[The Journal of Geology, 2006, volume 114, p. 501–510] ! 2006 by The University of Chicago. All rights reserved. 0022-1376/2006/11404-0007$15.00

501

Vertisol Carbonate Properties in Relation to MeanAnnual Precipitation: Implications for

Paleoprecipitation Estimates

Lee Nordt, Maria Orosz, Steven Driese, and Jack Tubbs1

Department of Geology, Baylor University, Waco, Texas 76798, U.S.A.(e-mail: lee_nordt@baylor.edu)

A B S T R A C T

Previous publications combining the properties of multiple soil orders show that depth to carbonate (DTC) increasessystematically between 350 and 1000 mm of mean annual precipitation (MAP). We hypothesize that carbonate inVertisols (clay-rich, shrink-swell soils) respond differently to water flux than other soil orders because of lowerpermeability. To test this hypothesis, we compiled soil description and characterization data from multiple publishedsources across a late Pleistocene climosequence of the coast prairie of Texas to assess the relationship between MAP(700–1400 mm) and DTC. The DTC of carbonate nodules represents an index of accumulation and the DTC ofcalcium carbonate equivalent (total carbonate !2.0 mm diam.) an index of leaching. The DTC for 1%, 2%, and 5%abundances were assessed using regression analysis. The R2 values were highest for the DTC of 2% nodules and of1% calcium carbonate equivalent in Vertisol microlows. Surprisingly, relatively high R2 values were calculated forregression between MAP and DTC in Vertisol microhighs, whereby the relationship is expressed as a parabolic curveand DTC is shallowest in the central part of the climosequence where gilgai expression is greatest. When comparedwith previous MAP-DTC relationships, it is clear that Vertisols retain carbonate into rainfall isohyets exceeding 1400mm, 1400 mm higher than the preservation of carbonate in other soil orders. When replotted, the use of DTC toestimate paleoprecipitation with previous equations underestimates MAP in a Mississippian paleo-Vertisol microlowby approximately 32% at a DTC of 100 cm for 5% nodules. Other paleosol proxies also project greater rainfall thanprevious DTC equations in this paleo-Vertisol.

Online enhancements: appendixes.

Introduction

Climofunctions are used to assess the relationshipbetween mean annual precipitation (MAP) anddepth to carbonate (DTC) in freely drained soilsbecause of the influence of soil-water interactionson the decalcification-calcification process (Jenny1941; Birkeland 1999; Retallack 2001). The first for-malized study of this nature was conducted byJenny and Leonard (1934) on grassland soils (Mol-lisols) in the central Great Plains of the UnitedStates, where it was discovered that DTC increasedsystematically from 350 to 1000 mm of MAP. Ark-ley (1963) performed a similar study on arid soils(Aridisols) extending the relationship into drier cli-

Manuscript received September 6, 2005; accepted February23, 2006.

1 Department of Statistics, Baylor University, Waco, Texas76798, U.S.A.

mates (!350 mm MAP) of the western UnitedStates. Retallack (1994) combined these two datasets and added data from other soil orders from thepublished literature, producing a MAP-DTC rela-tionship now commonly used for paleoprecipita-tion estimates from paleosols throughout the Phan-erozoic. Retallack (2005) recently updated thiswork by adding more DTC data from publishedcounty soil surveys, producing a similar curve butwith a lower coefficient of determination. The“universal” data set of Retallack (1994, 2005) isrobust in the sense that it can be applied to all soilsor paleosols. However, the data set has the follow-ing limitations: it combines most of the currentlyrecognized soil orders and their attendant proper-ties, it includes soil environments from polar todesert to tropical, it is biased toward soils with

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loamy textures, and it offers an unclear and incon-sistent definition of DTC. These factors undoubt-edly contribute to the moderate coefficient of de-termination obtained when quantifying therelationship between MAP and DTC, which hasbeen addressed by others on the grounds that thesoil-forming factors were not adequately con-strained (Ruhe 1984; Royer 1999).

