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| Titel |
On the origin and early diagenesis of early Triassic carbonate mud (Dolomites, Italy) |
| VerfasserIn |
Nereo Preto, Hildegard Westphal, Daniel Birgel, Raul Carampin, Jacopo Dal Corso, Giovanni Gattolin, Alice Montinaro, Jörn Peckmann  |
| 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 |
250106852
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| Publikation (Nr.) |
 EGU/EGU2015-6530.pdf |
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| Zusammenfassung |
| The earliest Triassic (early Induan) deposits of the Italian Southern Alps are shallow water
oolites, and lime-mudstone formed in an open shelf (mid to outer carbonate ramp)
sedimentary environment, deposited after the end-Permian extinction that killed all
carbonate producers. The origin of these lime-mudstones is thus enigmatic. We used a
multidisciplinary petrographic and geochemical approach to identify the origin
and early diagenesis of early Triassic lime-mudstones of the Dolomites (Northern
Italy).
This fine carbonate is made of pitted crystals of microsparite, ~ 25 μm in diameter,
exhibiting zonation both in fluorescence and cathodoluminescence. Field and standard
petrographic observations exclude an origin from fragmentation or abrasion of carbonate
grains. Strontium content, measured in-situ with electron microprobe, has a bimodal
distribution with values locally as high as > 4000 ppm.
Lipid biomarker analysis revealed molecular fossils of bacteria (terminally-branched alkanes,
hopanes, and scarce methylhopanes) along with compounds of low source specificity
(n-alkanes), whereas biomarkers of algae (steranes) were not detected. This suggests that,
differently from modern Caribbean shelfs, this fine carbonate did not originate from the
disgregation of green algae. A Pristane to Phytane ratio < 1 also suggests deposition under
anoxic conditions, in agreement with the known status of "superanoxia" of earliest Triassic
oceans.
Overall, our observations suggest an aragonitic mineralogy of the carbonate mud, followed
by calcite replacement and cementation in the marine burial early diagenetic environment.
Our data strongly suggest that the early Triassic carbonate mud of the Dolomites was
precipitated in the water column, similarly to the modern whitings of the Bahamas, and then
settled on a shelf bottom below wave base.
Our study shows that these lime-mudstones contain aragonite replaced by calcite
and calcite cement, in variable proportions. The δ13C of aragonite is expected to
be ~ 2.5‰ higher than calcite precipitated from the same water. Thus, even in
the simplified scenario of carbonate mud replacement in a semi-closed system,
followed by pore occlusion by syntaxial calcite overgrowth, the carbon isotopic
composition of this lime-mudstone cannot be straightforwardly interpreted as a
record of the δ13C of dissolved inorganic carbon (DIC) of early Triassic seawater. |
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