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| Titel |
Modelling of dead carbon fraction in speleothems: a step towards reliable speleothem 14C-chronologies |
| VerfasserIn |
Franziska A. Lechleitner, Robert A. Jamieson, Cameron McIntyre, Lisa M. Baldini, James U. L. Baldini, Timothy I. Eglinton |
| 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 |
250108724
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| Publikation (Nr.) |
 EGU/EGU2015-8491.pdf |
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| Zusammenfassung |
| Over the past two decades, speleothems have become one of the most versatile and promising
archives for the study of past continental climate. Very precise absolute dating is often
possible using the U-Th method, resulting in paleoclimate records of exceptional resolution
and accuracy. However, not all speleothems are amenable to this dating method for a variety
of reasons (e.g. low U concentrations, high detrital Th etc). This has lead researchers to
exclude many otherwise suitable speleothems and cave sites from further investigation.
14C-dating of speleothems has so far not been applicable, due to the “dead carbon” problem.
As drip water percolates through the karst, dissolving CaCO3, a variable amount of 14C-dead
carbon is added to the solution. This results in a temporally variable and site-specific
reservoir effect, ultimately undermining the development of speleothem 14C -chronologies.
However, a number of recent studies have shown a clear link between karst hydrology
and associated proxies (e.g., Mg/Ca and δ13C) and this “dead carbon fraction”
(DCF). We take advantage of this relationship to model DCF and its changes using
Mg/Ca, δ13C and 14C data from published speleothem records. Using one record
for calibration purposes, we build a transfer function for the DCF in relation to
δ13C and Mg/Ca, which we then apply to other 14C records. Initial model results
are promising; we are able to reconstruct general long-term average DCF within
uncertainties of the calculated DCF from the U-Th chronology. Large shifts in DCF related
to hydrology are also often detected. In a second step, we apply the model to a
speleothem from southern Poland, which so far could not be dated, due to very low
U-concentrations. To construct a 14C chronology, the stalagmite was sampled at 5 mm
intervals. CaCO3 powders were graphitized and measured by Accelerator Mass
Spectrometry (MICADAS) at ETH Zurich. Additional high-resolution (0.1 mm/sample) 14C
measurements were performed on the top 1mm of the stalagmite, using a gas ion source
(GIS) on the MICADAS. The resulting DCF-corrected chronology shows that the
stalagmite grew over much of the Holocene, and reveals that existing U-Th dates
strongly overestimated the real ages and thus are unreliable. In contrast, the new
14C-based chronology, when constrained using trace element ratios and stable isotopes,
enables confident reconstruction of broad climatic changes and trends over the
Holocene. |
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