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| Titel |
Record of Early Toarcian carbon cycle perturbations in a nearshore environment: the Bascharage section (easternmost Paris Basin) |
| VerfasserIn |
M. Hermoso, D. Delsate, F. Baudin, L. Le Callonnec, F. Minoletti, M. Renard, A. Faber |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1869-9510
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| Digitales Dokument |
URL |
| Erschienen |
In: Solid Earth ; 5, no. 2 ; Nr. 5, no. 2 (2014-08-06), S.793-804 |
| Datensatznummer |
250115326
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| Publikation (Nr.) |
 copernicus.org/se-5-793-2014.pdf |
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| Zusammenfassung |
| In order to understand the significance of the worldwide deposition of black
shale facies in the Early Toarcian (~ 183 Ma), considerable attention
has been drawn to this Early Jurassic sub-stage over the last 3 decades.
The discovery of a pronounced negative carbon isotope excursion (CIE) within
the black shales disrupting the generally positive trend in carbon isotopes
has stimulated many studies, particularly with a view to establish the local
versus global nature of this major geochemical phenomenon. Here we document
the sedimentological and chemostratigraphic evolution of a proximal
environment in the Luxembourgian sedimentary area. At Bascharage, Lower
Toarcian sediments record the isotopic signature of the Early Toarcian
oceanic anoxic event (OAE) by a pronounced positive trend that testifies for
widespread anoxia. The expression of the carbon isotope perturbation in this
section, however, is unusual compared to adjacent NW European sections. A
first −7‰ negative CIE, whose onset is recorded at the top of the
tenuicostatum zone, can be assigned to the well-documented and
potentially global Toarcian carbon isotope excursion (T-CIE) with confidence
using the well-constrained ammonite biostratigraphic framework for this
section. In this interval, facies contain only a limited amount of carbonate
as a result of intense detrital supply in such a proximal and shallow
environment. Stratigraphically higher in the section, the
serpentinum zone records a subsequent CIE (−6‰)
expressed as four negative steps, each being accompanied by positive shifts
in the oxygen isotopic composition of carbonate. The preservation state of
coccoliths and calcareous dinoflagellates in the second CIE is excellent and
comparable to that observed in under- and overlying strata, so this cannot be
an artefact of diagenesis. Considering the nature of this record, and the
lack of such a pronounced event in the serpentinum zone in coeval
sections in Europe, we hypothesise that this second CIE was caused by local
factors. The geochemical record of carbonate with a relatively light carbon
and relatively heavy oxygen isotopic composition is compatible with the
so-called Küspert model, by which a CIE can be explained by an influx of
12C-rich and cold waters due to upwelling bottom water masses. |
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