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Dep. of Geosciences, One University Plaza, Cape Girardeau, MO 63701
* Corresponding author (mtaide{at}semovm.semo.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: CEC, cation-exchange capacity • EDXRF, energy dispersive X-ray fluorescence • HIM, Hydroxy-Al interlayer mineral • HREE, heavy rare-earth elements • INAA, instrumental neutron activation analysis • LREE, light rare-earth elements • SOM, soil organic matter • REE, rare-earth element • XRD, X-ray diffraction spectroscopy
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Bartenfelder and Karathanasis (1989) used differential scanning calorimetry to evaluate the origins, environment of deposition, and the extent of quartz weathering. This approach represents the use of emerging technology to discriminate parent materials, particularly superimposed loess layers. Other approaches include 18O/16O and 13C/12C isotope ratios, trace mineral assemblages, and Zr/Ti ratios of the silt separates (Lindbo et al., 1997). The advent of commercially available INAA to estimate trace element abundances offers new possibilities for discriminating between parent materials by recognizing the unique signatures of trace metal assemblages.
The REEs span the Periodic Table from La to Lu. The light REEs (LREEs) consist of the lanthanide elements from La to Sm, whereas the heavy REEs (HREEs) consist of the lanthanide elements from Eu to Lu. Specific minerals have distinctive affinities for the individual REEs, thus the REEs' abundances in a particular mineral act as its signature. Weathering of REE-bearing minerals allows these elements to (i) form carbonate and phosphate complexes (Johannesson et al., 1996; Lee and Byrne, 1993), (ii) form organic complexes (Cantrell and Byrne, 1987; Schijf and Byrne, 2001), (iii) adsorb onto Mn oxyhydroxides (Ohta and Kawabe, 2001), and (iv) adsorb onto clays or clays coated with Fe and Mn oxyhydroxides (Aide et al., 1999; Aide and Smith, 2001). Although the REEs have a similar chemistry, the phenomenon known as the "lanthanide contraction" permits some differences in chemical reactivity (Cantrell and Byrne, 1987; Johannesson et al., 1996; Lee and Byrne, 1993; Schijf and Byrne, 2001) and the possible preferential migration of the individual REEs (Gouveia et al., 1993; Marsh, 1991; Prudencio et al., 1993; Braun et al., 1993; Braun and Pagel, 1994; Braun et al., 1998). While soil pedogenic processes may conceal the REE signatures, in certain instances whole soil REE signatures may be used to differentiate among various parent materials. In general, shale and clayey deposits typically have a REE signature reflective of its source area (Rollinson, 1993).
Marsh (1991) investigated REE mobility in soils developed in South African Jurassic dolerite sills and their weathered products. Marsh showed that (i) Si, K, Na, Mg, Ca, Ba, and V were mobilized and removed from the profile, (ii) Fe, Al, Ti, Zr, Hf, Zn, Cu, Co, and Ni were immobile, and (ii) the REEs were all mobilized, but formed new products within the profile and were conserved. The HREEs were mobilized to a greater extent than the LREEs. Prudencio et al. (1993) investigated REEs weathering of basalt-derived Portuguese soils, demonstrating that the REEs were mobilized and transported from the more acidic upper horizons and precipitated as secondary phosphates (Florencite). An exception was Ce, which was preferentially incorporated into Mn- and P-bearing Fe-rich nodules. Neodymium was the least reactive REE, followed by Sm, Eu, and Tb. The REE abundances in the particle-size fractions clearly established REE enrichment of the clay fraction. Gouveia et al. (1993) investigated REE weathering in Portuguese granite saprolites, demonstrating downward REE migration from the upper portions of the saprolite, with the HREEs having a greater degree of mobility. The REEs preferentially accumulated in the coarse silt fraction, followed by the fine silt and clay fractions, a feature attributed to the partial breakdown of apatite and zircon inclusions in biotite.
Braun et al. (1993) observed that phosphates effectively precipitate REEs, reducing their mobility. Phyllosilicates were observed to preferentially accumulate REE. Braun and Pagel (1994) investigated a lateritic profile derived from the weathering of syenite. Mass balance calculations of the host rock show that 50% of the Th, 70% of the LREEs, and 40% of the HREEs are found in allanite, epidote, titanite, and apatite. Using Th as an index element, many of the REEs were depleted in the Fe-rich upper horizons and were enriched in deeper, mottled, clayey horizons. The LREEs were found associated with phosphate minerals, whereas Ce formed fine coatings of cerianite (CeO2) in Fe-free domains composed of halloysite. Braun et al. (1998) studied a lateritic catena in the humid tropical region of Cameroon and documented the depletion of the REE in the acidic, ferruginous horizons in the soil's upper portion and the accumulation of Ce in the soil's lower portion, with other LREE accumulation in the underlying saprolite. As in their previous studies, phosphates were particularly effective in removing REEs from percolating soil solutions.
