Morphology, SEM, and stable isotopic analyses of pedogenic gypsum, USA and Jordan

Symposium no. 20 Paper no. 451 Presentation: oral

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Morphology, SEM, and stable isotopic analyses ofpedogenic gypsum, USA and Jordan

BUCK Brenda J. (1), VAN HOESEN John (1), KHRESAT Sa’eb (2) andRAWAJFIH Zahir (2)

(1) Department of Geoscience, University of Nevada Las Vegas, Las Vegas Nevada,USA

(2) Department of Natural Resources and the Environment, Jordan University ofScience and Technology, Irbid, Jordan

AbstractGypsic soils occur in over 200 million hectares worldwide. Pedogenic gypsum can

form in any type of parent material, and is most common in arid and semi-arid climates.The accumulation of gypsum in soil can affect all physical and chemical characteristicsand produce adverse effects on both agricultural and engineering uses. Four potentialorigins of gypsum accumulation in soils have been established: (1) in situ weathering ofexisting parent material (2) an oceanic source resulting in sulfate enriched precipitation(3) eolian or fluvial input of gypsum or SO4 rich sediment, and (4) in situ oxidation ofsulfate minerals. In this study, we examined pedogenic gypsum in soils from the USA:New Mexico, Nevada, and California; and from Northern Jordan. Macro- andmicromorphology were studied and the hydration water of gypsum in the New Mexicosoils was analyzed. The results indicate that gypsum accumulates in soils through timein a similar manner as calcium carbonate. It forms thin filaments, small nodules, andmassive, indurated horizons. Unlike calcium carbonate however, gypsum also formssmall snowballs (spherical accumulations of gypsum crystals) that form early inpedogenesis. SEM analyses indicate soil microorganisms and organic material may playa crucial role in the development of this morphology. However, no trends were found inthe crystal habits of gypsum suggesting the snowball morphology forms in a dynamicenvironment. Because the morphological stage of gypsum development in soils isdetermined by the flux of sulfate ions into the soil, pedogenic gypsum development insoils should not be the only criteria used to determine relative age of soil developmentor related geomorphic surfaces. Previous research suggested isotopic analysis of thehydration water of gypsum could be used as a paleoclimate indicator. Stable isotopicanalyses of the hydration water of pedogenic gypsum in this study indicated heavyenrichment of both δD and δ18O. Because gypsum precipitation in soils is driven byevaporation processes that result in isotopic fractionation, this enrichment cannot beused as a paleoclimatic indicator.

Keywords: gypsum, SEM analyses, stable isotopes, soil morphology

IntroductionThere are approximately 200 million hectares of gypsiferous soil covering the

earth’s surface (Nettleton, 1991), most of these occur in arid and semi-arid regions.Soils containing gypsic horizons can form in any type of parent material. These

BUCK ET AL. 17th WCSS, 14-21 August 2002, Thailand

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horizons can be composed of pedogenic as well as detrital gypsum. However, theformation of pedogenic gypsum is not thoroughly understood. The source of the sulfateion and the semi-soluble nature (~2.6 g L-1 at 25°C) of gypsum are the dominantcontrolling mechanisms for its formation and behavior in soils. Four potential originsfor gypsum accumulation in soils have been established: (1) in situ weathering ofexisting parent material (Carter and Inskeep, 1988), (2) an oceanic source resulting insulfate enriched precipitation (Podwojewski and Arnold, 1994), (3) eolian or fluvialinput of gypsum or SO4 rich sediment (Taimeh, 1992; Buck and Van Hoesen, in press),and (4) in situ oxidation of sulfate minerals (Mermut and Arshad, 1987; Podwojewskiand Arnold, 1994; Toulkeridis et al., 1998).

