Sulfur isotope geochemistry of gypsiferous Aridisols from central Iran

ELSEVIER Geoderma 80 (1997) 195-209

GZODZa,_m

Sulfur isotope geochemistry of gypsiferous Aridisols from central Iran

H. Khademi a, A.R. Mermut b,,, H.R. Krouse c

Department of Soil Science, College of Agriculture, Isfahan Universi~ of Technology, lsfahan, Iran

b Saskatchewan Centre for Soil Research, Department of Soil Science, 51 Campus Drive, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A8, Canada

Department of Physics and Astronomy, The Universi~ of Calgary, Calgary, Alberta T2N 1N4, Canada

Received 27 September 1996; accepted 15 July 1997

Abstract

Gypsum accumulation is one of the prominent pedogenic processes occurring in many arid regions of the world. Gypsiferous soils occur in large areas of the Iranian central plateau. The origin of gypsum in the Aridisols of central Iran and its distribution in different landscapes were studied using sulfur and oxygen isotopic composition of both solid and dissolved sulfates. The results strongly support the hypothesis that the area was cut off from the Tethys seaway at the end of the Mesozoic era and, as a result, the Lower Cretaceous sulfate has controlled the S geochemistry of the younger sediments, including the soils studied. The mean values of ~ 345 and

~80 of the soil gypsum and of dissolved sulfate match with the average isotopic values of the marine sediments which are the most common soil parent materials. The results also confirm that S and O in dissolved SO 4 and SO 4 in gypsum have not undergone any reduction and re-oxidation reactions. Dissolution and reprecipitation appear to be the possible mechanisms responsible for the observed minimal isotope fractionation. Both ~345 and 6 ~80 values of the soil gypsum decrease slightly with decreasing elevation in the study area. These findings are important for understand- ing the landform evolution in this arid region. © 1997 Elsevier Science B.V.

Kevwords: sulfur stable isotopes; oxygen stable isotopes; gypsum; Aridisols; Tethys Sea

* Corresponding author. Fax: + 1-306-966-6881; E-mail: [email protected]

0016-7061/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PI1 S0016-706 1(97)0009 1-8

196 H. Khademi et al. / Geoderma 80 (1997) 195-209

1. Introduction

There are about 200 million ha of gypsiferous soils on the earth's surface (Nettleton, 1991). With 28 million ha of gypsiferous soils, Iran appears to have the largest area of these soils in the world (Mahmudi, 1994). Whereas some general information is available about the morphology and some physico-chemical properties of Iranian gypsiferous soils, the source and the nature of gypsum have been a very controversial topic among local and international soil scientists and geologists (Khademi, 1997).

Numerous studies confirm that the stable isotope approach is a useful tool in tracing the source of sulfur and its mobility in soils and sediments. For example, Schoenau and Bettany (1989) examined the S isotope composition of sulfate in native and cultivated Chernozemic, Gleysolic, and Luvisolic soils in Saskatchewan. They found that mineral S (gypsum and pyrite) is the main source of S in weakly developed soils, but that atmospheric S becomes a more dominant source with increasing soil development. Dowuouna et al. (1992a,b) found that the S and O isotopic composition of sulfate in Solonetzic soils supported the hypothesis of Mermut and Arshad (1987) that pyrite oxidation and subsequent hydrolysis of natrojarosite is the major source of sulfate salts in Saskatchewan. Measurement of the 63~S of gypsum crystals from Vertisols occurring along the western coast of New Caledonia suggests that the gypsum in these soils is derived from two sources, an oceanic source where the major part of the sulfur is introduced as sulfate by atmospheric precipitation and a continental source where sulfides from subsurface deposits are oxidized and contribute to the formation of gypsum in the soil (Podwojewski and Arnold, 1994).

The stable isotopes of S and O in sulfates are useful tools to establish the degree and type of processes that occur within the earth's surface (e.g. Schwarcz and Cortecci, 1974; Ford and Schwarcz, 1981; Hendry et al., 1989; Dowuouna et al., 1993). Information on isotopic composition of sulfate in gypsiferous soils and their use in landscape studies in arid environments appears to be very limited (Kusakabe et al., 1976).

This study was initiated to determine the S and O isotope abundance variations of gypsum and dissolved sulfates in gypsiferous soils and their parent materials from central Iran. The objectives of this work were to: (1) identify the main source(s) of gypsum in the study area, and (2) understand the pathways and pattern of the sulfate movement in relation to landforms in the area.

