German-Brazilian Workshop on Neotropical Ecosystems – Achievements and Prospects of Cooperative Research
Hamburg, September 3-8, 2000
Session 6: Concepts and Paradigms for Management of Ecosystem Resources
Sedimentological, Geochemical and Geomorphological Studies of Sediment Deposits in the
Pantanal/Brazil
Buchas, H.1, Irion, G.1, Nunes da Cunha, C.2, Da Silva, C. J. 2, Kasbohm, J.3 and Junk, W. J. 4
1
Senckenberg - Institute, Wilhelmshaven/Germany
2
Institute of Biological Sciences, Federal University of Mato Grosso (UFMT), Cuiabá/Brazil
3
University of Greifswald/Germany
4
Max-Planck-Institut of Limnology, Plön/Germany
Keywords
Since surfaces act as substrate for plant communities the
information about its composition and its genesis may be
important contributions to vegetation studies. This is especially the case in the Pantanal where in the wetland plants
occur adapted to dry periods.
In our study we have given additional consideration to artificial earth-mounts, locally called "Aterros”, built up by the
indigenous population in precolumbian time. The earthmounts are, in contrast to the surrounding Pantanal plains,
characterized by a high fertility, which could originally not
be explained.
Pantanal, Quaternary geology, Clay mineralogy, Geochemistry
1
Introduction
One aim of our geological studies of the Pantanal of Mato
Grosso is to show the origin, the pathways and depositional
areas of sediments during Quaternary times. This knowledge
will help to get a better understanding of the formation of
the various structures on the surface of Pantanal plains.
Fig. 1: Map of the investigation areas
Buchas, H., Irion, G., Nunes da Cunha, C., Da Silva, C. J., Kasbohm, J. and Junk, W. J.:
Sedimentological, Geochemical and Geomorphological Studies of Sediment Deposits in the Pantanal/Brazil
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German-Brazilian Workshop on Neotropical Ecosystems – Achievements and Prospects of Cooperative Research
Hamburg, September 3-8, 2000
Session 6: Concepts and Paradigms for Management of Ecosystem Resources
2
Material and Methods
Sediment samples were taken predominantly by a small
coring device with which we reached maximum depths of
15 m. The field work was focussed on the Pantanal of
Poconé, on the area of the Transpantaneira and on the Rio
Cuiabá (Fig. 1). As well additional studies were done in the
southern Pantanal. Our studies were carried out by means of
sedimentology, mineralogy, geochemistry, scanning electron microscopy and by interpretation of satellite images.
Grain-size analyses were done using settling tubes and sieves. Mineralogy, mainly clay mineralogy, was carried out
by X-ray diffractometry. Chemical elements were determined by Atom absorption spectrophotometry (AAS) and
X-ray fluorescence (XRF). Clay mineral analyses were
done as well with a Transmission electron microscopy
(TEM) device at the Geological department of the
University of Greifswald/Germany. Due to a resolution in
the range of less than 100 nm one can get clear images of
clay minerals. The TEM is equipped with an energy
dispersive x-ray installation for chemical spot analyses,
used for the determination of the chemistry of single minerals.
Fig. 3: Clay mineral association of the sediments of Rio Cuiabá:
X-ray-diffraction diagrams of the <2 µm fraction of Rio Cuiabá
sediments. One can see high contents of illite and kaolinite and
lower contents of smectite. Typically for a tropical river sediment
load, there is no chlorite. Between Santo Antonio do Levergér and
Porto Cercado the clay mineral association is relatively constant.
The tributaries have no significant influence on the clay
mineralogy due to their low sedimentation load. Only the kaolinite
content increases most probably due to the erosion of intensively
weathered sediments on the river banks.
3
Results and Discussion
The plain of the Pantanal is built by large sediment cones,
created by rivers from the neighboring Planaltos formed by
highly weathered Precambrian, Paleozoic and Mesozoic
rock series. The sediment carried into the Pantanal is
already in an highly weathered stage (Fig. 2). The clay
mineral chlorite, which is part of most world river
sediments, does not occur in the sediments of rivers
crossing the Pantanal. Due to the intensity of tropical
weathering processes chlorite may be dissolved or altered
into other clay minerals. The sediment of the Pantanal
mainly consists of quartz and clay-minerals (Fig. 3). Since
the Planaltos are predominantly built up of sandstones the
Cuiabá River, and therefore the Pantanal as well, is rich in
sandy sediments.