Accumulations of gypsum in soil can affect all physical and chemicalcharacteristics and can have adverse effects for both agricultural and engineering uses.Currently, a thorough understanding of the genesis of gypsum accumulation in soils hasnot been established. However, the solubilities of gypsum and calcite are relatedbecause of the common ion effect (Reheis, 1987; McFadden et al., 1991). Therefore,parameters such as pCO2, availability of Ca+2, temperature, soil microorganisms, andvegetation can all effect and can be affected by gypsum dynamics within the soilsystem. Because of these reasons, more research is needed to understand the chemicaland physical processes of gypsum formation within soils.

Physically, the morphology of gypsum in soils can be very similar to calciumcarbonate morphology: it can appear as thin filaments, nodules, or as massive pluggedhorizons (Reheis, 1987; Buck and Van Hoesen, in press) (equivalent pedogeniccarbonate morphology of stages I, II, and III), (Gile et al., 1966). Recently, a newmorphology of pedogenic gypsum has been described; the ‘snowball morphology’(Buck and Van Hoesen, in press). This morphology is unique to pedogenic gypsum andis one of the earliest morphologies to form.

Pedogenic gypsum usually occurs as individual silt-and-sand sized euhedral tosubhedral crystals. The shape, size, and position of these crystals within the soil matrixcan help to differentiate whether they are pedogenic (Carter and Inskeep, 1988).Pedogenic gypsum in soils exhibits a variety of crystal forms that may representdifferent environments of formation (Amit and Yaalon, 1996; Jafarzadeh and Burnham,1992). These crystal habits are controlled by a variety of localized conditions. Factorssuch as alkalinity of the solution, pH, temperature, salinity and abundance of organicmaterial affect the rate of crystal nucleation and therefore the ultimate crystal habit. Thebroad distribution of crystal habits throughout a variety of soil types and soil conditionssuggests different habits may form under specific conditions but are not limited to onespecific environment.

Previous research suggests δ18O and δD ratios collected from the hydration water ofgypsum may be useful in paleoclimate reconstruction (Halas and Krouse, 1982;Dowuona et al., 1992; Podwojewski and Arnold, 1994; Khademi et al., 1997a, 1997b).This methodology assumes the hydration water of gypsum is in equilibrium withsurrounding soil moisture, which is dependent on atmospheric precipitation in aridregions. Surface water infiltrates the soil and combines with calcium and sulfate ions toform gypsum. Because the source for hydration water in gypsum is meteoric water andthe fractionation factor between the water of crystallization and soil water is known(Gonfiantini and Fontes, 1963). δ18O/δD values obtained from gypsum hydration water


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have the potential to reflect the δ18O/δD signature of past precipitation. Possible sourcesof error with the use of oxygen isotopes in paleoclimate studies of soil minerals include:(1) latitude and/or altitude effects (Dansgaard, 1964; Lamb, 1977), (2) source (andtemperatures of the source) of precipitation (Gulf of Mexico versus Pacific Oceanversus Mediterranean) (Lawrence et al., 1982; Amundson et al., 1996), (3) seasonalityof precipitation (Lawrence et al., 1982; Amundson et al., 1996), (4) soil temperaturewhere soluble salts precipitate, and (5) evaporation of meteoric water prior toprecipitation of soluble salts (Cerling, 1984; Amundson et al., 1989; Cerling and Quade,1993; Liu et al., 1996). This last factor is extremely difficult to quantify because somany other factors can effect it such as the depth of wetting-which is controlled by theamount of precipitation, soil texture, soil structure, surface cover, and presence ofsurface crusts (Liu et al., 1996). Other problems associated with this methodologyspecific to pedogenic gypsum include: (1) re-equilibration of gypsum with youngerwater during alternating wetting and drying cycles, (2) surface and sub-surfaceevaporation of soil water prior to crystallization of gypsum resulting in an enrichment ofδ18O, (3) rehydration of anhydrite to form gypsum, and (4) sampling diagenetic versuspedogenic gypsum. In this study, we use scanning electron microscopic analyses (SEM)of pedogenic gypsum in soils from the USA and northeastern Jordan to betterunderstand the genesis of gypsum in soils. In addition, we tested the hypothesis that δ18O and δD isotopic signatures of the hydration water of gypsum could be usefulpaleoclimate indicators.

