Tropical Spodosols in northeastern Amazonas State, Brazil

www.elsevier.com/locate/geoderma

Geoderma 119 (2004) 55–68

Tropical Spodosols in northeastern Amazonas State, Brazil

Adriana Maria Coimbra Horbea,*, Marco Antonio Horbeb, Kenitiro Suguioc

aDepartamento de Geociencias-Universidade Federal do Amazonas, Av. Gal. Rodrigo O. J. Ramos 3000, Japiim, 69077-000,

Manaus, AM, BrazilbCPRM-Servic�o Geologico do Brasil, Av. Andre Araujo 2160, Aleixo, 69060-001, Manaus, AM, Brazil

c Instituto de Geociencias-Universidade de Sao Paulo, Rua do Lago 562, Butanta, 05508-900, Sao Paulo, SP, Brazil

Received 26 March 2002; received in revised form 20 May 2003; accepted 3 June 2003

Abstract

The white sand formation found in northeastern Amazonas State in Brazil showed three horizons—E, B, and C. The

superficial horizon (E) was formed by sandy friable material of grayish to whitish colour with accumulations of organic matter

in the form of wavy bands. The B horizon remained essentially sandy but showed a pale yellowish to orangish colour. The C

horizon was friable with pink to creamy clayey sandy materials. In the contact zone between horizons B and C occurred an

enrichment of organic matter, forming ortsteins. The profile structure, the upward increasing of quartz grains corrosion, and the

predominance of quartz in the E horizon, and thus of iO2, instead of kaolinite, Al2O3, Fe2O3, and TiO2 that were more abundant

in the C horizon, suggest that the white sands are the product of podzolization. Soil horizonation and accumulation of organic

matter are governed by the active decomposition of the forest litter. These data also allow us to relate the Spodosols with the

mottled and the saprolitic horizons of truncated lateritic profiles of the Alter do Chao Formation. This process of podzolization,

which continues up to the present moment, is very aggressive since the studied profiles have developed in less than 3000 years

and beneath the modern forest.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Podzolization process; Oxisols; Weathering; Lateritic process

1. Introduction more intense than quartz dissolution, leads to a

Spodosols are essentially sandy and characterized

by quartz accumulation instead of clay minerals,

especially kaolinite. This accumulation of quartz is

attributed to the podzolization process, which assumes

the congruent destruction of the clay minerals and the

migration of the organic matter and the organometallic

complexes to the subsurface, since hydrolysis, being

0016-7061/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0016-7061(03)00233-7

* Corresponding author.

E-mail address: [email protected] (A.M.C. Horbe).

residual enrichment in this mineral (Bravard and

Righi, 1989). The migration of organic matter and

organometallic complexes and its deposition onto less

permeable materials during relative dry periods can

generate a hardened horizon, called humic ortstein, or

bands of black colouration (Schwartz, 1988).

They are typical of cold climates and are generally

covered by coniferous forests. When occurring in

humid tropical regions, they are found over rocks rich

in quartz (Schwartz, 1988; Lundstrom et al., 2000) or

over quartz saprolites of weathered profiles (Thomas

et al., 1999). Others, such as Schwartz (1988), sug-

A.M.C. Horbe et al. / Geoderma 119 (2004) 55–6856

gested that podzols develop in the lower portions of

the relief or in small depressions on plateaus. Accord-

ing to Thomas et al. (1999), they result from the

hydromorphic podzolization of weathered saprolites

in which the destruction of clays takes place in

previously leached profiles of the planar areas. The

lack of sedimentary structures and the abundance of

unworn quartz reinforce this possibility. Other possi-

bilities of accumulation are mentioned by Klinge

(1965), who suggested that white sands are fluvial

sediment, and by Roose (1980), who supposed a

selective erosion of clayey materials from a preexist-

ing substrate.

White sands in Amazonia occur in many places and

over different lithologies. Santos (1993) suggested that

they were produced by wind action in the north of

Amazonas State in Brazil, whereas Dubroeucq and

Fig. 1. Lithostratigraphic and location m

Volkoff (1998), studying the same area, and Lucas et

al. (1984), Chauvel et al. (1987), Bravard and Righi

(1989), and Lucas (1997), studying the area of the

current study, found that Oxisols and Spodosols were

part of a progressive transition along the slopes, where

the Oxisols represent the material being bleached by

organic compounds progressively from down to up

slope. This transformation is internal and related to

water percolation, intensifying solute migration, clay

neoformation, and quartz dissolutions along the

slopes.

From many outcrops near Manaus and along

highway BR-174, we observed that the huge white

sand deposits developed over Alter do Chao Forma-

tion were restricted to the first 60 km and were over or

near silicified quartz sandstones or a friable sandy

facies beneath truncated lateritic profiles, suggesting a

ap of the profiles in the study area.