2. Materials and methods

2.1. Study area and sampling

Soils located near the city of lsfahan, about 400 km south of Tehran in central Iran, were studied (Fig. 1). The study area is characterized by a dry climate with

H. Khademi et al. / Geoderma 80 (1997) 195-209 197

45 ° 50 ° 550 60 o 65 °

-ZC-Z:-E-'Z~?.'-" 1- - - ... l

) Tehran "N .~ -

Iraq \ S tl~y area X "~

< I_ _ I R A N

K u w a i t - T ~ ] ~ . ~ i ~ -x. x~.

Saudi ~ ~ Arabia ~ ~ ~ - i - . ~ . _ /

Fig. 1. The study area in central Iran.

40 °

35 °

30 °

25 °

hot temperatures. The mean annual atmospheric temperature is 14.7°C and the mean summer and winter temperatures are 25.8 and 5.8°C, respectively.

Bulk samples and gypsum crystals from different horizons of fifteen soil profiles (Fig. 2) were collected for the study. Pedons examined occur on three different landscapes: colluvial fans, plateau, and alluvial plains (Fig. 2). Gypsif- erous soils in these landscapes are classified as Calcic Argigypsids, Typic Haplogypsids, and Gypsic Haplosalids (Soil Survey Staff, 1994), respectively. While alluvial soils have a very low amount of gypsum accumulated only in the topsoil, subsoils of colluvial fans and both surface and subsurface soils on the plateau are moderately to extremely gypsiferous.

Rock samples from the dominant geological formations in the study area as well as the water samples from different locations of the Zayandehrud River were also taken for stable isotope analyses.

2.2. Stable isotope analyses

Hand-picked gypsum crystals were cleaned with acetone under ultrasonic agitation. All forms of sulfates (dissolved in water or soil extracts and mineral forms) were converted to pure BaSO 4 for •34S and ~ ~O analysis according to the method described by Dowuouna et al. (1992a).

Gas preparation for oxygen isotope analysis of BaSO 4 was carried out according to the method of Shakur (1982). Pure BaSO 4 purchased from Fisher Scientific Company with a 6 ~80 value of + 12%o (vs. V-SMOW) was prepared as a standard by the same method to calibrate the 6~So values. The repro- ducibility for the 6180 measurements was lo -= +0.1%~.


. . . . ~ n , ~ e ~ ~,~e~

TO Gavkhouni swamp Krrl ' ~ 30 Kms

Fig. 2. Block diagram of the study area and the locations of the pedons (solid circles), river waters (WI-W3) and rock samples (R1-R6) studied. Cross-section of A-A ' is shown in Fig. 3.

Sulfur dioxide gas was prepared from BaSO 4 for sulfur isotope analysis using the method by Yanagisawa and Sakai (1983). Standards including NBS-127 and BaSO 4 from Fisher Scientific Company with 634S values of +21.00 and + 13.76%~ (vs. V-CDT), respectively, prepared by the above-described method, were used to calibrate the samples. The standard deviation of 634S values measured using this technique was on average _+ 0.05%~, ranging from 0.03 to 0.09%0.

Stable isotope abundances are reported in the 6 notation. This notation specifies the deviation of the abundance ratio of two isotopes of the same element in a sample from that of an internationally accepted standard, in parts per thousand (%0):

R standard 1 6 sample= Rsample 1 X 1000

where R is the abundance ratio (34S/32S or lSo/160). The historical standard for reporting ~34S values is troilite (FeS) from the Cation Diablo Meteorite (CDT), while that for 6 JSO is Standard Mean Ocean Water (SMOW). The data


are reported on V-CDT and V-SMOW scales as established by the International Atomic Energy Agency, Vienna.

2.3. Rock analysis

Dried ground rock samples were digested with HF-HCIO 4 for total S measurement. After digestion was completed, the dried residue was dissolved in concentrated HC1, diluted and then filtered. Total S in the filtrate was measured using inductively coupled plasma atomic emission spectrometry (model ARL 3410, Applied Research Laboratories Inc., California).

The digestion method by HNO3-Br 2 (Krouse and Tabatabai, 1986) was used to extract sulfur from the parent-rock samples. The precipitate (BaSO~) prepared by this method was also analyzed for sulfur isotopes according to the method described above.

Due to the possible oxygen isotopic exchange between sulfate and water under the extremely acidic conditions during the extraction of sulfate from the rock samples, no attempt was made to measure the oxygen isotopic composition of sulfate in these samples.

