Geoderma 170 (2012) 378–389

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Evaluating the degree of weathering in landslide-prone soils in the humid tropics:The case of Limbe, SW Cameroon

Vivian Bih Che a,b,⁎, Karen Fontijn b,1, Gerald G.J. Ernst b, Matthieu Kervyn b,c, Marlina Elburg b,2,Eric Van Ranst b, Cheo Emmanuel Suh a

a Department of Geology and Environmental Science, University of Buea, P.O. Box 63, SW Region, Cameroonb Department of Geology and Soil Science, Ghent University, Krijgslaan 281 – S8, B - 9000 Ghent, Belgiumc Department of Geography & Earth System Sciences, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium

⁎ Corresponding author at: Department of GeologyUniversity of Buea, P.O. Box 62, SW Region, Cameroon.

E-mail address: hevivianbih@yahoo.com (V.B. Che).1 Present address: Earth Observatory of Singapore, Nan

50 Nanyang Avenue, N2-01b-30, Singapore 639798, Singa2 Present address; School of Geological Sciences,

Private Bag x54001, Durban 4000, South Africa.

0016-7061/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.geoderma.2011.10.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 March 2010Received in revised form 8 October 2011Accepted 30 October 2011Available online xxxx

Keywords:WeatheringVolcanic soilMineralogyTextural heterogeneitiesBasaltsCameroon

This study analyses the behaviour and mobility of major and some trace elements during the physical andchemical development of landslide-prone soil profiles in Limbe, SW Cameroon. The soils result from in situweathering of Tertiary basaltic and picrobasaltic rocks. Textural and chemical characterisations, togetherwith two mass balance models are applied to understand the mobility and redistribution of elements duringthe weathering of pyroclastic cones and lava flows. Weathering indices are used to estimate the extent ofweathering. The chemical composition of the samples is evaluated by Inductively Coupled Plasma–OpticalEmission Spectroscopy (ICP–OES) and their mineralogical composition by X-Ray Diffraction (XRD) analyses.It is observed that intensive weathering results in thick meta-stable soils in which significant loss of Ca, K, Mg,Na and Sr has taken place. There is a noticeable relative enrichment in all analysed trace elements (Ba, Zr, Y,Sc, V, Ni, and Co). Ti, Fe, Al, Mn, P and Ce tend to be leached in some horizons and concentrated in others. Zr,Ti, and Ce concentrations are greater in the soils than in the bedrock but show slight fluctuations in the soiland saprolites hence cannot be used as immobile elements for mass balance evaluations. Y increases progres-sively with advanced weathering. Major secondary mineral phases developed through weathering are amixture of expanding (smectites) and non-expanding clays (kaolinite, halloysite and mica). The profilesshow the presence of textural heterogeneities that can be exploited as slip surfaces. Data plotted in Si–Al–Fe diagram point out that the most advanced stage of weathering noted in these profiles is the kaolinisationstage.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The occurrence of landslides in tropical and sub-tropical regions isgenerally associated with weathered rock profiles characterised bychemical and mineralogical heterogeneities (Duzgoren-Aydin andAydin, 2006). These weathering profiles develop in response to phys-ical, chemical, and biological processes operating on the earth'ssurface (Anderson et al., 2002). Physical weathering results in themechanical breakdown of rock masses, thereby exposing fresh rocksurfaces and primary minerals to chemical weathering processes.Chemical weathering on the other hand, induces mineralogical, tex-tural and geochemical changes in rocks through dissolution, leaching,

and Environmental Science,

yang Technological University,pore.University of KwaZulu-Natal,

rights reserved.

precipitation, enrichment, and/or formation of secondary minerals atdiverse scales, thereby reducing rock strength.

Changes that accompany weathering processes are never uniformwith depth and thus result in heterogeneities that can act as weak-ness zones often exploited as slip surfaces for landslide occurrence.Hence, the identification and location of slip surfaces (Wen et al.,2004), and the understanding of the conditions and processes thatresult in the transformation of hard rock into soil and the develop-ment of heterogeneities within soil profiles are of paramount impor-tance in predicting and controlling landslides. Duzgoren-Aydin andAydin (2006) defined heterogeneities as sudden and substantialchanges in the mechanical and hydraulic characteristics across aprofile. Heterogeneities may develop as a result of non uniformweathering or may be associated with the mobilisation and redistri-bution of elements and the accumulation of clay horizons duringweathering. Previous studies attempted to link specific clay mineralsto landslide susceptibility (e.g. Azañón et al., 2010; Duzgoren-Aydinet al., 2002a; Shuzui, 2001) and others associated the occurrence oflandslides to the accumulation of clay in relict joints (e.g. Parry etal., 2000; Prior and Ho, 1972). It has also been shown that clay

379V.B. Che et al. / Geoderma 170 (2012) 378–389

mineralogy and soil chemistry provide indications for the existence ofpotential sliding planes (Kitutu et al., 2009; Shuzui, 2001; Wen et al.,2004; Zheng et al., 2002).

Elementmobilisation and redistribution in the course of weatheringresult from mineral breakdown (Eggleton et al., 1987; Hill et al., 2000;Jin-Long et al., 2007). Element redistribution may follow contrasted

Pit 1

a

Fig. 1. a) Location and general morphology of study area, Limbe and its surroundings on tridges; b) location of sampling pits. (For interpretation of the references to colour in this fi

pathways as different elements are affected differently by various ped-ogenic processes some of which include dissolution and transformationof primary minerals, formation of secondary minerals, redox processes,transport of material, and ion exchange (Middelburg et al., 1988). Ele-ments released by weathering may or may not be redistributed down-slope based on their mobility under constant or changing geochemical

Mt Cam

eroon

Pit 2

Pit 3

b

he SE foot slopes of Mount Cameroon characterised by the presence of E–W trendinggure legend, the reader is referred to the web version of this article.)

380 V.B. Che et al. / Geoderma 170 (2012) 378–389

environments along the slope (Brikeland, 1999). The mobility andredistribution of elements within the secondary environment havebeen widely used to estimate the degree of weathering and the behav-iour of elements duringweathering (Beyala et al., 2009). Venturelli et al.(1997) suggested that reliable information on element mobility duringweathering may be obtained by quantitative mass balance approaches.Inmostmass balancemodels, one ormore elements are isolated and as-sumed to be immobile (Anderson et al., 2002; Brimhall and Dietrich,1987; Brimhall et al., 1985; Nesbitt and Young, 1982).Weathering indi-ces (an approximation of the degree of weathering) represent one ofthe most widely used ways of quantifying chemical changes in rocks(Duzgoren-Aydin et al., 2002b; Hill et al., 2000; Patino et al., 2003;Price and Velbel, 2003).

