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| Titel |
The role of soil air composition for noble gas tracer applications in tropical groundwater |
| VerfasserIn |
Simon Mayer, Florian Jenner, Werner Aeschbach, Therese Weissbach, Bernhard Peregovich, Carlos Machado |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250126890
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| Publikation (Nr.) |
 EGU/EGU2016-6681.pdf |
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| Zusammenfassung |
| Dissolved noble gases (NGs) in groundwater provide a well-established tool for paleo
temperature reconstruction. However, reliable noble gas temperature (NGT) determination
needs appropriate assumptions or rather an exact knowledge of soil air composition.
Deviations of soil air NG partial pressures from atmospheric values have already been found
in mid latitudes during summer time as a consequence of subsurface oxygen depletion. This
effect depends on ambient temperature and humidity and is thus expected to be especially
strong in humid tropical soils, which was not investigated so far. We therefore studied NGs in
soil air and shallow groundwater near Santarém (Pará, Brazil) at the end of the
rainy and dry seasons, respectively. Soil air data confirms a correlation between NG
partial pressures, the sum value of O2+CO2 and soil moisture contents. During the
rainy season, we find significant NG enhancements in soil air by up to 7% with
respect to the atmosphere. This is twice as much as observed during the dry season.
Groundwater samples show neon excess values between 15% and 120%. Nearly all
wells show no seasonal variations of excess air, even though the local river level
seasonally fluctuates by about 8 m. Assuming atmospheric NG contents in soil air,
fitted NGTs underestimate the measured groundwater temperature by about 1-2˚ C.
However, including enhanced soil air NG contents as observed during the rainy season,
resulting NGTs are in good agreement with local groundwater temperatures. Our
presented data allows for a better understanding of subsurface NG variations. This is
essential with regard to NG tracer applications in humid tropical areas, for which
reliable paleoclimate data is of major importance for modern climate research. |
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