding Tucson are dominated by Precambrian to Cenozoic igneous rocks;

Paleozoic limestones constitute a small portion of exposed bedrock. Soils developed on alluvial fans and within basins commonly contain thick, indurated petrocalcic horizons below clay-rich argillic horizons, indicating that arid to semi-arid conditions have prevailed for ⬎500 ka (Birkeland et al., 1991). The Tucson basin is surrounded by four main mountain ranges (Fig. 1). The Tucson Mountains are mainly composed of Mesozoic felsic volcanic rocks with minor Mesozoic sedimentary rocks and intermediate to mafic Tertiary volcanic units. The Tortolita, Catalina, and Rincon Mountains contain a mix-

Recycling in soil carbonate

ture of Precambrian and Phanerozoic mid-crustal silicate rocks, with minor exposures of Paleozoic sediments. 3. SAMPLES AND SAMPLING STRATEGY The sampling strategy for this study was designed to examine the contribution of silicate mineral weathering to calcium in soil carbonates from the southwest United States. Specifically, three issues are examined: 1. variations in soil carbonate 87Sr/86Sr ratios with parent rock 87Sr/ 86 Sr ratios in the Tucson area, an inland continental setting where there is minimal carbonate bedrock available as a dust source; 2. the source of carbonate dust in the Tucson area; and 3. regional variations in soil 87Sr/86Sr ratios as a function of local carbonate dust sources. Two suites of samples were collected in order to address these issues. One suite includes a variety of sample types (soil carbonate, bedrock, dust, wash silts, flood-plain silts, playa silts, and rain) from the Tucson area, whereas the other includes soil and parent basalt samples from throughout Arizona, including one site in southeastern California. Soils from the Tucson area were selected for sampling in order to observe local variations in the 87Sr/86Sr ratio of soil carbonates due to variation in the parent material composition. Samples were obtained from soil carbonate and associated parent material which exhibits the widest possible range of 87Sr/86Sr ratios (Fig. 1, Table 1). The 87Sr/86Sr ratio of a rock is a time-integrated function of its Rb content. Old, Rb-rich silicate rocks (e.g., Precambrian granite) have high (radiogenic) 87Sr/86Sr ratios (⬎0.712) compared to young, Rb-poor silicate rocks (e.g., Quaternary basalt; ⬃0.703– 0.706) and marine limestone (⬍0.7096). Soil carbonates sampled for Sr analysis were not selected with intent to control for the variables of depth in soil, carbonate morphology, age of soil carbonate, or stage of soil carbonate development (Table 1). However, soil carbonates sampled for carbon and oxygen isotopic analysis (Table 2) were taken from ⬎1 meter depth, where the ␦13C value of soil CO2 is assumed to be constant (Cerling and Quade, 1993). Samples of soil carbonate developed on polylithic alluvium were collected to constrain the average Sr isotopic composition of soil carbonates from a given region where multiple rock lithologies are exposed. Dust deposited on a landscape can contribute strontium and calcium to soil carbonate in two ways: 1. the carbonate and labile (soil-water exchangeable) fraction of dust may be dissolved by meteoric waters, which re-precipitate the carbonate at depth; and 2. silicate phases in the dust may be weathered in-situ, releasing Ca which is translocated and precipitated at depth as calcium carbonate. The first mechanism is probably more important in Arizona because carbonate dust deposition fluxes are high, and carbonate weathering rates are rapid compared to weathering rates for silicates. We collected eleven dust samples from locations near Tucson, Arizona for isotopic analysis of the carbonate and labile fraction (Table 1). Dust can be sampled while in the air, from artificial dust traps, or from locations where dust collects naturally. We sampled naturally deposited dust in order to examine its time-averaged isotopic composition because the significance of various dust sources is likely dependent on time-related variables. For example, playas are much more important sources of dust during dry years following wet years (Reheis and Kihl, 1995). The cation (e.g., Ca, Sr) component in carbonate dust must ultimately derive from carbonate bedrock and/or soil carbonate. Carbonate dust may come from outcrops of bedrock or soils themselves, or from their alluvial derivatives, such as washes, floodplains, and playas. Carbonate bedrock was sampled from the Whetstone Mountains (location 20, Fig. 1a), one of the largest exposures of carbonate rocks in southern Arizona, and from the Rincon Mountains and the Tucson Mountains (locations 6a and 13, respectively, Fig. 1a). Silt-sized material was collected from washes which drain the mountain ranges surrounding the Tucson Basin, and from the Santa Cruz wash which drains the Tucson Basin. Material from Willcox Playa (Fig. 1b location