In the last two decades, the city of Limbe (Fig. 1) and its neigh-bourhoods located on the SE foot-slopes of Mt Cameroon, West Africa,were affected by numerous landslides. According to Fell's classifica-tion scheme (Fell, 1994), most of these slides are extremely small tosmall shallow translational earth and debris slides with the slipsurface occurring within saprolites or at the soil/saprolite interface(Che et al., 2011a, 2011b). Saprolite in this study refers to weatheredproducts in which secondary minerals form pseudomorphs of prima-ry mineral phases resulting in the preservation of the texture, fabricsand structure of the parent rock (Velbel, 1985). Soil, on the otherhand, refers to weathered material in which the texture and structureof the initial parentmaterial has been completely lost. The transformationof fresh rock into saprolites and soils in the study area principally involvesthe weathering of olivine, pyroxene, amphibole, calcic plagioclase andvolcanic glass. It is therefore vital to understand the chemical behaviourof the elements that make up these mineral phases during the formationof these soil profiles. Thiswill enhance our understanding of the processesinvolved in the transformation of parent rock to soil in this area charac-terised by high temperatures, intense and/or prolonged rainfall withcorresponding long periods of high soil moisture.

Apart from describing the weathering profiles, chemistry andmin-eralogy of the landslide prone soils, this paper traces the behaviourand distribution of major and some trace elements in the course ofthe weathering of basaltic rocks. It also provides insights into the de-gree of weathering that characterises the soil mantle affected by thelandslides, which might be significant for other landslide studies inthis area and other tropical volcanic areas around the world with ex-treme climate.

2. Geologic and physiographic characteristics of the study area

The study area lies on the SE foot-slope of Mt Cameroon (Fig. 1)characterised by two types of volcanic terrain that generate thickweathered blankets of soils on steep slopes:

• The lower flanks of Mt Cameroon, made up of numerous ~50–300 mhigh, ~20–40° steep degraded scoriaceous volcanic cones (Fig. 1), lavaflows and reworkedmaterial. Radiometric dates froma dyke and fromcores around Mt Cameroon suggest that the oldest rocks in this areaare about 4.7–9 Ma (Hedberg, 1968; Marzoli et al., 2000) or 10 Ma(Fitton et al., 1983).

• E–W trending, deeply dissected and eroded volcanic massif thatpredates the growth of Mt Cameroon generally referred to as theLimbe–Mabeta Massif (Géze, 1943; Hasselo, 1961; Hedberg, 1968).

Rocks within the study area either lie exposed at the surface or aremantled by several metres thick of dark brown, reddish brown and/orpale yellowish sticky, clayey soils derived from protracted intense ordeep weathering. Soil thicknesses vary greatly, ranging from zero toover 10 m in some areas. The major soil types are residual soils com-posed of an admixture of relict primary minerals and secondary min-erals produced from the weathering of primary basaltic mineral andglass phases.

The area is characterised by a humid tropical climate with meanannual temperature and precipitation controlled by altitude andproximity to the sea. This region is characterised by two distinct sea-sons—a four-month dry season that runs from November to mid-March and an eight-month rainy season from mid-March to Novem-ber. Maximum monthly rainfall occurs in June, July and August withmean monthly values ranging from 320 to 757, 606–990, and536–1090 mm, respectively, for different stations within the area.The total number of rainy days per year ranges from 104 to 212 andmean annual temperature is ca. 26 °C. Total annual rainfall is high toextreme with amounts between 1500 and over 6000 mm of rain inthe last 30 years (Che et al., 2011a, 2011b). These characteristics cor-respond to the Am or tropical Monsoon climate according to the Kop-pen climate classification scheme (Peel et al., 2007).

Because of the high rainfall, temperature and thick soil columns, theprimary vegetation cover is diverse and characterised by dense lowlandforest but most of it has been replaced by cropland and industrial palm,rubber and banana plantations.

3. Sampling and experimental procedures

The distribution and mobility of major and some trace elementswere assessed at three locations in the study area (Fig. 1). Threepits, 3 to 4 m deep, were dug into three landslide scars (two onweathered lava flows and one on a degraded pyroclastic cone). Soilhorizons were described in detail with focus on the colour, texture,and structure. Bulk samples of each soil horizon were collected fromthe walls of each pit by channel chipping, and put in plastic bags forsubsequent bulk density, mineralogical and chemical analysis. Dueto the fact that the pits did not go down to the bedrock, fresh rocksamples were collected from rock fragments within the slide debrisor from outcrops located at the lower reaches of the scar.

Soil colour characteristics of each horizon were obtained from theMunsell Soil Colour Chart. Sample bulk density was determined bywater displacement of paraffin coated samples based on Archimedesprinciple. Samples were weighed, coated with paraffin andreweighed. The coated sample was then immersed in water and thevolume of displaced water collected and measured. Particle specificgravity sometimes referred to as particle density (Gs) was obtainedwith the aid of a pycnometer. Natural moisture content was deter-mined as the weight loss measured after oven-drying at 110 °C for24 h. All the above values were characterised in replicates (n=3–4)and average values were used. Sieve and hydrometer analysis forsamples from each horizon were performed to determine the propor-tion of sand, silt and clay present.

Themineralogical composition of the soil and saprolite samples wasstudied by X-ray diffraction (XRD). XRD patterns were recorded with aPhilips diffractometer (PW 3710) using Kα radiation (40 kV, 30 mA) inthe 3° to 60° 2θ interval with 0.02° 2θ steps size and 2.5 s counting timeper step. XRD analysis was performed on non-oriented bulk powdersamples and on clay samples oriented on glass slides. Clay sampleswere analysed after Mg-saturation and Mg-saturation followed by gly-col treatment. These testswere done to identify swelling phases observ-able by a 00l peak shift towards lower 2θ angles.