Few pedons from the clayey and slowly perme-able Vertisol order were included in the universalDTC record (Retallack 1994, 2005), and there wasno distinction between microlow and microhighgilgai topography. This raises the possibility thatcarbonate in Vertisols responds differently to cli-mate than carbonate in other soil orders, and thiswarrants further study. Previous climosequencework in the coast prairie of Texas indicates that arelationship between Vertisol properties and cli-mate does exist. For example, Kunze et al. (1963)assessed three prominent Vertisol series by docu-menting general trends in physical, chemical, andmineralogical properties where weathering inten-sified from east (wetter) to west (drier). Stiles et al.(2001) calculated the DTC for eight pedons and dis-covered a strong positive correlation to MAP. Thesedata, however, provided no methodology for deter-mining DTC and included only a few samples.Driese et al. (2005) assessed an array of Vertisolgeochemical properties across the coast prairie asa climosequence, establishing general trends thatwere then applied to fossil soils in the rock record.None of these investigations, however, systemati-cally studied DTC in relation to climate.

We hypothesize that Vertisols, because of theirhigh clay content, slow permeability, and shrink-swell properties (Wilding and Tessier 1988; Cou-lombe et al. 1996; USDA Soil Survey 1999), havehydrological conditions and behaviors sufficientlydifferent from other soil orders as to produce aMAP-DTC curve distinct from the universal equa-tion of Retallack (1994, 2005). Consequently, westudied a well-constrained Vertisol climosequencealong the coast prairie of Texas and assessed theDTC of nodules and calcium carbonate equivalentfor both microlows and microhighs. We compareour results with previous MAP-DTC curves, dis-cuss implications for estimating paleoprecipita-tion, and provide a consistent and quantifiable def-inition of DTC.

Setting

The coast prairie Beaumont Formation of Texasprovides a unique setting to assess the relationshipbetween MAP and DTC of Vertisols because the

soil-forming factors—biota, parent material, topog-raphy, and time—can be held relatively constantand Vertisols are the dominant soil order coveringapproximately 4 million ha (Coulombe et al. 1996).The Beaumont Formation extends from the Texas-Louisiana border to southwest of Corpus Christi,with Vertisols forming on large expanses of flood-basin deposits capping the upper part of a fluvial-deltaic sequence (fig. 1). The soils have formed todepths of at least several meters in parent materialsthat are rich in smectitic clays. Detrital carbonatecontent in the fine-earth fraction of the parent ma-terial ranges from approximately 2% to 20% (Stileset al. 2001; Nordt et al. 2004; Driese et al. 2005).Neither dust nor rainfall, however, is thought tohave been a significant factor as a contributor ofcarbonate to the soils of the coast prairie region ofTexas (Rabenhorst et al. 1984). It is the combinationof minimal carbonate eolian influx and low parentmaterial carbonate content in humid to subhumidclimates that limits steady state carbonate nodulardevelopment to Stage II (Nordt 2002). Topograph-ically, the floodbasin deposits are flat and feature-less and on a slight gradient (!0.5 m km!1) towardthe Gulf of Mexico. The upper age (time) of theBeaumont Formation has been estimated to be asyoung as 35,000 radiocarbon yr B.P. (Gaston 1979;Birdseye and Aronow 1991) to as much as 70,000thermolumenescence yr B.P. (Durbin et al. 1997).Climax vegetation (biota) is warm-season grasslandthroughout the climosequence, although many ofthe areas have been converted to cropland duringhistoric times, accompanied by the encroachmentof woody species (Kunze et al. 1963; USDA-SCS1981).

The climate regime along the coast prairie ofTexas is described as subtropical humid in the eastto subtropical subhumid in the southwest (Bomar1995). Between the cities of Beaumont and CorpusChristi, MAP ranges from approximately 1400 to700 mm (fig. 1). The taxonomic soil moisture re-gime is udic to the east of the city of Victoria, wheremoisture deficit throughout the year is minimal(USDA Soil Survey 1999). The ustic soil moistureregion to the west experiences greater seasonalityof rainfall and greater moisture deficit during thesummer months. The taxonomic soil temperatureregime is hyperthermic, with mean annual tem-peratures slightly greater than 22"C (USDA SoilSurvey 1999).

Based on dating of the Beaumont Formation, Ver-tisol pedogenesis began in the late Pleistocene, thuspresenting the possibility that changes in climateregime have altered the MAP-DTC relationshipthrough time. Most information indicates that the

Journal of Geology V E R T I S O L C A R B O N A T E P R O P E R T I E S 503

Figure 1. Mean annual precipitation, taxonomic soil moisture (Udic, Ustic) and temperature (Hyperthermic, Ther-mic) regimes (USDA Soil Survey 1999), and Vertisol pedon localities along the coast prairie of Texas. Delineation ofthe coast prairie, late Pleistocene Beaumont Formation, is after Bernard and LeBlanc (1965).

late Pleistocene was cooler than present along theGulf Coast (Bryant and Holloway 1985; Nordt etal. 2002), but it is unclear whether precipitationwas higher or lower. Regardless, effective moisturewould have been higher per unit of rainfall withcooler temperatures. To avoid potential effects ofclimate change, we assessed the shallowest car-bonate peak encountered, if more than one existedin a profile, under the assumption that this DTCis in equilibrium with the modern climate regime.