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22) and the Gila Bend (location 15) flood plain were sampled for isotopic characterization of the major dust sources to the east and west of Tucson, respectively. Willcox Playa may be an important dust source in Tucson during the summer when storms blow in from the east and south. Gila Bend is likely an important dust source in the winter when storms arrive from the west (Pe´we´ et al., 1981). Rain water, which equilibrates with fine-grained, tropospheric dust, was collected in the late August of 1998 with acid-washed polyethylene funnels and containers. Soil profiles were sampled on a regional basis for strontium analysis (Fig. 1b). For this aspect of the study, soils developed on young basalt were sampled because young basalt typically has a very low and homogenous 87Sr/86Sr ratio. The Sr source partitioning for such soils is very simple, in contrast to soils developed on older, felsic rocks where the 87Sr/86Sr released by weathering depends strongly on which mineral is being weathered. At each location, samples were taken from soil A-horizons, continuous petrocalcic horizons, discontinuous soil carbonate at depth, and the parent basalt. These analysis place constraints on the Sr isotopic composition of dust at each location and provide a means to compare dust sources throughout the southwest United States. 4. ANALYTICAL PROCEDURES Soil carbonate rind coatings were scraped off parent cobbles with a dental tool, and dust samples deposited outdoors were sieved to remove large contaminants (mostly organic material). Soil carbonate, wash, flood-plain, and playa samples were ground to a fine powder before dissolution; dust and soil A-horizons were not ground before acid leaching. For strontium analysis, ⬃50 mg of each sample was reacted with ultra-pure 1 M acetic acid in an ultrasonic bath for half an hour. This dissolution technique allows for the measurement of only the carbonate and labile fraction (surfaces complexes on silicate phases and interlayer cations of clays) of each sample. Asahara et al. (1995) showed that weak acetic acid leaches insignificant amounts of Sr from silicate phases. Acetic acid solutions and filtered rain waters were evaporated to dryness, and precipitates were dissolved in 2.5 M HCl. Whole-rock samples of silicate bedrock were dissolved with HF and HNO3 in closed teflon containers, evaporated to dryness, and the precipitates were dissolved in 2.5 M HCl. All solutions were passed through cation exchange columns to separate strontium. Strontium isotopic ratios were measured on a VG Sector 54 mass spectrometer at the University of Arizona. During the time these samples were analyzed, measurements of NBS-987 Sr standard averaged 87Sr/86Sr ⫽ 0.710262 ⫾ 3 (2␴ standard error, n ⫽ 62). Most samples selected for carbon and oxygen isotopic analysis were treated in 6% H2O2 to remove organic contaminants. Samples were converted to CO2 using anhydrous H3PO4 in closed, evacuated vessels at 50°C. Per cent calcium carbonate was determined manometrically. Carbon and oxygen isotopic ratios were measured on a Finnigan Delta-S gas ratio mass spectrometer at the University of Arizona. Standard isotopic corrections were made and the isotopic values are reported in ␦ notation relative to the isotopic standard PDB. The precision of the laboratory carbonate standard is ⫾0.1‰ (1␴) for both carbon and oxygen isotope values. 5. RESULTS AND DISCUSSION

5.1. Plotting and Discussion of the Data This section describes the ways in which the Sr, O and C isotopic data are divided for presentation and interpretation. First, we have an extensive set of Sr data (Table 1; Figs. 2, 3 and 4) for soil carbonates, bedrocks of widely varying composition, alluvial silt and eolian dust in an area within approximately 150 km radius of Tucson, Arizona (most samples are within 50 km, see Fig. 1). Figures 2 and 3 also contain basaltic bedrocks and soil carbonates from the regional sample set, more than 150 km away from Tucson, when these are important for discussion of issues raised by the Tucson area Sr results. The Tucson-area data are used to discuss relationships between

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Z. Naiman, J. Quade, and P. J. Patchett Table 1. Sample names, locations, descriptions, and Sr isotopic ratios with standard errors. Soils and parent rocks, within 150 km of Tucson

Regiona

Lithology and locationb

Descriptionc

Sample

87

Sr/86Sr 2␴d

Tucson Stage IV soil carbonate on ⬃20 Ma Tumamoc Mountains (5a) basaltic andesite, Tumamoc Hill, Tucson

AK-1a AK-1b AK-3a TAK-1

clast rind exposed at surface near top of hill basaltic cobble from which AK-1a was derived clast rind exposed at surface ⬃10 m below AK-1 A-horizon

0.709215 0.707769 0.709509 0.709173

Tucson Stage I soil carbonate on Cretaceous algal Mountains (13) laminated marl of the Amole Arkose (Bisbee Group correlative) near Kinney Road ⬃4 miles N of Gates Pass Road

AK-20a AK-20b

marl clast rind from surface

0.710982 17 0.711478 13

Tucson Stage III soil carbonate on polylithic alluvium Mountains (5b) composed of late Mesozoic and Cenozoic rhyolitic to basaltic volcanics, exposed in wash at W Anklam Road

AK-11a

matrix carbonate ⬃3 m below surface, exposed in wash

0.709705 13

Tortolita Stage IV soil carbonate on monolithic alluvium Mountains (19) composed of mid-Tertiary Catalina quartz monozonite, exposed in wash

TK-1a TK-1b TK-1c

matrix carbonate from ⬃2 m depth clast rind from ⬃3 m depth clast rind from ⬃1 m depth

0.709536 7 0.709596 14 0.709759 20

Santa Catalina Mountains (1)

OK-1a OK-1b PK-1a PK-1b

surface carbonate on Oracle Granite Oracle Granite saprolite surface carbonate on San Manuel qtz monzonite San Manuel qtz monzonite saprolite

0.719024 0.788912 0.712696 0.722398

Surface carbonate development on saprolitic 1.4 Ga Oracle Granite and 60 Ma Laramide porphyry (in intrusive relationship), Purcell Window at San Manuel mine

20 18 16 14

40 19 34 13

Rincon foothills Stage II⫹ soil carbonate on polylithic alluvium RAK-2 matrix carbonate exposed by road cut, ⬃1 m depth 0.714885 24 (14) from Rincon Mtns., exposed by roadcut ⬃5 miles from termination of E. Speedway Blvd, overlies stage II⫹ soil carbonate on midRAK-1a Pantano Fm. (plus soil carbonate) 0.712878 17 Tertiary Pantano Formation (contains clasts of Paleozoic LS) Rincon Stage I soil carbonate on jasper (silica replaced Mountains (6a) LS) 50 m W of Old Spanish Trail, ⬃0.5 miles N of Colossal Cave Monument

