Element partitioning in sediment, soil and vegetation in an alluvial terrace chronosequence, Limagne rift valley, France: a landscape geochemical study

CATENA ELSEVIER

Catena 31 (1997) 91-117

Element partitioning in sediment, soil and vegetation in an alluvial terrace chronosequence, Limagne rift valley, France: a landscape geochemical study E.M. Korobova a,1 A. Veldkamp a,*, p. Ketner b, S.B. Kroonenberg a a Department of Soil Science and Geology, Agricultural University, P.O. Box 37, Wageningen 6700 AA, Netherlands b Departmem of Nature Conservation, Agricultural University, P.O. Box 9010, Wageningen, Netherlands

Received 13 June 1996; revised 16 April 1997; accepted 16 April 1997

Abstract The geochemical behaviour of specific elements in fluvial sediments, topsoils, soil profiles and plant material on a sequence of Quaternary alluvial terraces was reconstructed semi-quantitatively using indices of accumulation or depletion with respect to abundance in the lithosphere, local soils and sediments. Topsoils of different ages and soil profiles are most clearly differentiated by the degree of leaching of Ca, Mg and some minor elements with respect to less mobile Si and Ti. Clay translocation within soil profiles of older terraces leads to accumulation of AI and associated elements in deeper horizons and accumulation of Fe and Mn in pseudogley horizons in the upper part of the clay accumulation layers. P, K, Ca, Mn and Zn are strongly concentrated in plant materials relaUve to the topsoils. Ca is accumulated more strongly in oak bark than in grasses, and the relative accumulation is greater on the older terraces due to the low Ca content in their topsoils. Mn contents of both grasses and oak bark are greater on the older terraces than the younger. For most elements, topsoil and plant material contents show little correlation, probably because plant requirements exert a greater control on element uptake than does availability. The

* Corresponding author. i Current address: Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Kosygin Street 19, Moscow, Russian Federation. 0341-8162/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0341-81 62(97)00029-5

92

E.M. Korobova et al./ Catena 31 (1997) 91-117

results illustrate that landscape geochemical analysis is useful for rapid appraisal of long-term landscape processes, such as erosion/sedimentation, weathering and preferential plant uptake. © 1997 Elsevier Science B.V. Keywords:

Landscape;Geochemistry;Terrace; Weathering;Accumulation;Depletion

1. Introduction

Biogeochemical surveys have proved useful for both mineral exploration and the assessment of environmental quality. There are many ways to portray the biogeochemical relations between different components of the earth's geo-ecosystem. Landscape geochemistry, as developed in Russia based on the ideas of Dokuchayev and Vernadsky, is concerned with the distribution and migration of elements in the landscape. Landscape is seen as a three-dimensional entity that comprises both biotic components (vegetation, fauna) and their environment (rock, soil, water). It combines the concepts of ecosystem, as a specific life community in which chemical elements circulate, and of ecotope, referring to the site where this ecosystem exists. Landscape geochemical research has received little attention in the west, but in Russia it has proved helpful in different aspects of environmental study, such as geochemistry, soil science, biogeochemistry, medical geography and radioecology, etc. (Polynov, 1934, 1954; Perel'man, 1975, 1989; Glazovskaya, 1964, 1988; Dobrovorsky, 1983, 1988; Kovda, 1989). English-language reviews of Russian landscape (bio)geochemistry are given by Fortescue (1980) and Dobrovol'sky (1994). Landscape geochemistry is based on a comprehensive analysis of the vertical distribution between the main landscape components, (e.g., rocks, soils, plants, water) and lateral distribution (in toposequences within typical landforms) of chemical elements as a result of mechanical, chemical and biogenic migration by air, water and mass wasting. This broad approach distinguishes it from conventional biogeochemical approaches that are focused on chemical processes only (Velbel, 1985; Meybeck, 1987). The elementary landscape is the smallest terrain unit that can be discerned at the relevant scale by its relatively homogeneous lithology, soil and vegetation. Examples are plains, a particular slope or a valley floor. A regional combination of elementary landscapes is called a geochemical landscape (Perel'man, 1975). Landscape geochemical maps (see Fig. 1 as an example) portray the geochemical characteristics of specific land units as so-called water migration types, which are usually derived from existing data on lithology, soil and vegetation. The local and regional geochemical patterns they depict can be used to predict concentrations of specific elements in certain landscapes and their components. Landscape units where accumulation of specific elements occurs as a result of abrupt changes in environmental conditions are called geochemical barriers (Perel' man, 1989; Glazovskaya, 1988). An example of a geochemical barrier is given in Fig. 2. The most comprehensive way of understanding processes in landscapes is to make geochemical budgets, based on measured fluxes in precipitation, surface and subsurface waters. Many such studies have been done in the framework of soil and water acidification research (Sollins et al., 1980; FiSrstner and Miiller, 1981). However, the

93

E.M. Ko robooa et al. / Catena 31 (1997) 91-117

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Horphotogy

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S|opes

(Os)

Vattey bottoms (atluvium)

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H-Fe

H-Ca

H-Ca-Fe

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High terraces

Fu, Fv, Fw

Low terraces

Fx, Fy

Flood ptain

Fyz, Fz

/

Ob

boundaries are atso geochemical barriers study sites

Fig. 1. Simplified landscape geochemical map, including main landscapes with main water migration types (hatchings). The map portrays the geochemical characteristics of specific land units as so-called 'water migration types', which are derived as a rule from existing data on lithology, soil and vegetation(Perel'man, 1975).

limitation of this approach is that information can only be obtained on short time scales, whereas processes such as soil formation span time scales beyond those of instrumental monitoring. Processes in the different landscape components, moreover, take place simultaneously, but at widely differing rates.

E..M. Korobova et al. / Catena 31 (1997) 91-117

94

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Fig. 2. Cross-section through Randan terrace sequence. Classification of elementary landscapes and geochemical barriers according to the classification of Perel'man (1975).

In this paper, we assess the interaction between slower geochemical processes in the landscape (progressive soil formation on a chronosequence of depositional surfaces at time scales of 1 X 103-1 X 106 years and slope processes) and more rapid biological processes (the growth of forest and grassland at time scales of 1 × 10°-1 × 10 2 years on these surfaces). The vertical and lateral partitioning of major and minor elements among sediments, soils and vegetation was studied on river terraces of different ages in the Massif Central, France. The aim of the research was to investigate whether differences in weathering and soil development on terraces of different age are reflected in the chemical composition of specific plants (grasses and oak trees). This tests the application of the Russian landscape geochemical methodology for characterizing the current landscape.

2. M a t e r i a l s a n d m e t h o d s 2.1. Study area

The Allier river basin occupies the Limagne rift valley, an elongated tectonic depression in the Hercynian crystalline Massif Central, France. The drainage basin is underlain by Hercynian crystalline basement rocks (58%), with Cenozoic volcanic rocks

