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Pedogenesis in the Alnö carbonatite complex, Sweden
Geoderma 142 (2007) 127 – 135 www.elsevier.com/locate/geoderma
Pedogenesis in the Alnö carbonatite complex, Sweden E. Haslinger a,⁎, F. Ottner b , U.S. Lundström c a
b
Geological Survey of Austria, Dept. of Geochemistry, A-1030 Vienna, Austria Institute of Applied Geology, University of Natural Resources and Applied Life Sciences, A-1190 Vienna, Austria c Department of Natural Sciences, Mid Sweden University, SE-851 70 Sundsvall, Sweden Received 15 January 2006; received in revised form 23 April 2007; accepted 5 August 2007 Available online 19 September 2007
Abstract Soil and rock material was sampled from three soil profiles developed on the host rocks of Alnö Island, the migmatitic gneisses (profiles G2, G4 and G5) and three profiles with alkaline or carbonatitic parent material (A1, A3 and A6) to examine the differences in pedogenesis of soils on the two very different parent materials and the influence of parent material on pedogenetical processes. All soil profiles could be classified as Cambisols with partially extremely high rock contents up to 69% gravel in profile A3. Two soils on gneissic parent material (profiles G4 and G5) already show signs of podzolization processes, which seem to have been, slowed down’ by the influence of the alkaline rocks. Furthermore, the different parent material affects the vegetation considerably. The nutrients derived from alkaline/carbonatitic rocks are favourable for the growth of the vegetation in an otherwise rather acidic environment of the gneisses. © 2007 Elsevier B.V. All rights reserved. Keywords: Alno; Pedogenesis; Carbonatite; Cambisol; Vegetation
1. Introduction The alkaline and carbonatite complex of Alnö Island belongs to the best known complexes in the world, due to the extensive work and resulting classic memoir of the “The alkaline district of Alnö Island” by von Eckermann (1948). According to Streckeisen (1980), carbonatites are defined as igneous carbonate rocks with more than 50% modal carbonate minerals. Carbonatites are composed largely of carbonates of Ca, Mg, Na and Fe. Typical accessories are apatite, magnetite and rare minerals such as monazite and pyrochlore (McBirney, 1993). Carbonatites and related rocks have been investigated petrologically/magmatically or from an economic point of view with respect to their anomalous trace and rare earth element content. At Alnö, carbonatites always occur as dikes with thicknesses of some centimeters till several hundred meters. The term ‘alkaline rock’ refers to rocks ⁎ Corresponding author. Geological Survey of Austria, Dept. of Geochemistry, Neulinggasse 38, A-1030 Vienna, Austria. Tel.: +43 1 7125674 354; fax: +43 1 7125674 56. E-mail address: [email protected] (E. Haslinger). URL: www.geologie.ac.at (E. Haslinger). 0016-7061/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2007.08.014
containing feldspathoidal minerals or rocks not necessarily containing a feldspathoid but with low SiO2 and high alkali contents (Edgar, 1987). The complex is exposed in the northeast of Alnö (Fig. 1) and comprises pyroxenites, urtites, ijolites, carbonatites and fenites (Aspden, 1980). The alkaline and carbonatitic intrusions can also be found on the islands north of Alnö, with the most important occurrence at the island Långarsholmen. The complex was emplaced in the migmatitic and gneissic host rocks in the early Proterozoic to early Cambrian. The complex formed through repeated intrusive episodes during 80 Ma. The main complex at the northeast of Alnö Island was emplaced around 546 Ma BP. The northern ring complex of Långarsholmen was emplaced at about 600 Ma (Morogan and Lindblom, 1995). The aim of this investigation was to compare the pedogenesis at gneiss and alkaline parent material. This was studied at Alnö Island in central Sweden, where alkaline intrusions are interspersed into the bedrock of gneiss. The intrusions are narrow, well defined dikes which give an opportunity to study the impact of parent material on pedogenesis with other factors like tree stand, temperature and precipitation uniform throughout the sampling area.
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was made according to the World Reference Base for Soil Resources (WRB) (FAO, ISRIC & ISSS, 1998) and with the help of FAO (1998), Schoeneberger et al. (2002), and Zech and Hintermaier-Erhard (2002).
2. Methods 2.1. Field 2.1.1. Site The sampled profiles were chosen along a 500 m long transect in a 85-years old spruce stand (Picea abies) situated at Alnö Island SSW of the village Pottäng on Alnö Island in Sweden (62°24′N, 17°30′E) (Fig. 1). In this transect, it was very obvious from the assemblage of the vegetation, where the parent material is alkaline rocks or carbonatites and where the parent material is migmatitic gneiss. Six soil profiles were chosen – three on the gneissic and three on the alkaline/carbonatitic bedrock. The profiles developed on gneiss are labelled with the letter ‘G’ and the corresponding number; the profiles developed on alkaline/carbonatitic bedrock are labelled with the letter ‘A’ and the corresponding number. The result is the following designation of the profiles — A1, G2, A3, G4, G5 and A6 (Fig. 2). The geological map from this area is not very precise. Most of the soil profiles were supposed to have developed in a fenite area (Fig. 2), but during the field work it became obvious, that the vegetation reflects the parent material better than the geological map. The annual precipitation is around 600 mm and the average temperature is +3 °C. 2.1.2. Soil description Six soil profiles were excavated about 1 × 1 m wide. The depth was very much restricted by the increasing stone content with depth or the boulder size, respectively. The soil description
2.1.3. Soil and rock sampling Soil samples were taken from each horizon of each soil profile. Rocks were sampled both from the soil profile (parent material) and from the surface in the immediate vicinity of the profile. 2.1.4. Vegetation classification The ground vegetation was investigated and the properties and indicator functions of the different mosses, lichens, shrubs and trees concerning different preferred growth conditions in the habitat (e.g. high/low pH) were taken from Ellenberg et al. (1992), Frahm and Frey (1992), Ricek (1994), and Wirth (1995). 2.2. Soil physical and chemical parameters 2.2.1. Grain size analysis The grain size analysis was carried out in combination with the clay mineral analysis. The coarse parts of the samples were fractionated using sieves with mesh-sizes of 2000, 630, 200, 63 and 40 μm. The particle size distribution in the b40 μm-fraction was analyzed by means of a sedimentation analysis with a Micromeritics sedigraph 5000ET.
Fig. 1. Location of studied area.
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Fig. 2. Detail of the geological map of the Map of the Alnö Complex (modified and redrawn from: Kresten, 1986). The length of the transect is about 500 m.
2.2.2. pH-value Fine soil (b 2 mm) pH was determined with a WTW pH 196 Microprocessor pH-meter with a WTW pH-electrode SenTix 41 from air-dried samples in H2O and 0.01 M CaCl2-solution using a soil:solution ratio of 1:2,5 for both H2O and CaCl2.
2.2.3. Organic carbon (OC) The content of total carbon was determined with a LECO SC-444 at the Institute of Forest Ecology, University of Natural Resources and Applied Life Sciences, Vienna. The inorganic carbon was determined gasvolumetrically with the Scheibler-
Fig. 3. Sketch of the transect with the sampled soil profiles.
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apparatus (ON, 1989). The organic carbon content (OC) was calculated by substraction of inorganic carbon (IC) from total carbon (TC). 2.2.4. Cation exchange capacity (CEC) The CEC was measured for profiles A1, G4, G5 and A6 according to a modified method of Meier and Kahr (1999) with a 0.01 M Cu(II)trien sulfate solution. As this method was developed for clay minerals, it had to be modified for the measurement of fine soil samples. Instead of 200 mg of clay fraction (b2 μm) used in the original method, 500 mg of fine soil (b2 mm) were used. Simultaneously, two standards with known CEC were checked with both the original (200 mg) and the modified (500 mg) sample weight. The extinction of the samples was measured at 620 nm in a 10 mm cuvette against water as the blank. The CEC was then calculated with the equation CEC ðmmol=100g Þ ¼ ðEb Em Þ 100=Eb=2:5 Eb Em 2.5
Extinction without clay (blank) Extinction of the supernatant The correction factor for 500 mg sample
2.2.5. Soil colour The soil colour (moist and dry) was determined by Munsell® Soil Color Charts (2000). 2.2.6. Dithionite- and oxalate extractable Fe/Mn/Al The extraction of the pedogenic oxides of Fe, Mn and Al (Fed, Mnd and Ald) was carried out by citrate-bicarbonatedithionite (CBD) at pH 7,4 according to the method of Mehra and Jackson (1960). The determination of the free oxides of Fe, Mn and Al (Feo, Mno and Alo) was carried out by extraction with ammonium oxalate (NH4)2C2O4.H2O at pH 3,25 according to the method of Tamm (1932) and Schwertmann (1964). For soil profiles G2 and A3, these parameters were determined on samples of a geometrical sampling, due to technical reasons. 3. Results and discussion A sketch of the transect and the six sampled soil profiles is presented in Fig. 3. The soil profiles developed on alkaline/ carbonatitic parent material were named “Alkaline” (A1, A3 and A6) and the soil profiles developed on the host rock gneiss were named “Gneiss” (G2, G4 and G5). The results for the physical and chemical properties of the soil profiles are presented in Table 1, the results for the analyses of the dithonite extractable oxides of Fe, Mn and Al are presented in Table 2. 3.1. The soil properties of soil profile A1 Profile A1 is located on the toe of the slope and is the shallowest profile (Fig. 3). The parent rock for this profile is a calcite melanite ijolite. Ijolites are medium- to coarse grained magmatic rocks with a matrix of nepheline and augite (McBirney, 1993). The ijolite found in A1 also contains
