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| Titel |
40K-40Ca systematics as a Tracer of Silicate Weathering: A Himalayan case study |
| VerfasserIn |
Jesse Davenport, Guillaume Caro, Christian France-Lanord |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250106510
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| Publikation (Nr.) |
 EGU/EGU2015-6185.pdf |
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| Zusammenfassung |
| This study investigates the use of the 40K-40Ca system as a tracer to better quantify the
contributions of silicate and carbonate lithologies in the dissolved load of major Himalayan
rivers. Previous work using Sr isotopes as a proxy for silicate weathering has been
complicated by the redistribution of radiogenic 87Sr between silicate and carbonate
lithologies, particularly in the Lesser Himalaya, where dolomites exhibit 87Sr/86Sr ratios as
high as 0.85. The 40Ca signature of carbonates, on the other hand, appears to be remarkably
resistant to metamorphism and dolomitization [1]. It was therefore anticipated that the
40K-40Ca system could circumvent issues associated with such secondary events, and yield
more robust constraints on the relative contribution of silicate vs. carbonate lithologies in
dissolved river loads.
The main difficulty in applying the 40K-40Ca decay scheme as a tracer lies in the
analytical precision required to measure small variations (~1 É-unit) on the large
40Ca isotope (96.9%). This difficulty can now be overcome using the Finnigan
Triton TIMS, which allows measurements of the 40Ca/44Ca ratio with external
precision of 0.35 É-unit in multidynamic mode. Using this method, we generated
high-precision 40Ca data on carbonates/dolomites, bedload sediments, dissolved load, and
clay samples originating from and representing the main litho-tectonic units of the
Himalaya. Our results show that metamorphosed dolomites from the Lesser Himalaya
(LH) exhibit no radiogenic 40Ca excess despite highly variable 87Sr/86Sr signatures
(0.73-0.85). Thus, all Himalayan carbonates appear to be characterized by a homogeneous
É40Ca=0. In contrast, silicate material is radiogenic, with É40Ca averaging +1 in the
Tethyan Sedimentary Series (TSS), +1.6 in the High Himalaya crystalline (HHC)
and +4 É-units in the LH. Results obtained from a series of 35 Himalayan rivers
(including the Brahmaputra, Ganga and its main tributaries) show that É40Ca in
the dissolved load is significantly influenced by silicate weathering, with É40Ca
ranging from +0.1 in rivers draining carbonate dominated catchments to +1.6 É-units
in rivers draining predominantly gneissic catchments of the High Himalaya. No
simple relation exists between 87Sr and 40Ca systematics, which likely reflects the
decoupling of Rb-Sr and K-Ca systems in LH dolomites. In contrast, 40Ca signatures
correlate well with proxies of carbonate weathering such as Ca/Na or Mg/Na ratios.
Overall, our results indicate that the 40Ca signature of Himalayan rivers primarily
reflects the lithological nature of their erosional source, and highlight the significant
contribution of HHC gneisses to the dissolved calcium budget of the Ganga and
Brahmaputra.
[1] Caro et al. (2010) EPSL 296, 124-132 |
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