E.M. Korobovaet al./ Catena31 (1997) 91-117

95

(22%) mainly along the margins of the rift valley and partly calcareous Oligocene fluvio-lacusuine sediments (20%) as a rift valley fill. The Quaternary history of the region was marked by tectonically and climatically controlled fluvial deposition and erosion, which led to the formation of a flight of at least nine river terraces (Veldkamp, 1991; Veldkamp and Kroonenberg, 1993a). Furthermore, there was ongoing volcanism in the uplifted rim west of the rift valley. The nine terraces are covered by stratified fluvial sediments mainly deposited in braided river systems. These sands and gravels consist of volcanic rocks, granite, gneiss and quartz fragments. Volcanic rocks predominate in the Allier sediments, especially in the lower teIraces, as compared with those of the Dore river, its main right confluent (Kroonenberg et al., 1990; Veldkamp and Kroonenberg, 1993b). Oligocene calcareous rocks (sandstones and marls) underlie the terrace sediments and crop out in the scarps between them. The terrace soils exhibit polycyclic features corresponding to changing environmental conditions during the Quaternary. The Holocene soils of the lowest terraces are mainly well-drained homogeneous profiles. Older terraces show clay illuviation and prominent weathering features, which increase with age and result in marked textural contrasts between the upper and lower horizons (Jongmans et al., 1991). The vegetation is mainly anthropogenic and can be subdivided into four main units: arable land, pasture, shrubland arid woodland. Their spatial distribution is closely connected with the agricultural suitability of land. The flattest surfaces are used as arable land (oilseed rape, cereals, sunflower/maize) and steeper areas as grasslands, often used for hay and grazing; the :steepest areas are occupied by shrub and forest. Fragments of semi-natural forest (which have not been cut for more than 100 years) remain mainly in the valleys ( Robinia pseudoacacia-Alliaria petiolata and Carpinus betulus-Quercus robur types), although some woodland areas have been planted (Pseudotsuga menziesii, Pinus sylvestris types) (Bresoles and Salanon, 1971; Billy, 1988). The presence of extensive semi-natural woodlands (Quercus petrea-Carpinus betulus type) on the higher terraces near Randan, was the main reason for selecting this region for a detailed landscape-geochemical study. On the Allier river bed, flood forests occur. Based on available topographical, geological, soil and vegetation maps, an exploratory landscape geochemical map was made (Fig. 1). The different units are classified according to the so-called water migration types. To indicate the transitions between the different elementary landscapes and to illustrate the geochemical boundaries, a schematic cross-section was drawn (Fig. 2). The sample sites are indicated by arrows; the symbols used are partly derived from the legend of Fig. 1.

2.2. Sampling design Four fluvial terraces east of Randan were selected for a detailed landscape-geochemical study: the FL terrace (Plio-Pleistocene), which is the highest one at about 390 m above sea level; the Fv terrace at 325 m (800,000 yr BP); the Fw terrace at 300 m (300,000 yr BP) and the lowest terrace Fy (5000-100 yr BP) at 265 m, only 1-2 m above the present river level at 263 m (Veldkamp, 1991). Two transitional landscapes

96

E.M. Korobova et al. / Catena 31 (1997) 91-117

Table 1 Sampling sites (terraces and their soil-vegetation cover) Site no.

Terrace

Soil type (FAO-UNESCO, 1990)

Type of vegetation

0 2

FL Fv Fv

Dystric planosol (O-Ae-E-Bt-Btg) Eutric planosol (O-A-E-Bcfe-Btg) Eutric planosol (A-E-Btg)

3 4 5 6 7 8 9

FvFw FvFw Fw Fw FwFy Fy Fy

Eutric cambisol (A-Bw-Bt) Eutric cambisol (Ad-AB-Bw-Bt) Eutrie planosol (A-Eg-Bcsg-Btg) Eutrie planosol (A-AB-Bwg-Btg) Eutric cambisol (Ad-A-Bt-Btg) Mollie fluvisol (A-C) Mollic fluvisol (Ad-A-AC-C)

Quercetum sessiti- Florae occidentale (Lemee) Querco ~ Carpinetum Anthoxanthum odoratum- Ranunculus bulbosusAlopecurus pratensis Querco- Carpinetum Trifolium repens- Lolium perenne A. odoratum-T, repens Querco- Carpinetum T. repens- L. perenne Querco- Ulmetum Poa angustifolia- Elymus repens

1

(terrace scarps) between the terrace levels F v - F w and F w - F y were also studied (Figs. 1 and 2). On each terrace and in each transitional landscape, one site with woodland and one with grassland was selected. The F w - F y transitional zone lacked forest, whereas the FL level lacked grassland, so that in total, 10 sites were studied and sampled. All sites on the terraces were selected in flat to almost flat positions, the transitional sites in the middle of the slopes. Short descriptions of the sampling sites are presented in Table 1. Sample areas of 25 X 25 m in woodland and of 10 × 10 m on grassland were laid out. Based on previous experience, oak bark and litter were assumed to be long-term indicators of the local geochemical situation in woodlands. In the grasslands, aboveground biomass was separated into three major agrobiological groups (grasses, legumes and other herbs) to avoid interspecies variations in chemical composition. Within each sample area, topsoils, herbaceous biomass (grassland) and litter (woodland) were sampled in 5 plots (each 0.5 X 0.5 m) in an envelope pattern (one sample at each comer and one in the middle) to show the variations within the site. A soil pit of about 1 m depth was dug in the centre of the sample area for profile sampling. Duplicate samples of bark were collected throughout each woodland sample area as mixtures of equal-sized pieces taken at breast height from three individual trees of the same species and age (average for the site). The total sampling scheme is presented in Fig. 3. In this paper, geochemical data are given only for bark (woodlands) and grasses (grasslands). Bark accumulates over many years, and is therefore a long-term biogeochemical indicator, whereas grasses accumulate elements in only one year. Soil horizons were described in accordance with the Russian (Klassifikatsiya, 1977) and FAO (FAO-UNESCO, 1990) systems. Vegetation types were described according to the European system (cover abundance according to Braun Blanquet, 1964), except for the highest terrace, where the Tensley relative abundance scale was applied.

2.3. Analytical procedures The elements determined were chosen for their role in both geochemical and biogeochemical migration. Among the major elements, N, P and K were chosen as

E.M. Korobova et al. / Catena 31 (1997) 9 1 - 1 1 7

97

SAMPLING MODES

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TERRACE

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important for plants and as a test for the presence of fertilizers in grasslands. Ca, Na, Mg, Si, A1 and Fe were selected as differentiating elements (so-called typomorphic elements; Perel'man, 1975; Perel'man, 1989) in the present landscape. Mn is important in both bioh)gical and geochemical processes. Among the trace elements, Ti was used as a least mobile reference element to compare element migration properties, Cr as an indicator of geochemical migration of elements with little biological significance, and Cu and Zn as elements active in both geochemical and biogeochemical processes.

98

E.M. Korobova et al. / Catena 31 (1997) 91-117

After drying at 70°C and milling ( < 100 /xm), the plant samples were dissolved in glass flasks with sulphuric acid-salicylic acid-hydrogen peroxide and selenium for automated determination (Walinga et al., 1989). Mg, Mn and Zn were determined in an air-acetylene flame on a Spectr AA 300/400, N and P by colorimetry and K, Na, Ca in an acetylene flame on a flame photometer (MERC Eppendorf ELEX 6361). The ash content was obtained after drying at 105°C and then igniting at 450°C for 4 h. X-ray fluorescence analysis (XRF) was used to determine the bulk element concentrations in soils. XRF data for parent material (mostly sands) composition (over 500 samples) were taken from Veldkamp (1991). A1, Ti, Fe, Cu and Cr contents of plants were determined in 0.01 M CaC12 extracts (1:10 dilution) by ICP after thermal digestion with HNO 3 + HF and H202 in teflon beakers followed by evaporation to moist salts and dilution in 2 M HNO 3 (Houba et al., 1989). 2.4. Concentration coefficients

To portray geochemical variation without being hampered by the effect of widely differing natural abundance of elements, their concentrations with respect to specific materials (partitioning or concentration coefficients) are given in preference to actual concentrations, and the geochemical characteristics of the landscape components investigated (parent materials, topsoils, oak bark, grasses) are visualized in normalized geochemical diagrams as commonly used for rare earth element (REE) plots (Nance and Taylor, 1976). The most commonly used concentration coefficient is that of an element in a specific rock with respect to its natural abundance in the lithosphere, called the Clarke Concentration Coefficient (CCC x) by Vernadsky (1938). The CCC x for element ( x ) is calculated as the ratio of the average concentration in the object studied to the average concentration in the lithosphere. Instead of the composition of the whole lithosphere, an average regional parent rock can be taken as a reference material (Regional Clarke Concentration Coefficient). An advantage of CCCs is that one can easily see a relative enrichment (CCC > 1) or relative depletion (CCC < 1). A disadvantage is that mass percentages are used, so that changes in density influence the coefficients. For example, soil compaction can cause a relative enrichment. The CCC should not be confused with coefficients used in isovolumetric weathering studies to determine relative elemental mobilities (Velbel, 1985; Veldkamp et al., 1990). In Fig. 4, regional CCCs are shown for the sediments of the terraces studied. The way in which certain elements accumulate or are leached in a soil from a reference parent material is indicated by the Coefficient of Eluviation and Accumulation (CEA x) in the Russian landscape geochemical approach. An analogous coefficient can be calculated for the concentration of an element in plant material (Polynov, 1934) with respect to the underlying topsoil (Coefficient of Biological Accumulation, CBAx; Perel'man, 1975; Perel'man, 1989). The Coefficient of Lateral Differentiation (CLDx) is used to indicate downslope chemical differentiation in a toposequence. It is calculated as the ratio of element content (x) in the sample to the content in the most leached landscape unit, usually the eluvial landscape of the watershed. If there is a true catenary relationship between the elementary landscapes considered, the CLDx indicates the

