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| Titel |
Rainfall Control of Karst Solution and the Inter/Intra Annual Hydrogeochemical Evolution of Cave Dripwater: A Long-term, Site-specific Study, Soreq Cave, Israel |
| VerfasserIn |
Yuval Burstyn, Miryam Bar-Matthews, Avner Ayalon, Alan Matthews |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250121719
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| Publikation (Nr.) |
 EGU/EGU2016-549.pdf |
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| Zusammenfassung |
| Speleothem laminae preserve climate information transferred to the cave via
dripwater. High spatial resolution methods allow in situ measurement of
geochemical and isotopic proxies at seasonal resolution. Existing
hydrogeochemical calibration models suggest that high rainfall inhibits
karst water chemical evolution, resulting in low $\delta $18O values, and
low Mg/Ca and Sr/Ca ratios that are not necessarily correlated. Drought
periods display opposite chemical behaviour owing to lower infiltration
rates and increased Prior Calcite Precipitation (PCP).
This study aims to provide a site-specific, high-resolution hydrogeochemical
calibration for the Soreq Cave. We examine four sites that were continuously
sampled since 1990. Four main rainfall conditions are characterized: very
wet years, average, drought and very dry years. Two sites are fed by 'fast
drips', which only become active after $\sim $250mm rainfall has accumulated
since the beginning of the winter season. Two sites, located deeper in the
cave, are fed by 'slow drips' that are active all year round. Drip rate
measurements identify two main reservoirs -- fissure and matrix -- that
mainly differ in residence time. The $\delta $18O of fissure water is closer
to that of mean annual rainfall ($\sim $-6 \permil\VSMOW), while
matrix values are higher ($\sim $-3.5 \permil\VSMOW). Two main
Sr and Mg sources are identified -- dolomitic bedrock (Mg/Ca $\sim $700
mM/M, Sr/Ca $\sim $0.4 mM/M) and soil leachate (Mg/Ca $\sim $300 mM/M, Sr/Ca
$\sim $1.1 mM/M). Most cave dripwater evolves from $\sim $1:1 soil-bedrock
solution. PCP effect on dripwater solution at each site is estimated by
comparing the ln(Mg/Ca) vs ln(Sr/Ca) linear slope to the PCP slope
calculated using cave specific D(Mg) and D(Sr). Soreq Cave PCP slope is
similar to the global slope of 0.88$\pm $0.13.
The composition and chemical evolution of each reservoir and its
contribution to water influx at each site is primarily governed by annual
effective infiltration. Higher seasonal amplitude in $\delta $18O, Mg/Ca and
Sr/Ca in all sites is positively correlated to increase in rainfall. For the
deeper site, with rock cover of $>$40 m, the `classic' wet-dry model can be
applied - more soil input and less PCP in the wetter years and vice-versa.
Conversely, in the shallower sites, high PCP is observed in wetter years.
Results from this study are compared with high-resolution $\delta $18O and
trace element records of modern speleothems (age $\sim $20y). The speleothem
from the deeper site shows a good match with the hydrogeochemical data, thus
supporting the applicability of the model to palaeoclimate studies. However,
the speleothem from the shallow site shows a strong winter bias, which may
be due to complete secession of summer drip during dry years
(micro-hiatuses), or during wetter years, considerable winter calcite
precipitation resulting in minimal summer imprint on each seasonal lamina.
We plan to analyse a fast growing modern sample from the shallow site to
resolve this seasonal bias. Therefore, contemporaneous speleothem records
from different sites can be utilized to estimate past changes in annual and
decadal effective infiltration, allowing evaluation of water availability in
the region during periods of rapid climate change.
[1] Orland, I.J. et al. 2014. Chemical Geology, v. 363, p.
322--333. |
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