A.M.C. Horbe et al. / Geoderma 119 (2004) 55–68 57

lithological control. These evidences were not con-

sidered in the evolutionary processes of these material

reported by Lucas et al. (1984), Chauvel et al. (1987),

Bravard and Righi (1989), and Lucas (1997). These

lithological relations and results of 14C datings pro-

vided new contributions to the understanding of

Spodosol formation in this Amazonian region where

a broad extension of this material can be found.

2. Location and general aspects of the area

The study area is located along highway BR-174

between Manaus and President Figueiredo in the

northeastern part of Amazonas State, Brazil (Fig. 1).

This region is classified as Afi in the Koppen system,

tropical hot and humid, with an annual mean temper-

ature of 26 jC and mean rainfall of 1800 mm/year.

The rainy season extends from December to May and

the driest period normally occurs between August and

October. This aggressive climate favours intense

chemical weathering and helps the establishment

Fig. 2. Sketch section of the Spodosol profiles of the northeastern Am

and conservation of the exuberant vegetation covering

the area.

The morphosculptural units characterizing the

landscape in this region are flat hills of reduced

extension, up to 150 m high, with V-shaped valleys

and moderate drainage density. To the north and east

of the study area, there are extensive plateaus 1 to 2

km wide and 3 to 12 km long (Horbe et al., 2001;

Sarges, 2001). The degree of drainage carving

decreases from north to south, with maximum heights

of up to 180 m (in relation to sea level) in the north

and minimum heights around 60 m in Manaus.

The weathering process allowed for the develop-

ment of lateritic profiles. The hill profiles in this

region are usually truncated (the duricrust has been

eroded) and formed, from bottom to top, by saprolitic,

mottled, and soil horizons, with lateritic crust frag-

ments forming stone lines between the two upper

horizon. The Fe–Al lateritic crust is dismantled into

nodules and restricted to relicts in a few outcrops.

Although truncated, the profiles are well developed,

never less than 3 m thick. Intense iron depletion

azonas region. Three horizons are distinguished—E, B, and C.


affects the profiles and develops huge kaolin deposits

(Costa and Moraes, 1998). The truncation of the

profiles and the iron depletion created a new landform

in which the neotectonic movements interfered, fault-

ing the stone lines and placing saprolite horizons and

soils side by side. To the north and east of the study

area, the plateaus are sustained by laterites with iron

and iron–aluminium crusts, covered by yellow Oxi-

sols (Horbe et al., 2001).

The Alter do Chao Formation is the dominant

geological unit in the study area. It is made up by

arkoseous sandstones, pelites, mudstone, quartz sand-

stones, and intraformational breaches (Cunha et al.,

1994), forming strata deposited in fluvial-lacustrine

environments, as well as paleosols (Nogueira et al.,

1999). There occurs silicified quartz sandstone lens

with strong red colouring informally called Manaus

Sandstone (Albuquerque, 1922) made up basically by

quartz with aluminium–iron cement (Franzinelli and

Rossi, 1996). Through palynological and stratigraphic

studies, Dino et al. (1999) recently proposed a Creta-

ceous age (Aptian/Abian–Cenomanian—around 100

Fig. 3. (A) General view of the white sand deposits of the northeastern Am

differentiate the upper portion as the E horizon, with light gray colour, an

between horizons E and B and remainders of B in the E horizon. (D) Wavy

profile 3 in Fig. 2). (E and F) E horizon containing ochre-yellowish to gra

matter partially involving these fragments (see sketch of profile 3 in Fig.

million years) for the rocks deposited in the central

portion of the Alter do Chao Formation sedimentary

basin, in which the study area is located. The rocks are

deeply weathered turning Manaus Sandstones into

whitish coloured and kaolinitic and sandy-kaolinitic

materials formation.

Pedological units, clayey-sandy and clayey yellow

Oxisols, sandy and sandy-clayey yellow Ultisols, and

white sand (Spodosols) occur in the area. According

to Leal (1996), in the Manaus region, there is a

predominance of Oxisols over Spodosols.

3. Materials and methods

For 12 white sand occurrences identified between

kilometers 0 and 60 along highway BR-174, 5 with

the fullest profiles were selected, located on kilo-

meters 6.5, 10.5, 30, 35.5, and 56.5 and numbered

from 1 to 5. The mineralogical and chemical compo-

sitions were only quantified in profile 1. Mineralogy

was obtained by X-ray diffraction with a Phillips PW

azonas region. (B) Structure of the profile 2, where it is possible to

d the B horizon, darker, due to its ochre-yellow colour. (C) Contact

bands of organic matter in the E horizon of profile 3 (see sketch of

yish materials from the B horizon; observe the dark layer of organic

2).

Fig. 4. (A and B) Subvertical features developed in the B horizon of profile 2. The darkest portion represents the organic matter-enriched sand

that forms an external layer around the internal grayish-white sandy portion correlated to the E horizon.