3. Results and discussion

3.1. Geochemistry of sulfur in the sediments

Total S content of the dominant soil parent rocks in the study area is between 1812 and 3098 mg kg-~ (Table 1). This suggests that there is enough sulfur in the rock system to produce the gypsum found in these soils.

Cretaceous limestone is the most common soil parent material in the area (Geological Survey of Iran, 1978). Samples from this geological formation had a bulk ~345 value between + 11.45 and + 13.96%o (Table 1). It is well established that the 6 34S value of marine sulfate has changed considerably through-

Table 1 Total sulfur content and 6 345 values of different geological formations

Sample ~ Geological formation (location) Total S (~ 34S (mg kg ~) (%0, V-CDT)

RI R2 R3 R4 R5 R6

Cretaceous grey limestone (Kolahghazi) 1812 + 11.45 Cretaceous grey limestone (Hatmabad) 1978 + 12.15 Cretaceous grey limestone (Garghuyeh) 1859 + 12.42 Cretaceous marly bedded limestone (Kolahghazi) 1946 + 13.96 Oligo-Miocene foraminiferal limestone (Zefreh) 3098 + 12.72 Pyrite from Jurassic shale (Bama mine) nd - 2 1 . 9

The locations of samples R I - R 6 are shown in Fig. 2; nd: not determined.

200 H. Khademi et al . / Geoderma 80 (1997) 195-209

out geologic time. Measurement of 6 34S of marine evaporites (mainly gypsum) indicates a progressive change from about + 13.5%o in the Lower Cretaceous to + 22%0 in the Mid-Miocene with a subsequent slight reduction of 6 34S values from Mid-Miocene to the present (e.g. Claypool et al., 1980; Cecile et al., 1983). Thus, the isotopic composition of sulfur in the parent rocks studied (Table 1) is very similar to that reported for the Lower Cretaceous sea sulfate.

The Cretaceous sediments of the area belong to the Tethys geosyncline of Mesozoic time. Marine strata of this age occur in the Zagros Mountains in Iran including the study area (Krinsley, 1970). During the Late Cretaceous (about 75 million years ago) orogenic activities ended the history of the Tethys seaway (SengiSr et al., 1988). The 6348 value of a sample of Oligo-Miocene sediment was determined to be + 12.72%o (Table 1) which is similar to that of Cretaceous marine sulfate. Considering the typical Oligo-Miocene ocean ~34S values that range from + 20 to +22%e (Claypool et al., 1980), this may suggest that during the Tertiary, the area was already cut off from the adjacent oceans as a closed basin and the S geochemistry of central Iran was, therefore, controlled by the Lower Cretaceous sea water sulfate.

Jurassic shales in the study area have been reported to be pyritic (Thiele et al., 1968; Ghazban et al., 1994). A pyrite sample from a shale stratum in the Bama mine, close to profile No. 6 (Figs. 2 and 3), had a 634S value of -21.9%o (Table l), showing a great depletion in the heavy isotope as compared to the sulfur in the soil gypsum and also in comparison to sulfur in Cretaceous limestone. The 6348 values of metal sulfides from the study area reported by Ghazban et al. (1994) ranging from - 3 . 6 to -9.6%0. They attributed the depletion of 34S in metal sulfides to the partial reduction of marine sulfate. This process is very common in many present-day marine sediments (e.g. Kaplan et al., 1963; Krouse and Grinenko, 1991).

Elevation (m)

250O kandforms Sediments

Colluvlal fan ~ "" [ ~ Oligo- Miocene limestone

2250 ~ ~ Plateau ~Oligo-Miocene conglomerate

~ Alluvial plain ~Cretaceous limestone

2000 ~ ~ Cretaceous sandstone & conglomerate

1750 ~ x ~ Jurassic shale ~

1500 ~ 2 " ' " ' - " " I,

A A'

Fig. 3. Cross-section showing the sedimentary petrology (based on Geological Survey of Iran, 1978) and different landforms of the study area.


3.2. Sulfur and oxygen stable isotope abundances in soils

The S and O isotopic compositions of soil gypsum and soluble sulfate in three different landscapes (colluvial fans, plateau, and alluvial plains) are summarized in Table 2.

