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| Titel |
The nature of annual lamination in flowstones from non-karstic fractures, Vinschgau (northern Italy) |
| VerfasserIn |
Gabriella Koltai, Christoph Spötl, Hai Cheng |
| 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 |
250127952
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| Publikation (Nr.) |
 EGU/EGU2016-7884.pdf |
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| Zusammenfassung |
| The Vinschgau is an inneralpine valley in the Southern Alps. The region is built up by
metamorphic rocks characterised by a high degree of tectonic deformation. Although karst is
not known in the Vinschgau, calcite and calcite-aragonite flowstones are deposited from
supersaturated groundwater along the South-facing mountain slope as a result of strong
evaporation (Spötl et al. 2002).
Flowstone precipitation is strongly connected to fracture openings created by deep-seated
gravitational slope deformations. The carbonate-depositing springs are part of an extended
groundwater system controlled by the geometry of the deep-seated gravitational slope
deformation. Although the mean residence time of the groundwater is up to several decades
(Spötl et al. 2002), a few flowstones show macroscopically visible laminae whose annual
origin is confirmed by U-Th dating.
These laminae are composed of a darker and a lighter sublamina forming couplets whose
thickness ranges from 0.2 to 2 mm. In thin section, the darker sublaminae show a
higher abundance of opaque particles, whereas the light ones are inclusion-poor.
Strong epifluorescence confirms the organic origin of these dark inclusions. The
crystal fabric, dominated by the fascicular-optic type, shows no change across lamina
boundaries.
Laminated calcite shows δ18O oscillations with an amplitude of up to 1.4 permil. These
oscillations are also present in unlaminated calcite, albeit at much smaller amplitude. δ13C
lacks such a regular pattern in laminated samples, and only shows small variations which do
not correlate with δ18O in a consistent way. Changes in δ13C show smaller amplitudes than
δ18O. The extent of correlation between petrographic laminae and the stable isotopes varies
both in time and space. During the late Younger Dryas darker sublaminae mostly coincide
with δ18O lows, whereas in the Mid-Holocene they usually correlate with isotope
highs.
These data reflect the high degree of heterogeneity of such fracture zones in deep-seated
gravitational slope deformations. The annual signal is still preserved by the δ18O values, but
the development of laminae varies both in space and time.
Spötl, C., Unterwurzacher, M., Mangini, A., Longstaffe, F.J., 2002: Carbonate
speleothems in the dry, inneralpine Vinschgau Valley, northernmost Italy: Witnesses of
changes in climate and hydrology since the Last Glacial Maximum. Journal of Sedimentary
Research, 72, 793–808. |
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