considerable amounts of calcite and melanite. Gneisses can be found in the vicinity of profile A1 on the surface, but not in the profile. Therefore, it can be concluded, that the gneisses have been deposited recently and not during postglacial deposition. Therefore they were not included in the soil formation of profile A1. The Ah-horizon of profile A1 was too thin and too irregular to be examined and analysed. The macroscopically visible abundance of meso- and megafauna is also highest in A1 compared to the other soil profiles, which could be one explanation for the near absence of an Ah horizon. On the other hand, the Ah-horizon could also be missing due to erosion. The diagnostic cambic horizon of this profile is not very distinct. However, this soil profile was classified as Cambisol, since no other soil types come into question. The term ‘episkeletic’ was added to the soil type Cambisol, because of the high stone content throughout the soil profile. Only the soil material between bigger boulders or rocks was taken for particle size analysis. Therefore, the overall stone content of the two horizons easily reaches the N 40 wt.% stone content required for the lower level unit ‘skeletic’. The particle size analysis shows, that profile A1 is dominated by the sand fraction as well as the gravel fraction. The gravel and sand derives mainly from physical weathering of the parent rock (calcite melanite ijolite). The clay content in profile A1 is very low (Table 1). The pH-value changes rather abruptly between the two horizons. In the lower horizon, the high pH is due to the parent material. The Fe2O3values in the fine soil of the two horizons is rather high, which is due to the Fe-rich components of the parent rock — phlogopite, melanite and pyroxenes. The pedogenic oxides of Fe, Al and Mn are mainly concentrated in the upper horizon. There is no sign of migration of the pedogenic oxides or the clay, stressing the low age of the soil. The vegetation indicates acidic as well as alkaline soil properties. The indicators for alkaline parent material (Geranium sylvaticum, Hepatica nobilis and Ribes alpinum) obviously reflect the alkaline parent material – the calcite melanite ijolite – whereas the indicator plants for acidic soil properties – like Vaccinium myrtillus, Veronica officinalis, Hylocomium splendens and Frangula alnus – reflect the presence of gneissic rocks, which were found in the vicinity of profile A1. The mosses which grow in the vicinity of this profile – Dicranum scoparium and Hylocomium splendens – are indicators for acidic soils. However, the roots of the mosses in this profile just occur in the organic horizons of soils, therefore their ability to reflect the parent material is restricted. 3.2. The soil properties of soil profile G2 Like in profile A1, the term ‘episkeletic’ was added to the soil type Cambisol due to the high stone content in this profile. One of the main characteristics of profile G2 is the variety of rocks, which can be found in the profile. Mostly, it contains syenitic fenite, but also considerable amounts of gneisses, particularly in the lower part of the profile. This indicates that the profile has developed on gneiss, but was also largely influenced by syenitic fenite, which makes this profile an intermediate profile between alkaline and gneissic. The intermediate characteristics of this profile are stressed by both the mineralogical and the geochemical results for
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Table 1 Structure: abk = angular blocky, gr = granular, sbk = subangular blocky, sg = single grain; Transition, boundary: B = broken, C = clear, D = diffuse, G = gradual, I = irregular, S = smooth, W = wavy; Roots: 0 = no, 1 = few, 2 = common, 3 = many Profile
Soil type
Depth (cm)
Horizon
Soil colour (moist)
Structure
Transition, boundary
Roots
% Gravel
% Sand
% Silt
% Clay
pH CaCl2
OC (%)
CEC (mmol/100 g)
A1
Episkeletic Cambisol
G2
Episkeletic Cambisol
A3
Hyperepiskeletic Cambisol
G4
Dystric Cambisol
G5
Dystric Cambisol
A6
Episkeletic Cambisol
0–0(1) 0(1)–16 16–33 0–5 5–36 5–33 33(36)–70 0–1 1–6 6–18 18–42 0–10 10–20 20–42 42–74 0–4 4–20 20–50 50–70 0–10 10–25 25–40 40–54 70+
Ah BwC1 BwC2 Ah BwC1 Silt lens BwC2 Ah1 Ah2 BwC1 BwC2 Ah(E) BwC1 BwC2 BwC3 Ah(E) Bw1 Bw2 Bw3 Ah BwC1 BwC2 BwC3 BwC4
10YR/2/1 5YR/3/2 7.5YR/2.5/1 10YR/2/1 10YR/3/3 10YR/3/3 2.5Y/3/3 10YR/2/1 7.5YR/2.5/3 10YR/3/3.5 2.5Y/3/3 7.5YR/3/3 10YR/4/4 10YR/3/4 2.5Y/4.5/3 10YR/3/2 10YR/4/4 10YR/3/6 2.5Y/5/3 10YR/2.5/2 7.5YR/3/2.5 10YR/4/2.5 10YR/2/2 10YR/2.5/2
n.d. sbk, gr sbk, gr gr gr, sg sbk, gr sbk, gr gr gr gr sbk, gr gr, sg gr, sg sbk, gr sbk, sg sbk, gr sbk, gr abk, gr abk, gr gr abk, gr gr gr n.d.
I C, W Boulder C, W C, W C, W Boulder C, W D, I G, W Boulder C, S C, W C, S Boulder C, W C, I C, W Boulder D, I D, I D, B C, W n.d.
3 3 3 3 1 1 0 3 3 3 2 3 3 3 1 3 3 2 2 3 3 3 3 3
n.d. 30.4 23.6 n.d. 62.2 0.1 37.1 66.0 69.0 58.9 25.1 34.1 45.0 37.0 13.8 n.d. 12.8 4.9 8.2 31.1 63.8 18.0 19.5 33.1
n.d. 42.8 68.9 n.d. 32.5 34.9 33.5 22.2 26.8 38.4 43.9 47.2 37.5 37.3 44.1 n.d. 45.4 48.2 49.0 25.5 18.2 38.4 63.3 58.3
n.d. 22.2 6.2 n.d. 4.8 29.2 26.4 10.6 3.6 2.6 26.8 10.3 12.7 18.7 35.4 n.d. 35.1 41.2 38.8 31.9 12.0 37.4 14.4 7.1
n.d. 4.6 1.3 n.d. 0.5 5.8 3.0 1.2 0.6 0.1 4.2 8.4 4.8 7.0 6.7 n.d. 6.7 5.7 4.0 11.5 6.0 6.2 2.8 1.5
n.d. 4.9 7.0 n.d. 4.7 6.1 7.0 n.d. 4.3 5.1 4.5 4.0 4.4 4.5 4.9 3.6 4.3 4.3 5.2 4.8 6.0 6.4 6.5 6.6
n.d. 1.61 0.45 n.d. 1.13 1.01 0.80 16.77 3.49 0.91 1.40 1.28 0.96 0.88 0.20 3.50 1.08 0.67 0.22 4.97 3.65 2.77 0.72 0.21
n.d. 10.0 2.5 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.9 0.2 1.2 2.0 2.3 0.4 0.2 1.0 25.1 12.9 23.1 16.4 7.2
n.d. = not determined.
this profile, which will be published in separate publications. The mineralogical composition shows weathering products of the syenitic fenite (i.e. plagioclase, K-feldspar, phlogopite, pyroxene, fluorite, chlorite, hematite and melanite) and the fenitized gneiss
(plagioclase, quartz, phlogopite, monazite, xenotime, zircon, orthoclase, chalcopyrite, rutile, pyrite, apatite and magnetite). The geochemical analyses of fine soil samples of profile G2 show, that the lowermost horizon always groups with the horizons of profile
Table 2 Contents of Fe2O3, CBD-extractable Fe (Fed), Al (Ald) and Mn (Mnd), oxalate-extractable Fe (Feo), Al (Alo), Mn (Mno) in %, ratios of Fed to total Fe (Fet) and Feo/Fed in the fine soil of the six soil profiles Profile
Soil type
Depth (cm)
Fe2O3 [%]
Fed [%]
Feo [%]
Fed/Fet
Feo/Fed
Ald [%]
Alo [%]
Mnd [%]
Mno [%]
A1
Episkeletic Cambisol
G2
Episkeletic Cambisol
A3
Hyperepiskeletic Cambisol
G4
Dystric Cambisol
G5
Dystric Cambisol
A6
Episkeletic Cambisol
0–16 16–33 0–5 10–15 25–30 50+ 0–5 10–15 25–30 50+ 0–10 10–20 20–42 42–74 0–4 4–20 20–50 50–70 0–10 10–25 25–40 40–54 70+
22.88 20.26 15.25 12.99 13.75 6.92 9.70 16.50 14.44 12.35 4.79 5.25 6.54 3.56 3.63 4.17 3.82 3.79 18.60 18.29 17.48 17.42 14.47
3.59 1.01 3.28 2.62 3.24 1.33 2.31 1.46 1.87 2.11 0.75 0.78 1.07 0.52 1.09 0.96 0.67 0.50 3.06 3.57 2.64 1.72 1.08
0.83 0.59 1.31 0.46 0.65 0.53 1.15 0.65 0.84 0.64 0.80 0.64 0.86 0.46 0.70 0.78 0.36 0.22 2.81 2.31 2.03 0.50 0.31
0.16 0.05 0.22 0.20 0.24 0.19 0.24 0.09 0.13 0.17 0.16 0.15 0.16 0.15 0.30 0.23 0.18 0.13 0.16 0.20 0.15 0.10 0.07
0.23 0.58 0.40 0.17 0.20 0.40 0.50 0.44 0.45 0.30 1.07 0.82 0.80 0.88 0.64 0.81 0.54 0.43 0.92 0.65 0.77 0.29 0.29
0.27 0.05 0.28 0.35 0.21 0.13 0.21 0.22 0.21 0.21 0.12 0.16 0.22 0.09 0.17 0.21 0.26 0.09 0.38 0.48 0.24 0.15 0.10
0.13 0.08 0.21 0.14 0.11 0.11 0.19 0.17 0.21 0.14 0.17 0.22 0.26 0.10 0.15 0.21 0.24 0.09 0.50 0.52 0.36 0.21 0.16
0.26 0.03 0.22 0.11 0.15 0.10 0.26 0.17 0.21 0.16 0.02 0.02 0.03 0.02 0.07 0.01 0.01 0.02 0.26 0.25 0.31 0.13 0.06
0.13 0.05 0.13 0.04 0.13 0.09 0.20 0.14 0.20 0.11 0.05 0.02 0.05 0.03 0.05 0.01 0.00 0.03 0.25 0.24 0.33 0.10 0.05
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G4 and G5, which have developed on gneiss. The upper three horizons always group with the horizons of the alkaline profiles A1, A3 and A6. Concerning the particle size distribution, it is obvious, that profile G2 is absolutely dominated by the gravel fraction, which accounts for up to 62% in the upper 30 cm indicating that this profile was influenced by solifluction processes. The silt lens is dominated by the fine sand to middle silt fractions. The material mainly consists of fine flakes of weathered biotite (vermiculite). The lens cannot be described as an own horizon, because it wedges out on both sides. The silt lens of G2 shows considerably higher pH-values than the ‘surrounding’ BwC1 horizon (5 −33 cm). It seems that the weathering of the biotite/phlogopite – the main component of the silt lens – is responsible for this relatively high pH-value. This can be due to solifluction processes, where pre-weathered material of a biotiterich rock was caught in a cavern and then included in the soil formation. Like in profile A1, the pedogenic oxides of Fe, Al and Mn are mainly located in the upper parts of the profile and show no migrational trends. Pedogenic Fe (Fed) shows high values in the upper three horizons of G2 (between 2.6 and 3.3%) and decreases in the lowermost horizon to 1,3%. Also total Fe2O3, decreased downward in the soil profile from 15.25 to 6.92%. The Fe comes from the weathering of the Fe-rich minerals of the parent rocks of profile G2, phlogopites and pyroxenes in the syenitic fenite and phlogopite in the gneiss. The fraction of amorphous Fe (Feo) is highest in the uppermost and lowermost horizons of the profile. The oxides of Al and Mn are likewise concentrated in the uppermost horizon and are rather homogeneously distributed in the deeper soil profile. Therefore, concerning the migration of pedogenic oxides, profile G2 shows no advanced soil formation. The vegetation around profile G2 is dominated by indicators for acidic soils, like the Linnaea borealis, Maianthemum bifolium, Hylocomium splendens and Pleurozium schreberi. However, some plants indicating alkaline soils occur, such as Geranium sylvaticum and Hepatica nobilis, stressing the different types of rocks in this soil profile. Nevertheless, the indicator plants for acidic soil properties dominate this soil profile. 3.3. The soil properties of profile A3 The classification of profile A3 as ‘hyperepiskeletic’ Cambisol was done because of the extraordinarily high stone content regarding the whole soil profile. According to the WRB, the term ‘episkeletic’ requires a content of gravel or coarse fragments between 40 and 90%. The soil material sampled between the big boulders at the bottom of this profile, showed contents of gravel up to nearly 70%. When considering the boulders within the profile, the overall stone content will be up to 90 or even more %. The term hyper thus helps to take the much higher stone content of profile A3 into consideration, compared to the other two episkeletic cambisols G2 and A6. The parent material for profile A3 are big, angular to subangular boulders of the pyroxenite ijolite syenite and smaller rocks of borengite, i. e. a metasomatically altered magmatic rock, which consists nearly entirely of K-feldspar. Furthermore, some rocks of gneissic material can also be found in this profile, still, the