Whole rock (WR) major and trace element composition was de-termined by Inductively Coupled Plasma–Optical Emission Spectros-copy (ICP–OES). Soil samples were dried overnight at 40 °C whilerock samples were sawed to obtain fresh pieces and crushed intofiner fragments with a jaw crusher. Crushed rock and soil sampleswere pulverised in an agate ball mill. Ca. 4 g of each powder wasdried at 105 °C and loss on ignition (LOI) determined by heating thesamples at 850 °C for 2 h. 0.2 g of the sample was then homogenisedand fused with 1 g of 50/50 lithiummeta−/−tetraborate flux (AccuS-pec Ultrapure) in high purity graphite crucibles. The resulting glasswas dissolved in 2% HNO3 for analysis with a Spectro Arcos ICP–OES(for Al, Ca, Fe, K, Mg, Na, P, Mn, Ti, Si and selected trace elements:

381V.B. Che et al. / Geoderma 170 (2012) 378–389

Ba, Sr, Zr, V, Cr, Ni, Ce, Y, Sc and Co). Analyses of rock standards(BHVO-2, AGV-2, QLO-1 and GSP-2 from the US Geological survey,JSy-1 and JB-2 from the Japanese Geological Survey, and NIM-L fromMintek, South Africa) dissolved following the same procedure wereused to produce calibration lines. The compositional range of thestandards brackets that of the unknowns. Analytical accuracy wasmonitored with secondary rock standards, different from the onesused for calibration. Major elements are accurate within 2%. The accu-racy for trace elements above 10 ppm is better than 10%.

Chemical transformation and element losses and gains that accompa-ny physical breakdown of the parent rock into saprolites and soils werequantified by parent normalisation assuming that:

1. The system is open and all elements are mobile.2. The fresh rock sample can be taken as a reference.3. Relative change of a certain element can be calculated by normalising

the concentration of the element within the soils and saprolites tothat in the parent rock. If the normalised value is greater than 1, theelement is enriched; if it is less than 1 it is depleted.

The chemical mass balance model proposed by Brimhall andDietrich (1987) and Brimhall et al. (1985) was not applicable in thiscase because all the elements analysed in this study were either rela-tively enriched or depleted in the saprolite or soil. Instead, the densityof the samples was used as a proxy to the degree of weathering as-suming that the soils formed by isovolumetric processes. Percentagechanges (absolute change) were calculated according to the followingrelation (Millot and Boniface, 1955).

Absolute % change ¼ Cw:γw

Cp:γp−1

" #� 100

where C is the concentration of any element, γ is the bulk density, andw and p represent the weathered and parent rock, respectively.

The bulk rock magnesium number (Mg#), given by

Mg ¼ 100MgOMgOþ FeO

Macdonald et al. (2001)

with FeO calculated as 0.9×Fe2O3* (Fe2O3* is total iron), was also de-termined and used to estimate the degree of evolution of the magmasthat produced the various parent rock types (Macdonald et al., 2001).It is worth noting that Mg is calculated on a molecular basis.

Weathering indices (an approximation of the degree of weathering)such as the chemical index of alteration (CIA), the Vogt Residual Index(V), the silica/alumina ratio (Ruxton ratio) and the SiO2/(SiO2+Al2O3+Fe2O3*) ratio (S/SAF) given by the following equations were calculatedto estimate the degree of weathering within the profiles.

CIA ¼ 100 � Al2O3

Al2O3þ CaOþ Na2Oþ K2O

� �

Nesbitt and Young (1982)

V ¼ Al2O3þ K2OMgOþ CaOþ Na2O

� �

Vogt (1927)

Ruxton ratio ¼ SiO2

Al2O3

� �

Ruxton (1968)

SSAF

¼ Al2O3

SiO2þ Al2O3þ Fe2O3�

� �

Hill et al. (2000).The calculation of these indices is made using the molecular pro-

portion of the metal oxides based on the assumption that the distri-bution of elements along the profile is mainly regulated by thedegree of weathering (Duzgoren-Aydin and Aydin, 2006).

4. Results

4.1. Profile description

4.1.1. Profile 1The first profile (Fig. 2a) was obtained from a pit dug into a weath-

ered basaltic lava flow at Bonjo (Fig. 1) affected by landslides in 2005and later reactivated in August 2008. Fragments of unweathered rockfound within the slide debris suggest that the parent rock (P1HR1)is a dark-coloured porphyritic basalt with a bulk density (γ) of2.93 g/cm3. It is characterised by the presence of plagioclase, pyrox-ene and olivine phenocrysts in a microlite and glass-rich groundmass.Within this pit, the soil profile shows visible colour and textural gra-dation with 4 distinct horizons (Fig. 2a). However, no mineralogicalheterogeneities were observed from X-ray patterns for the soil andsaprolite samples obtained from this pit. Texturally, the soils andsaprolite are dominantly clay and silt with less than 25% sand. Theupper 20 cm is covered by a dark humus layer which was scrapedoff and discarded. Underneath, a 1 m thick pale yellow mottled plas-tic clayey loam horizon occurs, with a Gs of 2.83 g/cm³ and a γ of1.85 g/cm³, interrupted at 80 cm depth by a thin layer of decayingdebris, possibly representing the top material from the previousslide. At a depth of 1.2 m below the surface, there is an abruptchange in colour from yellow (5Y8/2) to brown (7.5YR5/2) claywith Gs 2.9 g/cm³ and γ 2.06 g/cm³. The material then graduallygrades into a light grey (2.5Y7/1) silty saprolite with alternatinggrey and brown strips.

XRD patterns from soil (Fig. 3a) and saprolite powders indicate thepresence of sanidine (6.50 Å, 3.7, 3.46, 3.30, 3.2 Å, 1.7 Å), anatase(3.51–3.53 Å, 1.89 Å), augite (3.2 Å), smectites and 1:1 clays (kaoliniteand halloysite) while the clay fraction (Fig. 3b) is dominated by non-expanding clays (kaolinite and halloysite) and some smectites. Themineralogy does not change significantly with depth. SiO2, Al2O3,Fe2O3 and CaO are the dominant oxides present in the fresh rock mak-ing up 83 wt.% of the major element content. As the rock transformsinto soil, CaO, MgO, Na2O and K2O are progressively leached, leavingbehind SiO2, TiO2, Al2O3 and Fe2O3, which constitute ca. 98% of themajor elements in the soils (Table 1). Along this profile, Al2O3, Ba, Zr,Ce and Y are most enriched in the soil compared to the saprolite andthe bedrock. TiO2, Fe2O3, V, and Co are more concentrated in the sapro-lite than in the soil and parent rock. All the other elements show higherabsolute depletion patterns in the soil relative to the saprolite and par-ent rock (Table 2). Calculated CIA varies from 98.6% for themostweath-ered horizon to 44.4% for the fresh rock sample. The Ruxton ratio andthe Vogt index evolve from the fresh rock to the weathered horizonfrom 2.7 to 1.3 and from 19.7 to 0.6, respectively (Table 3).