Values of 634S for soil gypsum in all three landscapes ranged between + 11.4 and + 14.4%e (Fig. 4a) with the average of + 13.58 and + 12.96%~ in colluvial fans and plateau, respectively (Table 2). Soils in alluvial plains are not gypsiferous. However, in the study area where the saline groundwater is close to the surface, appreciable amounts of gypsum can be found in the upper part of the soil profile. The only gypsum sample taken from an alluvial soil had a 634S value of + 12.1%c.

Values of 6180 (SO 4) for soil gypsum in all three landscapes studied ranged from + 10.8 to + 14.8%c (Fig. 4b) with averages of + 13.3 and + 12.4%e in colluvial fans and plateau, respectively (Table 2). The 6 ~80 value of one sample from the alluvial plain was + 11.1%~.

+15

+14-

+13-

+12-

+11-

I y = 0.91x - 0.18 r = 0.7"* []

I i |

+12-

+10.

+8

[]

[]

+1~1 J J +1~4 +10 +12 +13 +15 16

4 c S-Soluble sulfate (~cl

Fig. 4. Relationships between ¢~34S and 6 lSo values o f sulfates in (a) soil gypsum and (b) soluble sulfate.

+15 +10

+ 1'3 ' +i1 +12 +14

~34S-Gypsum (%~) +18 .

+16 t y = 0.76x+ 2.39 r = 0.68"*

rn

+14-


Table 2 Summary of isotopic data for soil gypsum and soluble sulfate in three different landscapes

Colluvial fans Plateaus Alluvial plains

mean%c SD%~ Nr. of mean%~ SD%~: Nr. of mean%c SD%e Nr. of samples samples samples

Gypsum (~34S + 13.58 0.6 10 + 12.96 0.8 20 + 12.11 a I 6~SO +13.26 1.1 10 +12.48 1.0 20 +11.12 ~ I

Soluble sulfate •34S + 13.47 0.6 21 + 13.17 1.0 27 + 11.45 0.4 9 6L80 +12.82 0.9 21 +12.09 1.1 27 +11.36 1.2 9

" One sample only.

In general, soluble sulfate had the same S and O isotopic compositions as soil gypsum (Fig. 5). This clearly shows that gypsum is the only sulfate mineral controlling the activity of sulfate in the soil solution. Slightly higher t~34S and

a +16

+15 1

+14-

~a

-~ +13- "6

+12-

+11

y = 0.81x + 2.63 r = 0.71"*

[]

[] []

I I I

+11 +12 +13 +14 +15

534S-Gypsum (%~) +15

y = 0.47x + 6.35 r = 0.49* []

[] +14- []

+13- [] I~ [] I ~ ~

+12- ~1 y GI

+11- [] NCil GI []

[]

+10 I I

+10 +11 +12 +1'3 +1'4 +15

~180_Gypsum (%c)

Fig. 5. Relationships of (a) sulfur and (b) oxygen isotope compositions between gypsum and soluble sulfate.

14. Khademi et al. / Geoderma 80 (1997) 195-209 203

10,

g6 4

2 o

834S (%0, vs. CDT)

~" 4

2 6

z 0

Dominant soil parent rocks

+ l 1 " +13 +15 +17 +19 8180 (%0, vs. SMOW)

Fig. 6. Histograms showing (a) ~34S and (b) 8~SO values of soil gypsum compared with corresponding values of marine sulfates of Cretaceous age (inset). Shaded area in (a) is the estimated precision of the curve (modified from Claypool et al.. 1980).

~ S O values of gypsum than those of soluble sulfate may be attributed to fractionation during dissolution (Claypool et al., 1980). The low amount of organic matter in the soils indicates that the contribution by decomposition of organic S to the sulfate pool is negligible.

Today's marine dissolved sulfate has a ~5~SO value of about +9.9%o (e.g. Longinelli and Craig, 1967; Rafter and Mizutani, 1967). This value has also been changing with geologic time. Lower Cretaceous marine sulfates have a mean ~ lSO value of about + 13%c (Claypool et al., 1980).

Histograms of both 634S and ~ 8 0 values of the soil gypsum along with values for marine sulfates of Cretaceous age are shown in Fig. 6. The mean values of both 634S and 6 ~80 in the soil gypsum match with the mean isotopic composition of the Lower Cretaceous marine sediments which are the most common parent rocks in the study area. This clearly indicates that marine sulfate of Cretaceous age is the main source of the soil gypsum. In contrast, sulfur in


sulfides, such as pyrite, which tends to be very depleted in 34S, does not appear to have been a major contributor to soil gypsum at any of the sites studied.