alkaline rocks predominate. The gneisses seem to have been deposited from profiles G4 and G5. Downwards profile A3, the transect becomes rather ‘steep’ towards profiles G2 and A1. Profile A3 is therefore absolutely dominated by the gravel and sand fraction. The clay content is extremely low-in both sampling types and in all horizons always under 5% and with the lowest contents in the horizons Ah2 and BwC1 of A3 with 0.6 and 0.1%, respectively. The Fe2O3-values in this profile are rather high (between 9.7 and 16.5%), due to the Fe-rich parent material. The pedogenic oxides of Fe, Al and Mn show only weak migrational trends, with a slight depletion of Fed in the second horizon (10– 15 cm). From the vegetation it can be said, that profile A3 predominantly shows alkaline soil properties, which can be seen primarily from the shrubs around this profile, i. e. Geranium sylvaticum, Hepatica nobilis, Luzula pilosa, Ribes alpinum and Sambucus racemosa. However, the tree Frangula alnus is an indicator for more acidic soils, which could reflect some influence of the ‘acidic’ soil profiles G4 and G5, which are located upwards the slope from profile A3. The same is true for Vaccinium myrtillus and Veronica officinalis, whereas these two shrubs are present along the whole transect. Nevertheless, in the vicinity of profile A3, most plants indicate alkaline soil properties. 3.4. The soil properties of profile G4 According to the WRB, soil profile G4 needs to be classified as Cambisol, whereas clear signs of podzolization are visible. There is no possibility in the WRB to exactly classify soils which stand between two soil types, like it is the case with profile G4. Concerning the soil classification, it becomes obvious, that the WRB system is somewhat restricted when forest soils need to be classified or when soils, which fall between two soil types, are met, like in the case of profile G4. Therefore the only possibility left is to classify this soil as Dystric Cambisol, which reflects the situation in this profile only inadequately. Pedogenic oxides clearly accumulate in the third horizon (20–42 cm, BwC2), even when this feature is still not strongly expressed. However, this horizon does not fully meet the requirements for a spodic horizon. First of all, the colour is not red enough for a spodic horizon. It contains at least 0.5% of Alo + 1/2Feo, as required by WRB, but does not contain two times or more Alo + 1/2Feo than the overlying horizon. Furthermore, the overlying horizon does not meet the requirements for an albic, umbric, ochric or anthropedogenic horizon, which must overlie a spodic horizon. The ‘bleaching’ of the mineral grains by eluviation of Al- and Fe-oxides and clay has just started at the border between the Ah(E) and BwC1. Nevertheless, the BwC2 horizon (20–42 cm) clearly develops towards a spodic horizon and the whole soil profile towards a podzol. This becomes obvious from the accumulation of the pedogenic oxides in this horizon. Furthermore, clay starts to accumulate in this horizon, which can also be seen in the increase of the CEC. In profiles G5 and G4, the lower parts consist of very homogeneous sands, which have a gneissic mineralogical composition (i.e. quartz, K-feldspar, plagioclase, biotite and vermiculite as main constituents) with gneissic boulders in the
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upper part of the profiles. The upper three horizons of profile G4 are dominated by the gravel and sand fraction. The lowermost horizon has a finer texture with a predominance of the sand and silt fraction. In this profile, the difference between the lower part and the upper part of the profile are very obvious. Whereas the lower part is sandy–silty and contains nearly no big rocks, the rock content is very high in the horizon BwC1 (10–20 cm). As primarily the material between the big rocks was sampled and also used for particle size analysis, the overall rock content of this horizon must be underestimated. Profile G4 shows signs for the mass movement processes which must have taken place in this transect. Whereas the horizons BwC2 and BwC3 consist of mainly weathered gneiss and which are rather homogeneous, the horizon BwC1 contains big, often very rounded gneissic boulders (up to 20 cm Ø). Around these boulders the soil material is very weathered. Furthermore, the boundary between BwC1 and BwC2 is very sharp. One possibility would be that the rocks were deposited together with already weathered soil material during solifluction. However, there are no abrupt textural changes or changes in mineralogy or geochemistry, which would indicate, that soil material from another place was placed on top of the original profile. On the contrary, profile G4 is rather homogenous concerning all parameters. It is likely, that the lower, rather sandy and homogeneous part of the profile could be sediments, which were deposited during uplift of the land and which are very common on Alnö (von Eckermann, 1948). Afterwards, the rocks were deposited rather early during post-glacial small-scale mass movements and were then included in the soil formation. The pH is very low, increasing from 4.0 in the uppermost horizon to 4.9 in the lowermost horizon, which reflects the acidic parent material. The CEC is extremely low with a minimum of 0.2 mmol/100 g in the second horizon (BwC1, 10–20 cm). The CEC thereby follows the trend of the clay content, which also has its minimum in this horizon. Concerning the pedogenic oxides of Fe, Al and Mn show weak migrational trends, as can be seen in the weakly expressed accumulation in the BwC2 horizon (20–42 cm). Profile G4 is dominated by plants, which indicate moderately to strongly acidic soil properties, like Linnaea borealis, Vaccinium vitisidaea, Vaccinium myrtillus and Veronica officinalis. Profile G4 and G5 are the only two profiles, where cowberry (Vaccinium vitis-idaea) occurs, which is an indicator for strongly acidic soil properties. 3.5. The soil properties of profile G5 Concerning the soil classification as Dystric Cambisol, the situation is the same as in profile G4. Profile G5 falls between the soil types Cambisol and Podzol, with very clear signs for podzolization. Profile G5 is the most homogeneous of all profiles concerning grain size distribution. It shows rather low gravel contents compared to the other soil profiles. About 90% of the fine soil is equally divided between the sand and silt fraction. The clay contents are well under 10%. Like profile G4, profile G5 seems to develop from the gneissic sediments, which have been deposited during land uplift, but lacking the big rocks and boulders in the upper part, which can be found in profile
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G4. The pH in the uppermost horizon (Ah(E), 0–4 cm) is strongly acidic (3.6), but increases to 5.2 in the lowermost horizon. The CEC is very low and seems to follow the trend of the clay content and the organic carbon, but increases then slightly in the lowermost horizon. This trend can be followed by neither clay content nor the organic carbon or the pedogenic oxides. But as the increase just accounts for 0.8 mmol/100 g, it is possible, that in these areas of extremely low CEC-values, the detection limit of the method has been reached. The migrational trends of the pedogenic oxides are as weak as in profile G4 and can be seen only with Fed and Alo in the Bw1 (4–20 cm) horizon and partially in the Bw2 (20–50 cm) horizon. Profile G5 is – like profile G4 – dominated by plants, which indicate acidic soil properties, i. e. Dryopteris carthusiana, Linnaea borealis, Lycopodium annotinum, Maianthemum bifolium, Vaccinium myrtillus, Vaccinium vitis-idaea and Veronica officinalis. On the other hand, there are also some plants, which indicate more alkaline soil properties, like Geranium sylvaticum, Hepatica nobilis and Luzula pilosa. It is possible, that profile G5 is somewhat influenced by alkaline parent material from profile A6, which is near profile G5. This would explain why the podzolization of this profile is less advanced than in profile G4, which is located at the same elevation. 3.6. The soil properties of profile A6 Profile A6 was classified as Episkeletic Cambisol. Most of the rocks found in this profile are alnöites, sövites and ijolites. It is hard to judge, whether these alkaline rocks really reflect the alkaline dike, which must be located under profile A6 with intruded alnöite, sövite and ijolite, or, if these rocks have been deposited from a nearby dike by short-distance deposition during post-glacial times. Nevertheless profile A6 is an alkaline profile, due to the predominance of alkaline rocks. This constitutes the main influence for the soil formation processes. This profile is dominated by the sand and silt fraction, whereas the second horizon BwC1 (10–25 cm) shows high rock contents, which can be due to the small-scale mass movements in the post-glacial, as was already discussed for profile G4. However, the rocks or boulders have been well incorporated into the soil formation of this profile, as – apart from the rock content – there are no abrupt changes in e. g. texture, pH, content of pedogenic oxides etc. In the second lowermost sampled horizon BwC3 the sand fraction is dominant with 63%. It also shows layering with alternating lighter and darker layers. This horizon consists of sediments, which were deposited during uplift of the land, comparable to profiles G4 and G5. The bigger rocks and boulders were then deposited on top of these sands. The pH in this profile increases from 4.8 in the uppermost horizon to 6.6 in the lowermost horizon. The CEC is rather high and follows the clay content, whereas the high gravel content in the second horizon (10–25 cm) is responsible for the sharp decrease of the CEC in this horizon. The pedogenic oxides show no migrational trend, but decrease from the uppermost to the lowermost horizon more or less gradually. However, even if this profile develops on very alkaline parent rocks (carbonatite and alnöite), which is indicated by Geranium