1729 with Cu tube and the textural properties by grain

size test (Wenthworth scale) and electron microscopy.

In profiles 1, 3, 4, and 5, the grain size tests were

made in all horizons, while in profile 2, only the

feature found in B horizon was studied.

Fig. 5. Aspect of the ortstein layers found in profiles 3 (A) and 4 (B). (C)

fractures filled with organic matter and the cement consisting of this sam

The chemical analyses for major elements and trace

elements (Ni, Co, Cr, V, Ce, Nd, Ba, La, Nb, Zr, Y, Pb,

Zn, S, Cl, and Th) in profile 1 were obtained by X-ray

fluorescence that, together with the X-ray diffraction,

allowed to determine mineralogical composition. The

and (D) Photomicrographs showing corroded quartz grains, where

e material can be observed; 40�.


organic carbon of the black wavy bands in the E

horizon was analysed by volumetry with potassium

bychromat solution. The infrared analyses using a

Perkin-Elmer 2000GC-IR were made in the organic

material to identify the organic groups. The enrichment

factors were obtained in relation to horizon C, consid-

ering zirconium (Zr) as the reference element. These

factors were calculated by taking the ratio of the

concentration of the elements in the E and B horizons

in relation to the C horizon (E/C and B/C), multiplied

by the ratio of Zr in C in relation to this element in the

other horizons (Zr in C/Zr in E and Zr in C/Zr in B).

The 14C datings were obtained by liquid scintillation

counting of synthesized benzene and 13C analyses by

isotope ratio mass spectrometry using CO2 generated

by sample combustion at 900 jC in an oxygen atmo-

sphere at the Radio Carbon Laboratory, of the Center

for Nuclear Energy in Agriculture, Sao Paulo Univer-

sity, Brazil (CENA-USP). The ages were expressed in

years before present (BP) and were corrected by

d13CPDB. One sample of charcoal and three of sand

rich in organic matter and ortstein were analyzed.

Fig. 6. (A) The circular structure found in horizon C, with core filled

by rich organic matter black sand representing, probably, a tree

trunk mold now filled. (B) Outcrop showing the Spodosol profile

over the Manaus Sandstone.

4. Results and discussion

4.1. Profile characterization

Three horizons, E, B, and C, were identified in the

five profiles studied (Fig. 2). The superficial horizon

(E) was up to 6 m thick, formed by sandy friable

materials, grayish in profiles 1 and 2 and whitish in

profiles 3, 4, and 5 (Figs. 2, 3A and B). Accumulations

of black organic matter generally occur in the form of

wavy bands, up to 4 mm thick (Fig. 3C and D). Some

of these bands were hardened by the presence of iron

oxy-hydroxides, which conferred a yellowish to

brownish colour to the adjacent sandy materials.

The formation of these wavy bands can be either

related to granulometric variation, with the alternation

of fine and coarse materials, to the action of termites,

or to the migration of colloids between the grains,

filling the intergranular spaces with materials concen-

trated from other horizons (Campy and Macaire,

1982). According to observations in the outcrops,

the bands are possibly related to alternation of erosion

and depositional processes near the surface of the

profiles that buried the lower portion of the horizon

since neither significant granulometric variation nor

termite action was observed. Nevertheless, the migra-

tion of organic colloids between the grains may be an

associated process specially when these bands cut the

remainders of B in E horizon. The irregularity in the

band (Fig. 3D) may be a consequence of a difference

in weight between the quartz grains and the organic

matter.

To depth, the soil texture remained essentially

sandy, but the colour changed to yellowish and

orangish, which distinguished the B horizon (Figs.

2, 3B, C, E, and F). The contact between horizons E

and B is generally clear (Fig. 3B and C), although

occasional remainders of B were found in the grayish

Table 1

Mineralogical composition of profile 1, in %

Horizon Quartz Kaolinite

E (0.5 m) 98 1

OM layer (0.8 m) 97 2

E (1.2 m) 91 8

B (2.3 m) 84 15

C (3.8 m) 74 25

Hematite + goethite, k-feldspar, chlorite with less than 1%;

OM=organic material.


and whitish E horizon (Fig. 3C, E, and F). Some of

these remainders presented accumulations of organic

matter on the upper surface, forming a black aureole

(Fig. 3E and F).

Subvertical features, up to 30 cm long, composed

of grayish to whitish material, and totally or partially

enveloped by a sandy external layer with a dark

colour due to the presence of organic matter, occurred

within the yellowish sandy material of the B horizon

(Fig. 4A and B). According to Lucas et al. (1984) and

Chauvel et al. (1987), these features were fingering

Fig. 7. Quartz grains with angular and subangular forms, silica phytoliths

potassium feldspar (4), and diatoms (5) found in the horizon A.

tongues of E horizon penetrating into the B horizon,

although near the surface, they seem to be related to

hollow spaces (Fig. 4) left empty by roots decompo-

sition. They are probably preferential channels for soil

water drainage; they were later filled by organic

matter accumulating in contact with the surrounding

material mixed with E horizon sand.