CK-10 CK-1a CK-1b

eolian silt from top of hill clast rind from colluvial apron, ⬃1.5 m from hill bottom matrix carbonate from same location as CK-1a

0.710060 20 0.710960 36 0.710986 24

Maricopa Mountains (3)

Stage IV soil carbonate on ⬃1.7 Ga Pinal Schist, XK-1a road cut on I-8, ⬃10 miles E of Gila Bend XK-1b

clast rind from ⬃2 m depth Pinal Schist cobble from ⬃2 m depth

0.712108 14 0.811899 16

Whetstone Mountains (2)

Stage I soil carbonate on monolithic alluvium composed of 1.4 Ga quartz monzonite, exposed by road cut on AZ-90, ⬃4 miles south of Benson

YgK-1a YgK-1b YgK-1c

clast rind from ⬃1 m depth matrix carbonate from ⬃1 m depth quartz monzonite cobble from ⬃1 m depth

0.717732 24 0.718162 23 1.879958 30

Rincon Stage II soil carbonate on Mississippian Mountains (6a) Escabrosa LS

CK-2 CK-11

clast rind exposed at surface 0.711539 26 fossiliferous LS with local hydrothermal alteration 0.710055 17

Whetstone Paleozoic strata with discontinuous surface Mountains (20) carbonate development, from French Joe Canyon, AZ-90, ⬃11 miles south of Benson

WMK-2a WMK-2b WMK-3a WMK-3b WMK-5 WMK-1b WMK-7

Abrigo LS (Cambrian) surface carbonate on Abrigo LS dolomitized Escabrosa LS (Pennsylvania) surface carbonate on Escabrosa LS Earp formation (Permian LS with silicic clasts) surface carbonate from Bolsa Qtzite (Cambrian) matrix carbonate from polylithic LS alluvium

Stage IV soil carbonate on polylithic LS alluvium Stage III⫹ soil carbonate from roadcut on AZ-90

MCK-1a matrix carbonate from polylithic LS alluvium

0.709957 0.709962 0.712173 0.710572 0.709748 0.710891 0.709440

13 14 16 13 16 14 16

0.709944 23

Silts from washes, Tucson area Locationa (16) Santa Cruz Wash at Ajo Way, south of Tucson (17) Santa Rosa Wash (Santa Cruz Wash) at I-8, NW of Tucson (23) Tanque Verde Creek (Catalina-Rincon Mountains) at Country Club Blvd. (14b) Tanque Verde Creek tributary, ⬃1 mile from end of E. Speedway Blvd. (5b) Anklam Wash (Tucson Mountains) at Anklam Road (19) Can˜ada Agua Wash (Tortolita Mtns.), ⬃300 m N of Tangerine Road

Sample

Sr/86Sr 2␴d

87

SCK-1 SCK-2 TVCK-1

0.709274 14 0.710257 20 0.714736 13

RAK-3

0.714247 13

AK-11b TK-10

0.709755 14 0.709957 28 (Continued)

Recycling in soil carbonate

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Table 1. (Continued) Playa and Flood Plain, within 150 km of Tucson Locationa

Descriptionc

Sample

87

Sr/86Sr 2␴d

Willcox Playa (22) by milepost 65 on US-191

WK-1 WK-2 WK-3

silts, ⬃500 m E of US-191 (central part of playa) 0.712360 37 sands, ⬃300 m E of US-191 0.711380 14 carbonate, ⬃100 m E of US-191 (edge of playa) 0.710489 16

Gila Bend flood plain, ⬃4 miles N of exit 111 on I-8 (15)

GBK-1 GBK-2

flood plain silts soil carbonate rind on clast from surface

0.710902 16 0.710308 16

Dust, Tucson area Locationa

Deposition

Precise location

Sample

Setting

Sr/86Sr

2␴d

87

Tucson Mtns (5)

⬎5 yrs ⬎5 yrs

U of A Desert Lab. on Tumamoc Hill

TDK-1 TDK-2

window ledge ⬃3 m off ground, exposed to precip. wooden shelf inside, ⬃1.5 m off ground

0.710260 0.709986

34 28

Tucson basin (18)

⬎40 yrs

721 N 2nd Ave

LTK-1

wooden eave in church attic, ⬃10 m off ground

0.710277

18

Tucson basin (7)

⬍3 yrs ⬍1 yr

9335 E Rosewood

RK-1a RK-1b

ceiling fan in covered porch, ⬃3 m off ground gutter, exposed to precipitation, ⬃4 m off ground

0.710550 0.710723

44 20

Tucson basin (8)

⬎2 yrs

4320 N Alvernon

CDK-1

wooden eave in open car port, ⬃3.5 m off ground

0.710954

26

Tucson basin (9)

⬎1 yr

3001 E Drachman St

MK-1

wooden shelf in shed, ⬃4 m off ground

0.711374

33

Catalina Mtns (10) Rincon Mtns (6b) Tucson Mtns (11) Catalina Mtns (12)

⬎5 yrs ⬍1 yr ⬎10 yrs ⬍1 yr

641 E Windward Circle Colossal Cave Monument 3251 N Six Bar Spur Summerhaven Post Office

VK-1 CK-12 JK-1 CHK-1

wooden eave in open car port, ⬃3.5 m off ground shingled roof of visitor center, ⬃5 m off ground metal surface in garage, ⬃6 m off ground wooden eave in open porch, ⬃3 m off ground

0.711925 0.712256 0.711360 0.710396

57 61 41 30

Rain, Tucson area Locationa

Description

Sample

Tucson Basin (21)

⬃800 ml from ⬃6 rain events over 8 days, filtered ⬃100 ml from 1 rain event filtered immediately