Methods

Vertisol pedon data from the Beaumont Formationof Texas were obtained from the U.S. Departmentof Agriculture–Natural Resource Conservation Ser-

vice (USDA-NRCS) soil description and character-ization website (USDA Soil Survey 2005) and fromthe publications of the USDA-SCS (1976) and Hall-mark et al. (1986; apps. A–D, available in the onlineedition or from the Journal of Geology office).These pedons were analyzed as part of formalizedresearch investigations and provide consistencyand thoroughness of description, sampling, and lab-oratory analysis. Pedons were divided into topo-graphic microlows and microhighs for abundancesof both calcium carbonate nodules and calcium car-bonate equivalent (total carbonate !2.0 mm di-ameter). Many of the pedon descriptions did notdesignate whether the data were collected from amicrolow or a microhigh. However, soil scientistsfrom the USDA-NRCS have traditionally described

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and sampled the microlow of a gilgai complex, evenif not stated as such (W. Miller, pers. comm., 2005).Nevertheless, we cross-checked each undesignatedprofile with accessory properties to ensure that thisimportant assumption was correct before assigninga pedon to the microlow position (e.g., dark colors,greater leaching depths).

Pedogenic calcium carbonate segregations, typi-cally identified by Bk horizons as an index of ac-cumulation, indicate a pathway of pedogenesis.Depth to calcium carbonate nodules was deter-mined based on the first encounter of abundancesof 1%, 2%, and 5%. We included any form of car-bonate segregation (nodule, soft mass, mass, lump,concretion) into the nodular category and allowedany size or shape. The 1% category was used toassess the first observation of visible pedogenic car-bonate; the 2% category approximates the few-common abundance descriptive boundary (USDASoil Survey 1993), and the 5% category approxi-mates the minimum needed as one criterion for adiagnostic calcic horizon in soil taxonomy (USDASoil Survey 1999). Problems were encounteredwhen abundance descriptions included qualitativeterms such as few, common, or many. “Few” wasarbitrarily assigned to a percentage of 1, and “com-mon” was assigned to a percentage of 5. The 2%category was problematic because it fell on the de-scriptive abundance boundary between “few” and“common,” so a decision was made for placementin the 2% or 5% abundance category based on othercontributing properties of the soil horizon. None ofthe Vertisols contained more than 20% nodules,thus excluding the descriptive category of “many.”

Calcium carbonate equivalent is commonly usedas a leaching index (USDA Soil Survey 1996). Thiscarbonate component consists of a combination ofdisseminated pedogenic carbonate and detrital car-bonate inherited from the parent material in the!2-mm particle size fraction (Nordt et al. 1998). Itis measured in the laboratory as a weight percent-age by treating samples with HCl, evolving the car-bon in carbonate to CO2, and converting to calciumcarbonate equivalent (Hallmark et al. 1986; USDASoil Survey 1996). Determination of depth to cal-cium carbonate equivalent was more straightfor-ward because of its quantitative nature as deter-mined in the laboratory. Depth to 1% calciumcarbonate equivalent records the actual depth ofleaching or decalcification, whereas the 2% and 5%abundances were designed to coincide with thenodular percentages.

MAP versus DTC for both calcium carbonatenodules and calcium carbonate equivalent were as-sessed with regression analysis, partitioned by mi-

crolow and microhigh, using the JMP statisticalsoftware package (SAS Institute 2001). Statisticalparameters presented are the coefficient of deter-mination (R2), root mean square error (RMSE), andprobability value (P) beginning with the 5% sig-nificance level (a). A 30-yr average based on 1961–1990 meteorological data was used for determiningMAP from weather stations between !1 km and nomore than 45 km from individual pedon localities.The MAP values among weather stations were sub-sequently interpolated using geographic informa-tion systems (Spatial Climate Analysis Service2004; Buttle and Tuttle 2005).