The C horizon was identified in profiles 1 and 5

(Fig. 2). It was friable and made up of pink to creamy

clayey sandy materials. In the contact zone, between

horizons B and C, there generally occurred enrich-

ments of organic matter, forming sandy accumulations

up to 10 cm thick of black colour that, in some places,

were hardened, forming ortsteins (Fig. 5A and B). The

framework of the ortsteins was formed by grains of

quartz with corroded edges and heterogeneous gran-

ulation, cemented by organic matter concentrations.

The organic matter also occurred filling fractures in

the grains of quartz (Fig. 5C and D).

In some places, the friability of horizons E and B

facilitated the natural removal of the upper levels,

causing the exposure of the C horizon. Where ex-

(1); ooliths of organic matter (2) and (6); kaolinite aggregate (3);


posed, some circular structures are found with up to

35 cm in diameter crossing the horizon to the bottom,

in which the core was filled by compact black sandy

material rich in organic matter, surrounded by the

massive white clayey sandy material of the C horizon.

These structures, except for the small and soft sub-

vertical features of the E horizon of Fig. 4, are remains

of tree trunks (Fig. 6A).

Manaus Sandstone parent rock only occurs in

profile 1 (Fig. 2). An outcrop of this rock was also

found in the eastern portion of highway BR-174, 10

km away from profile 1. There is a few-centimeters-

Fig. 8. Granulometric ana

thick transition in between Manaus sandstone and the

covering Spodosol (Fig. 6B).

4.2. Granulometric and mineralogical composition

Quartz was the most abundant mineral in all the

profiles. Mineralogy was quantified in profile 1 which

was considered to be the most representative in all the

area (Table 1). The amount of quartz increased from

the C (74%) to the E (98%) horizon. Kaolinite was

the most abundant clay mineral in the C horizon,

where it reached up to 25%. In addition to these

lysis of the profiles.

Table 2

Chemical composition of profile 1, in wt.%

Horizon SiO2 Al2O3 Fe2O3 MnO TiO2 MgO LOI

E 98.02 0.30 0.41 0.08 0.05 0.09 1.01

OM layer 95.67 0.90 0.59 0.08 0.08 0.11 2.54

E 95.29 2.95 0.45 0.08 0.13 0.11 0.95

B 90.78 5.84 0.42 0.08 0.20 0.14 2.50

C 86.89 9.91 0.34 0.08 0.26 0.16 2.34

CaO, MgO, Na2O, K2O, and P2O5 less than 0.01%; OM=organic

material.

Fig. 9. Correlations between SiO2, Al2O3, and TiO2 in profile 1 in

each horizon (E, B, and C).


minerals, grains of potassium feldspar, opaque min-

erals, diatoms, silicaceous phytoliths, and ooliths of

organic matter all occurred in quantities lesser than

1% (Fig. 7).

The most abundant granulometric class in all

horizons of the profiles was sand, with medium-sized

grains (0.50 to 0.25 mm) making up 20% to 50% of

the total (Fig. 8), followed by coarse sand (1.00 to

0.50 mm) and fine sand (0.25 to 0.125 mm). The

pelitic fraction represented less than 2% of the mate-

rial, and granules of quartz larger than 2 mm com-

posed up to 23% of the material in the E horizon of

profile 3.

Significant variation among profiles was observed,

but in those presenting C horizon, the sand ratio (0.50

to 0.25 mm) decreased from C to B (profiles 1 and 4).

In profile 5, which presented a smaller sand fraction

(31%), there was more (36%) coarse sand (1.00 to

0.50 mm). From the B to E horizons, the amount of

medium-sized sand also decreased in profiles 3, 4, and

5 and remained constant in profile 1. The remaining

fractions presented random distributions of sand along

the profiles.

The granulometric frequencies showed a tendency

to have a decreasing amount of the 0.35–0.25 mm

fraction downward from the top to the bottom of the

profiles, which may indicate corrosion of the quartz

grains. This corrosion is also evidenced by the angular

and subangular forms, and by the low sphericity of the

quartz grains (Fig. 7A and B).

4.3. Chemical composition

SiO2 was the main component in all profile 1

horizons (Table 2), reflecting their essentially quartz-

ous character. The Al2O3 amounts were variable, from

less than 1% in E to almost 10% in C horizon due to

the kaolinite increase down the horizons. Fe2O3 and

TiO2 were present at levels of less than 1%. The small

increase of Fe2O3 (0.59%) and the highest amount of

organic matter in the E horizon suggest that these two

components are related to each other, whereas the

decrease of TiO2 toward to the top profile shows the

destruction of the opaque minerals which are the

carriers of this element. Each remaining constituent

(CaO, MgO, Na2O, K2O, and P2O5) presented very

low values ( < 0.01%).

