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Recycling of Amazon floodplain sediment quantified by cosmogenic (super 26) Al and (super 10) Be

H. Wittmann, F. von Blanckenburg, L. Maurice, J. L. Guyot and P. W. Kubik
Recycling of Amazon floodplain sediment quantified by cosmogenic (super 26) Al and (super 10) Be
Geology (Boulder) (May 2011) 39 (5): 467-470


The modern Amazon trunk stream and tributaries contain substantial amounts of sediments that were formerly stored for 1-3 m.y. in floodplains. By reworking of floodplain deposits, this sediment is being incorporated into Amazonian streams. We conclude this from using the ratio of in situ-produced cosmogenic (super 26) Al to (super 10) Be in detrital sediment from rivers draining the sediment-producing Andean, Guyana, and Brazilian shield areas, as well as the largest Amazonian lowland rivers. Samples from the modern Amazon River yield (super 26) Al/ (super 10) Be ratios between approximately 3.8 and 5.5, and thus represent mixing between a non-buried sediment end member and a formerly buried sediment end member. The non-buried end member is sourced in the Andes with a ratio of approximately 6.5, reflecting recent erosion of hillslopes exposed to cosmic rays, and the buried end member is potentially represented by deeply stored floodplain sediment as old as Miocene age, with a ratio of approximately 3, that was partially shielded from cosmic rays. This simple picture of binary mixing is complicated by grain-size effects. Coarser-grained sediment from non-Andean settings (lowlands, cratonic shields) contains slightly more formerly buried material, whereas finer-grained lowland sediment mostly records non-buried (super 26) Al/ (super 10) Be ratios and nuclide concentrations that are indicative of an Andean source. Our assessment shows an incorporation of formerly buried, old sediment of central Amazonian floodplains to the fresh, Andean-derived sediment mainly carried in the Amazon trunk stream. We interpret our results to mean that the Amazon regularly avulses into these old deposits that crop out in more distal vicinities to the main river and temporarily leaves its current Holocene channel.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 39
Serial Issue: 5
Title: Recycling of Amazon floodplain sediment quantified by cosmogenic (super 26) Al and (super 10) Be
Affiliation: GeoForschungsZentrum German Research Center for Geosciences, Potsdam, Germany
Pages: 467-470
Published: 201105
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 25
Accession Number: 2011-046135
Categories: Quaternary geologyIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2011150
Illustration Description: illus. incl. 1 table, sketch map
S04°30'00" - N01°00'00", W73°30'00" - W49°30'00"
Secondary Affiliation: Universite de Toulouse Paul Sabatier, FRA, FranceInstituto de Pesquisa para o Desenvolvimento, BRA, BrazilEidgenoessische Technische Hochschule Zuerich, CHE, Switzerland
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201126
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