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| Titel |
Impact of flood events on lacustrine carbonate isotope records |
| VerfasserIn |
Lucas Kämpf, Birgit Plessen, Stefan Lauterbach, Carla Nantke, Hanno Meyer, Bernhard Chapligin, Hannes Höllerer, Achim Brauer |
| 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 |
250127890
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| Publikation (Nr.) |
 EGU/EGU2016-7814.pdf |
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| Zusammenfassung |
| Stable oxygen ($\delta ^{18}$O) and carbon ($\delta ^{13}$C) isotope
compositions of lacustrine carbonates are among the most frequently used
proxies in palaeolimnological / -environmental studies. Stable isotope
analyses are often carried out on bulk carbonate samples, which are prone to
contamination with detrital carbonates, transported into the lake by runoff
processes and carrying the isotopic signal of catchment rocks, thus
hampering the interpretation of the data in terms of past climatic and/or
environmental changes. Despite the awareness of a likely detrital bias, the
degree of contamination in most cases remains unknown and discrete
contaminated samples undetected due to a lack of methods to disentangle
endogenic and detrital carbonates in sediment records.
To address this issue and provide more comprehensive insights into effects
of flood-related detrital input on the bulk carbonate isotopic composition,
we conducted stable isotope measurements on sediments trapped on a 3--12 day
basis over a three-year period (January 2011 to November 2013) at two
locations in pre-Alpine Lake Mondsee, close to the inflow of the main
tributary and in the deepest part of the lake basin. Lake Mondsee was chosen
for the monitoring since the pelagic sediments are annually laminated
consisting of couplets of light calcite layers and dark layers made up by a
mixture of detrital clastic and organic matter.
Maximum calcite flux rates $>$1.5 g m$^{2}$ d$^{-1}$ were trapped between
May and September, indicating the seasonal endogenic precipitation of
calcite crystals. The comparison of the $\delta ^{18}$O composition of
trapped carbonates, rain and epilimnion lake water revealed equilibrium
calcite precipitation, allowing us to infer purely endogenic $\delta
^{18}$O (-9 to -11.3\permil\ VPDB) and $\delta ^{13}$C
values (-6 to -9\permil\ VPDB) throughout the summer season. The
endogenic calcite precipitation was interrupted by 14 peaks in carbonate
flux (4 to 175 g m$^{2}$ d$^{-1})$ triggered by runoff events of different
magnitudes (10-110 m$^{3 } $s$^{-1}$ peak runoff). The effect of these events
on mean seasonal $\delta ^{18}$O (0 to +3.5\permil\) and
$\delta ^{13}$C composition (0 to +3.6\permil\) is closely
related to sediment influx and, therewith, to runoff magnitude. However,
while the strongest floods could be detected in the sediment record as
discrete detrital layers (proximal: 2 events $>$50 m$^{3}$ s$^{-1}$, distal:
1 event $>$80 m$^{3}$ s$^{-1})$, lower magnitude floods ($>$30 m$^{3}$
s$^{-1})$ provide a 'hidden' detrital bias of 0 to 0.5\permil\. Evidence for detecting even this 'hidden' bias in sediment records is given
by the linear relation between detrital carbonate content and the isotopic
composition, resulting in a strong cross-correlation between $\delta
^{18}$O and $\delta ^{13}$C for samples containing $>$5{\%} detrital
carbonate whereas no relation was observed for purely endogenic samples.
Hence, cross-correlation between $\delta ^{18}$O and $\delta ^{13}$C in
bulk carbonate samples serves as a reasonable indicator for contamination
with detrital carbonates, applicable in sediment records from hydrologically
open lakes where evaporative effects on stable isotopes are negligible. |
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