Results

The three DTC graphs for calcium carbonate nod-ules in the microlow of Vertisols along the coastprairie reveal similar relationships whereby depthincreases at an increasing rate with MAP (fig. 2A–2C). The R2 is highest for DTC of 2% and 5% nod-ules and lowest for DTC of 1% nodules. The lowR2 for 1% nodules probably reflects the number ofpedogenic processes that can create or move a sin-gle nodule to various parts of a profile, not neces-sarily in equilibrium with rainfall. If projected tothe 600-mm rainfall belt, carbonate nodules wouldbe expected to occur at a depth of between 50 and100 cm, regardless of the carbonate percentageused. As extrapolated to 1600 mm MAP, carbonatenodules would be expected at a depth of between250 and 300 cm. The DTC of 2% nodules, for ex-ample, increases by approximately 18 cm per 100mm of rainfall.

The DTC of calcium carbonate equivalent in themicrolow of coast prairie Vertisols demonstratesthat microlows become calcareous to the surfacebelow 900 mm of MAP and that leaching depthsdo not approach 2 m until 11400 mm MAP (fig.2D–2F). The DTC for 1% calcium carbonate equiv-alent, which also marks the leaching front, has thestrongest relationship with MAP. Comparison ofDTC of 1% calcium carbonate equivalent with 2%and 5% nodular abundances reveals that at rela-tively low rainfall (e.g., 1000 mm MAP), the leach-ing depth is 150 cm higher in the profile than theaccumulation depth of nodules but that the twodepths begin to merge at higher rainfalls (e.g., 1400mm MAP). Assessment of parent materials in thestudy area reveals that the detrital carbonate con-tent typically ranges from 10% to 20% in the drierend and from 2% to 10% in the wetter end of theclimosequence. Thus, it takes longer to move theleaching front downward because of more detritalcarbonate and less water flux in the western part

Figure 2. Mean annual precipitation versus depth to carbonate of nodules (A–C) and calcium carbonate equivalent (D–F) in Vertisol microlows.

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of the climosequence. The thickness of the decal-cification zone, as measured by depth to 1% cal-cium carbonate equivalent, increases by about 28cm per 100 mm of MAP.

Different relationships emerge in Vertisol micro-highs of the coast prairie where DTC of nodules isdeepest in both the drier and wetter ends of theclimosequence (fig. 3A–3C). The weakest correla-tion for nodules is 1% DTC, and the strongest is5%. These curves show that carbonate nodules areaccumulating in the surface horizon of microhighsin the central part of the climosequence in responseto greater gilgai development and surface relief (20–40 cm). This occurs as a result of mechanical thrustzones discharging carbonate nodules laterally fromsubsurface horizons of the microlow to the near-surface horizons of the microhigh (Nordt et al.2004). Downward water movement is subsequentlylimited in the microhigh because of local runoffinto the microlow, which reduces leaching. Thisprocess also explains why more nodules are con-centrated in the microhigh than the microlow, al-though this is not from the typical calcification-decalcification process. Wet/dry cycles and thusgilgai formation are suppressed on the eastern sideof the climosequence, where high rainfall and per-sistent soil moisture limit the number of times theshrinkage limit is attained from drying. This is con-sistent with the taxonomic definition of the udicsoil moisture regime in which surface cracks mustremain closed for most of the year (USDA Soil Sur-vey 1999). In contrast, it appears that on the drierside of the climosquence, shrink-swell potential islimited by the presence of soil carbonate and gyp-sum. Here, calcium flocculates and strengthens thebonds between clay micelles (Wilding and Tessier1988), reducing the coefficient of linear extensibil-ity (COLE) because carbonate and gypsum displaceCOLE-bearing clay minerals per unit volume ofsoil. Collectively, these processes seem to suppressgilgai formation on the eastern and western endsof the climosequence, contributing to the shape ofthe curves. Excluding one deep, anomalous datapoint (fig. 3C), the DTC of nodules is similar be-tween the microhighs and microlows in the wettestand driest ends of the climosequence.

The DTC for 1% and 2% calcium carbonateequivalent demonstrates that little leaching occursin Vertisol microhighs and that the soils remaincalcareous to the surface until MAP exceeds ap-proximately 1100 mm, with only a gradual increasein leaching depth between 1100 and 1400 mm ofMAP (fig. 3D–3F). The DTC for 5% calcium car-bonate equivalent appears to follow the degree ofgilgai formation in the same way nodular distri-

bution marks the influence of subsurface lateralthrusting.