Negative SiO2–Al2O3 and TiO2–SiO2 and posi-

tive TiO2–Al2O3 correlations suggest gradual chem-

ical changes occurring in the profile. There was a

gradual increase in the amount of SiO2 and clear

reduction of Al2O3 and TiO2 in relation to Fe2O3

from the C to E horizons (Fig. 9).

The black-coloured wavy bands contained from

0.05% to 0.20% of organic carbon, between 50 and

Table 3

Concentration of trace elements, in ppm

Horizon Ni Co Cr V Ce Nd Ba La Nb Zr Y Pb Zn S Cl Th

E 7 1 340 2 2 < 1 26 2 3 326 < 1 2 1 22 20 < 2

OM layer 9 2 484 4 2 < 1 29 3 1 268 < 1 2 5 58 27 < 2

E 5 1 228 5 10 5 30 10 7 335 2 3 < 1 24 26 3

B 6 2 312 4 6 2 31 5 6 253 1 3 1 8 22 < 2

C 5 3 163 6 20 6 32 20 8 315 3 3 5 18 16 3

Sr, Rb, As, Cu, and Sc less than 1 ppm; OM=organic material.


1.160 ppm total iron, and less than 5 ppm of Mn

(Table 3). The possibility of iron immobilizing the

organic matter (quelation) as shown by De Coninck

(1980) may explain that these bands are preserved in a

heavily leached environment.

Only Cr, Zr, Ba, S, Cl, and, secondarily, Ce and La

appeared in significant amounts among the 21 trace

elements analyzed (Table 3) and allowed interpreta-

tion of their distributions along the profile. Zr occurs

in relatively high concentration and is related to the

presence of zircon. The presence of Cr is probably

related to chromite. The amounts of Cr, Ba, S, and Cl

are higher in the layer enriched by organic matter,

suggesting their relationship (Tables 2 and 3). Ce and

La appeared in somewhat higher amounts in kaolinite-

richer horizons, suggesting a positive correlation be-

tween these elements and the clay mineral.

The enrichment factors only point out an enrich-

ment of SiO2, Fe2O3, Mn, Ni, and Cr from horizon C

to E, showing the intense leaching that contributed for

the formation of this profile (Table 4). In A horizon,

the richest in organic matter, Ba, Zn, S, and Cl

Table 4

Enrichment factors of profile 1, in relation to zircon

Horizon SiO2 Al2O3 Fe2O3 MnO TiO2 MgO LOI

E 1.09 0.03 1.17 0.97 0.19 0.55 0.42

OM layer 1.30 0.11 2.05 1.18 0.36 0.81 1.28

E 1.03 0.28 1.24 0.94 0.47 0.65 0.38

B 1.31 0.74 1.54 1.25 0.96 1.09 1.34

C 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Horizon Ni Co Cr V Ce Nd Ba

E 1.36 0.32 2.02 0.32 0.10 * 0.79

OM layer 2.12 0.79 3.50 0.79 0.12 * 1.07

E 0.94 0.31 1.31 0.78 0.47 0.78 0.88

B 1.50 0.83 2.39 0.83 0.38 0.42 1.21

C 1.00 1.00 1.00 1.00 1.00 1.00 1.00

OM: organic matter; *: not calculated because the amounts were less than

additional enrichment suggested that these compo-

nents are forming organometallic complexes.

The chemical correlation and the similarity ob-

served in the amounts of the trace elements in the

different horizons suggest an in situ evolution of these

profiles, with gradual upward development of the

horizons, with pure quartz sands representing the

end product of the weathering/leaching process.

4.4. Organic matter chemical composition and its

influence on Spodosol formation

Humic and fulvic infrared spectra are shown in Fig.

10. They have features that are similar to those of Tan

(1977) and Gu et al. (1995). The two spectra have

strong absorption bands at 3400 cm� 1 indicating

strongly bonded –OH groups. The two spectra

showed an abundance of carboxylic groups at 1600

cm� 1 and in between 1600 and 1720 cm� 1. The band

in the 1000–1300 cm� 1 region represents alcohol,

phenol, ether, and carbohydrated compound related to

C–O group complexes. The bands at 1407 cm� 1 in

La Nb Y Pb Zn S Cl Th

0.10 0.36 * 0.65 0.19 1.19 1.21 *

0.18 0.15 * 0.79 1.18 3.80 1.99 *

0.47 0.82 0.63 0.94 * 1.25 1.53 0.94

0.31 0.94 0.42 1.25 0.25 0.56 1.72 *

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

1 ppm.