E.M. Korobova et al. / Catena 31 (1997) 91-117

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E.M. Korobova et a l . / Catena 31 (1997) 91-117

degree of lateral migration. Otherwise it can be used as an index of geochemical contrast, illustrating variability of parent material composition or slope processes causing mixing of different parent materials (Gavrilova and Kasimov, 1989). Various nonchemical processes are reflected in these coefficients, in contrast to other geochemical research on watershed mass balances (Velbel, 1985).

3. Results 3.1. Topsoil composition in relation to terrace level and vegetation As we wished to investigate to what extent differences in parent rock and soils are reflected in topsoil and vegetation, we concentrate on results for the last two. First we attempt to explain the distribution of elements in topsoils of the different terraces in terms of variations in parent material, landscape processes, soil formation and vegetation

Table 2 Mean values for elements in soil profiles Site

Terrace vegetation

Horizon

Si

Ti

A1

Fe

Mn

Mg

Ca

K

Na

P

0 0 0 0 0 0 0 0 0 1 1 I 1

FLforest FLforest FLforest FLforest FL forest FLforest FLforest FLforest FLforest Fv-forest Fv-forest Fv-forest Fv-forest Fv-forest Fv-forest Fv-grass Fv-grass Fv-grass Fv-grass Fv-grass FvFw-forest FvFw-forest FvFw-forest FvFw-forest

A A A A B B B B B A A B B B B A A A B B A A B B

37.28 38.02 37.18 36.60 33.80 32.38 32.80 32.39 32.12 36.17 34.32 33.74 33.36 29.41 30.89 33.44 33.72 33.03 30.04 30.04 35.66 35.91 32.70 34.65

0.45 0.42 0.47 0.52 0.68 0.75 0.70 0.77 0.72 0.61 0.82 0.94 0.91 0.96 0.73 0.91 0.83 0.91 0.83 0.84 0.69 0.62 0.65 0.64

5.27 5.14 5.56 5.92 8.03 9.10 8.95 8.96 9.05 5.66 6.73 7.09 7.25 10.72 9.51 7.31 7.22 7.96 10.87 10.40 5.71 5.86 8.36 7.00

1.55 1.48 1.63 1.86 3.44 4.35 3.91 4.36 4.36 2.57 3.37 3.59 3.89 5.40 4.71 3.30 3.22 3.58 4.88 5.09 2.41 2.42 3.89 3.07

0.24 0.09 0.05 0.07 0.03 0.02 0.02 0.02 0.02 0.12 0.13 0.11 0.18 0.03 0.10 0.16 0.16 0.18 0.02 0.30 0.11 0.11 0.06 0.08

0.20 0.16 0.17 0.17 0.33 0.45 0.38 0.43 0.38 0.19 0.23 0.23 0.24 0.57 0.91 0.31 0.29 0.28 0.53 0.53 0.32 0.31 0.68 0.47

0.45 0.10 0.05 0.06 0.07 0.08 0.10 0.11 0.19 0.27 0.25 0.24 0.24 0.59 0.51 0.46 0.40 0.31 0.42 0.51 0.37 0.29 0.30 0.26

0.71 0.61 0.78 0.85 0.76 0.61 0.69 0.73 1.07 1.05 1.29 1.40 1.45 1.26 0.77 1.40 1.40 1.40 1.09 1.33 0.95 0.91 0.77 0.83

3.14 2.94 3.45 3.47 3.04 2.75 2.80 2.80 2.81 3.10 3.18 3.18 3.14 2.48 2.96 3.25 3.24 2.94 2.40 2.48 3.35 3.42 3.17 3.05

0.07 0.04 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.05 0.06 0.06 0.06 0.04 0.05 0.08 0.07 0.05 0.02 0.03 0.05 0.04 0.04 0.03

1

1 2 2 2 2 2 3 3 3 3

Major elements are given in weight percentage, minor elements in ppm. Data normalized to volatile free and 100% totals.

101

E.M. Korobova et aL / Catena 31 (1997) 91-117

Table 3 Mean values for elements in soil profiles Site

Terrace vegetation

Horizon

Si

Ti

AI

Fe

4 4 4 4 4 4 5 5 5 5 5 5 6 6 6 6 6 6 7 7 7 7 7 8 8 8 9 9 9 9

FvFw-grass FvFw-grass FvFw-grass FvFw-grass FvFw-grass FvFw-gr~ss Fw-gras,~ Fw-grass Fw-grass Fw-grass Fw-grass Fw-grass Fw-forest Fw-forest Fw-forest Fw-forest Fw-forest Fw-fores! FwFy-gr~s FwFy-gr~s FwFy-gr~s FwFy-gr~s FwFy-gr~Lss Fy-forest Fy-forest Fy-forest Fy-grass Fy-grass Fy-grass Fy-grass

A A A B B B A A B B B B A A A B B B A A B B B A A C A A A C

37.00 37.61 36.84 36.77 37.01 36.13 37.05 37.60 37.04 33.14 32.65 33.42 36.94 36.42 36.44 35.36 32.59 30.11 33.68 33.78 31.05 30.32 30.54 29.51 29.88 36.40 28.58 29.89 34.52 35.28

0.40 0.32 0.39 0.38 0.34 0.48 0.33 0.30 0.34 0.51 0.53 0.48 0.41 0.46 0.45 0.51 0.62 0.77 0.53 0.52 0.58 0.70 0.73 0.84 0.82 0.36 0.97 0.83 0.58 0.49

5.49 1.63 5.24 1.36 5.73 1.55 5.82 1.67 5.75 1.58 6.22 1.96 5.74 1.19 5.44 1.09 5.88 1.23 7.04 4.50 8.74 3.87 8.30 3.55 5.83 1.38 6.22 1.61 6.06 1.75 6.72 2.22 9.00 3.58 10.61 4.65 7.64 2.98 7.48 3.07 9.06 4.26 9.20 4.94 8.96 4.91 8.82 4.86 8.72 4.77 5.07 1.70 9.12 5.57 8.78 4.83 5.81 2.99 5.40 2.48

Mn

Mg

Ca

K

Na

P

0.06 0.05 0.05 0.08 0.05 0.07 0.04 0.03 0.03 0.74 0.02 0.02 0.13 0.07 0.11 0.13 0.03 0.02 0.06 0.06 0.06 0.06 0.07 0.10 0.10 0.03 0.11 0.09 0.05 0.05

0.22 0.20 0.20 0.22 0.20 0.26 0.18 0.17 0.18 0.29 0.45 0.45 0.18 0.20 0.21 0.25 0.50 0.63 0.63 0.69 1.09 1.16 0.98 1.49 1.43 0.83 1.61 1.43 1.03 0.97

0.32 0.21 0.21 0.19 0.18 0.22 0.27 0.22 0.21 0.21 0.31 0.31 0.27 0.18 0.19 0.19 0.24 0.45 0.75 0.74 0.84 1.04 1.20 2.33 2.15 1.38 2.33 1.86 1.49 1.52