MethodsSoils in southern New Mexico (six profiles), southern Nevada (two profiles and

several surface crusts), Death Valley California (seven profiles), and northeasternJordan (two profiles and several surface crusts) were described and sampled accordingto the methods in Fieldbook for Describing and Sampling Soils (Schoeneberger et al.,1998) and soil color was defined using a Munsell Soil Color Chart. SEM and EDSanalyses were performed on a JEOL 5600 SEM and an Oxford ISIS EDS system onover 100 small soil peds to determine if the gypsum was pedogenic and to characterizethe micromorphology. Physical and chemical laboratory analyses for the New Mexicoprofiles can be found in Buck and Van Hoesen, in press. Data for the other profiles hasnot yet been published and is not included herein for brevity. Stable isotopic analyseswere performed on pedogenic gypsum from the New Mexico profiles. Sample selectionwas based on field data and SEM analysis indicating the presence of unalteredpedogenic gypsum. Samples were pre-treated and analyzed as described in Van Hoesen(2000). In addition, five surface water samples were collected for isotopic analyses andcomparison to the hydration water of the pedogenic gypsum. The δ18O and δD isotopicanalyses were conducted on 28 water samples at the University of New Mexico,Albuquerque in the Stable Isotope Laboratory during the spring of 2000. Five sampleswere local surface water and precipitation, and the remaining 23 samples werehydration water from pedogenic gypsum.


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Results

Pedogenic gypsum micromorphologyIn this study SEM analysis of soil gypsum crystals from over 100 small soil

samples indicate a variety of crystal habits including lenticular, tabular, pseudo-hexagonal, hexagonal, and lath (Van Hoesen, 2000; Van Hoesen et al., 2001).Lenticular gypsum crystals are lozenge, half-moon shaped in cross-section and resemblea convex lens (Figure 1a). They are spar sized, (ranging from 20-200 µm), disk shapedcrystals, with a strong development of the (111) crystal face and sometimes (103)crystal face. Tabular gypsum occurs as microspar-sized, (ranging from 10-20 µm),hexagonal crystals (Figure 1b) Tabular pseudo-hexagonal gypsum crystals aremicrospar-sized, (4-8 µm), six-sided crystals where one edge of the hexagon is longerthan the other edge. Pseudo-hexagonal lath gypsum crystals are found precipitating asmicrite to microspar-size, (2-10 µm), six-sided lath shaped crystals elongated to (101)and (111) where one axis of the hexagon is longer than the other. All of the gypsumcrystals are euhedral to sub-hedral, randomly oriented, and displace the soil matrix,indicating they grew in situ and haven’t undergone transportation or relocation. In allthe profiles studied, except the two in Jordan and the two in Nevada, all of the gypsumcrystals examined do not exhibit any re-crystallization “rings” associated with re-hydration of anhydrite or simple re-crystallization from alternating wetting and dryingevents. There was no evidence of corrosion, comb-shaped edges, or solution pits thatwould suggest dissolution. In contrast, many of the gypsum crystals from the profiles inJordan and in Nevada do exhibit dissolution features such as pits, and comb-shapededge forms. This is expected in the Nevada soils because of a shallow and fluctuatingwater table. The significance of this in the Jordanian soils is still being developed.