Aide et al. (1999), working in Missouri, demonstrated that the REEs were slightly depleted in acidic soils developed in rhyodacite. These well-drained soils, overlying thick sections of saprolite, were shown to have Th, Ti, Nb, and Zr accumulations in the argillic horizons, suggesting that the traditional indexing elements are somewhat mobile. The REEs were associated with greater clay contents, suggesting that lessivage and organic complexation may be important processes in REEs migration. Aide and Smith (2001) showed that REEs were largely conserved in Pachic Paleustolls and Lithic Haplustolls in southwestern Texas, whereas Si, Al, Na, K, Fe, and Mn were substantially depleted. Land et al. (1999), working with Swedish Spodosols, used a sequential extraction to partition the REEs into (i) exchangeable/adsorbed/carbonate, (ii) labile organic, (iii) noncrystalline Fe and Mn oxyhydroxides, (iv) crystalline Fe and Mn oxyhydroxides, and (v) non-labile organic and sulfide categories. The E horizons showed greater REE depletions than the Bs horizons and the LREE showed greater depletions than the HREE. The crystalline Fe and Mn oxyhydroxide fraction in the E horizon contained the major REE reserve. In the A horizon, the non-labile organic fraction provided the greatest REE content, whereas in the Bs horizon the non-labile organic fraction and the crystalline and noncrystalline Fe and Mn oxyhydroxide fractions were each important. These data indicate that appreciable inter-horizon transfer of REEs may be expected in soil systems exhibiting mobilization and migration of organic complexing materials (Land et al., 1999).
The purpose of this investigation is to assess the validity of using REEs to determine the presence of loess as parent material in soils developed primarily in rhyolite residuum. In this context we propose to use the mass distribution of the REE as a signature to indicate the parent material. In meeting this objective we will test the following assumptions: (1) the initial REE pool has not been appreciably depleted because of weathering; (2) the HREE concentrations are suggestive of loess mixed with rhyolite residuum; and (3) the REEs are preferentially concentrated in the clay fractions and clay eluviation-illuviation is not sufficiently significant to render the REE signature patterns ineffective.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The summer climate is hot and humid with July temperatures averaging 25°C, whereas winters are cold and humid with January temperatures averaging 0°C. Rainfall averages over 0.7 m and is evenly spread throughout the year, causing the upland soils in the study area to maintain a udic soil moisture regime (Brown and Gregg, 1991). Generally, the soils of the study area maintain mesic soil temperature regimes; however, south-facing slopes may have thermic soil temperature regimes (Brown and Gregg, 1991). Deciduous forests form a partially closed canopy cover over much of the study area, with vegetation composed mostly of white oaks (Quercus alba L.), red oaks (Quercus rubra L.), and an assortment of grasses and mosses.
Field and Laboratory Protocols for Soil Characterization
Two pedons representing the Knobtop series were sampled and described from excavated pits on Taum Sauk Mountain (T 33 N, R 3 E, Sec 4 and 5, Iron County, MO) (Fig. 1). Samples for chemical analysis were oven dried, lightly crushed and sieved to remove materials larger than 2 mm. The particle-size distribution was estimated by Na-saturation of the exchange complex, centrifuge washing with water-methanol mixtures to remove excess electrolyte, dispersion in Na2CO3 (pH 9.2), and centrifuge fractionation and wet sieving of the separates. Procedures for all methods are in Kunze and Dixon (1986) and Carter (1993).
Soil pH was determined using distilled water to establish a just-flowing paste and measured using a calibrated combination pH electrode. Exchangeable Ca, Mg, K, and Na were extracted with 1 M ammonium acetate (pH 7.0) and determined using flame atomic absorption spectroscopy. Total acidity was estimated by slow titration to pH 8.2 with 0.01 M NaOH. Soil organic matter (SOM) was estimated by preheating samples to 110°C, reweighing the samples and then heating at 360°C for 24 h; after which, the SOM was estimated by weight loss. The cation-exchange capacity (CEC) was calculated from the sum of the exchangeable cations and the total acidity.
The C horizon of the Menfro series (Fine-silty, mixed, active, mesic Typic Hapludalfs) was sampled with the assistance of NRCS personnel to provide loess-derived material. The Menfro series was selected because of its proximity to the study area, similar drainage characteristics, and relative purity (Festervand, 1981). The purity of the underlying loess implies the lack of other parent materials and minimal alteration or contamination by weathering processes.
