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| Titel |
Tracing climatic conditions during the deposition of late Cretaceous-early Eocene phosphate beds in Morocco by geochemical compositions of biogenic apatite fossils |
| VerfasserIn |
L. Kocsis, E. Gheerbrant, M. Mouflih, H. Cappetta, J. Yans, A. Ulianov, M. Amaghzaz |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250066634
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| Zusammenfassung |
| Morocco’s Western Atlantic coast was covered by shallow seas during the late
Cretaceous-early Eocene when large amount of phosphate rich sediments were deposited.
This time interval envelops a major part of the last greenhouse period and gives the
opportunity to study the event’s characteristics in shallow water settings.
These phosphate deposits are extremely rich in vertebrate fossils, while other types of
fossils are rare or often poorly preserved. Hence the local stratigraphy is based on the most
abundant marine vertebrate fossils, on the selachian fauna (sharks and rays). Our geochemical
investigations were also carried out on these remains, though in some cases frequently found
coprolites were involved as well. The main goal of our study was to test whether stable
isotope compositions (δ18OPO4, δ13C) of these fossils reflect any of the hyperthermal events
and/or the related perturbations in the carbon cycle during the early Paleogene (Lourens et al.
2005) and whether these geochemical signals can be used to refine the local stratigraphy.
Additionally, the samples were analyzed for trace element composition in order to better
assess local taphonomy and burial conditions. The samples came from two major
phosphate regions, the Ouled Abdoun and the Ganntour Basins and they were collected
either directly on the field during excavations (Sidi Chennane) or were obtained
from museum collections with known stratigraphical position (Sidi Daoui, Ben
Guerrir).
The phosphate oxygen isotopic compositions of shark teeth display large range across the
entire series (18.5–22.4 ) which can partly be related to the habitat of sharks. For instance the
genus Striatolamnia often yielded the highest δ18O values indicating possible deep water
habitat. Despite the large variation in δ18O values, a general isotope trend is apparent. In the
Maastrichtian after a small negative shift, the δ18O values increase till the Danian
from where the trend decrease till the Ypresian. The latter negative shift can be
linked to the globally recognized Early Eocene Climatic Optimum (Zachos et al.,
2001).
In terms of carbon isotopic composition, shark teeth enameloid yielded often positive
δ13C values, while dentine are always negative and sometimes following clear trend along the
series. Coprolites have similar values to dentine, however they display greater variation
reflecting the burial milieu and the special environment of phosphatization with the intensive
organic matter recycling. Bone-beds show even more variations that could be caused by
reworked specimens and also possible enhanced oxidation of organic matter at these levels.
Nevertheless, the Sidi Chennane section shows a negative δ13C trend in the early Ypresian,
which is compatible with global observations at the time. Moreover, the lowest δ13C values
are from the transitional layer between the Ypresian and Thanetian beds which
might relate to the Paleocene-Eocene boundary event, though it must be further
confirmed.
All the fossils display very similar rare earth element (REE) distribution that resembles
typical seawater pattern with negative Ce-anomaly and heavy REE enrichment.
However the large amount of analyses revealed a general drift in the magnitude of the
Ce-anomaly from the older to younger beds that can be used in paleoenvironmental
reconstruction.
References
Lourens et al. 2005. Nature, 435, 1083–1087.
Zachos et al. 2001. Science, 292, 686–693. |
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