4.1.2. Profile 2This profile describes a 4 m deep section obtained from a pit made

into soils developed on another basaltic flow (Fig. 2b) at Makuka(Fig. 1) also affected by a translational slide. It shows remarkable varia-tions in colour and texture but no significant changes in themineralogicalcompositionwith depth. The profile is topped by a 45 cm thick loose pur-plish horizon probably representing a humus layer underlain by a 50 cmthick light olive brown clayey layer with Gs 2.82 g/cm³, and γ 1.85 g/cm³.

Fig. 2. Sections through sampled pits. a) Section of 3 m deep pit at Bonjo; b) 4 m pit at Makuka; c) 3.8 m pit at Mabeta New layout. Sampled horizons are indicated, as well asassociated minerals. An: anatase; Au: augite; F: feldspars; G: goethite; Ha: halloysite; He: hematite; II: ilmenite; K: kaolinite; Mg: magnetite; Mh: meta-halloysite; Mt: titanomagnetite;O: olivine; Sa: sanidine; Sm: smectite; Gs: particle specific gravity; γ: bulk density. (For interpretation of the references to colour in this figure legend, the reader is referred to the webversion of this article.)

382 V.B. Che et al. / Geoderma 170 (2012) 378–389

Underneath, a 70 cm thick pale olive (5Y6/4) silty horizon occurs with aGs of 2.89 g/cm³ and γ of 1.96 g/cm³ which slowly transgresses towardsa grey (2.5Y6/1) stony and fractured layer with clays sandwiched inbetween the joints. This material has a Gs of 2.93 g/cm³ and γ of 2.20 g/cm³. The bottom of the pit is made up of a grey (10YR6/1) stony saprolitewhich still exhibits the textural characteristics of the parent rock. The par-ent rock (P2HR2) is a dark, dense porphyritic basalt with a bulk density of2.82 g/cm.

The mineralogical composition of the material (soils and saprolites)from this pit indicates that feldspar (sanidine), halloysite and titano-magnetite are the principal mineral phases, with small amounts of 2:1clays, mica and goethite (Fig. 3c and d). Chemically, SiO2, Al2O3, Fe2O3

and CaO make up 85 wt.% of the major elements in the rock. Al2O3,Fe2O3, Zr, V and Co are enriched in the saprolite compared to the soiland bedrock. TiO2, Ni, Y and Sc show higher absolute concentrationsin the soils relative to the saprolite (Table 2). Silica, together with thealkali and alkali earth metals shows significant depletion in both thesaprolite and soils. Calculated CIA varies from94.9% for themostweath-ered horizon to 49.5% for the fresh rock sample. The Ruxton ratio rangesfrom 1.7 to 3.2 and the Vogt index from 9.2 to 0.9 for the most weath-ered horizon and the fresh rock sample respectively (Table 3).

4.1.3. Profile 3The third profile describes the section of a 3.8 m deep pit dug into a

30–40° slope developed on a degraded pyroclastic cone atMabeta at an

elevation of 89 m a.s.l (Figs. 1, 2c). It shows no sharp colour differencesbetween horizons, but exhibits significant difference in terms of textureand humidity with the amount of water greatly increasing down theprofile. The first 70 cm are characterised by a loose reddish brown clay-ey loam soil with Gs 2.93 g/cm³ and γ 2.15 g/cm³, probably represent-ing loose debris from the previous slide. This depth corresponds to themaximum rooting system of plants within the slide. Under this horizondown to a depth of 2.8 m below the ground surface, a moist mottledsandy clay loam horizon with Gs 2.95 g/cm³ and γ 1.90 g/cm³ occurs.Below 2.8 m, the profile is water saturated, and characterised by loosesandy loam soils with visible olivine and pyroxene crystals togetherwith completely weathered rock blocks. This section has a Gs of2.94 g/m³ and γ 1.65 g/cm³. The parent rock (P3HR3) is a vesiculardark brown porphyritic picrobasaltic rock characterised by large pyrox-ene and olivine phenocrysts in a microlite- and glass-rich groundmass.It has a bulk density of 2.29 g/cm3.

The mineralogical composition is characterised by the presence oftitanomagnetite, goethite, hematite, olivine, anatase, halloysite andmeta-halloysite (Fig. 3e) and a subordinate amount of smectite (mont-morillonite) in the sample collected at the sliding surface. In terms ofchemistry, SiO2, Al2O3, Fe2O3, MgO and CaO make up 92 wt.% of themajor element oxides in the bedrock. In this profile TiO2, Fe2O3, MnO,Ba and Zr are more enriched in the saprolite while Cr, Ni, Y and Sc aremore enriched in the soil. SiO2, MgO, CaO, Na2O, K2O, P2O5 and Srshow significant depletion. Calculated CIA varies from 82.1 to 45.7%,

f

P1S01 P1S01 Clay fraction

cP2S03

dP2S04

eP3S05 P3S05 Clay fraction

a b

Fig. 3. X-ray diffractograms for bulk sample powders and some oriented clay fractions. An: anatase; Au: augite; F: feldspars; G: goethite; Ha: halloysite; K: kaolinite; M: mica; Mg:magnetite; Mh: meta-halloysite; Sa: sanidine; Sm: smectite. Sample locations are indicated on profiles in Fig. 2. P in the name above each diffractograph represents the pit numberand S the corresponding sample number.

383V.B. Che et al. / Geoderma 170 (2012) 378–389

the Ruxton ratio from 1.9 to 3.2 and the Vogt index from 1.6 to 0.5 forthe most weathered horizon and the fresh rock sample, respectively(Table 3).

4.2. Whole rock (WR) geochemistry

Results of WR chemistry for the rocks (HR) and soil (S) samples aregiven in Table 1. On a Total Alkali–Silica diagram (Fig. 4) after Le Bas etal. (1986), the fresh rock samples from the lava flows (P1HR1 andP2HR2) plot within the basalt field with Mg numbers that fall withinthe range of values calculated by Suh et al. (2003). The fresh rock samplefrom the pyroclastic cone (P3HR3) plots in the picrobasalt field with an

Mg number of 50.64 (Table 1) and thus represents a slightly moremafic magma composition than the basaltic lava flows (Fig. 4).

4.3. Variation in physical characteristics between parent rock and soilsand correlation with chemical elements

This study documents significant differences in the physical prop-erties of the soil and parent rock. Soil bulk densities are 2/3 those ofthe parent rock (Table 1) while those of the saprolite are about 3/4that of the parent. This decline in density probably results from a pro-gressive increase in porosity with weathering due to element trans-fer. For Profile 1 (Fig. 2a), the bulk density decreases from 2.93 to

Table 1Whole rock major and trace element composition of fresh rocks and soil samples from the Limbe area, at the SE foot slope of Mt Cameroon. HR and S indicate fresh rock and soilsamples analysed during this study, respectively. MC* are fresh rock samples from the 1959, 1982, 1999 and 2000 lava flows of Mt Cameroon analysed by Njome et al. (2008) andSuh et al. (2008). Note similarities in the major elements analysed in this study and the variation in the trace element composition. Fe2O3* is all iron expressed as Fe2O3, Mg#

(magnesium number) is given by 100MgOMgOþFeO, with FeO calculated as 0.9×Fe2O3*.