Discussion

Paleoprecipitation Estimates. In our initial as-sessment of DTC for Vertisols along the coast prai-rie of Texas, we compiled official series descrip-tions from all published soil surveys on theBeaumont Formation and found no correlationwith MAP. These data are commonly derived fromshovel pits and truck-mounted probes, with loca-tions selected in a way to fit a taxonomic constructcalled the soil series (L. Nordt, personal experi-ence). We therefore conclude that only careful pitstudies, as part of formalized investigations, gen-erate the kind of data needed to confidently assessMAP versus DTC. Consequently, we defer to theuniversal data set of Retallack (1994) for compari-son with our Vertisol data because his later com-pilation (Retallack 2005) was based exclusively onthe addition of data from published soil surveys.This also probably explains the lower R2 values be-tween MAP and DTC in the later work ( 2R p

, compared with 0.51).0.62Although the development of the universal DTC

equation of Retallack (1994) provides a powerfulmeans of estimating MAP in paleosols of the rockrecord, our results point to limitations for esti-mating rainfall if this equation is applied to paleo-Vertisols. It is clear that in loamy-dominated soilsof the universal data set, DTC increases dramati-cally at an increasing rate so that soil carbonatedisappears above a MAP of 1000 mm (fig. 4). Ver-tisol microlows, in contrast, retain carbonate nod-ules in areas receiving 11400 mm of MAP, at whichpoint the 2% nodule zone (strongest correlation)approximates DTC in the universal data set near aMAP of 1000 mm. It is unclear at what MAP car-bonate will disappear in Vertisols, other than it issomething beyond 1500 mm. Surprisingly, thecurves of the universal and Vertisol data begin tomerge near the 600-mm isohyet. Hence, we con-ducted a preliminary investigation of DTC for nod-ules in Vertisols of aridic soil moisture regimes inlate Pleistocene soils of south Texas along the RioGrande River and from west Texas near the BigBend region. Our preliminary analysis of charac-terization data from the USDA-NRCS Web site(USDA Soil Survey 2005) and from official seriesdescriptions (USDA-NRCS 2004) indicate that theDTC continues along the trajectory into drier cli-mates, as shown in figure 4. A possible explanationis that surface cracks remain open in Vertisols inaridic soil moisture regimes for significant portions

Figure 3. Mean annual precipitation versus depth to carbonate of nodules (A–C) and calcium carbonate equivalent (D–F) in Vertisol microhighs.

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Figure 4. Comparison of mean annual precipitation(MAP) and depth to carbonate (DTC) using the “univer-sal” data of Retallack (1994; open circles), and DTC of2% nodules (filled circles) and DTC of 1% calcium car-bonate equivalent (dashed line) from the study area.

Figure 5. Comparison of curves using depth to carbon-ate (DTC) of the “universal” equation and DTC of theVertisol equation (2% nodules) for paleoprecipitation es-timates. The vertical dashed line demarcates the depth(117 cm) beyond which the standard errors of the twocurves do not overlap.

of the year (USDA Soil Survey 1999). Rainfallevents may produce bypass flow to the basal depthof the cracks, which typically extend to 50–100 cm.This is precisely the depth at which most carbonateis located in these arid soils, which is near the DTCof loamy soils of similar climates.

Our results demonstrate that the universal equa-tion will produce spurious results if applied to Ver-tisols to estimate paleoprecipitation. For example,at approximately 600 mm of MAP, there is littledifference between the two curves, but at 1000 mmof MAP there is a 58% difference in DTC (fig. 4).If the universal data and our data for 2% nodulesare replotted with DTC as the independent variableto estimate MAP, rainfall for Vertisols is underes-timated by approximately 39% at a depth of 150cm and by approximately 45% at a depth of 200cm (fig. 5). Further, using the universal equation,the presence-absence boundary of carbonate nod-ules would occur at the 1000-mm MAP isohyet,whereas with Vertisols the presence-absenceboundary for carbonate nodules would occur some-where beyond 1400 mm of MAP (fig. 4). Anothernote of importance is that by assessing a single soilorder for DTC, the RMSE is reduced from #141mm in the universal equation to #109 mm in theVertisol equation for DTC for 2% nodules in mi-crolows; this is an error reduction of 29%.

It may also be possible to use leaching depthfrom calcium carbonate equivalent of paleosols inthe rock record to estimate paleoprecipitation,provided that diagenesis has not altered the orig-inal carbonate content. The DTC for 1% calciumcarbonate equivalent in Vertisol microlows pro-

vides the best means of estimating paleoprecipi-tation because it tracks the leaching front withthe strongest correlation and lowest RMSE (fig.2D–2F). Replotting these data with DTC as theindependent variable to estimate paleoprecipita-tion yields 2y p 7.00E ! 04x " 2.8796x " 798.6( , mm, ). Leaching2R p 0.81 RMSE p 95 n p 35depth as an indicator of rainfall, however, needsto be tested in the rock record for reliability.