Fig. 10. The infrared analyses of fulvic (A) and humic (B) acids of

the ortstein organic matter.


the fulvic acids and at 1261–1398 cm� 1 in humic

acids suggest that the carboxyl groups are complexed

(–COOH or –COO�) with iron-forming iron fulvate

as found by Gu et al. (1995). The bands 3694, 3612,

and probably 1031, 1008, and 912 cm� 1 may be

related to Si–OH on account of impurities in the

purification of the organic matter, although the last

three can also be associated to aliphatic and carbohy-

drate –OH. These organic groups, alcohol, carboxyl-

ic, phenol, ether, carbohydrate, and fulvate, are acidic

and hydrophilic, having negative energy and being

able to reduce the pH, facilitating the dissolution of

silicates and iron oxides, and corroding quartz through

the formation of organic complexes and colloids

(Chorover and Sposito, 1995).

According to De Coninck (1980), iron and alumin-

ium cations tend to immobilize the organic matter in

association with the clay fractions. Thus, clayey soils

with great amounts of these elements such as soils

developed over lateritic crust are richer in immobile

organic matter, while in soils less leached and with

lower amounts of iron and aluminium, the organic

matter may migrate downward destroying kaolinite

and corroding the quartz grains. Nevertheless, an

intense iron depletion was observed in some outcrops

in the study area (Fig. 11) suggesting iron migration.

Van Ranst et al. (1997) suggested that iron migra-

tion in soil profiles was due to humid climate and litter

layer that causes the reduction of the Fe3 + into soluble

Fe2 + in consequence of the lack in oxygen. This, along

with iron leaching, lessens both the microaggregated

structures and porosity, increasing the iron reductions

and turning the E horizon even more bleached. This Fe

leaching is not observed to the north of the area where

the Oxisols are developed over lateritic crust, suggest-

ing that in the truncated profiles, with low amount of

iron and aluminium, the organic compounds move

easily down the profile dissolving kaolinite and cor-

roding quartz. The organic matter is concentrated in

the contact zone between the more porous quartz-rich

E horizon and the more clayey C horizon, forming the

ortstein. The organic matter stabilization in this portion

of the profile may occur through binding with aromatic

and labile structures (Schmidt et al., 2000).

The organic matter migration suggests hydromor-

phic conditions in the development of profiles, and it

also reflects periods of relatively dry climatic con-

ditions that make cementing of quartz grains by

organic matter possible, as observed by Schwartz

(1988) in similar profiles. In profiles 1 and 2, the

ortstein was not individualized and the grayish colour

of the A horizon results from the organic matter lesser

leaching, indicating the presence of a less hydromor-

phic environment, with a lower water table or a less-

evolved profile than the others.

4.5. The carbon isotopes

Profile 1 horizon E coal sample 14C data point out

a nearly modern age (Table 5); hence, it is probably a

product from recent fires. The ages obtained for the

organic matter of ortsteins from profiles 3 and 4, and

for the friable organic sand of profile 5, varied

between 1960 and 2810 years BP (Table 5).

The values of d13c between � 28.6% and � 29.3%

were typical of organic matter generated by C3 veg-

Fig. 11. The intense desferruginous process in some outcrops on highway BR-174. The arrow indicates the desferruginous front.


etation and suggested the predominance of forest.

These data allow stating that the ortsteins and the

organic sand accumulations were formed at least

between 1960 and 2810 years BP under tropical forest

in a humid climate condition.

4.6. Genesis of white sands, their relations with

Oxisols, and the landscape

The structure of the profiles, the presence of

ortsteins and the organic matter in wavy bands, and

the predominance of quartz, and thus of SiO2 instead

of kaolinite, Al2O3, Fe2O3, and TiO2 more abundant

in the C horizon than in the E horizon, are all

suggestive of gradual evolution from the base to the

top of the profiles. These features allow to correlate

Table 5

Amount of modern carbon, age, and d13C of organic matter

Sample Material Age years

BP

d13C(x)

Profile 1—horizon E charcoal modern � 29.3

Profile 3—ortstein quartz +OM 2810F 70 � 29.0

Profile 4—ortstein quartz +OM 1960F 60 � 28.6

Profile 5—horizon E friable organic sand 2020F 60 � 28.59

OM=organic material; BP= before present.

the studied white sands with Spodosols. The E hori-

zon, being up to 6 m thick, could suggest the presence

of Entisols (quartz sands), but the geologic history of

the region, combined with the presence of both

organic matter layers and ortstein, confirms the hy-

pothesis that the white sands are related to vertical

pedogenetic processes with differentiation of horizons

and accumulation of organic matter generated by

decomposition of forest litter and not by fluvial sedi-

ments or selective erosion of clayey material.

The integration of available data with the geologic

units in the area allows the following considerations:

(1) although the Alter do Chao Formation extends

north to kilometer 90 on highway BR-174 (Nogueira

et al., 1997), Spodosols over this formation only occur

between Manaus and kilometer 60 on the same

highway; (2) the landscape features to the north of

the area show plateaus higher than 150 m sustained by

iron and iron aluminium lateritic crust and with

Oxisols, whereas in the study area, the small hills

up to 60 m high with truncated profile and stone lines

show Ultisols and Spodosols (Horbe et al., 2001).