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sylvaticum, Hepatica nobilis, Luzula pilosa and Ribes alpinum, the vegetation assemblage contains some indicators for acidic soil properties as well, e.g. Linnaea borealis and Frangula alnus. Profile A6 is located at a small hill, which is rather exposed to wind, leading to increased litter erosion. Therefore, the nutrient input is less with subsequent topsoil acidification and a development of the Cambisol towards ‘acidic Cambisol’ and eventually to a podzol. However, these processes can still not be seen in the chemical and physical properties, like migration of pedogenic oxides or clay. The vegetation reacts very sensitive to changes in the soil profile by changes in its assemblage. In the case of profile A6, the vegetation shows the beginning of the topsoil acidification. 4. Discussion All six soil profiles can be classified – according to the WRB – as Cambisols, whereas the biggest differences concern the rock content and the degree of podzolization. Four of the six sampled soil profiles – A1, G2, A3 and A6 – show big differences in the parent material, whereas the two sites on the more or less unaltered host rock — the migmatitic gneisses, i. e. G 4 and G5, show rather uniform mineralogy. Due to the very different parent material and the small-scale mass movements, which obviously have taken place in this transect, a comparison is very difficult. Furthermore, due to these deposition processes it is hard to judge, what the ‘real’ bedrock of the soil profiles is. The transect is roughly normal to the coastline. Thus, the six soil profiles also reflect six different ‘ages’ due to the isostatic uplift of the Baltic Shield. Profile A1, G2 and A3 have partially extremely high rock contents (up to 69%) and are restricted in depth by big boulders or increasing rock content. In contrast, in the lower parts of profiles G4, G5 and A6, which are located at somewhat higher elevations above sea level, the material is very sandy and rather homogeneous. It seems that this sandy material is constituted by sediments, which were deposited during the land uplift. In the upper three profiles – G4, G5 and A6 – there is a noticeably higher rock content in the upper 40 cm. The rocks are rather angular to subangular and show different composition. It seems that these rocks have been deposited over the gneissic sediments in the lower part of the profiles. However, the depositional processes can not have taken place over long distances, because of the angular shape of the rocks. Following, these depositional trends are discussed for the profiles from the uppermost (A6) to the lowermost profile (A1), following the slope of the transect. In profile A6, most of the rocks found in the profile are alnöites, sövites and ijolites. It is hard to judge, whether these alkaline rocks really reflect the alkaline dike, which must be located under profile A6 with intruded alnöite, sövite and ijolite, or, if these rocks have been deposited from a nearby dike by short-distance deposition. Nevertheless profile A6 is an alkaline profile, due to the predominance of alkaline rocks. This constitutes the main influence for the soil formation processes. In profiles G5 and G4, the lower parts consist of very homogeneous gneissic sands with gneissic boulders in the
upper part of the profiles. Most interesting concerning profile G4 and G5 is the rather low degree of podzolization. The distribution of Al and Fe are important indicators for the podzolization process (Melkerud et al., 2000). The E-horizons of podzols are depleted in Al and Fe, which are then enriched in the uppermost illuvial horizon (spodic, Bs horizon). As mentioned in the soil description for profiles G4 and G5, the contents of Al and Fe show no distinct depletion in the Ah(E)horizons or enrichment in the underlying horizons, but only beginning weak migrational trends. Therefore, the soil formation of profile G5 and G4 seems to be influenced by the surrounding alkaline rocks in the sense of a slowing of the podzolization process. In a study of the initial stages of podzolization in soils developed on Neoglacial moraines of Mendenhall Glacier in Alaska, Alexander and Burt (1996) showed, that the development of an E- and Bs-horizon took place within 240 years. Even when the situation at Mendenhall Glacier is very favourable for soil development due to the mild climate and the perhumid conditions, the investigation showed that the development of an E-horizon should be completed within some hundred years on acidic bedrock. In profiles G5 and G4, the formation of an E-horizon has only just begun in form of an Ah(E)-horizon. In the southern part of Alnö Island, where the parent rock is purely gneissic, soils at comparable elevation above sea level and therefore comparable relative age have developed to ‘real’ podzols with a distinct E-horizon and clear deposition of pedogenic oxides and organic material in the soil profile. One possibility would be a nutrient input in the two profiles G5 and G4 by lateral water flow from alkaline dikes. These nutrients then slow down the natural soil development (podzolization) on the gneissic parent material of G5 and G4, as they serve as buffers against acidification. Profile A3 has the highest rock content and is dominated by big, angular to subangular boulders of the pyroxenite ijolite syenite and smaller rocks of the borengite. Furthermore, some rocks of gneissic material can also be found in this profile, still, the alkaline rocks predominate. The gneisses seem to have been deposited from profiles G4 and G5. Downwards profile A3, the transect becomes rather ‘steep’ towards profiles G2 and A1. Profile G2 is the most mixed profile of all the profiles. Mostly, it contains syenitic fenite, but also big amounts of gneisses. The intermediate characteristics of this profile become obvious, when looking at the mineralogical and geochemical results of the soil profiles, which will be published in a separate paper. The geochemical analyses of fine soil samples of profile G2 show, that the lowermost horizon always groups with the horizons of profile G4 and G5, which have developed on gneiss. The upper three horizons always group with the horizons of the alkaline profiles A1, A3 and A6. Therefore, it can be concluded, that profile A2 has developed on gneiss, but was also largely influenced by syenitic fenite, which makes this profile an intermediate profile between alkaline and gneissic. Syenitic fenite could only be found in this profile. Therefore it is likely, that the original gneiss was both left unaltered and influenced by fenitization. Profile A1, as the relatively youngest profile, is also the shallowest with calcite melanite ijolite as parent material. This makes profile A1 a true alkaline profile. Gneisses
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can be found on the surface in the vicinity of profile A1, but not in the profile. Therefore, it can be concluded, that the gneisses have been deposited recently and not during postglacial deposition. Therefore they were also not included in the soil formation of profile A1. 5. Conclusions The soil formation processes in the soil profiles on alkaline/ carbonatitic parent material and gneiss, respectively, are the same, but differ in progress due to the different relative age of the soil profiles. The soils on gneissic parent material already show signs of podzolization. However, there seems to be an influence of the alkaline/carbonatitic rocks in the vicinity of these soils, as the podzolization process is slower than in comparable soil profiles in the southern part of the island. This fact stresses the importance of both the parent material and the dynamics within a catena on pedogenetical processes. The alkaline rocks and soil profiles differ strongly among one and another, thus making a comparison nearly impossible. The different parent material affects the vegetation considerably, which makes the location of alkaline dikes easier, also without a geological map. The nutrients derived from these rocks are favourable for the growth of the vegetation in an otherwise rather acidic environment of the gneisses. Acknowledgements The authors wish to thank Helene Pfalz-Schwingenschlögl from the Institute of Applied Geology at the University of Natural Resources and Applied Life Sciences Vienna for the drawing of Figs. 1 and 2. References Alexander, E.B., Burt, R., 1996. Soil development on moraines of Mendenhall Glacier, southeast Alaska. 1. The moraines and soil morphology. Geoderma 72, 1–17. Aspden, J.A., 1980. The mineralogy of primary inclusions in apatite crystals extracted from Alnö ijolite. Lithos 13, 263–268. Edgar, A.D., 1987. The genesis of alkaline magmas with emphasis on their source regions: inferences from experimental studies. In: Fitton, J.G., Upton, B.G.J. (Eds.), Alkaline Igneous Rocks. Geological Society Special Publication, vol. 30. Blackwell Scientific Publications, Oxford, pp. 29–52.
