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| Titel |
Lake Van carbonates: Implications for lacustrine stable isotope analysis |
| VerfasserIn |
Jeremy McCormack, Adrian Immenhauser, Ola Kwiecien |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250140936
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| Publikation (Nr.) |
 EGU/EGU2017-4391.pdf |
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| Zusammenfassung |
| Carbonate stable isotope (δ18O and δ13C) analysis is a commonly applied and powerful
proxy in lacustrine palaeoclimatology. In the absence of large quantities of detrital
carbonates, the bulk carbonate is assumed to mainly represent inorganic carbonates
precipitated in the epilimnion. In well-preserved, geologically young sediments (e.g.
varves), post-depositional processes affecting the mineralogy or geochemistry of
sedimentary carbonates are difficult to recognise. In case of terminal and alkaline
Lake Van, the interpretation of the δ18O and δ13C signals of bulk carbonates is, in
comparison to other proxies, far from straightforward when relying on traditional
interpretative approaches. Consequently, using a multi-component approach we
studied, individually and in detail, various components comprising Lake Van’s bulk
carbonates.
Samples investigated here cover the last glacial/interglacial period. Inorganic (< 63 μm)
and biogenic (> 63 μm) carbonates were isolated by wet-sieving and analysed by means of
XRD, SEM and isotope mass spectrometry. High-resolution mineralogical analysis revealed
variable amounts of aragonite and calcite as well as early diagenetic non-stoichiometric
(calcian) dolomite. The early diagenetic dolomite appears to be replacing the inorganic
aragonite/calcite and occurs within finely-laminated, organic-rich sediments. Isotopically the
dolomite differs significantly from the primary carbonates with typically heavier
δ18O and lighter δ13C values. Thus, in the case of bulk sediment isotope analysis
the presence of higher amounts of diagenetic dolomite is distorting the isotopic
pattern.
Ostracod valves represent biogenic carbonates. However, apart from well-preserved,
translucent carapaces we have found coated ones, reoccurring throughout the profile within
specific facies types. The coating, comprising mainly of aragonite has a significantly heavier
δ18O and δ13C signature compared to coeval inorganic carbonates, precipitated presumably
in the surface water. Additionally, the confinement of coated ostracod valves to
non-laminated sediments implies oxic bottom-water conditions during the, possibly
microbially-mediated, coating process.
Our results, though preliminary have wide implications for palaeolimnological analysis
and call for caution when using bulk and/or inorganic carbonates. |
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