This landscape is a consequence of neotectonic move-

ments that increased the carving and/or denudation in

the study area more than in contiguous north and east

areas, exposing sandy to sandy clayey saprolites and

mottled horizons and the Manaus Sandstone. These


conditions facilitated the podzolization in the area

such as observed by Schwartz (1988), Thomas et al.

(1999), and Lundstrom et al. (2000) in sandy materi-

als. Our suggestion of Spodosol originating only from

truncated profiles is corroborated by Dubroeucq and

Volkoff (1998) and Mafra et al. (2002) who studied

Spodosols that were also developed over profiles

without lateritic crust acting as a protective horizon

for the podzolization process.

We do not discard, however, a lateral process

starting from the leaching of Oxisols such as sug-

gested by Lucas et al. (1984), Chauvel et al. (1987),

Bravard and Righi (1989), and Lucas (1997) but

associated to truncated profiles with sandy clayey

and sandy materials. Given that these horizons are

richer in quartz, and that the organic matter attacks the

clay and iron minerals more easily, it is possible to

conclude that these horizons congregate the condi-

tions favourable for the formation of Spodosols in-

stead of Oxisols. Thus, the Spodosols are the

weathering end products in the study area. In the

well-developed profiles of the plateaus, the Oxisols

over the lateritic crust are richer in Fe and Al that help

to retain organic matter such as sesquioxide-organic

complexes, hindering their migration downward, thus

protecting them from intense podzolization processes.

5. Conclusions

The white sands of northeastern Amazonas State

are structured in horizons E, B, and C, in which

mineralogical and chemical characteristics show pre-

dominance of quartz, and thus of SiO2, in the E

horizon instead of kaolinite, Al2O3, Fe2O3, and TiO2

being more abundant in C horizon. These character-

istics and their occurrence in small hills up to 60 m

high showing truncated lateritic profile suggested that

the white sands in the study area are a product of

podzolization acting on either the sandy to sandy-

clayey mottled or the saprolitic horizon. These litho-

logical and geomorphological relations, associated

with progressive increment of water saturation along

the slopes, promote the development of the Spodosols.

This process depends on the organic matter gener-

ated by forest litter decomposition that migrates

downward associated with Fe2 +. This low pH and

highly chelating organic matter breaks up the micro-

aggregated structure and reduces the porosity of sandy

to sandy-clayey material that has low capacity to

immobilize the organic matter. Spodosols represent

and advanced the stage of weathering of truncated

profiles developed in tropical region. This podzoliza-

tion process, which continues up to date, is very

aggressive, developing the studied profiles in less

than 3000 years under modern forest conditions.

Acknowledgements

We thank the project ‘‘The Neocenozoic of western

Amazonia’’ (process no. 520243/98-6 PNOPG/

CNPq), Prof. Antonio Rossi and Prof. Gean Paolo

Signolfi from University of Modena, Italy, for the

chemical analysis, Raimunda Larangeira for the

granulometric analysis, and an anonymous referee

for the suggestion that improved the paper.

References

Albuquerque, O.R., 1922. Reconhecimento dos vales do Amazonas.

Bol. Serv. Geol. Min. Rio de Janeiro, Brasil, vol. 3, pp. 1–84.

Bravard, S., Righi, D., 1989. Geochemical differences in an oxi-

sol– spodosol toposequence of Amazonia, Brazil. Geoderma 44,

29–42.

Campy, M., Macaire, J.J., 1982. Geologie des formations super-

ficielles: geodynamique—facies—utilisations. Mason. 344 pp.

Chauvel, A., Lucas, Y., Boulet, R., 1987. On the genesis of the soil

mantle of the region of Manaus, Central Amazonia, Brazil. Ex-

perientia 43, 234–241.

Chorover, J., Sposito, G., 1995. Dissolution behavior of kaoli-

nitic tropical soil. Geochimica et Cosmochimica Acta 59,

3109–3121.

Costa, M.L., Moraes, E.L., 1998. Mineralogy, geochemistry and

genesis of kaolins from the Amazon region. Mineralium Depos-

ita 33, 283–297.

Cunha, P.R.C., Gonzaga, F.G., Coutinho, L.F.C., Feijo, F.J., 1994.

Bacia do Amazonas. Boletim de Geociencias da PETROBRAS

8, 47–55.

De Coninck, F., 1980. The major mechanisms in formation of spo-

dic horizons. Geoderma 24, 101–128.