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Ellenberg, H., Weber, H.E., Düll, R., Wirth, V., Werner, W., Paulißen, D., 1992. Zeigerwerte von Pflanzen in Mitteleuropa, 2nd ed. Scripta Geobotanica, vol. 18. Verlage Goltze, Göttingen. FAO, 1998. Topsoil characterization for sustainable land management. Land and Water Development Division (Draft). Soil Resources, Management and Conservation Service, Rome. FAO, ISRIC & ISSS, 1998. World reference base for soil resources. World Soil Resources Reports, vol. 84. International Society of Soil Science. Frahm, J.-P., Frey, W., 1992. Moosflora, 2nd ed. Eugen Ulmer GmbH & Co., Stuttgart. Kresten, P., 1986. Alnökomplexets Berggrund (Map of the Alnö Complex) — 1:10000. Sveriges Geologiska Undersökning Ba 31, Specialkarta 1. McBirney, A.R., 1993. Igneous petrology. Jones and Bartlett Publishers, Boston. Mehra, O.P., Jackson, M.L., 1960. Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Clays Clay Miner. 7, 317–327. Meier, L.P., Kahr, G., 1999. Determination of the cation exchange capacity (CEC) of clay minerals using the complexes of copper (II) ion with triethylenetetramine and tetraethylenepentamine. Clays Clay Miner. 47, 386–388. Melkerud, P.-A., Bain, D.C., Jongmans, A.G., Tarvainen, T., 2000. Chemical, mineralogical and morphological characterization of three podzols developed on glacial deposits in Northern Europe. Geoderma 94, 125–148. Morogan, V., Lindblom, S., 1995. Volatiles associated with the alkaline– carbonatite magmatism at Alnö, Sweden: a study of fluid and solid inclusions in minerals from the Långarsholmen ring complex. Contrib. Mineral. Petrol. 122, 262–274. Munsell Color, 2000. Munsell Soil Color Charts. Gretag MacBeth, New Windsor. ON (Austrian Standards Institute), 1989. ÖNORM L 1084 — Bestimmung von Carbonat. Ricek, E.W., 1994. Die Waldbodenmoose Österreichs mit Illustrationen. Abh. Zool.-Bot. Ges. Österr. 28. Schoeneberger, P.J., Wysocki, D.A., Benham, E.C., Broderson, W.D. (Eds.), 2002. Field book for describing and sampling soils, Version 2.0. Natural Resources Conservation Service, National Soil Survey Center, Lincoln, NE. Schwertmann, U., 1964. Differenzierung der Eisenoxide des Bodens durch Extraktion mit Ammoniumoxalat Lösung. Z. Pflanzenernähr. Bodenkd. 105, 194–202. Streckeisen, 1980. Classification and nomenclature of volcanic rocks, lamprophyres, carbonatites and melilitic rocks. IUGS subcommission on the systematics of igneous rocks. Geol. Rundsch. 69, 194–207. Tamm, O., 1932. Über die Oxalatmethode in der chemischen Bodenanalyse. Medd. Statens Skogsförsöksanst. 27, 1–20. von Eckermann, H., 1948. The alkaline district of Alnö Island. Sveriges Geologiska Undersökning. Ser. Ca, vol. 36. Wirth, V., 1995. Flechtenflora. 2nd ed. Eugen Ulmer GmbH & Co., Stuttgart. Zech, W., Hintermaier-Erhard, G., 2002. Böden der Welt–Ein Bildatlas. Spektrum Akademischer Verlag GmbH, Heidelberg.
Pedogenesis in the Alnö carbonatite complex, Sweden E. Haslinger a,⁎, F. Ottner b , U.S. Lundström c a
b
Geological Survey of Austria, Dept. of Geochemistry, A-1030 Vienna, Austria Institute of Applied Geology, University of Natural Resources and Applied Life Sciences, A-1190 Vienna, Austria c Department of Natural Sciences, Mid Sweden University, SE-851 70 Sundsvall, Sweden Received 15 January 2006; received in revised form 23 April 2007; accepted 5 August 2007 Available online 19 September 2007
Abstract Soil and rock material was sampled from three soil profiles developed on the host rocks of Alnö Island, the migmatitic gneisses (profiles G2, G4 and G5) and three profiles with alkaline or carbonatitic parent material (A1, A3 and A6) to examine the differences in pedogenesis of soils on the two very different parent materials and the influence of parent material on pedogenetical processes. All soil profiles could be classified as Cambisols with partially extremely high rock contents up to 69% gravel in profile A3. Two soils on gneissic parent material (profiles G4 and G5) already show signs of podzolization processes, which seem to have been, slowed down’ by the influence of the alkaline rocks. Furthermore, the different parent material affects the vegetation considerably. The nutrients derived from alkaline/carbonatitic rocks are favourable for the growth of the vegetation in an otherwise rather acidic environment of the gneisses. © 2007 Elsevier B.V. All rights reserved. Keywords: Alno; Pedogenesis; Carbonatite; Cambisol; Vegetation
1. Introduction The alkaline and carbonatite complex of Alnö Island belongs to the best known complexes in the world, due to the extensive work and resulting classic memoir of the “The alkaline district of Alnö Island” by von Eckermann (1948). According to Streckeisen (1980), carbonatites are defined as igneous carbonate rocks with more than 50% modal carbonate minerals. Carbonatites are composed largely of carbonates of Ca, Mg, Na and Fe. Typical accessories are apatite, magnetite and rare minerals such as monazite and pyrochlore (McBirney, 1993). Carbonatites and related rocks have been investigated petrologically/magmatically or from an economic point of view with respect to their anomalous trace and rare earth element content. At Alnö, carbonatites always occur as dikes with thicknesses of some centimeters till several hundred meters. The term ‘alkaline rock’ refers to rocks ⁎ Corresponding author. Geological Survey of Austria, Dept. of Geochemistry, Neulinggasse 38, A-1030 Vienna, Austria. Tel.: +43 1 7125674 354; fax: +43 1 7125674 56. E-mail address: [email protected] (E. Haslinger). URL: www.geologie.ac.at (E. Haslinger). 0016-7061/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2007.08.014
containing feldspathoidal minerals or rocks not necessarily containing a feldspathoid but with low SiO2 and high alkali contents (Edgar, 1987). The complex is exposed in the northeast of Alnö (Fig. 1) and comprises pyroxenites, urtites, ijolites, carbonatites and fenites (Aspden, 1980). The alkaline and carbonatitic intrusions can also be found on the islands north of Alnö, with the most important occurrence at the island Långarsholmen. The complex was emplaced in the migmatitic and gneissic host rocks in the early Proterozoic to early Cambrian. The complex formed through repeated intrusive episodes during 80 Ma. The main complex at the northeast of Alnö Island was emplaced around 546 Ma BP. The northern ring complex of Långarsholmen was emplaced at about 600 Ma (Morogan and Lindblom, 1995). The aim of this investigation was to compare the pedogenesis at gneiss and alkaline parent material. This was studied at Alnö Island in central Sweden, where alkaline intrusions are interspersed into the bedrock of gneiss. The intrusions are narrow, well defined dikes which give an opportunity to study the impact of parent material on pedogenesis with other factors like tree stand, temperature and precipitation uniform throughout the sampling area.
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was made according to the World Reference Base for Soil Resources (WRB) (FAO, ISRIC & ISSS, 1998) and with the help of FAO (1998), Schoeneberger et al. (2002), and Zech and Hintermaier-Erhard (2002).
2. Methods 2.1. Field 2.1.1. Site The sampled profiles were chosen along a 500 m long transect in a 85-years old spruce stand (Picea abies) situated at Alnö Island SSW of the village Pottäng on Alnö Island in Sweden (62°24′N, 17°30′E) (Fig. 1). In this transect, it was very obvious from the assemblage of the vegetation, where the parent material is alkaline rocks or carbonatites and where the parent material is migmatitic gneiss. Six soil profiles were chosen – three on the gneissic and three on the alkaline/carbonatitic bedrock. The profiles developed on gneiss are labelled with the letter ‘G’ and the corresponding number; the profiles developed on alkaline/carbonatitic bedrock are labelled with the letter ‘A’ and the corresponding number. The result is the following designation of the profiles — A1, G2, A3, G4, G5 and A6 (Fig. 2). The geological map from this area is not very precise. Most of the soil profiles were supposed to have developed in a fenite area (Fig. 2), but during the field work it became obvious, that the vegetation reflects the parent material better than the geological map. The annual precipitation is around 600 mm and the average temperature is +3 °C. 2.1.2. Soil description Six soil profiles were excavated about 1 × 1 m wide. The depth was very much restricted by the increasing stone content with depth or the boulder size, respectively. The soil description
2.1.3. Soil and rock sampling Soil samples were taken from each horizon of each soil profile. Rocks were sampled both from the soil profile (parent material) and from the surface in the immediate vicinity of the profile. 2.1.4. Vegetation classification The ground vegetation was investigated and the properties and indicator functions of the different mosses, lichens, shrubs and trees concerning different preferred growth conditions in the habitat (e.g. high/low pH) were taken from Ellenberg et al. (1992), Frahm and Frey (1992), Ricek (1994), and Wirth (1995). 2.2. Soil physical and chemical parameters 2.2.1. Grain size analysis The grain size analysis was carried out in combination with the clay mineral analysis. The coarse parts of the samples were fractionated using sieves with mesh-sizes of 2000, 630, 200, 63 and 40 μm. The particle size distribution in the b40 μm-fraction was analyzed by means of a sedimentation analysis with a Micromeritics sedigraph 5000ET.
Fig. 1. Location of studied area.
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Fig. 2. Detail of the geological map of the Map of the Alnö Complex (modified and redrawn from: Kresten, 1986). The length of the transect is about 500 m.
2.2.2. pH-value Fine soil (b 2 mm) pH was determined with a WTW pH 196 Microprocessor pH-meter with a WTW pH-electrode SenTix 41 from air-dried samples in H2O and 0.01 M CaCl2-solution using a soil:solution ratio of 1:2,5 for both H2O and CaCl2.
2.2.3. Organic carbon (OC) The content of total carbon was determined with a LECO SC-444 at the Institute of Forest Ecology, University of Natural Resources and Applied Life Sciences, Vienna. The inorganic carbon was determined gasvolumetrically with the Scheibler-
Fig. 3. Sketch of the transect with the sampled soil profiles.