0.85 0.81 0.96 0.92 0.90 1.01 1.17 1.07 1.23 1.27 0.97 0.97 1.04 1.08 1.06 1.10 0.90 1.17 1.05 0.98 0.94 0.91 1.05 1.22 1.21 1.24 1.22 1.32 1.35 1.39

3.38 3.38 3.53 3.42 3.37 3.27 3.32 3.21 3.14 3.25 3.00 2.76 3.12 3.23 3.23 3.38 3.01 2.76 2.94 2.94 3.28 3.08 2.85 2.51 2.48 2.68 2.52 2.55 2.49 2.50

0.01 0.06 0.04 0.04 0.03 0.04 0.08 0.05 0.05 0.50 0.04 0.03 0.07 0.05 0.05 0.05 0.04 0.05 0.09 0.08 0.06 0.09 0.11 0.18 0.16 0.08 0.17 0.13 0.09 0.08

Major elements are given in weight percentage, minor elements in ppm. Data normalized to volatile free and 100% totals.

type. T h e analytical data for soils, topsoils and v e g e t a t i o n are g i v e n in Tables 2 - 8 , respectively. A C C C plot o f the e l e m e n t c o n t e n t s in topsoils o f the different terrace levels and transitional zones irrespective o f v e g e t a t i o n (Fig. 5) s h o w s that there is usually a c l e a r d i v i s i o n into two groups o f sites: (1) the older terraces (FL, Fv, F w ) and (2) the y o u n g f l o o d p l a i n (Fy). T h e transitional sites ( F v F w and especially F w F y ) are often also transitional in c h e m i c a l characteristics. T h e topsoils on the older terraces are richer in residual elements, such as Si, K, Z r and Nb, and p o o r e r in m o r e m o b i l e e l e m e n t s such, as Ca, Mg, Fe, V, Sr, Ba, Ni, Cr, L a and Zn, but :no clear trends o c c u r in AI, Ti, Ga. Fig. 6 s h o w s that the terrace l e v e l s can be distinguished u s i n g the ratios M g : T i (indicative o f leaching: m o b i l e vs. i m m o b i l e e l e m e n t s ) and A I : S i ( i n d i c a t i v e o f clay vs. sand accumulation), and that the variabilities b e t w e e n five s a m p l e points f r o m e a c h site are less than those b e t w e e n the sites.

102

E.M. Korobova et al. / Catena 31 (1997) 91-117

Table 4 Mean values for elements in soil profile Site no.

Terrace vegetation

Horizon

Ba

Sr

Zn

Cr

0 0 0 0 0 0 0 0 0

FL forest FLforest FLforest FLforest FL forest FLforest FLforest FLforest FLforest Fv-forest Fv-forest Fv-forest Fv-forest Fv-forest Fv-forest Fv-grass Fv-grass Fv-grass Fv-grass Fv-grass FvFw-forest FvFw-forest FvFw-forest FvFw-forest FvFw-grass FvFw-grass FvFw-grass FvFw-grass FvFw-grass FvFw-grass Fw-grass Fw-grass Fw-grass Fw-grass Fw-grass Fw-grass

A A A A B B B B B A A B B B B A A A B B A A B B A A A B B B A A B B B B

582 540 592 584 485 454 468 444 489 489 577 541 636 507 595 593 591 649 526 580 582 595 558 583 593 654 663 651 675 657 612 578 618 874 582 560

163 141 147 147 137 129 140 143 152 163 177 197 199 228 181 213 205 219 199 234 183 173 172 175 170 167 183 174 172 187 183 164 181 162 163 167

35 24 27 24 45 60 50 55 48 20 2 44 43 62 77 34 26 43 75 79 27 22 50 38 23 0 0 0 0 0 13 0 0 30 49 42

37 19 40 26 28 23 35 18 56 17 60 20 58 17 137 19 59 17 36 30 60 26 61 24 58 22 84 40 62 29 52 31 60 30 61 21 66 26 62 22 44 26 30 28 52 19 48 16 28 31 27 34 25 21 27 20 23 14 32 16 24 29 23 29 25 11 43 22 51 25 48 24

1

1 1 1 1 1 2 2 2 2 2 3 3 3 3 4 4 4 4 4 4 5 5 5 5 5 5

Ni

Pb

V

Rb

Zr

Ga

La

Nb

55 51 55 54 52 79 52 55 56 51 62 63 57 64 152 102 63 82 80 74 68 71 107 87 60 55 55 62 58 53 49 45 47 112 82 73

83 81 86 99 138 161 151 154 155 102 141 189 190 200 155 149 133 197 183 187 104 102 140 139 63 51 71 73 60 89 45 40 60 114 106 102

163 151 170 172 188 198 195 195 194 125 139 146 149 165 222 143 142 161 168 172 160 169 208 196 163 157 175 175 175 162 148 141 154 160 189 175

288 263 293 283 358 333 306 331 307 245 357 437 391 318 300 416 426 517 483 587 411 377 396 395 287 203 249 278 244 266 223 196 217 229 185 192

13 13 15 11 20 20 20 21 20 0 14 15 12 23 20 16 13 18 25 22 11 14 19 17 13 13 13 13 11 12 13 11 10 12 19 19

25 25 18 12 16 18 12 13 16 22 25 34 48 55 61 42 35 47 52 72 38 38 44 49 21 29 36 23 25 45 0 18 35 43 20 28

0 21 13 12 16 18 12 13 16 0 14 16 0 0 18 21 15 19 15 23 16 19 16 17 14 0 11 13 14 13 13 11 11 12 0 0

Major elements are given in weight percentage, minor elements in ppm. Data normalized to volatile free and 100% totals.

G e o c h e m i c a l d i f f e r e n c e s b e t w e e n the t e r r a c e t o p s o i l s w e r e c a l c u l a t e d as C L D s (Fig. 5) in w h i c h the e l e m e n t s w i t h m o s t variable c o n t e n t s s h o w the l a r g e s t s p r e a d . C a a n d Z n s h o w m a x i m a l C L D r a n g e s ; t h e s e t w o are easily r e l e a s e d b y w e a t h e r i n g a n d are readily a c c u m u l a t e d in t h e b i o l o g i c a l cycle. M i n i m a l C L D r a n g e s are s h o w n b y e l e m e n t s p r e s e n t in less m o b i l e f o r m s , s u c h as Si, A1, K, Ba, Rb, Zr, N b , etc. W o o d l a n d t o p s o i l s o n the h i g h e r terraces h a v e s m a l l e r C E A v a l u e s f o r P a n d Ca, and larger v a l u e s f o r K a n d N b , t h a n g r a s s l a n d t o p s o i l s (Fig. 7), b u t in t h e F y sites the t r e n d s f o r t h e s e e l e m e n t s are o p p o s i t e . A1 is h i g h e r in w o o d l a n d t o p s o i l s than in g r a s s l a n d

E.M. Korobova et al. / Catena 31 (1997) 91-117

103

Table 5 Mean values fo:r elements in soil profiles Site no. Terracevegetation Horizon 6 6 6 6 6 6 7 7 7 7 7 8 8 8 9 9 9 9

Fw-forest Fw-forest Fw-forest Fw-forest Fw-forest Fw-forest FwFy-grass FwFy-grass FwFy-grass FwFy-grass FwFy-grass Fy-forest Fy-forest Fy-forest Fy-grass Fy-grass Fy-grass Fy-grass

A A A B B B A A B B B A A C A A A C

Ba 650 684 874 638 480 480 680 666 659 709 721 708 656 572 676 646 528 493

Sr Zn Cr Ni 189 185 189 173 145 176 234 225 223 238 281 342 328 289 346 306 299 293

0 0 66 0 51 71 50 50 69 78 75 188 185 0 204 147 33 0

30 35 55 32 56 71 50 50 62 74 63 96 91 36 110 92 68 51

Pb

V

Rb

25 65 23 70 38 89 13 76 21 73 31 55 28 101 23 103 33 88 36 89 35 157 48 101 44 101 37 36 48 120 42 101 35 42 30 36

79 84 113 91 128 163 120 114 127 159 157 203 187 68 239 188 118 91

165 169 184 180 218 202 187 187 226 222 119 173 173 115 185 174 12 105

Zr Ga La Nb 315 347 351 318 266 259 218 262 260 302 314 340 332 107 369 363 200 134

14 16 22 15 19 24 19 20 19 22 20 25 22 0 24 23 11 12

41 35 46 46 38 41 38 44 46 54 51 56 62 16 58 49 34 24

20 20 13 16 0 0 11 14 15 19 20 20 22 0 24 23 0 0

Major elements are given in weight percentage, minor elements in ppm. Data normalized to volatile free and 100% totals.

topsoils at all sites for which data are available. For most elements, there are few C E A trends, however, and the contrasts related to vegetation type are usually much less than those between sites.