Bonjo (slide 1) Makuka (slide 2) Mabeta (slide 3) Lava from MC

Sample number P1S01 P1S02 P1HR1 P2S03 P2S04 P2HR2 P3S05 P3S06 P3HR3 MC* 1959 MC* 1982 MC* 1999 MC* 2000Depth (cm) 80 200 50 290 160 290Bulk density (g/cm3) 1.85 2.06 2.93 1.85 2.20 2.82 1.90 1.65 2.29Gs (g/cm3) 2.83 2.90 2.82 2.93 2.95 2.94

Major element concentration (wt.%)SiO2 38.52 43.41 44.71 38.57 40.92 46.82 38.68 39.23 43.91 46.58 44.71 46.47 45.99TiO2 4.02 5.20 3.21 4.51 5.18 3.24 3.99 4.39 2.91 3.24 3.5 3.21 3.15Al2O3 25.89 18.87 13.86 24.32 20.60 16.01 15.30 17.47 11.54 16.31 15.21 15.75 15.17Fe2O3* 18.46 20.40 12.77 18.07 17.35 11.11 21.54 20.66 12.98 10.94 12.84 11.57 11.83MnO 0.04 0.26 0.19 0.16 0.22 0.21 0.33 0.32 0.19 0.2 0.2 0.2 0.2MgO 1.02 2.09 6.91 1.58 2.70 5.16 8.40 5.50 11.98 5.44 6.24 6.29 7.08CaO 0.04 0.35 11.31 0.66 2.70 10.26 4.22 4.16 12.09 9.97 12.03 10.58 11.03K2O 0.31 0.61 1.40 0.53 1.58 1.71 0.00 0.00 0.92 4.45 3.51 4.02 3.79Na2O 0.00 0.00 3.37 0.07 0.81 3.14 0.00 0.01 1.04 1.84 1.26 1.65 1.49P2O5 0.49 0.54 0.62 0.61 0.97 0.74 0.05 0.06 0.40 0.84 0.54 0.73 0.67Total 88.79 91.72 98.35 89.09 93.02 98.42 92.52 91.80 97.95 99.81 100.04 100.49 100.4LOI 12.28 8.91 1.66 11.88 7.43 1.59 7.94 8.67 2.09 −0.43 −0.39 −0.58 −0.44Mg# 37.55 34.04 59.63 34.7 36.57 40.49 38.94

Trace element concentration (ppm)Ba 730 375 384 548 630 470 537 836 361 516 370Sr 300 262 792 186 284 979 26 35 474 1140 917 1055 1063Zr 477 387 235 459 462 324 328 380 244 386 348 404 410V 377 467 275 405 412 212 349 359 314 246 331 266 272Cr 91 255 184 142 116 101 2581 1664 800 44 37 106 151Ni 56 114 82 99 51 44 806 460 279 48 65 74 88Ce 229 146 111 224 158 162 117 192 102 163 143 168Y 61 48 28 64 44 33 53 41 27 37 31 34 33Sc 29 41 27 37 28 16 100 84 43 18 31 22 25Co 43 82 48 69 68 41 135 117 60 44 46 38 43

384 V.B. Che et al. / Geoderma 170 (2012) 378–389

2.06 g/cm3 in P1S02 and then to 1.85 g/cm3 in P1S01, i.e. a 30% and36% change, respectively. Pit 2 (Fig. 2b) shows a 24–34% variationwhile Pit 3 (Fig. 2c) shows a 17–28% change in bulk density betweenthe parent rock and the weathered products. The porosity tends to di-minish with depth from the surface. Gs range from 2.8 to 3.0 g/cm³ anddoes not vary significantly for the different pits and weathering degree(Table 1). These values can be attributed to the mafic composition ofthe parent rock rich in iron and magnesium silicates (olivine, pyroxene)

Table 2Absolute mass changes calculated for the soils and saprolite using individual elementconcentrations and corresponding density values for the soil, saprolite and parent rock.

Element P1S01 P1S02 P2S03 P2S04 P3S05 P3S06

Mass change (%)

SiO2 −45.60 −31.73 −42.09 −44.82 −38.07 −43.58TiO2 −20.94 13.91 −2.19 0.96 −3.40 −4.72Al2O3 17.90 −4.31 6.81 −18.76 −6.78 −4.42FeOtot −8.74 12.31 14.39 −1.41 16.70 0.50MnO −85.48 −5.25 −45.54 −35.54 22.94 9.54MgO −90.64 −78.74 −78.51 −66.99 −50.68 −71.03CaO −99.80 −97.84 −95.47 −83.39 −75.46 −78.29K2O −85.80 −69.16 −78.17 −41.47 −99.75 −100.00Na2O −100.00 −100.00 −98.37 −83.78 −100.00 −99.17P2O5 −50.25 −39.62 −41.95 −18.15 −90.34 −89.86LOI −60.13 −50.57 −45.15 −56.12 −41.67 −40.44Ba 20.05 −31.36 −18.01 −15.32 4.73 46.36Sr −76.06 −76.77 −86.68 −81.66 −96.22 −95.31Zr 28.12 15.77 −0.57 −10.11 −5.45 −1.57V −13.57 19.25 34.06 22.46 −21.76 −27.77Cr −68.89 −2.59 −1.64 −27.61 126.79 31.28Ni −57.06 −2.84 58.40 −26.22 103.00 4.13Ce 30.03 −7.55 −2.87 −38.41 −19.93 18.28Y 37.86 20.84 35.39 −16.35 37.32 −4.77Sc −33.78 5.19 57.53 8.23 63.67 23.38Co −44.84 19.28 19.56 7.18 58.23 23.42

and containing Fe–Ti oxides (e.g. hematite, titanomagnetite andgoethite) which constitute the major opaque mineral phases in rocksof the Mount Cameroon region (Njome et al., 2008; Suh et al., 2003,2008). Variations in the textural properties observed within these pro-files, illustrated by the evolution in bulk density, grain size,water contentand permeability documented in details elsewhere (Che et al. in prep),might result in heterogeneities that can act as slip zone as observed byNgole et al. (2007) for the Mabeta area.