The sequence of sand bodies and floodplain fines can be
explained as fluvial deposits (Fig. 4a). The rivers
accumulate sand forming dams and levées. The fine
Fig. 2: Sand particles.- Highly weathered (corroded surfaces)
quartz grains from Rio Cuiabá near Porto Cercado. The sand
origins from weathering profiles formed in the old sandstones of
the drainage area of the Rio Cuiabá. There the corrosion took place
during the long lasting tropical climate. The SEM image shows
grains of different stages of corrosion. Etching and pitting of the
surfaces results in the segmentation of the grains (see highly
segmented grain at the lower left of the image).
Buchas, H., Irion, G., Nunes da Cunha, C., Da Silva, C. J., Kasbohm, J. and Junk, W. J.:
Sedimentological, Geochemical and Geomorphological Studies of Sediment Deposits in the Pantanal/Brazil
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German-Brazilian Workshop on Neotropical Ecosystems – Achievements and Prospects of Cooperative Research
Hamburg, September 3-8, 2000
Session 6: Concepts and Paradigms for Management of Ecosystem Resources
Fig. 4a: An idealised cross-section through the Pantanal of Poconé.- The cross-section demonstrates the main phases of dislocation and of
accumulation of sediments. It shows sand bodies - cordilheiras and levées - rising above the plain which are subjected to erosion. Their sand
contributes to the surface sediments of the Pantanal plain but it may be removed and redeposited again by smaller creeks.
Fig. 4b: Clay mineralogy and some geochemical aspects of two sediment profiles from the cross-section of Fig. 4a.- Left: profile 1 is taken
from a Cordilheira which is most probably an old levée of the Rio Cuiabá. Its mineralogy of the <2 µm-fraction shows kaolinite and illite
together with smectite in the lower part; above the depth of 1.5 m smectite disappears and only kaolinite and illite are still present. In the upper
0.5 m Al-chlorite dominates the <2 µm fraction. Geochemical analyses show an increase in Aluminium due to an increase in quartz (not shown
in the Fig.), resulting from weathering of the clay minerals, in higher sections of the profile. As well due to weathering the Ti values increase
by the factor 2 in the profile. Right: Profile 2 is taken from the plain. Here the smectite is present as well in the upper part of the sequence, the
higher Al content points to an generally lower alteration of the clay minerals with the exception of the surface sample.
Buchas, H., Irion, G., Nunes da Cunha, C., Da Silva, C. J., Kasbohm, J. and Junk, W. J.:
Sedimentological, Geochemical and Geomorphological Studies of Sediment Deposits in the Pantanal/Brazil
739
German-Brazilian Workshop on Neotropical Ecosystems – Achievements and Prospects of Cooperative Research
Hamburg, September 3-8, 2000
Session 6: Concepts and Paradigms for Management of Ecosystem Resources
Fig. 6: Schematic cross-section of the "Aterro” P-040 on the banks of the Rio Paraguay (16° 50’S).- The "Aterro” was piled
up by taking Pleistocene floodplain sediments together with shells of limnic molluscs. The composition of the sediments
created an calcium rich environment that favors the formation of beidellite, a clay mineral of the smectite group
Fig. 5: Clay mineral distribution in a 4 m sediment sequence taken
in an area within the meander bend of the Rio Bento Gomes (see
Fig. 4a, profile 3).- The clay mineral association of the lower
section shows that the sediments originate from the Rio Cuiabá,
whereas the upper sediment section corresponds to the Rio Bento
Gomes. At 0,8 m depth high amounts in anatase and rutile (both
TiO2), which are geochemical markers of intensive weathering
processes, show an old land surface.
Fig. 7: TEM image of a beidellite-rich specimen.- The clay
minerals are fine-grained and similar to a normal smectite. Only
chemical spot analyses reveals its different mineralogical
composition.
Buchas, H., Irion, G., Nunes da Cunha, C., Da Silva, C. J., Kasbohm, J. and Junk, W. J.:
Sedimentological, Geochemical and Geomorphological Studies of Sediment Deposits in the Pantanal/Brazil
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German-Brazilian Workshop on Neotropical Ecosystems – Achievements and Prospects of Cooperative Research
Hamburg, September 3-8, 2000
Session 6: Concepts and Paradigms for Management of Ecosystem Resources
sediments, clays and silts, are deposited by overbank flows.
In a later stage the plain is topped by a thin cover of sand
from the erosion of the dams and levées by small creeks.
