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| Titel |
The Himalayas of Nepal, a natural laboratory for the search and measurement of CO2 discharge |
| VerfasserIn |
Frédéric Girault, Bharat P. Koirala, Mukunda Bhattarai, Sudhir Rajaure, Patrick Richon, Frédéric Perrier |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035593
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| Zusammenfassung |
| Large CO2 flux has been found in the Trisuli Valley, North of Kathmandu, Central Nepal, in
2005. This leakage zone is located in the vicinity of the Syabru-Bensi hot springs, and is
characterized by an average flux of CO2 of 6500±1100 g m-2 day-1 over an area of 15 m Ã
15 m (Perrier et al., Earth and Planetary Science Letters, 2009). The site is also located close
to the Main Central Thrust Zone (MCT Zone), one of the large Himalayan thrust, connected
at depth to the Main Himalayan Thrust, the main thrust currently accommodating the
India-Tibet collision (Bollinger et al., Journal of Geophysical Research, 2004). Isotopic
carbon ratios (δ13C) indicate that this CO2 may come from metamorphic reactions at about
15 km of depth (Becker et al., Earth and Planetary Science Letters, 2008; Evans et al.,
Geochemistry Geophysics Geosystems, 2008). Actually, this zone was originally found
because of the large δ13C found in the water of the hot springs suggesting degassing (Evans
et al., Geochemistry Geophysics Geosystems, 2008). In 2007, another zone of CO2 discharge
was discovered 250 m away from the main Syabru-Bensi hot springs. This new zone, located
next to the road and easy to access all over the year, was intensely studied, from
the end of 2007 to the beginning of 2009. In this zone, an average value of CO2
flux of 1700±300 g m-2 d-1 was obtained over an area of about 40 m à 10 m.
Using CO2 flux data from repeated measurements, similar flux values were observed
during the dry winter season and the wet summer period (monsoon) (Girault et al.,
Journal of Environmental Radioactivity, 2009). Thus, in addition to fundamental
issues related to global CO2 balance in orogenic belts and tectonically active zones,
these small scale (100-meter) CO2 discharge sites emerge as a potentially useful
laboratory for detailed methodological studies of diffusive and advective gas transport.
Recently, the search for further gas discharge zones has been carried out using various
clues: the presence of a hot spring with high δ13C, of H2S smell, of hot spots in
thermal images, of a geological contact, of self-potential anomalies (Byrdina et al.,
Journal of Geophysical Research, 2009) or of large radon-222 flux. Preliminary
results about the failures or successes of the various methods will be given in the
Trisuli and Langtang valleys (Central Nepal), in the Kali Gandaki valley (Western
Nepal) and in the Thuli Bheri valley (Lower Dolpo, Far Western Nepal). These
various sites also offer an opportunity to test the optimal estimation of total CO2
flux, using the least amount of experimental measurements. Preliminary results
complemented by simulations will also be given on the total CO2 flux. In parallel,
monitoring methods are being studied in the Syabru-Bensi pilot site. First, CO2 flux has
been studied as a function of time using repeated measurements. Furthermore, the
high radon content of the geological CO2 allows cost-effective monitoring using
BARASOL-¢ probes. More than two years of data are already available and give hints on
the use of radon to follow CO2 discharge as a function of time. These first results
show how experimental studies carried out in natural discharge zones provide a rich
laboratory to test the methodological approaches useful for CO2 leakage and monitoring. |
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