3.2. Topsoil--parent material vertical differentiation Concentrations of chemical elements in topsoils were compared with those in the terrace deposits sampled at 2 - 5 m beneath the surface (data from Veldkamp, 1991). The coefficients of eluviation and accumulation (CEA ) obtained reflect not only migration processes during soil formation, but also variations in primary clay content. Grain size distribution o f soil profiles in the same chronosequence (Jongmans et al., 1991) showed that all soils are developed in a similar parent material, a fining upward top layer o f the terrace deposits. The influence o f variations in organic matter and moisture contents was excluded by recalculation o f the concentrations on a loss on ignition basis. C E A patterns are shown for the three terrace levels for which both grassland and woodland plots were available (Fig. 7). Transitional sites are not discussed here, as their parent materials consist o f m i x e d marl and terrace sediments. The topscfils o f all older terraces show very similar geochemical spectra. Generally there is a relative accumulation in topsoils of Mn > Pb > Zr > Nb > Ti ( C E A > 1; Fig. 7), with a corresponding depletion of Zn > Ca > M g > Ni > Sr > Ba with respect to the parent material ( C E A < 1). Accumulation is thought to be mainly residual, especially for elements such as Zr, Nb and Ti, which occur mainly in stable heavy minerals. The

104

E.M. Korobova et al. / Catena 31 (1997) 91-117

Table 6 Mean values and standard deviations (n = 5) for elements in topsoils Forest

Means Si

FL Fv FvFw Fw Fy

34.94 32.88 34.29 34.28 26.70

Ti

AI

Fe

Mn

Mg

Ca

Na

K

P

0.46 0.66 0.58 0.42 0.74

5.09 5.71 5.51 5.60 8.06

1.47 2.51 2.32 1.41 4.35

0.07 0.12 0.11 0.09 0.09

0.16 0.19 0.28 0.19 1.32

0.09 0.22 0.27 0.19 2.00

0.71 1.06 0.91 1.03 1.14

3.24 3.00 3.21 3.16 2.29

0.033 0.046 0.039 0.055 0.146

0.77 0.42 0.30 0.51 0.80 Sr

6.51 5.59 5.31 6.85 8.19 Zn

2.91 1.77 1.07 2.88 4.65 Cr

0.14 0,05 0,03 0.06 0.09 Ni

0.28 0.24 0.17 0.61 1.37 Pb

0.43 0.30 0.22 0.71 1.99 V

1.28 0.81 1.04 0.87 1.11 Rb

2.91 3.16 3.13 2.64 2.30 Zr

0.075 0.086 0.067 0.086 0.144 Ga

17 169 27 18 50 169

35.87 37.72 29.11 29.18 83.96 47.88 31.40 22.66 52.30 92.92

25 20 24 30 42 27 26 29 24 40

48 55 66 58 96 62 57 43 112 103

Grassland Fv FvFw Fw FwFy Fy

31.07 34.19 36.00 30.19 26.18 Ba

La

Nb

22 27 32 31 52 42 29

10 14 17 17 17 17 11

37 47

12 20

Forest FL Fv FvFw Fw Fy Fv FvFw Fw FwFy Fy

465 474 553 593 616 504 574 573 590 611

133 162 169 168 304 190 160 164 199 302

68 104 89 68 174 126 71 43 111 192

137 128 150 156 162 127 154 136 170 165

264 351 351 310 295 390 264 187 225 319

9 9 12 11 22 14 13 12 17 22

Absent values are below detection limits. Major elements are given in weight percentage, minor elements in ppm.

elements Si, AI, Fe, Na, K, P, V, Rb and La are roughly similar in topsoils and parent material in the higher terraces. That does not necessarily indicate that they are inert; instead, their expected increase by residual or biological concentration may be counteracted by a loss through clay eluviation (cf. Jongmans et al., 1991). Pronounced depletion of Fe, Mn, Ti, Ca and Mg from the Fw terrace grassland topsoil compared with that of the woodland site seems to be related to intense gleying. Fv grassland topsoil is also depleted of Mn, although not to such an extent. This agrees with soil data showing that grassland soils in the study area have more gley features than those of woodlands (Bornand et al., 1968). It remains to be seen whether the use of land for grass stimulated gleying or whether the occurrence of gley conditions played a role in determining its use.

Except for Si, Ni and K (CEA < 1), the Fy terrace topsoil contains more of all elements than the parent material (CEA > l) (Fig. 7). This seems to reflect the larger amount of primary fines in the topsoil compared to the sandier subsoil in the fining upward terrace deposit. Grassland and woodland CEA spectra for the Fy terrace topsoils

E.M. Ko roboua et al. / Catena 31 (1997) 91-117

105

Table 7 Standard deviations (n = 5) for elements in topsoils Forest

Mean:; Si

Ti

Standard deviations Forest Si Ti FL 0.77 0.07 Fv 1.29 0.12 FvFw 3.15 0.26 Fw 0.68 0.03 Fy 1.00 0.03 Grassland Fv 0.71 FvFw 8.32 Fw 0.31 FwFy 0.38 Fy 0.38 Forest FL Fv FvFw Fw Fy Fv FvFw Fw FwFy Fy