Isotopic resultsThe δ18O values of the local meteoric water range from-3.4 to-10o/oo and the δD

values ranged from-12.9 to-63.8o/oo. The δ18O values from the hydration water rangefrom-7.2 to 17.5o/oo and the δD values range from-24.9 to-91o/oo. The mean δ18O and δDvalues were 10.90 and-48.44o/oo. The δ18O values from the water of crystallization rangefrom-11.6 to 13.5o/oo and the δD values range from-5 to-72.45o/oo. The mean δ18O and δD values of the water of crystallization were 6.87 and-29.02o/oo. The trends between δ18O and δD of the hydration water of gypsum are similar in the same profile, howeverthe δ18O and δD trends between different soil profiles are highly variable. Both thewater of crystallization data and the hydration water data were plotted using a best-fitsimple regression function. The slope of these equations is consistent with previousresearch that found slopes of 2.5 to 4, characteristic values for arid regions (Sofer,1978). Global and local meteoric water lines were plotted using isotopic values fromsurface and precipitation water using a best fit linear regression producing a correlationequation of D = 7.22 18O=-8.13. The hydration water and water of crystallization forgypsum plot well below the local and global meteoric water lines (Figure 2). The δ18Oand δD values were separated into three relative age categories based on degree of soildevelopment: Late Pleistocene, Early Holocene, and Middle to Late Holocene. Whenthese values were plotted by their corresponding age, they overlapped and showed noapparent trend or distinct isotopic signature based on depth in the soil profile or with


age. The only consistent trend in the data is the hydration water of gypsum is alwaysenriched in heavier isotopes with respect to the water of crystallization.

Figure

PecarbonVan Hgypsummicrooin presused a1981).must bsulfatelocatioas a rmorphproperand the

a

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1 SEM images of the crystal habitslenticular gypsum crystals (New Megypsum crystal (Nevada) (photo by crystals (Nevada) (photo by Van Hoexhibiting dissolution features (Jorda

Discussidogenic gypsum has been reported to oate, except for the ‘snowball’ morpholooesen, in press). This morphology has b accumulation in soils and may at

rganism actinomycetes and/or other orgs). The stages of development in pedos an indicator for the degree of soil dev The potential exists to use gypsum ine exercised because gypsum accumulati ion input and other chemical interactionns, the flux of eolian gypsic dust has presult of climatic changes. Therefore ology alone to indicate the degree of soilties, in addition to gypsum morphology,ir related geomorphic surfaces. The wid

b

c
d
of pedogenic gypsum: (a) cluster ofxico) (photo by Van Hoesen), (b) tabularWolff), (c) lenticular and tabular gypsumesen), and (d) lenticular gypsum crystaln) (photo by Buck).

onccur in similar morphologies as calciumgy that is unique to gypsum (Buck and

een shown to represent an initial stage of least partly, owe its genesis to theanic constituents (Buck and Van Hoesen,genic calcium carbonate have long beenelopment (Gile et al., 1966; Gile et al.,

a similar manner. However, great careon in soil is controlled by the amount ofs that may change through time. In mostobably varied throughout the Quaternaryit would be difficult to use gypsum development. We suggest that other soil be used to estimate relative ages of soilse range of crystal habits for pedogenic


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δ18O-20 -15 -10 -5 0 5 10 15 20

δ D

-120-110-100-90-80-70-60-50-40-30-20-10

01020

GMWL: δD = 8δ18O-10LMWL: δD = 7.22δ18O-8.13HWL: δD = 2.39δ18O-74.5WCL: δD = 8δ18O-10

Figure 2 Comparison of δ18O and δD values from the hydration water of gypsum(HWL), water of crystallization (WCL), local meteoric (LMWL), and globalmeteoric water (GMWL).

gypsum observed in the same soil horizons suggest they may have formed underdifferent environmental conditions. Previous research has indicated the crystal habit ofpedogenic gypsum can be linked to the presence of organic material and soil impuritiesand the degree of soil solution supersaturation with respect to gypsum (Cody, 1979;Jafarzadeh and Burnham, 1992). Therefore, the wide variety in crystal habits forpedogenic gypsum suggests a dynamic environment subject to frequently changingconditions.