TR-1 TR-2

87

Sr/86Sr

2␴d

0.710712 30 0.711151 18

Soils and parent basalt, Arizona and adjacent California a

Region

Lithology and locationb

Sample

Descriptionc

87

Sr/86Sr 2␴d

Geronimo Volcanic Field (4)

GK-1a GK-1b GK-2a GK-10 GK-11 GK-12 GK-13 GK-15 Quaternary alluvium, ⬃2 miles south of cinderconeGK-14

clast rind from surface near top of cindercone 0.705183 clast rind from surface near top of cindercone 0.704886 surface carbonate from fracture exposed in road cut 0.705830 A-horizon on grey/maroon mottled cinder 0.704927 continuous petrocalcic horizon, ⬃35 cm below GK-10 0.705102 discontinuous surface carbonate, ⬃1.75 m below GK-10 0.705164 discontinuous surface carbonate, ⬃2.75 m below GK-10 0.705122 basalt from ⬃2 m below GK-10 0.703314 A-horizon 0.707700

44 17 24 11 16 15 12 27 14

Flagstaff (24)

Stage II soil carbonate on Quaternary basalt, I-40 E, 0.2 miles west of mile marker 211

FK-5 FK-6 FK-7 FK-10

A-horizon continuous petrocalcic horizon, ⬃60 cm below FK-5 discontinuous surface carbonate, ⬃2 m below FK-5 basalt from ⬃2 m below FK-5

0.706202 0.706470 0.707127 0.703792

11 10 14 12

Springerville Stage II⫹ soil carbonate on Quaternary basalt, Volcanic US-60 E, 0.1 mile east of mile marker 368 Field (25)

SVK-3 SVK-4 SVK-5 SVK-6 SVK-7 SVK-10 SVK-11

A-horizon 0.706624 continuous petrocalcic horizon, ⬃25 cm below SVK-3 0.706809 discontinuous surface carbonate, ⬃75 cm below SVK-3 0.707161 continuous petrocalcic horizon, ⬃1.25 m below SVK-3 0.707231 discontinuous surface carbonate, ⬃1.75 m below SVK-3 0.707170 basalt from ⬃25 cm below SVK-3 0.703610 basalt from ⬃1.75 m below SVK-3 0.703601

12 14 12 13 12 17 22

Cima, CA (26)

CBTK-1 soil carbonate coating on basalt CBTK-2 A-horizon CBTK-5 basalt

a

Stage II soil carbonate on Quaternary Geronimo Basalt (87Sr/86Sr ⫽ 0.7031, Menzies et al., 1985) AZ-80 N near mile marker 391

Black Tank flow (Turrin et al., 1984)

0.710268 12 0.710205 15 0.702869 13

Numbers correspond with Figure 1. Highway descriptions are Interstate (I), U.S. (US), and state (AZ). Street names are within Tucson or immediate environs. Soil carbonates are described using the six-stage development terminology from Birkeland et al. (1991). c Clast rind refers to laminar carbonate coatings from soil cobbles; matrix carbonate refers to carbonate from soil pore space, and surface carbonate is from weathered faces or fractured bedrock; LS ⫽ limestone; m, cm ⫽ meters, centimeters. d Standard errors (*10⫺6). b

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Table 2. Carbon and oxygen isotopic data and percent calcium carbonate of selected samples. Sample type

Sample

␦13C(PDB)a

␦18O(PDB)a

% CaCO3

Dust

TDK-1 TDK-2 LTK-1 VK-1 CK-12 JK-1 CDK-1

⫺0.4 ⫺7.6 ⫺4.9 ⫺3.0 ⫺1.5 0.0 ⫺3.4

⫺7.8 ⫺6.8 ⫺5.1 ⫺7.5 ⫺8.3 ⫺7.3 ⫺10.0

1.9 3.8 7.2 6.6 2.7 5.6 0.9

Wash

SCK-1 SCK-2 TVCK-1 AK-11B RAK-3

⫺5.3 ⫺4.7 ⫺4.5 ⫺5.8 ⫺4.0

⫺7.8 ⫺6.9 ⫺8.9 ⫺7.9 ⫺9.5

0.8 0.7 0.8 4.4 4.4

Flood Plain

GBK-1

⫺2.8

⫺5.6

2.3

Playa

WK-1

1.6

⫺1.5

9.4

Carbonate Bedrock

WMK-2a WMK-3a CK-11

⫺0.3 1.8 0.9

⫺10.6 ⫺8.7 ⫺11.6

78.9 74.9 90.9

Soil Carbonateb

WB-2a WB-2b WB-2 WB-4a WB-4b WB-4c WB-3c Tum1 Tum2

⫺5.6 ⫺5.6 ⫺5.3 ⫺4.4 ⫺3.5 ⫺5.8 ⫺6.9 ⫺5.6 ⫺5.4

⫺8.1 ⫺8.3 ⫺6.3 ⫺6.2 ⫺7.1 ⫺6.2 ⫺6.2 ⫺6.4 ⫺6.5

a Uncertainties are from precision of the laboratory carbonate standard, and are ⬍0.1‰ for both carbon and oxygen (1␴) b Wb-samples are from near location 19, and Tum samples are from location 5a; all samples are from ⬎1 meter depth.

bedrock lithology, dust inputs and soil carbonate 87Sr/86Sr ratios. Sometimes the figures contain averages of 87Sr/86Sr analyses, where individual results are similar in context and magnitude. Of the Sr data in Table 1, just three analyses are not explicitly used in plots and discussion: these are the results from location 6a, where limestone bedrock was almost 100%

Fig. 2. Range in 87Sr/86Sr ratios of bedrock, soil carbonate (Soil), dust, wash, playa, and rain samples from the Tucson area (including bedrock and soil samples from the Geronimo Volcanic Field, location 4). Range in bedrock ratios extends to 1.88. Numbers in columns refer to the number of samples which constitute each range. Range of carbonate bedrock Sr ratios are shown labeled LS within the range of bedrock ratios.