LOI values obtained for the fresh rock samples range from 1.5 to2.1 wt.% whereas those for the soil and saprolite range from 7.4 to15.2% (Table 1). This decreasing pattern with depth can be attributedto decreasing weathering intensity and can also be attributed to the

Table 3Weathering indices for fresh rock and soil samples from landslide scars in Limbe, SWCameroon. Volcanic rocks on Mt Cameroon and world average values. * values calculatedfrom the chemistry of fresh rock samples from the Mt Cameroon region by Njome et al.(2008) and Suh et al. (2008) and, ** values provided by Price and Velbel (2003). # valueprovided by Ruxton (1968). CIA: Chemical Index of Alteration, S/SAF: Silica/(silica+aluminium+iron oxide) ratio.

Sample number CIA Ruxton ratio Vogt ratio S/SAF

P1S01 98.6 1.26 19.74 0.37P1S02 95.1 1.92 6.60 0.28P1HR1 44.4 2.74 0.63 0.23P2S03 94.9 3.23 9.16 0.35P2S04 78.9 1.69 3.10 0.31P2HR2 49.5 2.48 0.85 0.25P3S05 79.9 2.14 1.06 0.25P3S06 82.1 1.91 1.62 0.27P3HR3 45.7 3.23 0.45 rMean fresh rock(Suh et al., 2003)

48.8±1.1

2.94±0.07

0.82±0.07

0.21±0.01

Optimum fresh value** b 50 >10 (4.0–4.5#) b1Optimum weathered value ** 100 0 (2#) Infinite

Fig. 4. Plots of the chemical composition of fresh rock samples on the Total Alkali–Silicadiagram after Le Bas et al. (1986). Compositions from other Mt Cameroon lavas aretaken from Njome et al. (2008) and Suh et al. (2003, 2008).

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incorporation of water (hydrous phases) into the secondary mineralphases formed during weathering. LOI also shows a negative correla-tion with density, silica, the alkali and alkali earth metals with Pear-son's correlation coefficient (r) between −0.65 and −0.80. It alsoshows a positive correlation with Al2O3, Fe2O3, TiO2, Ce and Y(r=0.74–0.93; Fig. 5a and b).

SiO2, Na2O, K2O, CaO and Sr show a positive correlation with bulkdensity (Fig. 5c and d). Elements of the Iron group (Fe, Sc, Co, Ti, Ni,and Cr), together with Y and Zr, show a negative correlation withdensity whereas Ce, Cr, Sc, SiO2 and MgO show no correlation withdensity. These correlations suggest that silica, the alkali and alkaliearth metals are depleted with increasing weathering while the irongroup elements are enriched. The other elements show an irregularpattern. Silica correlates negatively with Ba and Zr. Ba correlatespositively with Zr and negatively with Sr. Cr, Ni, Sc and Co correlatepositively with MnO and shows a strong negative correlation withP2O5 (Fig. 5e and f). All the alkali and alkali earth metals correlatepositively with silica and negatively with Fe2O3 and Al2O3. MgO

Fig. 5. Plots of a) SiO2 vs. LOI; b) Y vs. LOI; c) Sr vs. γ; d) Fe2O3* vs. γ; e) Co vs. MnO; and f) Crvs. MnO and the negative correlation between SiO2 and LOI, Fe2O3 and γ, and Cr and P2O5.

correlates negatively with Zr and Y. Negative correlations are alsoobserved between Ca and Zr, Ca and V, Ca, Ce and Y.

4.4. Relative element mobility and absolute mass changeduring weathering

Relative enrichment calculations (i.e. ratio of element concentrationin weathered material divided by the element concentration in theparent rock (Cjw/Cjp)) illustrate that the weathering pattern in eachprofile is unique (Fig. 6). However, some patterns can be deduced forthe weathering of basaltic lava flows within the study area. Parentnormalised plots (Fig. 6) suggest that Al2O3, Fe2O3 and TiO2 are relative-ly enriched within these profiles. All the alkali and alkali earth metaloxides (K2O, Na2O, MgO, and CaO) are leached and silica is relativelyunchanged. Sr is themostmobile trace element and tends to be leached.Most of the trace elements analysed for (Ba, Y, Zr, V, Ni, Co and Ce) tendto be relatively enriched asweathering progresseswith highest concen-trations measured in the most weathered portions of the profile.

When the density of the parent rock and soil is considered in eval-uating enrichment and depletion patterns (i.e. absolute change),some elements that exhibit enrichment patterns in the parent nor-malised evaluation, now show depletion patterns as observed inFig. 7. This implies that the enrichment or depletion of an elementis largely dependent on whether the density measurements areconsidered or not. It is suggested in this study that more realistic pat-terns are obtained if density measurements are considered in massbalance calculations for weathering profiles since weathering is al-ways accompanied by mass loss that must be taken into considerationrather than relying only on relationships that exist between elementconcentration in the soil and parent rock. Mass changes calculatedusing rock and soil density according to Millot and Boniface (1955)are shown in Table 2. From this table, we note significant depletionin all the major alkali and alkali earth metal oxides, P2O5, silica and Sr.

Sr shows far lower concentrations in the soils relative to the par-ent than other trace elements. This is probably due to its chemicalsimilarity to Ca (e.g. it is a common substitute for Ca in plagioclase).It shows between 75 and 96% absolute loss with the maximum lossoccurring within the pyroclastic material. There is >70% mass lossin all the alkali and alkali earth metal oxides. Depletion patterns gen-erally follow the order Na=Ca>K>Mg>Sr>P>Si. All the other

vs. P2O5. Symbols are shown in 5a. Note the positive correlation for Y vs. LOI, Sr vs. γ, Co

Fig. 6. Parent normalised element distribution patterns (relative element mobility diagrams) for soils and saprolites from landslide scars in Limbe. a) Pit 1 at Bonjo; b) Pit 2 atMakuka; c) Pit 3 at Mabeta New Layout. Note significant depletion in Ca, Na and Sr and a corresponding enrichment in all the other trace metals.

386 V.B. Che et al. / Geoderma 170 (2012) 378–389

elements show more erratic patterns with enrichment in some hori-zons and depletion in others with the noticeable exception of Ywhich is enriched with increasing intensity of weathering. Fe2O3,

Al2O3, Zr, and V show b20% depletion in some horizons and up to48% enrichment in others.

Fig. 7. Absolute mass change a) Pit 1 at Bonjo; b) Pit 2 at Makuka; c) Pit 3 at Mabeta New Lapattern with the trace elements. Relatively low silica variation in the parent normalised diagevaluations.

