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Ferrallitisation in an Oceanian Tropical Environment in : Joseph K.T

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Ferrallitisation in an Oceanian Tropical Environment in : Joseph K.T Reprint Ferrallitisation in an Oceanian Tropical Environmnet MARC LATHAM* Abstract Two well drained ferrallitic soil types have been observed on Lakeba, an andesitic island located in the South East of the Fiji group, exposed to a tropical climate characterized by a mean annual rainfall of20000 mm: - Kaolinic ferrallitic soils with a low gibbsite content on the erodedsummits and slopes of the island. - Ferrallitic soils rich in metallic oxydes and hydroxydes on plateaux, relicts of an older gemor­ phology. On comparison, it is observed that, in the Suua region, soils on the same type of andesite as those ofLakeba but under a mean annual rainfall of3.5 m, though highly rejuvenated by erosion, have shown a cleargibbsiteferrallitic evolution. These facts allow to establish WL andesite a pedogenetic limit betweenferrallitic soils with a moderate allitic evolution ofLakeba and ferrallitic soils with a high allitic evolution of the Suva region. There­ fore, a paleoclimatic explanation is considered to elucidate the formation on the Lakeba plateaux of ferrallitic soils rich in metallic oxydes and hydroxydes. Lakeba is located in the southeast of the varying from 50 m to 200 m. According to Fiji Islands in the Lau archipelago at 178° East Twiford and Wright (1966) the majority of by 18° South. It is a subcircular andesitic the soils could be classified as ferrugineous mass of eight to nine kilometers in diameter latosols or "Talasiga soils". Talasiga means overlain in its western and southern parts by in Fijian sunburnt land (Parham, 1972). Miocene limestone scraps (Ladd and Hoff­ Taking part in the UNESCO/UNFPA meister, 1945). Brookfield and Hart (1966) population and environment project in the give the annual mean rainfall for Lakeba as eastern island of Fiji, a recent pedological about 2000 mm. with no strongly marked dry prospecting of this island has found season. As the highest point is at 230 m, among Talasiga soils, in well drained changes of climate throughout the island conditions, two major pedogenetic units: may not be very important. This island ferrallitic soils with kaolinite and low looks like a single massif with round shaped gibbsite individualization and ferrallitic soils summits, drained by a sub radial hydrographic with high metallic oxydes and hydroxydes network. Some plateaux - relicts of an older individualization (Latham et al., 1977). morphology occur on the island, at altitudes In this note after indicating the major • Soil Scientist. Centre DRSTOM - BP. AS Noumea (New Caledonia) characteristics of these soils we will try to 12 clear up their genesis with regard to their pre­ zons, an important microporosity and very sent evolution as well as their past evolutions low bulk density has been noted, giving to under perhaps different climates. That is the soil a smeary consistance, Chemically why we will do a compared study with the these soils are acid. Their base saturation soils located in the Suva surroundings, on the except for A horizon is very low. The cation main Fiji island, in Mendrausutu range. This exchange capacity of their mineral matter is area which has an andesitic substratum similar very weak. Triacid total analysis indicate to Lakeba's receives a mean annual rainfall low to very low amounts of silica and high of more than 3500 mm. iron and very high aluminium percentage in A and B horizons. From a mineralogical Main Characteristics ofthe Soils point of view, gibbsite and hematite are the predominating minerals; boehmite and Kaolinic soils of Lakeba occupy the goethite have also to be noticed. Among eroded slopes and summits which shape the lattice clay, when present which is exceptional, frame of the island. Their depth is variable, metahalloysite is the only mineral identified· their A and B horizons do not exceed 20 Its percentage increases with the depth. X ray to 30 cm in the most truncated areas, where diagrams indicate also a certain amount of elsewhere their C horizon may occur at about amorphous products. So these soils are 1 m or 1.5 m. Their brownish red colour is developed soils in which metallic oxydes and normally redder than 5 YR. Truncated hydroxydes either predominate over phyllites, soils have a medium texture while the deepest or are the only soil constituent. have a fine one. They are acid soils with a low, highly unsaturated, cation exchange The soils of the