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Titel |
Linking the isotopic composition of monthly precipitation, cave drip water and tree ring cellulose - 15 years of monitoring and data-model comparison |
VerfasserIn |
Inga Labuhn, Dominique Genty, Valérie Daux, François Bourges, Georg Hoffmann |
Konferenz |
EGU General Assembly 2013
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 15 (2013) |
Datensatznummer |
250076106
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Zusammenfassung |
The isotopic composition of proxies used for palaeoclimate reconstruction, like tree ring
cellulose or speleothem calcite, is controlled to a large extent by the isotopic composition of
precipitation. In order to calibrate and interpret these proxies in terms of climate, it is
necessary to study water isotopes in rainfall and their link with the proxies’ source
water. We present 10 to 15-year series of stable hydrogen and oxygen isotopes in
monthly precipitation from three sites in the south of France, along with corresponding
REMOiso model simulations, a monitoring of cave drip water from two of these
sites (Villars cave in the south-west and Chauvet cave in the south-east), as well as
measurements of oxygen isotopes in tree ring cellulose from oak trees growing in the same
area.
The isotopic composition of monthly precipitation at the three sites displays a typical annual
cycle. At the south-west sites, under Atlantic influence, the interannual variability is much
more pronounced during the winter months than during the summer, whereas the
south-eastern Mediterranean site shows the same variability throughout the year. The model
simulations are able to reproduce the annual cycle of monthly precipitation δ18O as well as
the intra-seasonal variability. Compared to the data, however, the modelled average
isotopic values and the seasonal amplitude are overestimated. Correlations between
temperature and precipitation δ18O are generally weak at all our sites, on both the
monthly and the annual scale, even when using temperature averages weighted
by the amount of precipitation. Consequently, a proxy which is controlled by the
δ18O of precipitation cannot be directly interpreted in terms of temperature in this
region.
The isotopic composition of cave drip water in both caves remains stable throughout the
monitoring period. By calculating different weighted averages of precipitation δ18O
for time periods ranging from months to years, we demonstrate that the cave drip
water isotopic composition is the result of several years of rainfall mixing. The
precipitation of every month must be considered in order to attain the drip water
values, which means that rain water infiltrates throughout the year. There is no
modification of the soil water isotopic composition by evaporation and no seasonal bias
introduced by transpiring plants; they use water from reserves which represents
several months or years of mixing. For the interpretation of tree ring cellulose δ18O,
this implies that – at least for the monitoring period of 15 years – the source water
signal is more or less constant. Therefore, the variability of cellulose δ18O must be
mainly due to evaporation at the leaf level, which is strongly dependent on summer
temperature.
Insights on the variability and temperature correlations of stable isotopes in precipitation and
on the origin and composition of cave drip water are important for the interpretation
of proxies. Long-term monitoring is needed for model validation, and the locally
validated and corrected model can provide longer time series for a reliable proxy
calibration. |
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