The creeks are mostly periodically streams which are active
only during the annual flood-season. In the Pantanal of
Poconé, as can be assumed for the whole Pantanal, the
surfaces additionally underwent a long-lasting period of
strong weathering as it is detected by increases in TiO2 and
quartz in surface sediments (Fig. 4b). The migration of the
river channels on the plain results in the deposition of
sediments of different origins which are accumulated one
above the other. One example is a profile taken near Rio
Bento Gomes (Fig. 5). The Rio Bento Gomes originates
from the abandoned Quaternary floodplain of the Cuiabá
River whose main channel is recently located about 30 km
eastwards from the former floodplain. It receives its water
predominantly from the Pantanal plains where illite content
is relatively low and kaolinite content high, whereas the
sediment of Rio Cuiabá show higher illite and lower
kaolinite contents. In the profile shown in Fig.5 Rio Cuiabá
sediments is overlain by deposits of the Bento Gomes
showing the mobility of the water courses in Pantanal plain.
As mentioned above the "Aterros" are one point of the
present investigations (Fig. 6). Main interest was given to
clay mineralogy of their sediments. The predominant clay
mineral in "Aterro” sediments is beidellite, a mineral
belonging to the smectite group. Its neoformation requires
badly drained conditions and is favored by tropical climate
as well as by high Ca contents which result from the dilution
of large amount of shells of limnic molluscs and snails left
behind by the indigenous population. This explains that
during a comparatively short period of time, between the
time when the "Aterros” were piled up not more than 1000
years ago and today, beidellite was formed. It is to be
excluded, that the beidellite was a compound of the
sediments from which the "Aterros” have been built, since
in the surrounding plain there is no smectite or beidelliterich sediment to be found and the indigenous population
may not have carried the material for the mounts over a long
distance. The beidellite (Fig. 7), together with the high content of calcium is responsible for the high fertility of the
"Aterros”.
4
Conclusion
The applied methods of investigations, mainly the
sedimentology, clay mineralogy, geochemistry and
scanning electron microscopy are suitable tool to analyze
the complex situations of Pantanal surfaces. They show that
most of the surface structures in the Pantanal are fluvial
deposits (see as well IRION et al. this volume). With the help
of clay mineralogy one can differentiate the source areas of
the sediments. During the deposition the sediments are
already in an highly weathered stage, an additional
weathering takes place in all of the surface sediments.
According to these findings the low fertility of the Pantanal
plains can be explained. Sedimentation on the plain is characterized by several phases. There may be deposits from
different source areas one above the other. The
"Cordilheiras” are sand bars deposited by major water
courses. Small temporary active creeks are an additional
factor for the leveling of the Pantanal plain.
The sediments of the man made "Aterros” as well
underwent a strong alteration. Beidellite a smectite group
mineral that occurs in "Aterro” sediments is formed in a
comparatively short period. The neoformed clay mineral
together with high calcium contents, is responsible for the
fertility of the earth mounts in contrast to the surrounding
plains.
Acknowledgement
The research project was carried out under the auspices of the agreement
on scientific-technological cooperation signed by the governments of
Germany and Brazil. The German partner was sponsored by the Federal
Ministry of Education and Research (BMBF – Förderkennzahl 0339373B).
Buchas, H., Irion, G., Nunes da Cunha, C., Da Silva, C. J., Kasbohm, J. and Junk, W. J.:
Sedimentological, Geochemical and Geomorphological Studies of Sediment Deposits in the Pantanal/Brazil
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German-Brazilian Workshop on Neotropical Ecosystems – Achievements and Prospects of Cooperative Research
Hamburg, September 3-8, 2000
Session 6: Concepts and Paradigms for Management of Ecosystem Resources
742
Reinhard Lieberei 1, Helmut K. Bianchi 2, Vera Boehm 1, Christoph Reisdorff 1
Editors
1
Universität Hamburg, Institut für Angewandte Botanik, Ohnhorststr. 18, 22609 Hamburg, Germany
2
GKSS-Forschungszentrum Geesthacht GmbH, Max-Planck-Straße 1, 21502 Geesthacht Germany
Layout
Helmut K. Bianchi, GKSS, Karsten Bittner, Documedia, Geesthacht, Germany
Printing
GKSS-Forschungszentrum Geesthacht GmbH, Geesthacht, Germany
ISBN 3-00-010691-X
Lieberei, R., Bianchi, H-K., Boehm, V., Reisdorff, C., (eds.) 2002:
Neotropical Ecosystems, Proceedings of the German-Brazilian Workshop,
Hamburg 2000. GKSS-Geesthacht .
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