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| Titel |
Stable carbon isotopes and lipid biomarkers provide new insight into the formation of calcite and siderite concretions in organic-matter rich deposits |
| VerfasserIn |
Lydia Baumann, Daniel Birgel, Michael Wagreich, 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 |
250102726
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| Publikation (Nr.) |
 EGU/EGU2015-2122.pdf |
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| Zusammenfassung |
| Carbonate concretions from two distinct settings have been studied for their petrography,
stable carbon and oxygen isotopes, and lipid biomarker content. Carbonate concretions are in
large part products of microbial degradation of organic matter, as for example by
sulfate-reducing bacteria, iron-reducing bacteria, and methanogenic archaea. For these
prokaryotes certain lipid biomarkers such as hopanoids, terminally-branched fatty
acids (bacteria) and isoprenoids (archaea) are characteristic. Two different types
of concretions were studied: a) Upper Miocene septarian calcite concretions of
the southern Vienna Basin embedded in brackish sediments represented by partly
bituminous calcareous sands, silts and clays; b) Paleocene-Eocene siderite concretions
enclosed in marine, sandy to silty turbidites with varying carbonate contents and
marl layers from the Upper Gosau Subgroup in northern Styria. Calcite concretions
consist of abundant calcite microspar (80-90 vol.%), as well as detrital minerals
and iron oxyhydroxides. The septarian cracks show beginning cementation with
dog-tooth calcite to varying degrees. Framboidal pyrite occurs in some of the calcite
concretions, pointing to bacterial sulfate reduction. Siderite concretions consist
of even finer carbonate crystals, mainly siderite (40-70 vol.%) but also abundant
ferroan calcite, accompanied by iron oxyhydroxides and detrital minerals. The δ13C
values of the calcite concretions (-6.8 to -4.1‰ ) most likely reflect a combination
of bacterial organic matter oxidation and input of marine biodetrital carbonate.
The δ18O values range from -8.9 to -7.8o agreeing with a formation within a
meteoric environment. The surrounding host sediment shows about 1-2‰ higher δ13C
and δ18O values. The siderite δ13C values (-11.1 to -7.5‰ ) point to microbial
respiration of organic carbon and the δ18O values (-3.5 to +2.2‰ ) agree with a
marine depositional environment. In contrast to the calcite concretions, the stable
isotope composition of the host sediment differs significantly from the siderite
concretions. The δ13C values of the Gosau host sediment reflect marine conditions,
whereas the oxygen isotope values are best explained by meteoric overprint. Lipid
biomarkers have been extracted before and after dissolution of the concretions in
order to assess their authenticity and to exclude recent surface contamination. In the
following, only the biomarkers extracted after dissolution are discussed, since they are
thought to be related to concretion formation. The calcite concretions comprise
abundant plant wax derived long-chain n-alkanes, reflecting high terrestrial input.
Bacterial, terminally-branched fatty acids were found, but in overall low abundance.
The siderite concretions did not yield biomarkers due to their high maturity. No
archaeal biomarkers were found in any of the concretions. Considering the presence of
framboidal pyrite, the moderately low δ13C values, and the biomarker inventory,
bacterial sulfate reduction apparently contributed to the formation of the calcite
concretions in a brackish environment. In contrast, ongoing sulfate reduction and resultant
hydrogen sulfide production inhibit siderite precipitation. Therefore, the low δ13C
values of the siderite concretions are best explained by bacterial iron reduction. |
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