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apparatus (ON, 1989). The organic carbon content (OC) was calculated by substraction of inorganic carbon (IC) from total carbon (TC). 2.2.4. Cation exchange capacity (CEC) The CEC was measured for profiles A1, G4, G5 and A6 according to a modified method of Meier and Kahr (1999) with a 0.01 M Cu(II)trien sulfate solution. As this method was developed for clay minerals, it had to be modified for the measurement of fine soil samples. Instead of 200 mg of clay fraction (b2 μm) used in the original method, 500 mg of fine soil (b2 mm) were used. Simultaneously, two standards with known CEC were checked with both the original (200 mg) and the modified (500 mg) sample weight. The extinction of the samples was measured at 620 nm in a 10 mm cuvette against water as the blank. The CEC was then calculated with the equation CEC ðmmol=100g Þ ¼ ðEb Em Þ 100=Eb=2:5 Eb Em 2.5
Extinction without clay (blank) Extinction of the supernatant The correction factor for 500 mg sample
2.2.5. Soil colour The soil colour (moist and dry) was determined by Munsell® Soil Color Charts (2000). 2.2.6. Dithionite- and oxalate extractable Fe/Mn/Al The extraction of the pedogenic oxides of Fe, Mn and Al (Fed, Mnd and Ald) was carried out by citrate-bicarbonatedithionite (CBD) at pH 7,4 according to the method of Mehra and Jackson (1960). The determination of the free oxides of Fe, Mn and Al (Feo, Mno and Alo) was carried out by extraction with ammonium oxalate (NH4)2C2O4.H2O at pH 3,25 according to the method of Tamm (1932) and Schwertmann (1964). For soil profiles G2 and A3, these parameters were determined on samples of a geometrical sampling, due to technical reasons. 3. Results and discussion A sketch of the transect and the six sampled soil profiles is presented in Fig. 3. The soil profiles developed on alkaline/ carbonatitic parent material were named “Alkaline” (A1, A3 and A6) and the soil profiles developed on the host rock gneiss were named “Gneiss” (G2, G4 and G5). The results for the physical and chemical properties of the soil profiles are presented in Table 1, the results for the analyses of the dithonite extractable oxides of Fe, Mn and Al are presented in Table 2. 3.1. The soil properties of soil profile A1 Profile A1 is located on the toe of the slope and is the shallowest profile (Fig. 3). The parent rock for this profile is a calcite melanite ijolite. Ijolites are medium- to coarse grained magmatic rocks with a matrix of nepheline and augite (McBirney, 1993). The ijolite found in A1 also contains
considerable amounts of calcite and melanite. Gneisses can be found in the vicinity of profile A1 on the surface, but not in the profile. Therefore, it can be concluded, that the gneisses have been deposited recently and not during postglacial deposition. Therefore they were not included in the soil formation of profile A1. The Ah-horizon of profile A1 was too thin and too irregular to be examined and analysed. The macroscopically visible abundance of meso- and megafauna is also highest in A1 compared to the other soil profiles, which could be one explanation for the near absence of an Ah horizon. On the other hand, the Ah-horizon could also be missing due to erosion. The diagnostic cambic horizon of this profile is not very distinct. However, this soil profile was classified as Cambisol, since no other soil types come into question. The term ‘episkeletic’ was added to the soil type Cambisol, because of the high stone content throughout the soil profile. Only the soil material between bigger boulders or rocks was taken for particle size analysis. Therefore, the overall stone content of the two horizons easily reaches the N 40 wt.% stone content required for the lower level unit ‘skeletic’. The particle size analysis shows, that profile A1 is dominated by the sand fraction as well as the gravel fraction. The gravel and sand derives mainly from physical weathering of the parent rock (calcite melanite ijolite). The clay content in profile A1 is very low (Table 1). The pH-value changes rather abruptly between the two horizons. In the lower horizon, the high pH is due to the parent material. The Fe2O3values in the fine soil of the two horizons is rather high, which is due to the Fe-rich components of the parent rock — phlogopite, melanite and pyroxenes. The pedogenic oxides of Fe, Al and Mn are mainly concentrated in the upper horizon. There is no sign of migration of the pedogenic oxides or the clay, stressing the low age of the soil. The vegetation indicates acidic as well as alkaline soil properties. The indicators for alkaline parent material (Geranium sylvaticum, Hepatica nobilis and Ribes alpinum) obviously reflect the alkaline parent material – the calcite melanite ijolite – whereas the indicator plants for acidic soil properties – like Vaccinium myrtillus, Veronica officinalis, Hylocomium splendens and Frangula alnus – reflect the presence of gneissic rocks, which were found in the vicinity of profile A1. The mosses which grow in the vicinity of this profile – Dicranum scoparium and Hylocomium splendens – are indicators for acidic soils. However, the roots of the mosses in this profile just occur in the organic horizons of soils, therefore their ability to reflect the parent material is restricted. 3.2. The soil properties of soil profile G2 Like in profile A1, the term ‘episkeletic’ was added to the soil type Cambisol due to the high stone content in this profile. One of the main characteristics of profile G2 is the variety of rocks, which can be found in the profile. Mostly, it contains syenitic fenite, but also considerable amounts of gneisses, particularly in the lower part of the profile. This indicates that the profile has developed on gneiss, but was also largely influenced by syenitic fenite, which makes this profile an intermediate profile between alkaline and gneissic. The intermediate characteristics of this profile are stressed by both the mineralogical and the geochemical results for
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Table 1 Structure: abk = angular blocky, gr = granular, sbk = subangular blocky, sg = single grain; Transition, boundary: B = broken, C = clear, D = diffuse, G = gradual, I = irregular, S = smooth, W = wavy; Roots: 0 = no, 1 = few, 2 = common, 3 = many Profile
Soil type
Depth (cm)
Horizon
Soil colour (moist)
Structure
Transition, boundary
Roots
% Gravel
% Sand
% Silt
% Clay
pH CaCl2
OC (%)
CEC (mmol/100 g)
A1
Episkeletic Cambisol
G2
Episkeletic Cambisol
A3
Hyperepiskeletic Cambisol
G4
Dystric Cambisol
G5
Dystric Cambisol
A6
Episkeletic Cambisol
0–0(1) 0(1)–16 16–33 0–5 5–36 5–33 33(36)–70 0–1 1–6 6–18 18–42 0–10 10–20 20–42 42–74 0–4 4–20 20–50 50–70 0–10 10–25 25–40 40–54 70+
Ah BwC1 BwC2 Ah BwC1 Silt lens BwC2 Ah1 Ah2 BwC1 BwC2 Ah(E) BwC1 BwC2 BwC3 Ah(E) Bw1 Bw2 Bw3 Ah BwC1 BwC2 BwC3 BwC4
10YR/2/1 5YR/3/2 7.5YR/2.5/1 10YR/2/1 10YR/3/3 10YR/3/3 2.5Y/3/3 10YR/2/1 7.5YR/2.5/3 10YR/3/3.5 2.5Y/3/3 7.5YR/3/3 10YR/4/4 10YR/3/4 2.5Y/4.5/3 10YR/3/2 10YR/4/4 10YR/3/6 2.5Y/5/3 10YR/2.5/2 7.5YR/3/2.5 10YR/4/2.5 10YR/2/2 10YR/2.5/2
n.d. sbk, gr sbk, gr gr gr, sg sbk, gr sbk, gr gr gr gr sbk, gr gr, sg gr, sg sbk, gr sbk, sg sbk, gr sbk, gr abk, gr abk, gr gr abk, gr gr gr n.d.
I C, W Boulder C, W C, W C, W Boulder C, W D, I G, W Boulder C, S C, W C, S Boulder C, W C, I C, W Boulder D, I D, I D, B C, W n.d.
3 3 3 3 1 1 0 3 3 3 2 3 3 3 1 3 3 2 2 3 3 3 3 3
n.d. 30.4 23.6 n.d. 62.2 0.1 37.1 66.0 69.0 58.9 25.1 34.1 45.0 37.0 13.8 n.d. 12.8 4.9 8.2 31.1 63.8 18.0 19.5 33.1
n.d. 42.8 68.9 n.d. 32.5 34.9 33.5 22.2 26.8 38.4 43.9 47.2 37.5 37.3 44.1 n.d. 45.4 48.2 49.0 25.5 18.2 38.4 63.3 58.3
n.d. 22.2 6.2 n.d. 4.8 29.2 26.4 10.6 3.6 2.6 26.8 10.3 12.7 18.7 35.4 n.d. 35.1 41.2 38.8 31.9 12.0 37.4 14.4 7.1
n.d. 4.6 1.3 n.d. 0.5 5.8 3.0 1.2 0.6 0.1 4.2 8.4 4.8 7.0 6.7 n.d. 6.7 5.7 4.0 11.5 6.0 6.2 2.8 1.5
n.d. 4.9 7.0 n.d. 4.7 6.1 7.0 n.d. 4.3 5.1 4.5 4.0 4.4 4.5 4.9 3.6 4.3 4.3 5.2 4.8 6.0 6.4 6.5 6.6
n.d. 1.61 0.45 n.d. 1.13 1.01 0.80 16.77 3.49 0.91 1.40 1.28 0.96 0.88 0.20 3.50 1.08 0.67 0.22 4.97 3.65 2.77 0.72 0.21
n.d. 10.0 2.5 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.9 0.2 1.2 2.0 2.3 0.4 0.2 1.0 25.1 12.9 23.1 16.4 7.2
n.d. = not determined.
