Evaluation of terra rossa geochemical baselines from Croatian karst regions

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Abstract

In karst regions of Croatia, regolith is the only favourable medium for geochemical mapping. Mediterranean climate and good drainage due to hard, fissured, permeable limestones and dolomites result in a spacious distribution of terra rossa (FAO-luvisols and cambisols) — a polygenetic type of soil. Samples of terra rossa from coastal and inland Croatian Dinaric karst terrains were collected during the initial studies for the Geochemical Map of Croatia at a density of 1 site/25 km2. A total of 87 terra rossa soil samples taken from a depth of 5–25 cm together with 27 samples from deeper profiles (down to 850 cm) were analysed for total Al, Ba, Ca, Co, Cr, Cu, Fe, Ga, La, Mg, Mn, Ni, Pb, Na, Sr, Ti, V and Zn concentrations. A stoichiometric approach was applied by modeling of terra rossa baselines on the basis of linear regressions of metals on Al and the calculation of enrichment factors (EF and CEFs) on the basis of soil standards. A noticeable enrichment of Pb was found in surface samples compared to the terra rossa deeper in the soil profile. Using these baseline relationships, an attempt is made to partition terra rossa metal concentrations into natural and anthropogenic fractions. Also, the models from both polluted and less polluted (uninhabited) karstic terrains improve the comparability of element contents through correction of variable background concentrations. A comparison of elemental concentrations revealed that due to contributions of bauxite debris, a number of studied samples is enriched in Cr and Ni (also with variable amounts of boehmite). The corrections will serve to reduce data variability and to increase the detection of spatial and temporal differences presented on the geochemical maps.

Introduction

For most continental areas, natural trace element concentrations can range over two orders of magnitude causing confusion in the interpretation of anthropogenic data (Windom et al., 1989). However, the natural value or background is more a range than a single value, and it strongly depends on local factors and particular environments. A background range, or soil (sediment) baseline, which is probably more correct, is thus close to the definition of the threshold in the exploration geochemistry, which is defined as a value that separates anomalous samples (or areas) from background (Sinclair, 1991).

Karst terrain environments affect the geochemistry of soils developed on carbonate rocks (Pirc and Maksimović, 1985; Pirc et al., 1991; Prohić, 1989; Prohić and Juračić, 1989; Pirc, 1993; Prohić et al., 1995). A critical examination for establishing the geochemical baselines in karst environments is thus required.

Terra rossa is a reddish, clayey to silty–clayey (red and yellowish-red, 2.5YR and 5YR, dry) material especially widespread in the Mediterranean region, which covers limestone and dolomite as a discontinuous layer ranging in thickness from a few centimeters to several meters. Its colour (5YR to 10R Munsell hues) is a diagnostic feature of terra rossa and is the result of rubification, i.e., the formation of Fe-oxides. The nature and relationship of terra rossa to underlying carbonates is a long-standing problem which has resulted in different opinions with respect to the parent material and the origin of the soil. In this work terra rossa refers to reddish materials (5YR to 10R dry colors after Munsell color charts) overlying hard and permeable limestone and dolomite.

In order to use trace elements to determine the effects of anthropogenic activity, it is necessary to establish their values in the precursor rocks. Although the validity of a regional definition of baseline may tend to decrease as the region becomes larger, we use this approach to describe a uniform baseline for soils in Sinjsko polje, which is the representative of a typical karst polje, using different approaches given in the literature. Our goal was to establish the best approach of data normalisation that would prove applicable for specific karst environments.

Section snippets

Study areas

The Istrian Peninsula, located on the rim of the northeastern Adriatic Sea (Fig. 1), is composed of Upper Jurassic and Cretaceous shallow water carbonate rocks, Paleogene carbonate and clastic rocks, and Neogene and Quaternary sediments (Velić et al., 1995). It belongs to the northwestern part of the Adriatic carbonate platform. Since the Eocene/Oligocene, the surface has been affected by karstic processes and weathering which has led to the development of both surficial and underground

Materials and methods

Red and yellowish-red (2.5YR and 5YR, dry) terra rossa situated on limestone and dolomite were collected as single samples (i.e., 5 homogenised subsamples) on a 5×5 km regular grid as defined by the Geochemical Map protocol for Croatia (Prohić et al., 1998), or along profiles based on structure and colour variations. Top soil samples were taken from the upper 15 cm of the profile, air dried, and sieved to <0.063 mm for analyses. Mineralogical analyses were performed by XRD; trace elements were

Mineral composition of terra rossa soils

Particle size analysis showed that the terra rossa is clay to silty clay with a very low sand content (below 2%). It consists of quartz, plagioclase, K-feldspar, micas, kaolinite, chlorite, vermiculite, mixed-layer clay minerals, hematite, goethite, boehmite and XRD-amorphous inorganic compounds. The main mineral phases in the clay fraction are kaolinite, illite, Fe-oxides and XRD amorphous inorganic compounds.

Geochemistry

The Cr, Pb, Zn, Ni, Fe2O3 and Al2O3 contents in the terra rossa soils of the Istrian

Conclusions

A comparison of normalisation procedures which may give an insight into geochemical baseline values and enrichment of studied elements (Pb, Zn, Cr, Ni, Co, Fe, Al) in terra rossa soils that cover a large part of the mainly carbonate bedrock coastal area of Croatia, has enabled us to draw the following conclusions:

(1) Element concentrations vary considerably within and among the terra rossa soils.

(2) Enrichment factors (EFs) normalized on average soil data (Martin and Whitfield, 1983)

Acknowledgements

This study was funded by the Ministry of Science and Technology, Republic of Croatia (projects #1-09-087 and 1-09-072). Their support is gratefully appreciated.

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