4.4.1. Weathering indicesThe CIA, Ruxton ratio, Vogt residual index and the S/SAF ratio are

shown in Table 3. They are compared with those calculated for freshrock samples of the Mt Cameroon region and optimum fresh andweathered values given by Price and Velbel (2003). The CIA and

yout. Note significant depletion in all alkali and alkali earth metals, and more irregularram is more significant when density measurements are considered in the mass balance

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Vogt indices for the fresh rock samples are lower than the maximumthreshold value for fresh samples reported by Price and Velbel (2003)so they appear as a good representation of the unaltered parentmaterial to which the soils and saprolites are normalised. The Ruxtonratio on the other hand are lower than the minimum value reportedby Price and Velbel (2003) and by Ruxton (1968), probably becausethe parent rocks are enriched in ferromagnesian minerals with rela-tively low amounts of silica and aluminium when compared withmore evolved igneous rocks. The Ruxton ratio is therefore not appro-priate for use in profiles generated from mafic (basic) igneous rocks.S/SAF for the fresh rock samples ranges between 0.20 and 0.25 forall the Mt Cameroon rocks and increases progressively with increas-ing degree of weathering. P1S01, P1S02 and P2S03 show weatheringindices close to the optimum weathered value suggesting near-complete weathering. P2S04, P3S05, and P3S06 show intermediatevalues (Table 3) which can be interpreted as having undergonelower weathering intensities. These results may indicate that thelava flows are older than the pyroclastic materials and thus haveundergone more weathering-induced leaching than the pyroclasticmaterials. This hypothesis is made on the basis that the climatic con-ditions under which weathering is taking place is the same. Howeverthe texture and structure (porosity, permeability) of both materialsare significantly different thus it is expected that pyroclastic materialswould be more intensely weathered than lava flows of the same age.The analysed samples plotted on a Si–Al–Fe ternary diagrammodifiedby Hill et al. (2000) suggest that the most advanced stage of weather-ing in this study area is the kaolinisation stage (Fig. 8).

5. Discussion

In this study, the mobility and redistribution of major and sometrace elements in the profiles of landslide prone soils are evaluated.Results show that major differences exist in the physical propertiesof soils and parent rock from which they were derived. These changesare not uniform along the profile, as observed in Fig. 2, but result intextural heterogeneities. Soil bulk densities are more than half thoseof the parent. This decline in density results from a progressiveincrease in porosity with weathering due to element losses duringweathering. These results are similar to observations made byAnderson et al. (2002) and Jersak et al. (1995) in the Oregon CoastalRange, USA. Other textural variations were documented based on the

Kaolinisation

Fig. 8. Si–Al–Fe ternary diagram modified by Hill et al. (2000) for the parent rocks andsoil samples from landslides scars in Limbe. 1: average composition of fresh basalts; 2:lithomarge (saprolite); 3: laterite; 4a: bauxite, and 4b: iron ore crust.

grain size analysis, textural classification, porosity and permeability.These results are presented in detail elsewhere (Che et al. in prep).

During the study, bedrock was not observed within any of theslide scars. It is therefore concluded that the slip plane lies withinthe saprolite or at the soil/saprolite boundary rather than along thesaprolite/bedrock boundary as has been reported in previous studies(e.g.Wen et al., 2004). This can be attributed to textural heterogeneitiesat the soil/saprolite boundary or within the saprolite imposed by non-uniform weathering.

It is also observed that each profile shows a unique element distri-bution pattern. However some generalisation can be made for theweathering of basaltic material under humid climatic conditions. Var-iation trends in the weathering patterns of major elements (particu-larly, the alkali and alkali earth metals) along these profiles forexample strongly indicate the unstable nature of their primary min-erals (olivine, pyroxene, amphibole and plagioclase), the formationof secondary minerals, and mobility of the alkali and alkali earth ele-ment during weathering. Strong depletion in the alkali and alkaliearth metals (K2O, Na2O, and CaO) reflect intense and even completedecomposition of plagioclase which represent the principal primarymineral host of these elements in basic igneous rocks. Depletion inthe alkali earth metals and the occurrence of kaolinite and halloysite to-gether with lower smectite concentrations within the profiles indicateprolonged and/or intense weathering under well drained conditions(Scarciglia et al., 2007). Good drainage conditions enhance rapid flush-ing of water through the weathering profile (Noack et al., 1993) thusresulting in the depletion of the more mobile elements. Absolute deple-tion of silica along the profile is indicative of well drained conditions asprevious studies indicate that the dissolution of silica is favoured underwell drained conditions during weathering (Scarciglia et al., 2007).Derry et al. (2005) noted that in strongly weathered soils, biogenic silicacontrols silica leaching while direct mineral–water reactions accountfor a small fraction of the exported silica.

Unlike other reports of iron and aluminium enrichment alongprofiles within the humid tropics (Middelburg et al., 1988), absolutemass change calculations in this study reveal that Al and Fe are notalways concentrated but are leached from some horizons and concen-trated in others (Table 2). The migration rates of Al and Fe are rela-tively small. This low migration can be linked to the precipitation ofpoorly crystalline solid phases from supersaturated solutions(Chadwick et al., 2003) or can be associated with the low solubilityof Al and Fe3+ hydroxides. Slight enrichment in Al2O3, Fe2O3* andTiO2 might be associated with the formation of secondary minerals(Wen et al., 2004) that can host these elements during the earlyphase of weathering, particularly non swelling clays, hydroxides andgoethite which were observed in the diffraction patterns. Variationsin Ti, Fe and Al may be explained by their low mobility and theirprominent occurrence in secondary mineral phases. The presence ofanatase in these profiles may be attributable to the precipitation ofTi released from primary mineral phases.

The lava flow samples from this study are chemically similar to lavasamples analysed by Njome et al. (2008) and Suh et al. (2003, 2008) forthe 1954, 1959, 1999 and 2000 lava flows of the Mt Cameroon area(Fig. 4). As expected, both fresh rock and soil samples from the picroba-salt cone, show significantly higher Ni and Cr concentrations than freshbasalt samples and soils developed on basaltic lava flows (Table 1;Chauvel et al., 2005; Dia et al., 2006; Njome et al., 2008; Suh et al.,2003), due to the more primitive nature of the former magmas. Uponmagmatic differentiation, Ni and Cr are preferentially fractionated intoMg-rich olivine, pyroxene and amphibole (Deer et al., 1993; Sato etal., 1990). The weathering of these minerals would liberate theseelements, which then become enriched in the soil because of theirlow mobility.