this profile, which will be published in separate publications. The mineralogical composition shows weathering products of the syenitic fenite (i.e. plagioclase, K-feldspar, phlogopite, pyroxene, fluorite, chlorite, hematite and melanite) and the fenitized gneiss
(plagioclase, quartz, phlogopite, monazite, xenotime, zircon, orthoclase, chalcopyrite, rutile, pyrite, apatite and magnetite). The geochemical analyses of fine soil samples of profile G2 show, that the lowermost horizon always groups with the horizons of profile
Table 2 Contents of Fe2O3, CBD-extractable Fe (Fed), Al (Ald) and Mn (Mnd), oxalate-extractable Fe (Feo), Al (Alo), Mn (Mno) in %, ratios of Fed to total Fe (Fet) and Feo/Fed in the fine soil of the six soil profiles Profile
Soil type
Depth (cm)
Fe2O3 [%]
Fed [%]
Feo [%]
Fed/Fet
Feo/Fed
Ald [%]
Alo [%]
Mnd [%]
Mno [%]
A1
Episkeletic Cambisol
G2
Episkeletic Cambisol
A3
Hyperepiskeletic Cambisol
G4
Dystric Cambisol
G5
Dystric Cambisol
A6
Episkeletic Cambisol
0–16 16–33 0–5 10–15 25–30 50+ 0–5 10–15 25–30 50+ 0–10 10–20 20–42 42–74 0–4 4–20 20–50 50–70 0–10 10–25 25–40 40–54 70+
22.88 20.26 15.25 12.99 13.75 6.92 9.70 16.50 14.44 12.35 4.79 5.25 6.54 3.56 3.63 4.17 3.82 3.79 18.60 18.29 17.48 17.42 14.47
3.59 1.01 3.28 2.62 3.24 1.33 2.31 1.46 1.87 2.11 0.75 0.78 1.07 0.52 1.09 0.96 0.67 0.50 3.06 3.57 2.64 1.72 1.08
0.83 0.59 1.31 0.46 0.65 0.53 1.15 0.65 0.84 0.64 0.80 0.64 0.86 0.46 0.70 0.78 0.36 0.22 2.81 2.31 2.03 0.50 0.31
0.16 0.05 0.22 0.20 0.24 0.19 0.24 0.09 0.13 0.17 0.16 0.15 0.16 0.15 0.30 0.23 0.18 0.13 0.16 0.20 0.15 0.10 0.07
0.23 0.58 0.40 0.17 0.20 0.40 0.50 0.44 0.45 0.30 1.07 0.82 0.80 0.88 0.64 0.81 0.54 0.43 0.92 0.65 0.77 0.29 0.29
0.27 0.05 0.28 0.35 0.21 0.13 0.21 0.22 0.21 0.21 0.12 0.16 0.22 0.09 0.17 0.21 0.26 0.09 0.38 0.48 0.24 0.15 0.10
0.13 0.08 0.21 0.14 0.11 0.11 0.19 0.17 0.21 0.14 0.17 0.22 0.26 0.10 0.15 0.21 0.24 0.09 0.50 0.52 0.36 0.21 0.16
0.26 0.03 0.22 0.11 0.15 0.10 0.26 0.17 0.21 0.16 0.02 0.02 0.03 0.02 0.07 0.01 0.01 0.02 0.26 0.25 0.31 0.13 0.06
0.13 0.05 0.13 0.04 0.13 0.09 0.20 0.14 0.20 0.11 0.05 0.02 0.05 0.03 0.05 0.01 0.00 0.03 0.25 0.24 0.33 0.10 0.05
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G4 and G5, which have developed on gneiss. The upper three horizons always group with the horizons of the alkaline profiles A1, A3 and A6. Concerning the particle size distribution, it is obvious, that profile G2 is absolutely dominated by the gravel fraction, which accounts for up to 62% in the upper 30 cm indicating that this profile was influenced by solifluction processes. The silt lens is dominated by the fine sand to middle silt fractions. The material mainly consists of fine flakes of weathered biotite (vermiculite). The lens cannot be described as an own horizon, because it wedges out on both sides. The silt lens of G2 shows considerably higher pH-values than the ‘surrounding’ BwC1 horizon (5 −33 cm). It seems that the weathering of the biotite/phlogopite – the main component of the silt lens – is responsible for this relatively high pH-value. This can be due to solifluction processes, where pre-weathered material of a biotiterich rock was caught in a cavern and then included in the soil formation. Like in profile A1, the pedogenic oxides of Fe, Al and Mn are mainly located in the upper parts of the profile and show no migrational trends. Pedogenic Fe (Fed) shows high values in the upper three horizons of G2 (between 2.6 and 3.3%) and decreases in the lowermost horizon to 1,3%. Also total Fe2O3, decreased downward in the soil profile from 15.25 to 6.92%. The Fe comes from the weathering of the Fe-rich minerals of the parent rocks of profile G2, phlogopites and pyroxenes in the syenitic fenite and phlogopite in the gneiss. The fraction of amorphous Fe (Feo) is highest in the uppermost and lowermost horizons of the profile. The oxides of Al and Mn are likewise concentrated in the uppermost horizon and are rather homogeneously distributed in the deeper soil profile. Therefore, concerning the migration of pedogenic oxides, profile G2 shows no advanced soil formation. The vegetation around profile G2 is dominated by indicators for acidic soils, like the Linnaea borealis, Maianthemum bifolium, Hylocomium splendens and Pleurozium schreberi. However, some plants indicating alkaline soils occur, such as Geranium sylvaticum and Hepatica nobilis, stressing the different types of rocks in this soil profile. Nevertheless, the indicator plants for acidic soil properties dominate this soil profile. 3.3. The soil properties of profile A3 The classification of profile A3 as ‘hyperepiskeletic’ Cambisol was done because of the extraordinarily high stone content regarding the whole soil profile. According to the WRB, the term ‘episkeletic’ requires a content of gravel or coarse fragments between 40 and 90%. The soil material sampled between the big boulders at the bottom of this profile, showed contents of gravel up to nearly 70%. When considering the boulders within the profile, the overall stone content will be up to 90 or even more %. The term hyper thus helps to take the much higher stone content of profile A3 into consideration, compared to the other two episkeletic cambisols G2 and A6. The parent material for profile A3 are big, angular to subangular boulders of the pyroxenite ijolite syenite and smaller rocks of borengite, i. e. a metasomatically altered magmatic rock, which consists nearly entirely of K-feldspar. Furthermore, some rocks of gneissic material can also be found in this profile, still, the
alkaline rocks predominate. The gneisses seem to have been deposited from profiles G4 and G5. Downwards profile A3, the transect becomes rather ‘steep’ towards profiles G2 and A1. Profile A3 is therefore absolutely dominated by the gravel and sand fraction. The clay content is extremely low-in both sampling types and in all horizons always under 5% and with the lowest contents in the horizons Ah2 and BwC1 of A3 with 0.6 and 0.1%, respectively. The Fe2O3-values in this profile are rather high (between 9.7 and 16.5%), due to the Fe-rich parent material. The pedogenic oxides of Fe, Al and Mn show only weak migrational trends, with a slight depletion of Fed in the second horizon (10– 15 cm). From the vegetation it can be said, that profile A3 predominantly shows alkaline soil properties, which can be seen primarily from the shrubs around this profile, i. e. Geranium sylvaticum, Hepatica nobilis, Luzula pilosa, Ribes alpinum and Sambucus racemosa. However, the tree Frangula alnus is an indicator for more acidic soils, which could reflect some influence of the ‘acidic’ soil profiles G4 and G5, which are located upwards the slope from profile A3. The same is true for Vaccinium myrtillus and Veronica officinalis, whereas these two shrubs are present along the whole transect. Nevertheless, in the vicinity of profile A3, most plants indicate alkaline soil properties. 3.4. The soil properties of profile G4 According to the WRB, soil profile G4 needs to be classified as Cambisol, whereas clear signs of podzolization are visible. There is no possibility in the WRB to exactly classify soils which stand between two soil types, like it is the case with profile G4. Concerning the soil classification, it becomes obvious, that the WRB system is somewhat restricted when forest soils need to be classified or when soils, which fall between two soil types, are met, like in the case of profile G4. Therefore the only possibility left is to classify this soil as Dystric Cambisol, which reflects the situation in this profile only inadequately. Pedogenic oxides clearly accumulate in the third horizon (20–42 cm, BwC2), even when this feature is still not strongly expressed. However, this horizon does not fully meet the requirements for a spodic horizon. First of all, the colour is not red enough for a spodic horizon. It contains at least 0.5% of Alo + 1/2Feo, as required by WRB, but does not contain two times or more Alo + 1/2Feo than the overlying horizon. Furthermore, the overlying horizon does not meet the requirements for an albic, umbric, ochric or anthropedogenic horizon, which must overlie a spodic horizon. The ‘bleaching’ of the mineral grains by eluviation of Al- and Fe-oxides and clay has just started at the border between the Ah(E) and BwC1. Nevertheless, the BwC2 horizon (20–42 cm) clearly develops towards a spodic horizon and the whole soil profile towards a podzol. This becomes obvious from the accumulation of the pedogenic oxides in this horizon. Furthermore, clay starts to accumulate in this horizon, which can also be seen in the increase of the CEC. In profiles G5 and G4, the lower parts consist of very homogeneous sands, which have a gneissic mineralogical composition (i.e. quartz, K-feldspar, plagioclase, biotite and vermiculite as main constituents) with gneissic boulders in the
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upper part of the profiles. The upper three horizons of profile G4 are dominated by the gravel and sand fraction. The lowermost horizon has a finer texture with a predominance of the sand and silt fraction. In this profile, the difference between the lower part and the upper part of the profile are very obvious. Whereas the lower part is sandy–silty and contains nearly no big rocks, the rock content is very high in the horizon BwC1 (10–20 cm). As primarily the material between the big rocks was sampled and also used for particle size analysis, the overall rock content of this horizon must be underestimated. Profile G4 shows signs for the mass movement processes which must have taken place in this transect. Whereas the horizons BwC2 and BwC3 consist of mainly weathered gneiss and which are rather homogeneous, the horizon BwC1 contains big, often very rounded gneissic boulders (up to 20 cm Ø). Around these boulders the soil material is very weathered. Furthermore, the boundary between BwC1 and BwC2 is very sharp. One possibility would be that the rocks were deposited together with already weathered soil material during solifluction. However, there are no abrupt textural changes or changes in mineralogy or geochemistry, which would indicate, that soil material from another place was placed on top of the original profile. On the contrary, profile G4 is rather homogenous concerning all parameters. It is likely, that the lower, rather sandy and homogeneous part of the profile could be sediments, which were deposited during uplift of the land and which are very common on Alnö (von Eckermann, 1948). Afterwards, the rocks were deposited rather early during post-glacial small-scale mass movements and were then included in the soil formation. The pH is very low, increasing from 4.0 in the uppermost horizon to 4.9 in the lowermost horizon, which reflects the acidic parent material. The CEC is extremely low with a minimum of 0.2 mmol/100 g in the second horizon (BwC1, 10–20 cm). The CEC thereby follows the trend of the clay content, which also has its minimum in this horizon. Concerning the pedogenic oxides of Fe, Al and Mn show weak migrational trends, as can be seen in the weakly expressed accumulation in the BwC2 horizon (20–42 cm). Profile G4 is dominated by plants, which indicate moderately to strongly acidic soil properties, like Linnaea borealis, Vaccinium vitisidaea, Vaccinium myrtillus and Veronica officinalis. Profile G4 and G5 are the only two profiles, where cowberry (Vaccinium vitis-idaea) occurs, which is an indicator for strongly acidic soil properties. 3.5. The soil properties of profile G5 Concerning the soil classification as Dystric Cambisol, the situation is the same as in profile G4. Profile G5 falls between the soil types Cambisol and Podzol, with very clear signs for podzolization. Profile G5 is the most homogeneous of all profiles concerning grain size distribution. It shows rather low gravel contents compared to the other soil profiles. About 90% of the fine soil is equally divided between the sand and silt fraction. The clay contents are well under 10%. Like profile G4, profile G5 seems to develop from the gneissic sediments, which have been deposited during land uplift, but lacking the big rocks and boulders in the upper part, which can be found in profile