Fig. 3. 87Sr/86Sr ratios of bedrock (squares) and associated soil carbonate (circles) and dust (triangles), by location. All samples are from within 150 km of the Tucson basin. Shaded region labeled dust refers to the range in 87Sr/86Sr ratios of dust samples directly measured in this study, with the exception of location 4. Numbers in parentheses refer to the 87Sr/86Sr ratios of bedrock samples plotted at the top of the figure which are off the scale of this plot. The 87Sr/86Sr ratio of dust from location #1 is an average of the Sr ratios from dust samples CK-12 (from the Rincon Mountains) and VK-1 and CHK-1 (from the Catalina Mountains) because similar lithologies are exposed in these regions. The Sr ratio of dust from location #3 is from sample GBK-1 (Gila Bend silts) because location 3 is near the Gila Bend flood plain, a major dust source (Fig. 1). The 87Sr/86Sr ratio of dust from location #4 is from an A-horizon leach of a soil near the volcanic field.

silicified, obscuring the logic of the soil carbonate-bedrock relationship. All O and C isotope data derive from the Tucson region (Table 2). They are plotted in Figure 5, and used, in conjunction with Sr data, to constrain the origin of dust contributions to soil

Fig. 4. 87Sr/86Sr ratios of carbonate detritus in washes compared to local soil carbonate and local carbonate bedrock. Note that the 87Sr/86Sr ratio of carbonate detritus in washes seems to correlate with the 87 Sr/86Sr ratio of the local soil carbonate, not the local carbonate bedrock. Washes which drain the Catalina-Rincon Mountains (samples TVCK-1 and RAK-3), the Tortolita Mountains (TK-10), and the Tucson Mountains (AK-11b) were sampled. Carbonate bedrock samples are from the Rincon Mountains (RAK-1a and CK-11) and the Tucson Mountains (AK-20a). Soil carbonate samples are from polylithic alluvium from the Rincon Mountains (RAK-2), the Tucson Mountains (AK-11a), and the Tortolita Mountains (average ratio of TK-samples).

Recycling in soil carbonate

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Fig. 6. 87Sr/86Sr ratios of soil A-horizon leaches, soil carbonate, parent basalt, and local dust (where available) by location. The variation in soil 87Sr/86Sr ratios is due to the different carbonate dust sources in the different regions. Note that soil carbonate 87Sr/86Sr ratios are always higher than the basalt parent rock and, where constrained, appear to be a mixed isotopic signal between parent rock and dust. SVF ⫽ Springerville Volcanic Field; GVF ⫽ Geronimo Volcanic Field.

5.2. Soil Carbonates, Parent Rocks, and Dust from the Tucson Area

Fig. 5. (a) Predicted ␦18O(PDB) and ␦13C(PDB) values of authigenic carbonate, which may precipitate from meteoric waters (rain and nonevaporated surface waters) or evaporated surface waters (flood plains and playas), and detrital soil carbonate. We assume that the temperature of carbonate precipitation has ranged from the modern mean annual temperature (20.2°C) to 5° cooler during glacial periods. Fractionation factors were determined with the following equations: 103ln␣calcite-water ⫽ 18.03 * 103/T(C) ⫺ 32.42 (Kim and O’Neil, 1997), and 103(␣calcite-CO2 ⫺ 1) ⫽ 11.98 ⫺ 0.12 * T(K)/1000) (Romanek et al., 1992). (b) ␦18O(PDB) vs. ␦13C(PDB) of carbonate bedrock, dust, wash, soil carbonate, flood plain, and playa samples. Shaded regions are as labeled in (a). Note that the dust samples significantly overlap with the soil carbonate samples, but not with the carbonate bedrock samples.

carbonate. These data result in the important conclusion that previously-existing soil carbonate, not limestone bedrock, must comprise the main source of carbonate dust in the Tucson region. Lastly, we have a set of Sr data (Table 1) collected from a much larger region that includes most of Arizona and some adjacent areas of California. These data, where the bedrock is always basalt, are plotted in Figure 6, and include one locality from the Tucson area (Tumamoc Hill) that fits with criteria for this discussion. The data are used to discuss regional variations in 87Sr/86Sr ratios of dust, which display smooth regional variations on a 200 –300 km geographic scale.

In this section we use the quite heavily sampled Tucson area to deduce our general concept for mixing of dust and parentrock weathering in soil carbonate production, and to make broad inferences about production and cycling of soil carbonate in the area. Parent rock samples exhibit a large range in 87 Sr/86Sr ratios (0.7029 –1.88) compared to soil carbonates (0.7049 – 0.7190) (Table 1; Fig. 2). Dust samples yielded a narrow range of 87Sr/86Sr ratios (0.7100 – 0.7123) within the range of soil carbonate and parent rock 87Sr/86Sr ratios. At all locations, the 87Sr/86Sr ratio of soil carbonate lies between that of dust and the parent rock (Fig. 3), supporting the conclusion of many previous studies that the Sr in soil carbonate derives from both parent rock and dust sources. The proportional contributions of Sr from parent rock and dust sources may be quantified in simple weathering situations where 87Sr/86Sr ratios of parent rock minerals are both homogeneous and do not change on Quaternary timescales. The 87 Sr/86Sr ratio of a parent rock can be divided into two components: initial Sr, which is incorporated into a rock at the time of its formation, and post-formational Sr, which a rock acquires with time due to decay of 87Rb. Rubidium-rich minerals (e.g., K-feldspars and micas) will develop higher 87Sr/86Sr ratios with time than Rb-poor minerals (e.g., plagioclase) in the same rock. These minerals weather at different rates, so a rock containing these minerals (e.g., a granitoid) will weather to produce fluids with a 87Sr/86Sr ratio that changes with time. Mafic minerals (e.g., olivine, pyroxene, and plagioclase) have low Rb/Sr ratios, so much time is needed for a mafic rock to develop Sr-isotope heterogeneities. Quaternary basalt is therefore dominated by initial Sr and will probably weather to produce a constant 87Sr/86Sr ratio with time. Marine limestone is close to mono-mineralic and contains almost no Rb, so it will also weather to produce a constant 87Sr/86Sr ratio with time.

