 |
| Titel |
Black shale deposition during Toarcian super-greenhouse driven by sea level |
| VerfasserIn |
M. Hermoso, F. Minoletti, P. Pellenard |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1814-9324
|
| Digitales Dokument |
URL |
| Erschienen |
In: Climate of the Past ; 9, no. 6 ; Nr. 9, no. 6 (2013-12-04), S.2703-2712 |
| Datensatznummer |
250085269
|
| Publikation (Nr.) |
 copernicus.org/cp-9-2703-2013.pdf |
|
|
|
|
|
| Zusammenfassung |
| One of the most elusive aspects of the Toarcian oceanic anoxic event
(T-OAE) is the paradox between carbon isotopes that indicate intense global
primary productivity and organic carbon burial at a global scale, and the
delayed expression of anoxia in Europe. During the earliest Toarcian, no
black shales were deposited in the European epicontinental seaways, and most
organic carbon enrichment of the sediments postdated the end of the
overarching positive trend in the carbon isotopes that characterises the
T-OAE. In the present study, we have attempted to establish a sequence
stratigraphic framework for Early Toarcian deposits recovered from a core
drilled in the Paris Basin using a combination of mineralogical (quartz and
clay relative abundance) and geochemical (Si, Zr, Ti and Al) measurements.
Combined with the evolution in redox sensitive elements (Fe, V and Mo), the
data suggest that expression of anoxia was hampered in European
epicontinental seas during most of the T-OAE (defined by the positive
carbon isotope trend) due to insufficient water depth that prevented
stratification of the water column. Only the first stratigraphic occurrence
of black shales in Europe corresponds to the "global" event. This interval
is characterised by >10% Total Organic Carbon (TOC) content
that contains relatively low concentration of molybdenum compared to
subsequent black shale horizons. Additionally, this first black shale
occurrence is coeval with the record of the major negative Carbon Isotope
Excursion (CIE), likely corresponding to a period of transient greenhouse
intensification likely due to massive injection of carbon into the
atmosphere–ocean system. As a response to enhanced weathering and riverine
run-off, increased fresh water supply to the basin may have promoted the
development of full anoxic conditions through haline stratification of the
water column. In contrast, post T-OAE black shales during the serpentinum and
bifrons Zones were restricted to epicontinental seas (higher Mo to TOC ratios)
during a period of relative high sea level, and carbon isotopes returning to
pre-T-OAE values. Comparing palaeoredox proxies with the inferred sequence
stratigraphy for Sancerre suggests that episodes of short-term organic
carbon enrichment were primarily driven by third-order sea level changes.
These black shales exhibit remarkably well-expressed higher-frequency
cyclicities in the oxygen availability in the water column whose nature has
still to be determined through cyclostratigraphic analysis. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|