The soils have derived from the weathering products of the sulfate-rich sediments of the surrounding area. Extremely high amounts of gypsum in soils on the plateau (i.e. the center of the basin before the development of an alluvial plain) also confirm this view. Considering the geological time period, the amount of S found in the rocks (about 3000 mg kg -1) and the area being a closed basin with an arid climate, the accumulation of gypsum appears to be reasonable. Gypsum accumulation, in the presence of carbonates, is inevitable, when SO 4- activity is more than 0.02 M 1-I(Lindsay, 1979).

3.3. Catenarv variations in the sulfur and oxygen isotopic composition of gypsum

Both (~34S and 6 ~O values of soil gypsum significantly decrease (P < 0.1) from colluvial fans to plateau and from plateau to alluvial plain. This may be related to the movement of sulfate from colluvial fans to the center of today's basin (alluvial plain) as also supported by the isotopic composition of the gypsum hydration water (Khademi et al., 1997). Reduction of sulfate does not seem to have been significant as this process causes a large enrichment in both 34S and ~SO of remaining sulfate (e.g. Krouse and Tabatabai, 1986; Hendry et al., 1989) resulting in an opposite trend along the toposequence. Dry soil conditions are not conducive for sulfate-reducing anaerobic bacteria. Adsorption of sulfate by soil particles and dissolution-recrystallization are among the other fractionating processes of S and O isotopes which need to be discussed.

The possibility of any sulfate adsorption by the soil particles under the conditions prevailing in the soils studied seems to be insignificant. Even though palygorskite as the dominant clay mineral (authors' unpublished data) has a relatively high adsorption capacity for anions (Singer, 1989), the high pH of the soils (7.5-7.9) is not favorable for an appreciable anion adsorption (Zhang et al., 1987).

During crystallization of gypsum, 3as and ~O are favored in the precipitate by 1.65 _+ 0.12 and 3.6 +_ 0.9%c, respectively (e.g. Holser and Kaplan, 1966). Thus, remaining sulfate becomes depleted in the heavier isotope along a positive slope on a 6~80 versus ~34S diagram. As shown in Fig. 4a, there is a positive correlation between 6 ~80 and 634S values in soil gypsum with a slope of 0.91, suggesting that dissolution and recrystallization are responsible tbr fractionation of S and O as sulfate moves from higher landscape positions to the center of the basin. This is in line with our findings on the isotopic composition of gypsum hydration water in the same soils (Khademi et al., 1997).

During partial reduction and re-oxidation, the S isotopic composition of sulfate remains unchanged, whereas soil water contributes to the O isotopic composition of sulfate (e.g. Schwarcz and Cortecci, 1974; van Everdingen et al.,


1982). The correlation between ~ lSO and ~348 in the soil gypsum as well as in dissolved sulfate (Fig. 4) signifies that S and O in the structure of dissolved sulfate and gypsum have not undergone any reduction and re-oxidation cycle. The O isotope exchange between water and sulfate is very slow (e.g. Lloyd, 1968; Chiba and Sakai, 1985) and should not have changed the O isotopic composition of SO 4 in the soil gypsum.

3.4. Sulfate geochemistr3' of surface waters and rain

As shown in Table 3, the salinity level as well as the sulfate concentration increase along the course of the Zayandehmd River. These changes are due to the facts that river water is used for irrigating agricultural lands in the area and sulfate-enriched drainage water returns to the river.

Although only a few water samples from the fiver were analyzed for S and O isotopic composition of sulfate, it appears that there is a very little isotopic difference of these two elements in sulfate along the river. In general, the isotopic composition of the sulfate in river water resembles that of the soil and parent rocks.

Water from the Zefreh hot spring, located in the northeastern part of the area, has an appreciable amount of sulfate with a t~34S value of +21.0%o (Table 3). The enrichment in 34S of the sulfate in this water when compared with the S isotopic signature already discussed for the sediments, soils and river waters, could be the result of partial reduction of sulfate as can be inferred from the H z S smell around the outlet of this spring. Since the temperature of the water is below 80°C, both thermo-chemical and bacterial reduction can be operative (Krouse et al., 1988). Partial reduction of sulfate brings about the enrichment of remaining sulfate in both 34S and JSO (e.g. Kemp and Thode, 1968). If we assume Cretaceous marine sulfate present in the sediments, through which hot water passes, to be the source of sulfate in this spring, a lack of change in the

