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| Titel |
Reactive and dissolved meteoric 10Be/9Be ratios in the Amazon basin |
| VerfasserIn |
Hella Wittmann, Nadine Dannhaus, Friedhelm von Blanckenburg, Julien Bouchez, Annette Suessenberger, Jean-Loup Guyot, Laurence Maurice, Naziano Filizola, Jérôme Gaillardet, Marcus Christl |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091760
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| Publikation (Nr.) |
 EGU/EGU2014-6069.pdf |
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| Zusammenfassung |
| Recently, the ratio of the meteoric cosmogenic nuclide 10Be to stable 9Be has been
established as a weathering and erosion proxy where meteoric 10Be/9Be ratios in reactive
phases of secondary weathering products leached from detrital Amazonian river sediment
were measured[1]. For this dataset, we derived a new 10Be-based mass balance,
which compares the fluxes exported during erosion and weathering, Fout, calculated
by the sum of [10Be]reac multiplied by gauging-derived sediment discharge and
[10Be]dissmultiplied by water discharge, to the meteoric depositional flux Fin. This
assessment allows evaluating the weathering state of the Amazon basin. Further, in
order to assess equilibration of reactive phases in the water column, we measured
(10Be/9Be)reac ratios leached from suspended sediments for two depth profiles of
the Amazon (55m depth) and Madeira (12m depth) Rivers, their corresponding
surface dissolved 10Be/9Be ratios, as well as dissolved ratios of smaller Amazon
tributaries (Beni, Madre de Dios) to compare with published reactive ratios[1]. In these
rivers, modest pH and salinity fluctuations help to constrain a “simple” system
that might however still be affected by seasonally changing isotopic compositions
between water and suspended sediment[2] and seasonal fluctuations of TSS and
TDS[3].
The 10Be-based mass balance shows that in Andean source areas Fout/Fin -1, indicating
a balance between ingoing and exported flux, whereas in the Shield headwaters,
Fout/Fin=0.3, indicating a combination of decay of 10Be during storage and little export of
10Be associated with particulate and dissolved loads. In central Amazonia, the export of 10Be
decreases slightly relative to its atmospheric flux as evidenced by Fout/Fin=0.8 for the
Amazon and Madeira Rivers. This value is interpreted as being close to steady state, but its
modification could be due to additions of Shield-derived sediment to sediment carried in the
main river[4].
Regarding the depth profiles, our preliminary findings stress that the (10Be/9Be)reac for
the Amazon River (n=3, Avg.= 5.4x10-10with SD=3.7x10-11) and the Madeira River (n=3,
Avg.= 4x10-10with SD=2.1x10-11) do not change significantly within the water
column. These depth-dependent reactive ratios compare well with 10Be/9Be ratios of
surface waters and sediments and with published data available for the Negro and
Orinoco[5]: For all these large rivers, surface (10Be/9Be)reac vs. (10Be/9Be)dissagree very
well (R2 -1). For smaller tributaries like the Apure, La Tigra, Beni and Madre de
Dios, (10Be/9Be)reacare 2-3 times lower than (10Be/9Be)diss. As pH values are
similar for all these rivers, one possibility is that in smaller river systems mixing of
sediment and water between the channel and the floodplain is less thorough, potentially
resulting in reactive and dissolved phases that are not fully equilibrated. For large
rivers, however, our depth-invariant (10Be/9Be)reac data indicate consistent and
probably early equilibration of Be with depth. We also do not observe potentially
divergent 10Be/9Be ratios due to e.g. floodplain remobilization or different erosion rates
in the source area. From this, we infer a thorough mixing of the clay/silt fraction
within large rivers, with the different 10Be/9Be ratios of Madeira and Amazon Rivers
fingerprinting the different prevailing denudation rates of the source areas (Andes and
Brazilian Shield). The here presented results suggest that one surface sample, either
reactive or dissolved, would be sufficient to determine denudation rates of an entire
catchment.
[1] F. von Blanckenburg et al., EPSL, 351-352 (2012) 295-305.
[2] J. Viers, et al., EPSL, 274 (2008) 511-523.
[3] J. Bouchez, et al., Geochem. Geophys. Geosys. 12 (2011) Q03008.
[4] H. Wittmann et al., Geology 39 (2011) 467-470.
[5] E. T. Brown et al., GCA 56 (1992) 1607-1624. |
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