0.05 0.11 0.02 0.02 0.02

A1

Fe

Mn

Mg

Ca

Na

K

P

AI 0.38 0.54 2.30 0.14 0.14

Fe 0.15 0.62 1.03 0.11 0.14

Mn 0.02 0.02 0.05 0.01 0.05

Mg 0.03 0.02 0.11 0.01 0.05

Ca 0.04 0.05 0.14 0.02 0.14

Na 0.08 0.15 0.42 0.03 0.05

K

P

0.21 1.35 0.08 0.07 0.11

0.32 0.45 0.03 0.08 0.12

0.03 0.07 0.01 0.05 0.04

0.03 0.07 0.03 0.07 0.07

0.03 0.17 0.04 0.03 0.02

0.01 0.02 0.003 0.01 0.003

0.21 0.06 1.24 0.16 0.06

0.004 0.01 0.02 0.01 0.01

0.09 0.77 0.03 0.05 0.03

0.01 0.03 0.01 0.01 0.01

Ba

Sr

Zn

Cr

Ni

Pb

V

Rb

Zr

Ga

La

Nb

28 17 38 26 8 34 23 15 29 12

13 10 13 7 11 5 4 5 5 8

-

3 7 4 5 6 5 4 2 7 3

13 8 4 13 3 6 3 1 1 2

9 4 7 5 6 6 7 2 33 3

11 25 20 12 11 14 16 2 4 8

10 5 6 5 6 5 4 2 3 3

58 57 35 10 17 33 34 14 12 19

1 2 1 3 1 1 2 1 1 1

7 6 4 5 5 7 6

4 4 1 3 3 2 2

4 3

2 3

9 17 5 8 4 9

Absent values are below detection limits. Major elements are given in weight percentage, minor elements in ppm. are a l m o s t the same. T h e s e topsoils are p r o b a b l y too y o u n g to s h o w clear b i o g e n i c and g e o c h e m i c a l differentiation. 3.3. Eluuiation and accumulation in individual soil profiles In all soft profiles e x c e p t those in the y o u n g alluvial plain, Si f o r m s 3 0 - 3 6 % in the topsoil ( T a b l e 2), d e c r e a s i n g with depth usually to values around 2 8 - 3 2 % ; K also d e c r e a s e s d o w n w a r d s . T h e s e decreases are c o m p e n s a t e d by strong d o w n w a r d increases in A1 and F e and w e a k e r increases in M g and C a ( f r o m 0.2 to 0 . 5 % ) in the B horizons. T h e i m p o r t a n c e o f clay a c c u m u l a t i o n in the B h o r i z o n s (higher A I : S i ratios) and o f l e a c h i n g in the topsoils ( l o w e r M g : T i ratios) are s h o w n in Fig. 8. M n has a c c u m u l a t e d in all topsoils, a b o v e i m p e r m e a b l e g l e y e d B g horizons, especially w h e r e F e - M n c o n c r e tions h a v e :formed u n d e r strongly r e d u c i n g c o n d i t i o n s (Feijtel et al., 1988). N a and K s h o w rather u n i f o r m distributions u n d e r both w o o d l a n d s and grasslands (Tables 2 and 3). T h e r e is not m u c h differentiation b e t w e e n different older terrace levels, nor b e t w e e n d i f f e r e n t v e g e t a t i o n types. L o c a l l y h i g h C a v a l u e s are f o u n d in the topsoils ( 0 - 5 c m ) on

106

E.M. Korobova et al. / Catena 31 (1997) 91-117

Table 8 Analytical data plant material (ppm) Ti

AI

Fe

Forest FL 84 Fv 64 FvFw 7 Fw 12 Fy 11 Mean 13 Standard deviation 16

60039 2553 8650 344 5982 356 23988 1233 19018 951 2353 1087 19405 809

Grassland Fv Fw FwFy Fy Mean Standard deviation

53524 32070 16466 83178 46309 25024

52 19 20 6 24 17

Mn 6988 5177 3686 8792 456 5020 2854

Mg

Ca

Na

2990 3639 4629 4719 4447 4083 669

301500 381259 335095 207212 240528 293119 62844

773 687 773 601 601 687 77

5074 3666 13214 2455 3710 11290 3334 2556 16191 1342 155 6607 3051 25221 1826 1 3 6 5 t442 3482

34421 26234 41532 27788 32494 6057

1649 1443 3677 172 1735 1256

K

P

Zn

Cr

Cu

11830 13143 17378 19861 16648 15772 2916

1150 1385 1731 2077 2193 1707 397

37 45 26 37 30 35 7

8 44 5 4 3 13 16

32 150 27 24 17 50 50

159528 171678 159411 112856 150868 22596

30140 21050 17773 18142 21776 4993

209 237 201 158 207 28

3 0.4 0.5 0.2 1 1

316 268 243 167 249 54

Forest samples are bark samples.

some older terraces (FL, Fv(grassland), FvFw), probably because of bioconcentration or seepage of calcareous groundwater along the slope. On the young alluvial plain, the loamy topsoils are poorer in Si and K than the sandy subsoil, and much richer in Mg and Ca (1.8-2.1%) than any other terrace soil in the toposequence. These data confirm that, in all terraces above the floodplain level, most of the Mg and Ca from the original sediment have been translocated by soil formation. Moreover, translocation of clay from the A (E) horizons has led to a relative increase of Si in the A horizons and of AI and Fe in the B horizons. Locally large Fe and Mn accumulations are found on top of the clay illuviation horizon, probably as a result of pseudogley formation (Jongmans et al., 1991). Ba and Ti concentrations may result from physical processes because no obvious chemical process is evident. P shows maximum concentrations in grassland topsoils, probably because of fertilizing as the topsoil concentrations are less pronounced in the forest profiles. In the FwFy transition, P is evenly distributed throughout the profile due to sediment mixing during the mass movements. Ba is to some extent correlated with Mn, which accumulates in F e / M n concretions above and within B horizons of the older terraces. It is proposed that Ba is involved in the processes leading to these concretions as observed for M n / F e concretions elsewhere (Pruissen and Zuurdeeg, 1988; Huisman et al., 1997). In sandy sediments, Sr in general follows Ba, as can be expected in feldspar-containing sediments, but Sr is not correlated with Mn in Bg horizons (Jongmans et al., 1991). The same holds true for Zn. Cu concentrations above the detection limit ( > 10 ppm) occur only in the lower parts of B horizons and in the topsoil of the youngest terrace. Co was detected only in forest topsoils on the older terraces and in B horizons in the grasslands. Pb is usually most abundant in B horizons. Only on the Fv terrace grassland is it more abundant in the

E.M. Korobova et al. / Catena 31 (1997) 91-117

107

.=.

v

z

8

x

.~_ ~.~ o c:~

~ ~

~ c~

~ ~

,-:

o --:

~ c5

,~o

c5

c5

c5

o.

o

o

o

m

o

(a,aqdsoql!l u! 9~l~JOAV/llosdol tlOl.lP-d'lua:)uo~)) :(l"I~)

, •

o

o

~'~

..= -~

4- v o

0

~

e~ •

~ o

E.M. Korobova et al. / Catena 31 (1997) 91-117

108 0.33 0.32 0.31 0.3 0,29 0.28 0.27 0.26 0.25

'< =¢

0.24 0.23 0.22 0.21 0.2 0.19

9

0.18

0.17 0.16 0.15

0.14 0.13

0.12

I

0.2

I

0.4

[

1

J

0.6

I

0.8

1

1.2

1.4

1.6

1.8

Ratio: Mg/Ti Fig. 6. Differentiation of topsoils according to Mg:Ti and AhSi ratios. Data labels refer to sites (Table 1). Young terraces (Fy and sites 8 and 9) plot upper right, older terraces (F1, Fv, Fw, and F v - F w transition sites 1 to 6) at lower left. Site 7 is the transition zone between Fw and Fy.

topsoil, which suggests that atmospheric (industrial) contamination is slight relative to the natural Pb content. Ni accumulates in topsoils and above the B horizon in the profiles on the Fw terrace and in the FwFy transitional zone. We conclude on our own observation and those of Jongmans et al. (1991) that redistribution of elements within the soil profiles results from several processes: (a) weathering and leaching of Mg and Ca and residual accumulation of Si, K, Rb, Zr and Nb associated with quartz, K-feldspar and heavy minerals in topsoils of the higher terraces; (b) secondary accumulation of A1, Fe, Mg, Ti, K, St, Ga, V, Cr and La in the B horizon of the higher terraces by clay neoformation, illuviation or adsorption; (c) redox changes caused by waterlogging in and above the B horizon with accumulation of Fe and Mn accompanied by Ba and Pb; and (d) biological accumulation in topsoils, most pronounced for P and Mn. 3.4. Grass and oak bark composition

Average element concentrations in oak bark and grasses (Table 3) were obtained from the five grass samples per site and oak bark samples for the whole plot (see above). Large ( > 40%) within-site variations in Na, Ti, A1 and Cr contents of grasses were not due to contamination with soil, as the ash content of the grass samples within sampling sites was almost constant. Concentrations of other elements do not vary by more than 20%, except that Mn shows large variation (34-38%) in both transitional sites. In