Dino, R., Silva, O.B., Abrahao, D., 1999. Caracterizac�ao palino-

logica e estratigrafica de estratos cretaceos da Formac�ao Alter

do Chao, Bacia do Amazonas. Simposio sobre o Cretaceo do

Brasil, Rio Claro, vol. 5. Boletim de resumos, Rio Claro,

pp. 557–565.

Dubroeucq, D., Volkoff, B., 1998. From Oxisols to Spodosols and

Histosols: evolution of the soil mantles in the Rio Negro Basin

(Amazonas). Catena 32, 245–280.

Franzinelli, E., Rossi, A., 1996. Contribuic�ao ao estudo petrografico


e geoquımico do Arenito Manaus. Simp. Geol. Amaz., vol. 5.

Belem, Brasil, pp. 209–211.

Gu, B., Schmitt, J., Chen, Z., Liang, L., McCarthy, J.F., 1995.

Adsorption and desorption of different organic matter fractions

on iron oxide. Geochimica et Cosmochimica Acta 59, 219–229.

Horbe, A.M.C., Nogueira, A.C.R., Horbe, M.A., Costa, M.L., Su-

guio, K., 2001. A lateritizac�ao na genese das superfıcies de

aplanamento da regiao de Presidente Figueiredo-Balbina, nor-

deste do Amazonas. Contribuic�oes a Geologia do Amazonas 2,

145–176.

Klinge, H., 1965. Podzol soil in Amazon Basin. J. Soil Sci., 96–103.

Leal, P.C., 1996. Caracterizac�ao e interpretac�oes geneticas de algunssolos da regiao de Manaus-AM. Dissertac�ao de Mestrado. UFPe.

109 pp.

Lucas, Y., 1997. Biogeoquımica em ambiente equatorial: exemplo

dos sistemas Latossolos-Podzois da Amazonia. Cong. Intern.

Geoq. 4j, 9–12.Lucas, Y., Chauvel, A., Boulet, R., Ranzani, G., Scatolini, F., 1984.

Transic�ao latossolos-podzois sobre a Formac�ao Barreiras na

regiao de Manaus, Amazonia. Revista Brasileira de Ciencias

do Solo 8, 325–335.

Lundstrom, U.S., Van Breemen, N., Bain, D., 2000. The Podzoli-

zation process. A review. Geoderma 94, 91–107.

Mafra, A.L., Miklos, A.A.W., Volkoff, B., Melfi, A.J., 2002. Pedo-

genese numa sequencia Latossolo-Espodossolo na regiao do

Alto rio negro, Amazonas. Revista Brasileira de Ciencias do

Solo 26, 381–394.

Nogueira, A.C.R., Souza, V.S., Soares, E.A.A., 1997. Contribuic�aoa tectonica cenezoica da regiao de Presidente Figueiredo, norte

do Amazonas. Simposio Nacional de Estudos Tectonicos, 6j.Pirinopolis-GO, Brasil, pp. 123–125.

Nogueira, A.C.R., Vieira, L.C., Suguio, K., 1999. Paleossolos da

Formac�ao Alter do Chao, Cretaceo-Terciario da Bacia do

Amazonas, regioes de Presidente Figueiredo e Manaus. Sim-

posio sobre o Cretaceo do Brasil, vol. 5. Rio Claro, Brasil,

pp. 261–266.

Roose, E., 1980. Dynamique actuelle des sols ferrallitique et ferru-

gineux tropicaux d’Afrique Occidentale. These Doct. Univ. Or-

leans. 586 pp.

Santos, J.O.S., 1993. O pantanal setentrional e os campos de dunas

da Amazonia Ocidental. International Symposium of Quaternary

of Amazonia, Resumos, Brasil, p. 110.

Sarges, R.R., 2001. Geologia e compartimentac�ao geomorfologica

da regiao de Presidente Figueiredo. Relatorio Interno. FUA/ICE/

DEGEO. 37 pp.

Schwartz, D., 1988. Some podzol on Bateke Sands and their ori-

gins, People’s Republic of Congo. Geoderma 43, 229–247.

Schmidt, M.W.I., Knicker, H., Kogel-Knaber, I., 2000. Organic

matter accumulation in Aeh and Bh horizons of a Podzol—

chemical characterization in primary organo-mineral associa-

tions. Org. Geochem. 31, 727–734.

Tan, K.H., 1977. Infrared spectra of humic and fulvic acids, con-

taining silica, metal ions, and hygroscopic moisture. Soil Sci-

ence 123, 235–240.

Thomas, M., Thorp, M., Mcalister, J., 1999. Equatorial weathering,

landform development and the formation of white sands in north

western Kalimantan, Indonesia. Catena 36, 205–232.

Van Ranst, E., Stoops, G., Gallez, A., Vandenberghe, R.E., 1997.

Properties, some criteria of classification and genesis of upland

forest Podzols in Rwanda. Geoderma 76, 263–283.

Documents

Tropical Spodosols in northeastern Amazonas State, Brazil