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We use the following simple mass balance to calculate the relative contributions of dust and parent rock to soil carbonate developed on basalt and limestone lithologies: x(87Sr/86Sr)dust ⫹ (1⫺x)(87Sr/86Sr)parent rock⫽(87Sr/86Sr)soil carbonate (1)

Table 3. ␦18O values of water reservoirs and ␦13C values of dissolved CO2 from near Tucson. Water reservoirs

␦13C(PDB)(‰)

␦18O(SMOW)(‰)

Meteoric waters Soil waters

⫺7a ⫺11.9 to ⫺19.8b

⫺5.9 to ⫺10.1c ⫺5.9 to ⫺10.1c

a

where x is the proportion of Sr contributed by dust. We can apply eqn. (1) to calculate that 39% of the Sr in soil carbonate developed on the Tumamoc Basalt (sample AK-1b) is derived from the basalt. The 87Sr/86Sr ratio of dust used in this calculation is an average of two samples (TDK-1 and TDK-2), and the 87Sr/86Sr ratio of soil carbonate is from sample AK-1a. In the Rincon Mountains (location 9), the parent limestone has contributed about 33% of the Sr to soil carbonate. We also calculate that 58% of the soil Sr from the Geronimo Volcanic Field (GK-samples) derives from basalt. At this location, we assume the 87Sr/86Sr ratio of dust from the 87Sr/86Sr ratio of a leach from a nearby soil A-horizon (sample GK-14). Calculation of Ca inputs requires a knowledge of the Sr/Ca ratios of the sources, and involves assumptions about the partitioning of Sr and Ca between solutions and clay mineral surface exchange sites. Silicate rocks have higher Sr/Ca ratios than carbonates, so the proportions of Ca derived from silicate weathering of the Tumamoc and Geronimo basalts would be less than the calculated proportions of Sr (39% and 58%, respectively) derived from the basalts. The correlation between soil carbonate and parent rock 87Sr/ 86 Sr ratios (Fig. 3) further demonstrates the importance of silicate weathering in the generation of soil carbonate. This correlation is not apparent within the group of felsic parent rocks (Fig. 3) because felsic rocks contain minerals which have different 87Sr/86Sr ratios and different weathering rates, and our samples were taken from soils in various stages of soil development; and because there are probably differences in the carbonate content of dust and in dust fluxes between sample locations. Two parent rock and soil carbonate pairs (samples OK-1a and OK-1b; PK-1a and PK-1b) were sampled from location 1 (Fig. 1) where felsic plutons of different ages are in intrusive contact. The dust flux, dust carbonate content, and stage of soil development on each rock are assumed to be equal here, and the soil carbonate and parent rock 87Sr/86Sr ratios do indeed correlate (Fig. 3). 5.3. Dust Sources in the Tucson Area In this section we use Sr in conjunction with O and C isotope data to constrain the sources of dust in the area around Tucson. The cation component of carbonate dust must derive directly from exposures of carbonate bedrock and soil carbonate, or from their alluvial derivatives, such as washes, flood-plains, and playas. The most significant exposures of carbonate bedrock near the Tucson area have 87Sr/86Sr ratios between 0.7097 and 0.7122, higher than Phanerozoic ocean water, indicating that limestones and dolostones have undergone post-depositional alteration. This range in values overlaps with the ranges exhibited by all of our surface and soil carbonate samples, so the ultimate source of carbonate dust cannot be easily resolved by Sr data alone. However, features of our Sr data set support a soil carbonate origin of carbonate dust. The carbonate fraction

Clark and Fritz, 1997 Wallick, 1973; Turin, 1986 c Kalin, 1994; Eastoe, personal communication; range in values of mean annual summer and winter precipitation in the Tucson Basin and the Catalina-Rincon Mountains b