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G4. The pH in the uppermost horizon (Ah(E), 0–4 cm) is strongly acidic (3.6), but increases to 5.2 in the lowermost horizon. The CEC is very low and seems to follow the trend of the clay content and the organic carbon, but increases then slightly in the lowermost horizon. This trend can be followed by neither clay content nor the organic carbon or the pedogenic oxides. But as the increase just accounts for 0.8 mmol/100 g, it is possible, that in these areas of extremely low CEC-values, the detection limit of the method has been reached. The migrational trends of the pedogenic oxides are as weak as in profile G4 and can be seen only with Fed and Alo in the Bw1 (4–20 cm) horizon and partially in the Bw2 (20–50 cm) horizon. Profile G5 is – like profile G4 – dominated by plants, which indicate acidic soil properties, i. e. Dryopteris carthusiana, Linnaea borealis, Lycopodium annotinum, Maianthemum bifolium, Vaccinium myrtillus, Vaccinium vitis-idaea and Veronica officinalis. On the other hand, there are also some plants, which indicate more alkaline soil properties, like Geranium sylvaticum, Hepatica nobilis and Luzula pilosa. It is possible, that profile G5 is somewhat influenced by alkaline parent material from profile A6, which is near profile G5. This would explain why the podzolization of this profile is less advanced than in profile G4, which is located at the same elevation. 3.6. The soil properties of profile A6 Profile A6 was classified as Episkeletic Cambisol. Most of the rocks found in this profile are alnöites, sövites and ijolites. It is hard to judge, whether these alkaline rocks really reflect the alkaline dike, which must be located under profile A6 with intruded alnöite, sövite and ijolite, or, if these rocks have been deposited from a nearby dike by short-distance deposition during post-glacial times. Nevertheless profile A6 is an alkaline profile, due to the predominance of alkaline rocks. This constitutes the main influence for the soil formation processes. This profile is dominated by the sand and silt fraction, whereas the second horizon BwC1 (10–25 cm) shows high rock contents, which can be due to the small-scale mass movements in the post-glacial, as was already discussed for profile G4. However, the rocks or boulders have been well incorporated into the soil formation of this profile, as – apart from the rock content – there are no abrupt changes in e. g. texture, pH, content of pedogenic oxides etc. In the second lowermost sampled horizon BwC3 the sand fraction is dominant with 63%. It also shows layering with alternating lighter and darker layers. This horizon consists of sediments, which were deposited during uplift of the land, comparable to profiles G4 and G5. The bigger rocks and boulders were then deposited on top of these sands. The pH in this profile increases from 4.8 in the uppermost horizon to 6.6 in the lowermost horizon. The CEC is rather high and follows the clay content, whereas the high gravel content in the second horizon (10–25 cm) is responsible for the sharp decrease of the CEC in this horizon. The pedogenic oxides show no migrational trend, but decrease from the uppermost to the lowermost horizon more or less gradually. However, even if this profile develops on very alkaline parent rocks (carbonatite and alnöite), which is indicated by Geranium
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sylvaticum, Hepatica nobilis, Luzula pilosa and Ribes alpinum, the vegetation assemblage contains some indicators for acidic soil properties as well, e.g. Linnaea borealis and Frangula alnus. Profile A6 is located at a small hill, which is rather exposed to wind, leading to increased litter erosion. Therefore, the nutrient input is less with subsequent topsoil acidification and a development of the Cambisol towards ‘acidic Cambisol’ and eventually to a podzol. However, these processes can still not be seen in the chemical and physical properties, like migration of pedogenic oxides or clay. The vegetation reacts very sensitive to changes in the soil profile by changes in its assemblage. In the case of profile A6, the vegetation shows the beginning of the topsoil acidification. 4. Discussion All six soil profiles can be classified – according to the WRB – as Cambisols, whereas the biggest differences concern the rock content and the degree of podzolization. Four of the six sampled soil profiles – A1, G2, A3 and A6 – show big differences in the parent material, whereas the two sites on the more or less unaltered host rock — the migmatitic gneisses, i. e. G 4 and G5, show rather uniform mineralogy. Due to the very different parent material and the small-scale mass movements, which obviously have taken place in this transect, a comparison is very difficult. Furthermore, due to these deposition processes it is hard to judge, what the ‘real’ bedrock of the soil profiles is. The transect is roughly normal to the coastline. Thus, the six soil profiles also reflect six different ‘ages’ due to the isostatic uplift of the Baltic Shield. Profile A1, G2 and A3 have partially extremely high rock contents (up to 69%) and are restricted in depth by big boulders or increasing rock content. In contrast, in the lower parts of profiles G4, G5 and A6, which are located at somewhat higher elevations above sea level, the material is very sandy and rather homogeneous. It seems that this sandy material is constituted by sediments, which were deposited during the land uplift. In the upper three profiles – G4, G5 and A6 – there is a noticeably higher rock content in the upper 40 cm. The rocks are rather angular to subangular and show different composition. It seems that these rocks have been deposited over the gneissic sediments in the lower part of the profiles. However, the depositional processes can not have taken place over long distances, because of the angular shape of the rocks. Following, these depositional trends are discussed for the profiles from the uppermost (A6) to the lowermost profile (A1), following the slope of the transect. In profile A6, most of the rocks found in the profile are alnöites, sövites and ijolites. It is hard to judge, whether these alkaline rocks really reflect the alkaline dike, which must be located under profile A6 with intruded alnöite, sövite and ijolite, or, if these rocks have been deposited from a nearby dike by short-distance deposition. Nevertheless profile A6 is an alkaline profile, due to the predominance of alkaline rocks. This constitutes the main influence for the soil formation processes. In profiles G5 and G4, the lower parts consist of very homogeneous gneissic sands with gneissic boulders in the
upper part of the profiles. Most interesting concerning profile G4 and G5 is the rather low degree of podzolization. The distribution of Al and Fe are important indicators for the podzolization process (Melkerud et al., 2000). The E-horizons of podzols are depleted in Al and Fe, which are then enriched in the uppermost illuvial horizon (spodic, Bs horizon). As mentioned in the soil description for profiles G4 and G5, the contents of Al and Fe show no distinct depletion in the Ah(E)horizons or enrichment in the underlying horizons, but only beginning weak migrational trends. Therefore, the soil formation of profile G5 and G4 seems to be influenced by the surrounding alkaline rocks in the sense of a slowing of the podzolization process. In a study of the initial stages of podzolization in soils developed on Neoglacial moraines of Mendenhall Glacier in Alaska, Alexander and Burt (1996) showed, that the development of an E- and Bs-horizon took place within 240 years. Even when the situation at Mendenhall Glacier is very favourable for soil development due to the mild climate and the perhumid conditions, the investigation showed that the development of an E-horizon should be completed within some hundred years on acidic bedrock. In profiles G5 and G4, the formation of an E-horizon has only just begun in form of an Ah(E)-horizon. In the southern part of Alnö Island, where the parent rock is purely gneissic, soils at comparable elevation above sea level and therefore comparable relative age have developed to ‘real’ podzols with a distinct E-horizon and clear deposition of pedogenic oxides and organic material in the soil profile. One possibility would be a nutrient input in the two profiles G5 and G4 by lateral water flow from alkaline dikes. These nutrients then slow down the natural soil development (podzolization) on the gneissic parent material of G5 and G4, as they serve as buffers against acidification. Profile A3 has the highest rock content and is dominated by big, angular to subangular boulders of the pyroxenite ijolite syenite and smaller rocks of the borengite. Furthermore, some rocks of gneissic material can also be found in this profile, still, the alkaline rocks predominate. The gneisses seem to have been deposited from profiles G4 and G5. Downwards profile A3, the transect becomes rather ‘steep’ towards profiles G2 and A1. Profile G2 is the most mixed profile of all the profiles. Mostly, it contains syenitic fenite, but also big amounts of gneisses. The intermediate characteristics of this profile become obvious, when looking at the mineralogical and geochemical results of the soil profiles, which will be published in a separate paper. The geochemical analyses of fine soil samples of profile G2 show, that the lowermost horizon always groups with the horizons of profile G4 and G5, which have developed on gneiss. The upper three horizons always group with the horizons of the alkaline profiles A1, A3 and A6. Therefore, it can be concluded, that profile A2 has developed on gneiss, but was also largely influenced by syenitic fenite, which makes this profile an intermediate profile between alkaline and gneissic. Syenitic fenite could only be found in this profile. Therefore it is likely, that the original gneiss was both left unaltered and influenced by fenitization. Profile A1, as the relatively youngest profile, is also the shallowest with calcite melanite ijolite as parent material. This makes profile A1 a true alkaline profile. Gneisses
E. Haslinger et al. / Geoderma 142 (2007) 127–135
can be found on the surface in the vicinity of profile A1, but not in the profile. Therefore, it can be concluded, that the gneisses have been deposited recently and not during postglacial deposition. Therefore they were also not included in the soil formation of profile A1. 5. Conclusions The soil formation processes in the soil profiles on alkaline/ carbonatitic parent material and gneiss, respectively, are the same, but differ in progress due to the different relative age of the soil profiles. The soils on gneissic parent material already show signs of podzolization. However, there seems to be an influence of the alkaline/carbonatitic rocks in the vicinity of these soils, as the podzolization process is slower than in comparable soil profiles in the southern part of the island. This fact stresses the importance of both the parent material and the dynamics within a catena on pedogenetical processes. The alkaline rocks and soil profiles differ strongly among one and another, thus making a comparison nearly impossible. The different parent material affects the vegetation considerably, which makes the location of alkaline dikes easier, also without a geological map. The nutrients derived from these rocks are favourable for the growth of the vegetation in an otherwise rather acidic environment of the gneisses. Acknowledgements The authors wish to thank Helene Pfalz-Schwingenschlögl from the Institute of Applied Geology at the University of Natural Resources and Applied Life Sciences Vienna for the drawing of Figs. 1 and 2. References Alexander, E.B., Burt, R., 1996. Soil development on moraines of Mendenhall Glacier, southeast Alaska. 1. The moraines and soil morphology. Geoderma 72, 1–17. Aspden, J.A., 1980. The mineralogy of primary inclusions in apatite crystals extracted from Alnö ijolite. Lithos 13, 263–268. Edgar, A.D., 1987. The genesis of alkaline magmas with emphasis on their source regions: inferences from experimental studies. In: Fitton, J.G., Upton, B.G.J. (Eds.), Alkaline Igneous Rocks. Geological Society Special Publication, vol. 30. Blackwell Scientific Publications, Oxford, pp. 29–52.
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