109

E.M. Korobova et aL / Catena 31 (1997) 91-117 _~ es

6.31

•~,

5.Ol

~

3.98 3.16

~

2.51 2.00

~

1.58 1.26 1.00

0.79

'~

0.63 0.50 0.40 032

o~

j

0.25 0.20 0.16 0.13 0.10

I

I

I

I

I

I

I

I

I

I

Si Ti AI F e M n M g C a N a K P B a S r Z n C r Ni P •

a

Fv-forest Fv-grass

+ x

Fw-forest Fw-grass

o v

V R ZrG

La

Fy-forest Fy-grass

Fig. 7. Coefficientsof eluviation and accumulation(CEA) for topsoils relative to parent materials for types of vegetation on Fv, Fw and Fy terraces (transitional sites and FI omitted). Values > 1 indicate relative enrichment and values < 1 relative depletion. Vertical scale is exponential. general, variations of element concentrations in grasses are highest in the FwFy transitional landscape. Na and K show large variation at the FL level, Ti and A1 at the Fw level. On the three terrace levels for which both oak bark and grass results are available, the differences in most elements between grass and bark are greater than between the terrace levels (Fig. 9). Na, AI, Fe, K, P, Mg and Zn in ash are all an order of magnitude more abundant in grasses than in oak bark. Oak bark, on the other hand, concentrates Ca, Cu and Cr. Ti is also more abundant in bark than in grass, and it decreases regularly with terrace level. The only element less abundant, in both grass and oak bark from the young flood plain than in those from the older terraces, is Mn. This is probably due to the lower mobility of Mn compared with the higher terraces, where redox reactions are more important. Therefore, as far as absolute abundances in vegetation composition are concerned, lVln (and to some extent Ti) are the best biogeochemical indicators of terrace age. 3.5. Biological accumulation: topsoils and plant material

The degree to which grasses and oak bark accumulate elements with respect to the topsoils is expressed as the CBA (Coefficient of Biological Accumulation) (Ax of Perel'man, 1975). Fig. 10 shows that P, Ca, and Zn and to a lesser extent K, Mg, Mn and Zn are concentrated in plant material relative to the underlying topsoils (CBA _> 1),

E.M. Korobova et al. / Catena 31 (1997) 91-117

110 0.38

l B2

0.36

B2

0.34

B6

B-horizons order ferroces

0.32

BI

0.3

! B0 B0 B0

0.28 ~

0.26

B3

FwFy

A

0.24

BO BO

B5 / A7

=g 0,22 B3

0.2

B7

A1

B6 B5

A/L -

Fy A7

B6 0.18 A9 0.16 ~ . ~ 9

0.14 0.12

~ " 0.2

- ~ - 0.4 0.6

- I 0.8

-

1

1.2

~

l 1.4

~ ~ 1.6

- " 1.8

q

CI 2

-

- ~ 2.2

2.4

Ratio: Mg/T| Fig. 8. Soil profile horizon differentiation (A and B horizons) in individual soil profiles on different terraces as shown by AI:Si ratios indicative of clay alluviation, and Mg:Ti ratio indicative of the degree of weathering. First symbols refers to horizon designation and second to site number in Table 1.

whereas uptake of Na, Fe, Ti and Cr are limited, causing slight to strong depletion in plant material (CBA < 1). Two factors affect the CBA values: the type of plant material sampled, and the site characteristics. Fe and K are about 10 times more abundant in grass ash than in oak bark ash, irrespective of terrace level or soil age. For both elements the type of plant material la0rgely overrules the site characteristics. For Ca both influences are clearly visible: oak bark samples are about 10 times more enriched than grass samples, and samples from the older terraces are about 10 times more enriched relative to the youngest terrace. Similar influences may affect other elements. Grass and oak bark from the Fy terrace have the lowest degree of biological accumulation for most mobile elements (P, Ca, Mg, Zn), but the highest relative contents of these elements in the topsoils (Fig. 7, CEA). This reflects their predominant occurrence in nonweathered minerals in these young soils. 4. D i s c u s s i o n

4.1. Lateral geochemical differentiation Lateral geochemical differentiation in the area can be considered at two levels: at the terrace level and at the slope level in between the different terraces.

E.M. Korobova et al. / Catena 31 (1997) 91-117

111

1000000

100000 10000

1000

100

~

lO

0.1

P •

]

I

]

I

K

Ca

Mn

Zn

Mg

AI

Na

I

I

Fe

Ti

Fv-GR

+

Fw-GR

~'

Fy-GR

zx Fv-OB

x

Fw-OB

v

Fy-OB

Cr

Fig. 9. Chemical composition (wt.%) of ashes of grass (GR) and oak bark (OB) from terrace levels Fv, Fw, Fy.

The terrace sediments consist of a mixture of granitic and basaltic mineral grains and rock fragme~ats, and therefore have a similar bulk chemical composition to the (granodioritic) lithosphere (cf. Perel'man, 1975; Ronov, 1980; Taylor and McLennan, 1985). However, they differ slightly from one terrace to another (Fig. 4), probably as a result of climate con~trolled changes in sediment fluxes and active volcanism in the Allier headwaters (Kroonenberg et al., 1988; Veldkamp, 1991; Veldkamp and Kroonenberg, 1993a). The terraces differ in age by several 100,000 years; hence, in degree of weathering and soil formation (Fig. 5). The Fv terrace is about 800,000 years old, the Fw terrace about 300,000 years, and the Fy terrace is Holocene. In spite of the large age difference between Fw and Fv, the chemical differences in the topsoils between them are less than between either of them and the Fy terrace, and are often overruled by local secondary factors, such as the development of the planosol/pseudogley at the Fw-grassland site. Apparently weathering and soil formation are not linear with time, as was illustrated by modelling o:f subsoil weathering in the Allier chronosequence (Veldkamp and Feijtel, 1992). During previous soil formation research on the Allier terraces (Jongmans et al., 1991), additiional soil chemical properties were measured. The Fy soils have pH (H20) ranges from 8 (subsoil) to 6.8 (topsoil), indicating slightly acid conditions for the topsoil only. The soils on the older terraces are more acidic, favouring leaching of soluble elements: Fw (pH (H20) from 5.1 to 6.7), Fv (pH (H20) from 6.0 to 6.3) and Ft (pH

E.M. Korobova et al. / Catena 31 (1997) 91-117

112

1000.000

'~ lOO.OOO 1 o.ooo o

o=

1 .ooo

o.1oo ~=

O.OLO

.<

0.001

0.000 P • Fv-grass ,x Fv-oak bark

i

i

i

I

i

1

i

Ca

Mn

Zn

Mg

AI

Na

Fe

+ x

Fw-grass Fw-oak bark

o v

i

Ti

Cr

Fy-grass Fy-oak bark

Fig. 10. Coefficient of biological accumulation (CBA) for selected elements in ashes of grass (GR) and oak bark (OB) for Fv, Fw and Fy terraces, normalized to topsoil composition at each site. Values > 1 indicate relative enrichment and values < 1 relative depletion.

(H20) from 3.7 to 4.5). These acid soil conditions are reflected in the losses by leaching of most Ca, Mg, Zn and P from the topsoils. In topsoils of the young, almost non-weathered Fy terrace, these elements are still at their primary levels or have been enriched by biological accumulation. Fresh supply of these elements to the soil still occurs during exceptional floods. However, oak bark and grass from all terrace levels contain similar amounts of Ca, Mg and Zn irrespective of the extent of weathering of the topsoils. This explains the low CBA values for Fy in Fig. 10, which could result either from different degrees of availability of these elements to the plants (only bulk content was measured in the topsoils) or extraction of Ca, Mg and Zn by the plants according to their needs, irrespective of their abundance in the soil. In the young Fy terrace Mn seems to be less available to plants, probably as a result of the high pH. Its contents in the topsoils are similar to most topsoils in higher terraces, but the absolute Mn content in Fy grass and oak bark is much less, leading to low CBA values (Fig. 10). Summarizing, the best (bio)geochemical indicators for distinguishing the older terrace landscapes from the younger ones in the Allier chronosequence are as follows. In topsoils: CCCs and CEAs for Ti, AI, Fe, Mg, Ca, P, Ba, Sr, Zn, Cr, Ni, Pb, Rb, Ga, La and Nb; CEAs for Na and Rb are large in Fy topsoils, and less in Fw and Fv topsoils (Figs. 5 and 7).