of wash silts from the Tucson Basin have 87Sr/86Sr ratios that match that of local soil carbonate, not carbonate bedrock, within their drainage areas (Fig. 4). Because washes deposit material in flood plains and playas, it is likely that the Ca in carbonates in these settings is also derived from soil carbonate. The washes which drain the mountains surrounding Tucson have a notable effect on the 87Sr/86Sr ratio of detrital carbonate in the Santa Cruz Wash (compare the ratios of samples SCK-1 and SCK-2), supporting the notion that reworked soil carbonate is transferred long distances in washes. We measured the ␦13C and ␦18O values of dust, carbonate bedrock, soil carbonate, and wash, playa, and flood-plain silts in order to track carbonate through these surface systems. Calcium carbonate may dissolve/re-precipitate between the time it is derived from its ultimate sources (i.e., soil carbonate or carbonate bedrock) and the time the dust is sampled. Unlike Sr isotopes, C and O isotopes fractionate during dissolution and precipitation of calcium carbonate. The oxygen and carbon isotopic compositions of carbonate are a function of the oxygen isotopic composition of the water from which the carbonate precipitates, and the carbon isotopic composition of aqueous carbon dioxide, respectively. In addition, the temperature at which the carbonate precipitates will affect its isotopic composition. Dust samples may have a detrital carbonate component, which has not been isotopically modified since derivation from its ultimate source, and an authigenic component, which has re-precipitated from surface waters. It is therefore necessary to predict the isotopic composition of the authigenic component in order to determine the origin of the detrital component. Authigenic carbonate is expected to precipitate in equilibrium with surface waters. The ␦18O value of pertinent water reservoirs and the ␦13C value of their dissolved CO2 (Table 3) were used to determine the expected ␦18O and ␦13C values of carbonates precipitated from surface water reservoirs (Fig. 5a). The following assumptions were made: 1. all surface waters are in equilibrium with atmospheric CO2, and soil waters are in equilibrium with soil CO2; 2. the ␦18O values of surface waters, especially in flood plains and playas, may be higher than meteoric water values due to evaporation; 3. the ␦18O value of soil water is the same as the ␦18O value of meteoric water (i.e., soil waters are not significantly enriched in 18O by evaporation); and 4. carbonate precipitates in oxygen isotopic equilibrium with water, and carbon isotopic equilibrium with the dissolved CO2 of that water.

Recycling in soil carbonate

Soil carbonate is predicted to have a lower ␦13C value than authigenic carbonate (Fig. 5a) because the ␦13C value of soil CO2 is a function of plant respiration and decay, and plants have lower ␦13C values than the atmosphere. We expect that playa and flood-plain samples have the largest authigenic carbonate component because repeated flooding and evaporation in these environments promotes the precipitation of carbonate. Carbonate bedrock samples have low ␦18O(PDB) values and high ␦13C(PDB) values compared to dust, washes, and soil carbonate (Fig. 5b, Table 2). Willcox playa silts yielded high ␦13C(PDB) and ␦18O(PDB) values. Gila Bend flood-plain silts have higher ␦13C(PDB) and ␦18O(PDB) values than wash and soil carbonate samples. Dust, wash silts, and soil carbonate ␦13C and ␦18O values plot in overlapping fields (Fig. 5b), whereas carbonate bedrock samples plot in a distinct field with higher ␦13C and lower ␦18O values. This results in the important first-order conclusion that in the Tucson area, soil carbonate, rather than carbonate bedrock, is the most significant source of carbonate dust. Pedogenic carbonate, present in soils throughout a high proportion of the terrain, must be eroded, transported by water and wind, and recycled as dust into new and developing petrocalcic horizons. The large variation in ␦13C and ␦18O values of dust samples compared to soil carbonate samples can be attributed to mixing between soil carbonate, carbonate bedrock, and authigenic carbonate sources. The contribution of carbonate bedrock to carbonate dust is probably small relative to the contribution from soil carbonate. Of the three dust samples with high ␦13C values, two were exposed to precipitation after deposition, and are likely to have an authigenic component with a high ␦13C value. Two dust samples from Tumamoc Hill have very different ␦13C values; the sample with the high ␦13C (TDK-1) was exposed to precipitation whereas the other (TDK-2) was not. The dust sample with a high ␦13C value which was not exposed to precipitation (JK-1) does not have ␦18O as low as carbonate bedrock. The dust samples which have ␦18O values similar to carbonate bedrock samples are from near the Catalina and Rincon Mountains where rain, and by inference soil waters, have similarly low ␦18O values (Kalin, 1994; Eastoe, personal communication). Gila Bend flood-plain silts have higher ␦13C and ␦18O than any wash sample, indicating that these silts contain an authigenic component. Willcox Playa silts plot near the field expected for carbonate precipitated from evaporated surface waters, indicating that there is a large component of authigenic carbonate in playa silts. Willcox Playa does not appear to be a significant source of dust in the Tucson area. 5.4. Spatial Variations in the 87Sr/86Sr Ratio of Carbonate Dust, Southwestern USA In this section we use the soil carbonates developed on basaltic bedrock to compare 87Sr/86Sr ratios of dust sources on a regional basis across Arizona and adjacent regions of California. Our data provide insight into the distance of carbonate dust sources from sites of dust deposition. Pe´we´ (1981) suggested that local dust (5–50 ␮m particles) is derived from less than 100 km away, whereas regional dust (2–10 ␮m particles) is transported in the troposphere, deposited by precipitation,