Isotopic analysis of the hydration water of gypsum produced values enriched in theheavier 18O and D isotopes. In comparison, isotopic analysis of pedogenic carbonatefrom southern New Mexico, specifically the Hueco Basin, has established a pattern ofdepletion in the Late Pleistocene with increased enrichment into the late Holocene(Buck and Monger, 1999). However, the isotopic values from the hydration water ofgypsum lack any evidence for this shift. The enriched isotopic values suggest highevaporation rates prior to gypsum crystallization leading to an erroneous isotopicsignature. Previous work illustrates evaporation is the primary controlling process overthe genesis of gypsum in arid soils (Buck and Van Hoesen, in press). Therefore,enriched isotopic values should be expected from crystals precipitating from anevaporating soil solution. Extensive evaporation of surface and soil water induced bylocalized environmental factors and possible seasonal and daily temperaturefluctuations, introduce too much error into the δ18O and δD signature of pedogenicgypsum to be used as a paleoclimate proxy. Unfortunately, quantifying the magnitude


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of isotopic enrichment for each gypsum crystal will prove difficult since soil conditionsare so variable. Therefore, contrary to previous research by Dowuouna et al. (1992),Khademi et al. (1997a), that suggests δ18O and δD isotopic values from the hydrationwater of pedogenic gypsum are useful paleoclimatic indicators, this study suggestsisotopic analysis of pedogenic gypsum is not an applicable paleoclimatic proxy. Futureinvestigations may be able to quantify evaporation and the amount of fractionation foreach individual crystal, but until then, this method is not applicable. Furthermore,without evaluating whether sampled gypsum crystals are pedogenic it is impossible todifferentiate between the isotopic signature of hydration water reflecting paleo-precipitation, and hydration water of gypsum that experienced re-precipitation, re-hydration, and/or extensive dissolution. It is essential to conduct micromorphologystudies on gypsum crystals prior to isotopic analysis, regardless of the soil environment.

ConclusionPedogenic gypsum has been reported to occur in several different morphologies

that are very similar to the morphologies formed by pedogenic calcium carbonate.However, the snowball morphology is unique to gypsum, and is not seen withpedogenic calcium carbonate or silica. This morphology has been shown to represent aninitial stage of gypsum accumulation in soils (Buck and Van Hoesen, in press). Inaddition, it may at least partly owe its genesis to the microorganism actinomycetesand/or other organic constituents (Buck and Van Hoesen, in press). Although the stagesof development in pedogenic calcium carbonate have long been used as an indicator forthe degree of soil development, pedogenic gypsum should not be used in a similarmanner unless there is additional data present to support it. Ancillary data is requiredbecause gypsum accumulation in soil is controlled by the amount of sulfate ion inputand other chemical interactions that may change through time. We suggest that othersoil properties, in addition to gypsum morphology, be used to estimate relative ages ofsoils and their related geomorphic surfaces.

The wide range of crystal habits for pedogenic gypsum observed in the same soilhorizons suggest they have formed under different environmental conditions. Thus, thisdata further substantiates that the soil is a dynamic environment and the precipitation ofgypsum is primarily driven by evaporation, but under frequently changing micro-conditions. Unfortunately for paleoclimatic research, these frequently changing degreesof evaporation lead to numerous differences in the amount of enrichment in δ18O and δDin the hydration water of gypsum. This introduces too much error into the δ18O and δDsignature of pedogenic gypsum for it to be a useful paleoclimate proxy. However, thisstudy shows how essential it is to conduct micromorphologic analyses of soil mineralsin order to determine if these crystals experienced re-precipitation, re-hydration, and/orextensive dissolution prior to conducting isotopic analyses.

AcknowledgementThanks to Doug Merkler, Leon Lato, Bruce Harrison, Ilsa Schiefelbein, Katie

Wolfe, Tim and Diana Lawton, Greg Arehart, and Simon Poulson.

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Morphology, SEM, and stable isotopic analyses of pedogenic gypsum, USA and Jordan