Elemental analysis of whole soil and separated clay, silt, and sand fractions was determined by INAA (As, Ba, Br, Ce, Co, Cr, Cs, Eu, Fe, Hf, La, Lu, Na, Nd, Rb, Sb, Sc, Sm, Ta, Tb, Th, U, Yb, and Zr). Chondrite normalization values for normalizing the soil horizons, loess and rhyolite samples are those of Taylor and Gorton (1977). The chondrite normalization values are: La (0.315), Ce (0.813), Nd (0.597), Sm (0.192), Eu (0.072), Tb (0.049), Yb (0.208), Lu (0.0323). XRAL Laboratories (Toronto, Ontario) crushed the composited bulk rock samples to obtain representative samples and performed the INAA procedures.
A classical total element analysis for Si, Al, Ca, Mg, Na, K, Ti, Fe, Mn, P, Rb, Sr, Nb, Zr, and V was performed on multiple rhyolite rock samples and soil horizons using energy dispersive X-ray fluorescence (EDXRF) to verify the INAA data and to estimate the elemental abundance of Al, Mn, Si, and Ti. Sample preparation for EDXRF involved grinding and pelleting.
Oriented whole clay (<2 µm) samples were prepared for XRD to identify the clay mineralogy and provide qualitative estimates of clay mineral abundance. Potassium-saturated and Mg-saturated glycerol-solvated samples were air-dried by sedimentation onto glass-slides, producing oriented mounts. X-ray diffractograms were obtained with a Scintag diffractometer using CuK
radiation(Thermo, Franklin, MA). Spectra were scanned from 2 to 30° 2
at 0.02° s-1. Potassium-saturated specimens were heat treated at 150, 300, and 550°C to estimate the extent of the hydroxy-Al interlayer minerals (HIM). Peak areas were computed using Scintag software using Pearson algorthms within a profile engine. Peak positions of 1.77 to 1.8, 1.4 to 1.5, 0.99 to 1.01, and 0.71 to 0.72 nm using Mg-saturated and glycerol-solvated samples and peak positions at 1.0 and 0.7 nm using K-saturated and heat treated samples were used to identify smectite, HIM, nonexpanding 1.0-nm minerals (clay mica) and kaolinite (Moore and Reynolds, 1989).
Titanium Indexing
An element loss estimate is obtained by indexing the total element concentration in a particular soil horizon to Ti, then dividing this quotient by a similarly produced index for the parent material (White, 1995). The rhyolite extracted from the BC horizons was selected as the parent material.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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-quartz. The rhyolite has 3.4% excess Al, after meeting the feldspar requirements. Important minor and trace element concentrations include: 0.07 g CaO kg-1, 0.4 g MgO kg-1, and 1.69 g TiO2 kg-1.
Clay Mineralogy
The clay mineralogy is composed of HIM, 1.0 nm of nonexpandable minerals, kaolinite, and smectite (Table 3 and Fig. 2)
. Smectite is an important mineral in the Bt and BC horizons and absent elsewhere. HIM, kaolinite and clay mica abundances are relatively consistent throughout the soil profiles when allowances for the presence of smectite in the lower solum is considered. Heating of the K-saturated samples to 300 and 500°C demonstrated a high degree of hydroxy-Al interlayering (Barnhisel and Bertsch, 1989; Moore and Reynolds, 1989). The XRD patterns for the silt separates established the nearly complete dominance of -quartz, with minor to trace quantities of orthoclase (Moore and Reynolds, 1989). Karathanasis et al. (1983) reported that HIM and kaolinite are thermodynamically stable in loess-derived Kentucky soils.
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Ti Estimated Elemental Losses
Zirconium and Nb were not acceptable indexing elements because a significant portion of these elements were affiliated with the mobile clay fraction. Using Ti indexing, Si, Al, K, Na, and Fe loss estimates are >80%, suggesting that either weathering has been exceptionally intensive or Ti is not an acceptable indexing element. White et al. (1998) reported substantially smaller Si and Al loss estimates for intensely weathered Puerto Rican soils. Titanium is an unacceptable indexing element for the Knobtop series because (1) the soil profile descriptions and the mixed clay mineralogy appear incompatible with Si, Al, and Fe loss estimates greater than 80%; and (2) the parent material is not be exclusively composed of rhyolite residuum. The effect of a loess component, with a greater Ti content, would accentuate the elemental loss estimates.
Central and Lower Mississippi River Valley loess deposits generally have between 3 to 5 g Ti kg-1 (Lindbo et al., 1997; Muhs et al., 2001), a Ti concentration range consistent with the 3.9 to 4.3 g Ti kg-1 in the Knobtop soil horizons, but substantially greater than the 1.1 g Ti kg-1 associated with the rhyolite. Using the 4.4 g Ti kg-1 reported in the Muhs et al. (2001) study for the Ellis Grove (Illinois) loess site or the 4.2 g Ti kg-1 from the C horizon of the unweathered Peoria loess from the Menfro site, the elemental loss estimates for Si, Al, and Fe in the Knobtop pedons range from 10 to 25%, values considered reasonable for these Ultisols. Unfortunately, loess mantles within the study area are intimately mixed with the underlying residuum and the closest deposits of fresh unweathered loess only exist adjacent to the Mississippi River.