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Smaller M g / T i and A1/Si ratios in topsoils of the older terraces than of the young terraces (Fig:. 6). Absolute Mn content in oak bark and grass is small in Fy and greater in Fv, Fw (Fig. 9). CBAs for P, Mn, Zn, Mg (all oak bark and grass) and Ca (grass only) are small for Fy and greater for Fv and Fw (Fig. 10). Lateral differentiation is evident in the transitional landscapes, which are subject to slope proce~ses and groundwater flow. Slope processes, such as mass wasting, have mixed terrace sediments in different stages of weathering with the underlying Oligocene marls. Together with ground water flow, this leads to the formation of a variety of intermediate landscapes characterized by distinct lateral enrichment in several elements. The FvFw lransition is a landslide with a considerable admixture of terrace sand, as shown by the relative enrichment in Si and K in the topsoil compared with that in the upper terraces. Mg, Ba, Pb and Rb contents and pH are all greater (visible reaction with 1 M HCI) in the transition, possibly due to admixture of less weathered material. Topsoils in the FwFy transition are enriched in Ca, Mg, Na and Fe, possibly by groundwater seepage along the slope, in turn stimulating biological accumulation (P). Enrichment of Cr, Pb, V and Rb at this site may be due to the admixture of less weathered terrace material. 4.2. Vertica.I geochemical differentiation

The disaibution of elements within individual soil profiles depends on four main processes: weathering and clay neoformation, clay illuviation, formation of stagnogley features and bioaccumulation. Well-drained flat terrace surfaces were subject to progressive weathering of parent material. This led to the formation of acid leached topsoils with residual enrichment of two groups of elements: Si, K, Rb, Ga, Pb, corresponding to quartz and K-feldspar, and Fe, Ti, Zr and Nb, probably corresponding to stable heavy minerals, such as magnetite, rutile, zircon and ilmenite (Kroonenberg et al., 1988). Weathering was accompanied by intense clay neoformation and clay illuviation into the B horizon (Fig. 8). This horizon is consequently enriched in a large group of elements, including AI, Ti, Mg, Ca, Na, Ga etc., which are partly in the clay mineral lattices and partly in adsorbed forms. Clay illuviation leads to water stagnation on top of the less permeable B horizon; hence, development of temporary reducing conditions and redistribution of Fe and Mn (Feijtel et al., 1988) with coprecipitation of accessory minor elements. "l~e associations of Fe with Mn and Ba are most typical. This process is most pronounced in grasslands where it may be accentuated by the smaller evapotranspiration capacity of grasses compared to natural forest vegetation (Bornand et al., 1968). Mn is concentrated in the form of hard concretions and pans, which severely impede agricultural land u~e. Biological processes resulted in the accumulation of Na, K, P, Ni and Zn in the topsoils. 4.3. Biological differentiation

Fig. 11 portrays the Clarke Concentration Coefficients for all chemical elements in parent mate:dais, topsoils, and plant material in the area averaged over all terrace sites. It

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31.62 10

.~ 3.162 .=

~ g ~

0.316

i

0.031

{,,)

0.1

0.01 0.003 0.001 P •

sand

t

I

I

I

1

I

i

i

I

K

Ca

Mn

Zn

Mg

AI

Na

Fe

Ti

+

grass-soil grass

o x

Cr

forest soil oak bark

Fig. 11. Clarke ConcentrationCoefficients(CCC) for selected elements in different landscape components, irrespective of terrace level, normalized to average lithosphere composition. Values > 1 indicate relative enrichmentand values < 1 relativedepletion.Vertical scale is exponential. shows the much greater activity of biological differentiation processes compared with abiotic ones. The enrichment in P, Ca, K, Mn and Zn and the depletion of Na, Fe, Ti and Cr in plant material with respect to topsoils are clearly visible, as well as the consistent differences in geochemical behaviour between oak bark and grasses with respect to K and Ca. The CBA coefficients of biological accumulation of chemical elements: (P) > (K, Ca) > (Zn,Mn) > (Mg,A1) > (Fe,Na) > (Ti,Cr)) (Fig. 11), mainly reflect their biological significance and geochemical mobility, rather than their bulk concentrations in the soils. Phosphorus in oak bark shows a direct link between soil pool and plant with accumulation in both components from Fv to Fy. The relatively large P content in grasses may be attributed to fertiliser use. Biological accumulation also influenced Zn and Mn, both of which show high CBA values and relative enrichment in the topsoils. In comparison with average CBA x values (Pererman, 1989), both oak bark and grasses of the older terraces show remarkably strong biological accumulation of Mn (CBA Mn = 5.0 and 2.4, respectively), which may be related to the pseudogley development. Mn accumulation is therefore an important biogeochemical process. P, K, Mn, Zn and Mg are actively being accumulated in plants. K, Mg, P, and Zn are important for several enzymes used in photosynthesis (Alloway, 1990; Kabata-Pendias and Pendias, 1984). Therefore, they are concentrated in the green parts (grasses), whereas Ca (utilized for cell membranes) and elements with less biological function (Cr)

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concentrate in dead organs (bark). Oak bark is a long-term accumulator of biologically less important elements. Ti seems to have no particular biological significance, as oak bark and grasses have similar Ti concentrations. A1 accumulation in vegetation could be caused by interaction of A1 with P due to formation of Al-phosphates and/or internal adsorption or precipitation of A1 and P in plants (Kabata-Pendias and Pendias, 1984). P, Zn and most of the Mn accumulation in the topsoils also seems to be of biological origin. Bornand et al. (1968) explained the enrichment of P in the topsoils of the lower terraces by admixture of volcanic material. However, taking into consideration the biological importance of phosphorus, its relatively high content in the parent material of the lower terrace and the absence of significant differences in the P content of topsoils under woodland and grassland, one may conclude that its enrichment in topsoils is biogenic. 5. Conclusions

The geochemistry of the Randan terrace landscapes is closely connected with their geological and ecological history. Among the elements investigated, Si, K, Ca, Mg, P, Fe and Mn, the typomorphic elements of Perel'man (1975), largely determine the characteristics of the present geochemical environment. There are no mineral deposits, no significant sources of industrial or agricultural contamination (with the exception of P in grasses from fertilisers); therefore, the main geochemical differentiation is due to three groups of natural processes which act at different scales of space and time. (1) Long-term, Quaternary, geological processes account for the composition of the soil parent naaterials and for the formation of terraces. (2) Medium-term processes, such as weathering and soil formation, led to chemical differentiation of the terraces, both relative to each other in the toposequence and within the individual soil profiles. Erosion, mass movements and surface and groundwater flow caused differentiation in the transitional sites between the terraces. (3) Short-term biological processes, annually for grass and over decades for bark, leads to accumulations of specific elements in topsoils and plants. Our results refer to a single sampling campaign giving a rapid overview of the long term effecl~s of various processes. To study short term temporal variation in element concentrations in vegetation and soil due to seasonal plant growth, meteorological conditions etc., one should start a monitoring campaign, but this method is not feasible for long-telm processes. Ideally, budgets should be studied at the scales influenced by all these processes. The coefficients used do not allow the reconstruction of such an elemental h,udget, but the differences in coefficients give a semi-quantitative indication of the relative importance (e.g., the relative rates) of the various processes in shaping the landscape. However, the greater the detail in quantification of processes, the less the period and the area to which such quantifications can be extrapolated. The landscape geochemic-~d approach, therefore, has the advantage of presenting a rapid geochemical inventory of large scale landscape components valid for longer time spans. Such an appraisal is useful, for example, to judge the environmental qualities of areas with similar natural characteristics, or to designate representative uncontaminated standard areas against which to judge the environmental quality of other areas.

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A major disadvantage of the Russian landscape geochemical approach is that it is impossible to make real mass balances for landscape element partitioning. Only semiquantitative impressions of the relative enrichment/accumulation or leaching/depletion can be obtained. Despite these disadvantages, the integrated geochemical approach can give a rapid insight into the relative importance of several landscape processes involved in element partitioning, and thus allow semi-quantitative reconstruction of landscape evolution.

Acknowledgements Thanks are due to Bram Kuijper, Ab Jonker, Vic Houba and Wil van Vark for their assistance with laboratory analysis.

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