Ni concentration measured in the picrobasalt profile ranges from279 ppm in the parent rock to 806 ppm in the soil. Cr concentrationsrange from 800 ppm in the parent rock to 2581 ppm in the soil. Ni

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and Cr concentration for other fresh rock samples from Mt Cameroonare typically lower, ranging from 5 to 99 and 40–213 ppm, respective-ly (Chauvel et al., 2005; Njome et al., 2008; Suh et al., 2003, 2008).Deruelle et al. (1987) and Sato et al. (1990) however measuredhigher Ni and Cr concentrations similar to those obtained in thisstudy in some pricritic rocks samples from the Mt Cameroon region.Sato et al. (1990) noted that Ni and Cr concentrations in rocks fromthe Mt Cameroon increased with decreasing Fe2O3*/MgO as estimat-ed in fractional crystallisation models, an observation that is alsotrue for the present study. The picrobasalt analysed here also hashigher MgO, and lower Al2O3 and K2O contents than other rocks ofthe Mt Cameroon region.

Zhang et al. (2007) observed an increase in the enrichment ratio ofTi, V and Cr with soil age, which might also be the case in this areathough absolute rock ages are not known. Liu et al. (1996) foundthat soils developed on basalts normally have higher concentrationsof elements that belong to the Fe family, such as Ti, V, Co, Cr, and Nirelative to soils developed on more evolved rocks such as graniteand rhyolite. These trace elements become relatively enriched in thesoils formed from basaltic rock due to preferential loss of the othermajor elements during weathering. These observations were alsonoted in this study as metals of the iron family all show a positiverelative enrichment with intense weathering. Strong positive correla-tions were also observed between these elements.

By comparing the concentrations of trace elements in the soils andthe parent rocks, it is observed that the analysed trace elements showsignificant relative enrichment to the noticeable exception of Sr whichis significantly depleted. Rocks on Mt Cameroon are generally charac-terised by high Ba (400 to 609 ppm) and Sr (927 to 1216 ppm) concen-trations and depleted in K (Chauvel et al., 2005; Njome et al., 2008; Suhet al., 2003, 2008). Consequently, high Ba concentrations in the soilsresult from high initial concentration in the parent rock that becomesenriched in the soils asweathering proceeds. Even though Sr is eventual-ly leached out with increasing weathering intensity, its concentration inthe soil is still high. Similar results have been noted by Dia et al. (2006)who measured Ba concentrations between 200 and 619 ppm, and Srconcentrations between 374 and 1021 ppm in multiple soil samplesfrom Mt Cameroon. Nchia (2010) also measured Ba concentrationsbetween 136 and 1117 ppm, and 906–1196 ppm Sr in top soils withinthe Limbe area. Eggleton et al. (1987) also noted Ba enrichment in theweathering profiles of Australian basalts.

Significant Sr depletion is likely related to the decomposition ofplagioclase in which Sr substitutes for Ca. Sr is a highly mobile elementwith similar chemical behaviour to Ca and K implying that soils willgenerally have lower concentrations of Sr than the parent rocks. Batoo, is a highly mobile element. Studies have shown that Ba is easilyscavenged by Mn-minerals, showing a high affinity for Mn in diverseredox-active environments (Wen et al., 2004). However there is no cor-relation between Ba and MnO within the profiles studied here. Ba canalso be present in sanidine, which is still present in profiles 1 and 2.Thus Ba accumulation may be associated with the presence of sanidinein the weathered profile. In absolute terms, Ba shows erratic patternswithin these profiles.

From the parent normalised patterns, it is noted that all elementsare relatively mobile as some are depleted and others enriched. Thisobservation particularly for Profile 1 is contrary to the common as-sumption that Ti and Zr are nearly always immobile as assumed inmany mass balance calculations (e.g. Anderson et al., 2002; Beyalaet al., 2009) within the tropical and temperate zones. Y is observedin this case to increase progressive with increasing weathering inten-sity although Hill et al. (2000) noted that Y can be mobile at the veryearly stages of the weathering. It is also observed that all the elementsshow diverse levels of absolute mass change. An immobile element isexpected to have a 0 absolute mass change which is not the case inthis study. These observations advocate that all the elements are mo-bile at least for one of the profiles. A wider suite of trace and rare

earth element analyses is recommended to further constrain themost appropriate inert element that can be used in mass balance cal-culations in this area. Ti, Zr and Y have been used as inert element inseveral mass balance models (e.g. Anderson et al., 2002; Zhang et al.,2007). It is possible that these elements are conserved within thetemperate climate but are mobile under extreme climatic conditions(Braun et al., 1993, this study) that operate within the humid tropicalregion. Braun et al. (2005) also suggest that dissolved organic mattercan significantly improve the transfer of commonly insoluble ele-ments such as Al, Fe, Zr, Th and Ti. The study area lies within the tro-pics characterised by dense vegetation and heavy rainfall. Theseconditions favour the development of thick layers of organic matterthat might account for the mobility of some of these elements insome profiles within the study area.

6. Conclusions

We present data on the density, chemistry, and mineralogicalcomposition of slide prone residual soils derived from the weatheringof basaltic rock under extreme tropical conditions in SW Cameroon.Our research helps to elucidate the behaviour of some major andtrace element during the weathering of basaltic rocks under humidtropical climate. Key conclusions include:

• Weathering of basaltic and picrobasaltic rocks that characterise theLimbe municipality is accompanied by depletion in all the alkali andalkali earth metals with Sr as the most mobile of all the trace ele-ments analysed.

• The profiles are characterised by the presence of textural heteroge-neities but there are no obvious mineralogical or chemical hetero-geneities at the profile scale, although differences exist in themineralogical composition of the pits analysed in this study.

• The most advanced stage of weathering in this study area is thekaolinisation stage.

• Ti, Zr and Y generally considered as immobile elements for massbalance evaluation, are mobile in some cases under extreme climaticconditions.

• CIA shows a positive correlation with bulk density and S/SAF ratioshows a negative correlation with density implying that chemicalweathering is associated with density decrease and porosityincrease.

• More realistic mass change patterns are observed when the densityof soils and saprolite are used in mass balance evaluations.

Acknowledgement

This work was compiled as part of CVB's PhD thesis sponsored by agrant from the Vlaamse Inter-Universitaire Raad (Flanders, Belgium)in the framework of the project entitled ‘Capacity building in geohazardmonitoring in volcanically active areas of South-West Cameroon’. KFand GGJE were supported by the Belgian Research Foundation - Flan-ders (Fonds voor Wetenschappelijk Onderzoek –Vlaanderen). We aregrateful for constructive comments and criticism from two anonymousreviewers.

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