Mendrausutu range occur capacity (Table 1). Triacid total analysis on a very dissected landscape. They are indicates high amount of silica, aluminium highly truncated. Weathered rock elements and iron. Silica and alumina are mainly occur since 30 to 50 cm deep, when continuous tied up in phyllites. X-ray diffraction weathered rock becomes visible at 80 cm. allows to state them as pseudo monoclinic Their (B) horizons have brownish red (5 YR kaolinite or fire clay (Table 2). In the C 5/6) colour. They have a medium texture horizons, fire clay may be associated with and a clear blocky structure. Chemically metahalloysite. Few gibbsite have been these soils are acid, with a mean, highly un­ evidenced in A and B horizons. This mineral saturated, cation exchange capacity. Among may lack in the C weathered horizon. Iron total elements, silica, alumina and iron are individualizes itself as magnetite, hematite abundant. Silica percentages increase with and goethite, So these kaolinite rich depth and the Si0 ratio which is ferrallitic soils have only little amounts of 2/A1203 gibbsite. close to 1 in A and Breaches 1,67 in the C horizon. Hence free alumina amounts are .f Ferrallitic soils with high metallic oxydes high in A and B horizons. X ray diffraction and hydroxydes individualization are observed emphasizes an abundance of gibbsite and a on plateaux, as indicators of an older morpho­ rather high quantity of hematite, goethite and logy. They are deep (5 to 6 m deep), dusky magnetite. Among phyllites, metahalloysite red (10 R) soils, showing a very fragile struc­ is the only discernible mineral. A certain ture in their upper horizons. Ferromanganesi­ amount of amorphous products is also present. ferous concretions and a coarse texture rich Unlike soils of Lakeba plateaux these in pseudo sands are observed in these upper soils are poorly developed. However, the horizons. Some small millstone fragments importance of metallic oxydes and hydroxydes have also been noticed. In the deeper hori- and particularly gibbsite must be emphasized. 13 TABLE 1 - PHYSICO-CHEMICAL CHARACTERISTICS Kind of profile 1 Lakeba plateaux, soils Lakeba eroded soils Mendrausutu range eroded soils I Sample LS 43 LS 71 LS 73 LS 75 LS 101 LS 103 LS 104 LS 113 VL 11 VL 12 VL 13 VL 14 Deepness in cm 80-100 0-5 90-100 180-200 0-10 50-60 120-130 90-100 0-8 30-40 60-70 150-160 Horizon B2 Al B2 B3C Al (B) (B) C Weather- Al (B) (B) C C ed rock Clay % 14,0 10,5 52,0 66,0 28,S 33,0 19,0 - 21,0 40,0 28,0 19,5 Organic matters % 0,3 4,6 0,3 - 7,2 0,5 - - 16,9 1,0 0,6 0,2 pH 5,3 5,5 5,9 5,8 5,6 4,8 4,5 4,0 4,5 4,5 4,6 4,5 Cation exchange capacity in me/l00g 5,1 20,8 5,1 11,4 26,6 7,8 10,6 5,2 88,3 15,8 15,8 15,9 Base saturation in me/lOOg 0,31 5,23 1,29 8,65 13,5 1,7 1,6 0,84 6,2 0,6 1,0 0.9 I Triacid total analysis % Loss on ignation 22,0 20,2 18,3 16,4 20,0 13,9 14,3 13,3 33,6 18,8 18,9 15,4 Insoluble 0,95 0,90 0,45 0,40 0,50 0,40 0,50 0,70 2,0 0,20 0,15 0,20 Si 02 1,50 15,8 12,5 22,9 34,0 36,7 39,6 38,4 18,4 23,2 24,4 31,0 Al2 03 33,S 30,S 35,6 32,0 28,5 32,0 34,5 31,2 29,4 34,4 36,0 31,5 Fe203 23,S 22,S 25,0 23,8 12,5 14,0 9,1 13,5 13,5 19,8 17,5 18,5 Ti 02 1,90 2,40 2,57 2,7 2,02 1,67 1,90 1,42 1,57 1,87 1,59 1,56 Mn 02 16,8 5,9 4,75 0,04 0,27 0,24 0,18 0,08 0,14 0,11 0,17 0,33 P2 05 0,01 0,1 0,1 0,03 0,2 0,2 0,1 0,05 0,3 0,3 0,2 0,3 Si 02/Al2 03 0,08 0,88 0,60 1,21 2,02 1,94 1,94 2,08 1,06 1,14 1,15 1,67 , I I I I I I ·' .. TABLE 2 - MINERALOGICAL CHARACTERISTICS Kind of profile Lakeba plateaux soils Lakeba eroded soils Mendrausutu range eroded soils Sample I LS 42 LS 71 LS 73 LS 7S LS 101 LS 103 LS 104 LS 113 VL 11 VL 12 VL 13 VL 14 Seepness in cm 20-30 o-S 90-100 iso-zoo 0-10 S0-60 120-130 90-100 o-S 4Q-SO 60-70 lS0-160 Horizon B2 Al B2 B3C Al (B) (B) C Weather- Al (B) (B) C C I ed rock Gibbsite ttt ttt tttt ttt tt tt t tttt tttt tttt ttt Boehrnite I tt tt tt Hematite I ttt ttt ttt tttt tt ttt ttt ttt ttt Goethite I t tt tt tt tt tt t t tt tt tt tt Magnetite ttt ttt t tt ttt t Metahalloysite tt tt ttt tt tt ttt ttt ttt ttt Fire clay ttt tt tt ttt Montmorillonite Amorphous products tt tt tt tt t t tt tt tt tt I tttt abundant ttt rather abundant tt a little t traces Therefore we can notice the difference The nearly complete desilicification of soils between the mainly kaolinic composition of of the Lakeba plateaux and the individuali­ eroded soils of Lakeba and the essentially zation of metallic oxydes and hydroxydes metallic oxyde and hydroxyde constitution raise in these conditions the problem of their of Mendrausutu range eroded soils and of genesis.
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