Sensitivity Test: Late Mississippian Paleo-Vertisol,Tennessee, United States. As a sensitivity test forVertisol DTC climofunctions and to illustrate theeffect of our study on paleoprecipitation estimatesfor paleo-Vertisols, we can consider the PenningtonFormation (Late Mississippian) paleosol describedin Caudill et al. (1996). This paleosol exhibited gil-gai topography so well preserved that microhighand microlow profiles included no evidence of ero-sional truncation before burial. The horizon se-quence described in the microlow, for example, isA-Bss-Bssk-BC, attesting to slickenside formation(Bss) and pedogenic carbonate accumulation (Bssk)in the subsoil. The depth (corrected for a modest10% compaction; Caudill et al. 1997) to the top ofthe Bssk horizon with 5% carbonate nodules was100 cm. Using Retallack’s (1994) DTC climofunc-tion, Late Mississippian paleoprecipitation is esti-mated as mm, which is comparable with648 # 141the MAP of present-day Brownsville, Texas (fig. 1).In contrast, the Vertisol microlow DTC climo-

Journal of Geology V E R T I S O L C A R B O N A T E P R O P E R T I E S 509

function of the present study for 100 cm and 5%nodules (Fig. 2C) predicts mm MAP857 # 117( , )2 2y p !4.00E ! 05x " 4.251x " 432.27 R p 0.68for the Late Mississippian in Tennessee, which ap-proximates the MAP 200 km to the north ofBrownsville (fig. 1). The Retallack (1994) DTC cli-mofunction, which did not include Vertisols, un-derestimates MAP by 34% relative to our estimatein the microlow by this procedure.

The carbonate nodules in the microhigh of theLate Mississippian paleo-Vertisol occurred at adepth of approximately 50 cm. To make a roughestimate of paleoprecipitation in the microhigh,we replotted figure 3C using only those datapoints falling below 1200 mm of MAP. Accord-ingly, the paleoprecipitation estimate is 977 #

mm ( ,257 y p !8.00E ! 04x ! 1.362x " 1047.40). The estimated MAP for the same Late2R p 0.80

Mississippian paleosol using Fe (wt%) in Fe-Mnconcretions (Stiles et al. 2001) is 989 mm, and theestimated MAP using the chemical index of al-teration minus potassium (CIA!K) of Sheldon etal. (2002) is mm based on bulk geo-1219 # 182chemical data from Caudill et al. (1992). Theseother MAP proxy indicators for late Mississippianpaleoprecipitation are also substantially higherthan those predicted by the Retallack (1994) DTCclimofunction.

Conclusions

We offer a detailed analysis of DTC and MAP forcalcium carbonate nodules and calcium carbonateequivalent in Vertisol microlows and microhighsalong the coast prairie Beaumont Formation ofTexas. Based on our study, we conclude that for

freely drained Vertisol microlows in ustic and udicsoil moisture regimes, DTC of 2% carbonate nod-ules provides the strongest correlation with MAP.Previous DTC equations developed from other soilorders will underestimate rainfall substantiallyif applied to Vertisols, particularly above a MAPof 600 mm. This is an important point becausethe abundance of clayey alluvial deposits in therock record is unquestionably replete with paleo-Vertisols (Retallack 2001). Our results also suggestthat DTC of 1% calcium carbonate equivalent haspotential as a paleoprecipitation estimator if car-bonate diagenesis has been minimal or absent.

More work is needed conducting detailed andcomprehensive field analysis of the other 11 soilorders (USDA Soil Survey 1999) and for Vertisolsin other climate regimes. But it is imperative thatthese important relationships be established as partof detailed pedological investigations. Althoughour data set was compiled from investigations con-ducted in this way, inconsistencies undoubtedlycontributed to some of the scatter in the data be-cause the descriptions and analyses were performedover a period of some 50 yr by many differentscientists.

A C K N O W L E D G M E N T S

W. Miller and J. Wiedenfeld (U.S. Department ofAgriculture–Natural Resources Conservation Ser-vice), provided invaluable support and suggestionsfor the data compilation of this project. C. Moraand an anonymous reviewer also provided helpfulsuggestions. L. Zygo assisted in the preparation offigure 1.

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