Evaluation of terra rossa geochemical baselines from Croatian karst regions
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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2018, Applied GeochemistryCitation Excerpt :CIA is calculated as moles using the following equation:CIA= Al2O3/(Al2O3+CaO*+Na2O + K2O) × 100where CaO* is the Ca content of silicates. CIA ranges 1 to 100, the latter indicating highly weathered soils (‘terra rossa soils’: 60–90 CIA in Croatia; Miko et al., 1999). In order to better understand the compositional behaviour of Be and the natural grouping of the 23 investigated elements, we used a clr-biplot (Fig. 6) which is a graphical display of the clr-transformed compositional data (Aitchison and Greenacre, 2002).