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and may be derived from up to thousands of kilometers away. We assume that dust derived from regional sources is better mixed than locally derived dust. The spatial variation in dust 87 Sr/86Sr ratios is then an indication of the proportions of locally and regionally derived dust. We observe significant variation in 87Sr/86Sr dust ratios (⬎1%) over a scale of kilometers, which indicates that local sources of carbonate dust are important. However, the total range in dust 87Sr/86Sr ratios is significantly narrower than that of soil carbonate (Fig. 2), suggesting that dust must contain a well-mixed, regionally derived component. We estimate the regional 87Sr/86Sr ratio from a sample of rainwater (TR-2) which was filtered immediately after collection, and has an 87Sr/86Sr ratio of 0.71115. This ratio is almost identical to the Sr ratio which is at the center of the distribution of dust samples (0.71112). We analyzed soil A-horizon leaches, soil carbonate, and parent basalts throughout Arizona, including one site in south eastern California, to help constrain the 87Sr/86Sr ratio of dust at these locations. Soil A-horizon samples yield similar 87Sr/ 86 Sr ratios to soil carbonate samples (Fig. 6); these values do not vary systematically with depth. Parent basalts have much lower 87Sr/86Sr ratios (Fig. 6). The soil 87Sr/86Sr ratios provide a minimum value for the dust 87Sr/86Sr ratios at each location because any contribution to soil Sr from the parent basalt would lower the soil 87Sr/86Sr ratio. We estimate the 87Sr/86Sr ratio of carbonate dust from the Geronimo Volcanic Field from the acetic acid leach of a nearby soil A-horizon. This soil, developed on recent alluvium composed of mostly intermediate volcanic rocks derived from the Chiricahua Mountains, probably has a weaker signal from the local dust generated by the cinder field, so the A-horizon leach is more representative of the regional dust 87Sr/86Sr ratio. There are significant variations in the 87Sr/86Sr ratios of dust (as assessed from the soil carbonate 87Sr/86Sr ratios) from throughout the southwest United States (Fig. 7). At inland sites (removed from sea-spray) where exposures of carbonate bedrock are not abundant (Tucson, AZ region, southeast California, and southwest Nevada), dust can exhibit high 87Sr/86Sr ratios which derive from felsic silicate bedrock. Regions which have major exposures of carbonate bedrock clearly have lower 87 Sr/86Sr ratios in dust. Dust from the edge of the Colorado Plateau (Flagstaff and Springerville locations) has 87Sr/86Sr ratios similar to the abundant limestone (mostly Mesozoic) in the region, and dust from New Mexico has similar 87Sr/86Sr ratios to the abundant limestone (mostly Paleozoic) in this region (Capo and Chadwick, 1993; Stewart et al., 1998; Van der Hoven and Quade, 1994). Locations near the Gulf of California, in the Pinacate Volcanic Field and the lower Colorado River region, have dust 87Sr/86Sr ratios similar to modern ocean water, probably showing a significant sea-spray influence at these sites. The 87Sr/86Sr ratio of carbonate dust varies significantly over a scale of 200 –300 km in the southwest United States (Fig. 7). The isotopic heterogeneity observed in carbonate dust is attributed to the high dust production rates which characterize this arid to semi-arid region, coupled with regional and local variations in carbonate dust sources. On the other hand, there is a very clear progressive narrowing of the range of 87Sr/86Sr ratios from bedrock to soil carbonate, and through fluvial sediment and dust to the tropospheric dust sampled by rain

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inate carbonate dust. Carbonate dust from inland settings with only minor exposures of carbonate bedrock has high 87Sr/86Sr ratios derived from silicate bedrock, via soil carbonate. In the southwest United States, it appears that the 87Sr/86Sr ratio of dust varies significantly on a scale of 200 –300 km. Recycling of pedogenic carbonate by erosion, fluvial and wind transport tends to smooth out these differences, but does not achieve full homogenization because of continued input from silicate weathering. Acknowledgments—This research was supported by NSF grants EAR 97-25607 (Quade) and EAR 95-26536 and 98-14885 (Patchett), and student fellowships from NSF and the University of Arizona (Naiman). We thank Clark Isachsen and David Dettman for their instruction in analytical procedures, and Jon Spencer for helpful discussions regarding sampling locations. REFERENCES Fig. 7. Mean 87Sr/86Sr ratio of carbonate dust, by region. Unlabeled locations are from this study. a: Zuni-Bandera Volcanic Field; Van der Hoven and Quade, 1994. b: Las Cruces; Capo and Chadwick, 1993; Stewart et al., 1998. c: Pinacate Volcanic Field; Slate et al., 1991. The dust 87Sr/86Sr ratio is assumed from soil carbonate ratios because these volcanic rocks have thick eolian silt caps which, in similar settings (e.g. the Zuni-Bandera Volcanic Field), dictate the soil carbonate 87Sr/86Sr ratio. d: Lower Colorado River region; Spencer and Patchett, 1997. The dust 87Sr/86Sr ratio is assumed from soil carbonates developed on a variety of lithologic parent rocks from this region. These soil carbonates yield nearly identical 87Sr/86Sr ratios, regardless of substrate rock. e: Yucca Mountain region; Marshall and Mahan, 1994. This dust value is assumed from the 87Sr/86Sr ratio of silt-sized eolian sediment.

(Fig. 2). This is evidence that the production of pedogenic carbonate and its recycling by erosion, fluvial and wind transport are capable of smoothing 87Sr/86Sr variations from weathering over large regions. 6. SUMMARY AND CONCLUSIONS

Southeast Arizona is a semi-arid continental setting where soils are not influenced by sea-spray, and where limestone is not abundant enough to dominate surface carbonate reservoirs. The 87Sr/86Sr ratio of soil carbonates from the Tucson area correlate with the ratios of the respective parent rocks; soil carbonates contain an isotopic signal from silicate weathering reactions. The similar ␦18O and ␦13C values of carbonate dust, soil carbonate, and washes, and the distinctly different ␦18O and ␦13C values of carbonate bedrock, indicate that pre-existing soil carbonate is the most important source of carbonate dust in the Tucson area. Thus, soil carbonate is recycled through petrocalcic horizons by erosion, fluvial transport, and eolian redistribution. Playa silts have higher ␦18O and ␦13C values than dust, indicating that these silts contain a large authigenic carbonate component, and that playas are not a significant source of regional dust in southeast Arizona. Small variations in the 87Sr/86Sr ratio of dust from near Tucson indicate that local sources of carbonate dust are important. The 87Sr/86Sr ratio of carbonate dust varies more significantly on a regional scale due to the regional variation of Sr sources. In coastal settings, and inland regions where marine carbonate exposures are common, marine 87Sr/86Sr ratios dom-

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