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| Titel |
A millennial hydrogen isotope chronology from tree-ring cellulose contradicts the mechanistic model describing the incorporation of stable water isotopes into cellulose |
| VerfasserIn |
Sarah Hangartner, Anne Kress, Matthias Saurer, Markus Leuenberger |
| 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 |
250040747
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| Zusammenfassung |
| Abstract
In the present study we investigated deuterium (δD) isotopes on a millennial larch (Larix
decidua) tree–ring chronology from alpine sites in Valais, Switzerland. Cellulose in annual
tree rings is formed from atmospheric CO2 and soil water which is eventually derived
from meteoric water. Due to fractionation processes in the atmosphere, meteoric
water contains a temperature signal. Climate induced signals such as the isotopic
composition of cellulose is stored in annual increments of trees: source water is
assimilated by the roots without any isotopic fractionation and further on transported to
the leafs where isotopic ratios of the leaf water are changed by evapotranspirative
enrichment and further biochemical fractionations. Finally, cellulose is synthesized from
photosynthates and medium water. δD and stable oxygen isotopes (δ18O) in tree–ring
cellulose are therefore expected to reflect ancient humidity and temperature in annual
resolution. We applied a conventional isotope ratio mass spectrometry technique
to analyse δD in α–cellulose (Filot, 2006). The investigated δD series cover the
period 1000–2004 AD in three cohorts (each consisting of five trees) with a 50–year
gap around 1200 AD. This required the development of methods to merge these
tree–ring isotope series to assess the common signal within the different cohorts. A
comparison of the δD series with the corresponding δ18O chronology revealed a
common variance of around 20% in the different cohorts, which is lower than expected
from the mechanistic model (Roden, 2000) – the model assumes similar pathways
and fractionation processes of δD and δ18O from source water uptake to cellulose
synthesis. Assessing the sensitivity of δD to changing climate variables leads to
conflicting results: while temperature, sunshine duration and precipitation signals
in δ18O are clearly visible, climate signals in δD are hardly detectable. Note that
isotopic signals in tree–ring cellulose are not controlled by one dominating factor
but are usually a combination of several climate variables such as temperature,
relative humidity or precipitation. Further quantified meteoric data such as relative
humidity, barometric pressure or wind speed are not imprinted in the δD series neither
and δD from meteoric water from a proximate meteo station reveals no significant
correlations with δD from cellulose in the period 1984–2004 AD. These results lead to the
conclusion that δD and δ18O fractionation processes in trees differ — undiscovered
biochemical fractionations in δD after leaf water enrichment are likely to account for
deviating signals in the water isotopes of cellulose. The lack of climate signals in our
millennial δD series raises the questions if (a) a more detailed analysis method
concerning the different positions of hydrogen isotopes in cellulose molecules such as
suggested by Augusti et al. (2008) would be more appropriate to detect climate
signals in δD or (b) not climatically induced δD fractionation processes in trees are
superimposing the climate signal from source water and leaf water enrichment in such
a way that the original climate signals can not be retrieved from δD in tree–ring
cellulose.
References
Augusti, A., Betson, T.R. and Schleucher, J. (2008), Chemical Geology.
Filot, M. (2006), Rapid Commun. Mass Spectrom..
Roden, J.S. (2000), Geochimica et Cosmochimica Acta. |
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