Table 3 Geochemical data for water samples from different sources

Sample ~ EC dS m - i [SO 4] meq 1- i 8180(SO4 ) %0, V-SMOW 8348(804)%~, V-CDT

W1 0.21 0.34 nd nd W2 1.58 5.48 + 9.96 + 13.58 W3 4.80 14.86 + I 1.81 + 12.81 GSW 28.34 42.86 + 11.90 + 12.33 RN 0.20 0.99 nd + 21.61 ZHS 4.40 25.58 + 13.05 + 21.00

W1-3: river water taken at 30 km west, 10 km east, and 50 km west of Isfahan, respectively (see Fig. 2); GSW: Gavkhuni Swamp water (about 80 km west of Isfahan); RN: rain sample taken on April 2nd, 1994 at a weather station close to pedon 1; ZHS: Zefreh hot spring located in the northeastern part of the study area; rid: not determined.


6 tSO value of sulfate could be attributed to the counteractive process of oxygen isotopic exchange between the remaining sulfate and isotopically light water in the region ( 6 1 8 0 = - 7 % e , Khademi et al., 1997). Even though the rate of exchange of O atoms between SO 2 and H20 is extremely low under ambient temperatures, it could be quite substantial in the hot environment of the spring, as experimentally shown by Lloyd (1968) and Chiba and Sakai (1985).

Atmospheric sulfur has been mentioned in some cases as a significant input of sulfur to the soil (e.g. Schoenau and Bettany, 1989; Podwojewski and Arnold, 1994). A rain sample from the study area was found to have about 1 meq 1-1 of sulfate with a ~34S value of +21.6%o (Table 3), which is similar to that of today's marine sulfate. This is much higher than the 6 34S value obtained for the soil gypsum.

From the standpoint of S geochemistry, it appears that both soil and water systems in the area are controlled by the geological formations with only very little associated S and O isotope fractionations.

4. Conclusions

The stable isotopes of sulfur and oxygen provided insight into the source and distribution of sulfate in the Isfahan region, central Iran.

Cretaceous sea-water sulfate appears to play a significant role in controlling the sulfate concentration and isotopic abundances in the soil and water systems in the area. Whereas the 634S values of the Cretaceous sediments are consistent with those in other regions in the world, those from the Oligo-Miocene show a great deviation from the •34S age curve (Claypool et al., 1980). The similarity in S isotopic composition between the Cretaceous and Oligo-Miocene sediments supports the hypothesis that the area was cut off from the Tethys seaway at the end of the Mesozoic. Therefore, SO 4 in the Lower Cretaceous sediments controls the geochemistry of the younger geological deposits, including the soils studied.

Although the mean 634S and 6~80 values of soil gypsum are almost the same as those in the parent sediments, they decrease in going from higher to lower positions of the toposequence. Aridity of the region does not allow the sulfate to undergo any measurable fractionation by microbial reduction. Prefer- ential dissolution and movement of sulfate ions with lighter isotopes in the toposequence have likely resulted in a small decrease of ~534S and 6 ~80 values from higher to lower elevations. Isotope fractionation during adsorption and desorption of sulfate by clay minerals appear to be insignificant.

The results support the hypothesis that the study area has received substantial amounts of water with high sulfate concentration derived from parent rock evaporite minerals, which resulted in an unusually high amount of gypsum accumulation in the center of the basin in the past (plateau) and at its edge (deep

H. Khademi et aL / Geoderma 80 (1997) 195-209 207

colluvial soils). Further accumulation of coarse materials, derived from the weathering products of mountains and hills, at the edge of this basin formed the colluvial fans. Today's lowest soil landscape (alluvial plain) was created by the cutting action of the Zayandehrud River.

Acknowledgements

The first author wishes to thank the Iranian Ministry of Culture and Higher Education for providing him with a postgraduate scholarship for his Ph.D. program, part of which is presented in this work. We thank Mrs. N. Lozano, Mrs. J. Pontoy and Mrs. M. Michaelescu of the Stable Isotope Laboratory, Department of Physics and Astronomy, The University of Calgary, for their excellent technical assistance. The study was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) funds awarded to Drs. A.R. Mermut and H.R. Krouse. Comments by G. Cortecci, K. Hattori and an anonymous reviewer are appreciated. This is contribution No. R808 of the Saskatchewan Centre for Soil Research, University of Saskatchewan.

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Sulfur isotope geochemistry of gypsiferous Aridisols from central Iran