Rare-Earth Element Distributions in Whole Soil, Loess, and Rhyolite
The REE concentrations of the Peoria loess underlying the Menfro pedon and the rhyolite extracted from each Knobtop pedon were chondrite normalized to procure REE signatures (Fig. 3)
. The loess and the rhyolite REE signatures are similar, with rhyolite having a slightly greater REE abundance and a prominent negative Eu anomaly. The loess sample lacks any evidence of an Eu anomaly. In many soils the LREEs are frequently concentrated in apatite and the HREEs are frequently concentrated in zircon (Henderson, 1984). Apatite and zircon have been shown to be common accessory minerals in the rhyolites of the Van East Group (Tolman and Robertson, 1969).
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Rare-Earth Elements and their Partitioning Among the Particle-Size Fractions
The REE concentrations were determined for the sand, silt, and clay fractions and expressed as a percentage of whole soil (Table 4). The silt fractions contain the largest allotments of the total REE pool, reflecting the silt loam textures of these soil horizons. The clay fractions have appreciably greater REE concentrations than the whole soil, inferring a preferential REE accumulation. The preferential REE accumulation, on a unit clay basis, is particularly notable for the clay-poor A horizons and any subsequent clay eluviation-illuviation would promote a REE redistribution among the various soil horizons. However, if the relative affinities of the majority of the various REEs for interacting with the clay fraction are similar, the REE signature would be largely preserved.
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Validity of Using the REE Signatures for Isolating Lithologic Discontinuities
The moderately deep Knobtop soils are Ultisols having highly weathered profiles, somewhat masking the mutual contributions of the loess and rhyolite residuum. Evidence for loess includes silty textures, appreciable amounts of HIM overlying nearly micaeous-free rhyolite, and an apparent Ti enrichment beyond that expected if rhyolite residuum was the only parent material.
The REE signatures are consistent with a loess contribution. The validity of using REE signatures to isolate lithologic discontinuities depends on two issues: (1) reproducible and verifiable REE parent material signatures that are sufficiently distinct to be effective baseline standards, and (2) the REE signatures are not markedly altered by pedogenic processes.
In the present study, an unweathered loess sample was obtained from a soil just outside of the study area, because an unweathered loess sample in the study area does not exist. The Knobtop pedons are presumed to be developed in Peoria loess deposited contemporaneously with the Peoria loess extracted from the Menfro site. We acknowledge that physical and chemical changes in the loess occur with increasing distance from its source area and that REE differences may exist along this distance transect; however, the Knobtop REE signatures share a commonality with the loess and are distinctly different from those of the rhyolite residuum.
The Knobtop REE signatures were altered to a limited extent, particularly Nd and Eu because of their preferential accumulation in clay and lessivage. The HREE abundances in the whole soil are largely between the greater abundances in the rhyolite and the estimated abundances in the proxy loess, a condition best explained by a mixture of the two parent materials. Substantial weathering and depletion of Si, Al, and other macro-elements would relatively enrich the whole-soil REE concentrations, presuming that the soil REE are conserved. However, macro-elemental loss estimates are modest and the overall REE signatures are largely preserved. The REE are frequently reside in selected primary minerals and that on weathering the REE generally recrystallize as inert REE-phosphates (Braun et al., 1993; Braun et al., 1998; Gouveia et al., 1993; Marsh, 1991). Thus, the majority of the REE is generally conserved in the soil horizons, except in cases where clay migration is substantial or weathering has been pronounced and sustained. In the present study, the degree of clay eluviation-illuviation and weathering has been insufficient to alter the whole soil REE signatures, thus the presence of loess may be inferred.
The use of REEs as a diagnostic tool for recognizing lithologic discontinuities should only be attempted in cases where (1) the REE signatures among the verified parent materials are sufficiently distinctive to offer credible differences; and (2) soil processes do not interfere by altering the REE signatures. The second point remains an area of future research until the method may be implemented with full confidence.
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ACKNOWLEDGMENTS |
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Received for publication June 5, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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-MnO2) and Fe oxyhydroxide: Ce(III) oxidation by -MnO2. Geochim. Cosmochim. Acta 65:695–703.This article has been cited by other articles:
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  P. M. Jacobs and J. A. Mason Late Quaternary climate change, loess sedimentation, and soil profile development in the central Great Plains: A pedosedimentary model GSA Bulletin, March 1, 2007; 119(3-4): 462 - 475. [Abstract] [Full Text] [PDF]
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