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| Titel |
New light on the Metacopina (Crustacea, Ostracoda) Toarcian extinction event: integrated data from the Lusitanian Basin, Portugal |
| VerfasserIn |
M. Cristina Cabral, Isabel M. Loureiro, Susana Pinto, Luís V. Duarte, Ana C. Azerêdo |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048080
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| Zusammenfassung |
| The extinction of the Suborder Metacopina (Ostracoda, Crustacea) has been rather
discussed by several authors (e.g. Lord, 1982; Boomer et al., 2008). The precise age of
this extinction event has been differently defined according to the various authors
and geographical regions, ranging from topmost Pliensbachian to middle part of
the Lower Toarcian (lowermost Falciferum =Serpentinum=Levisoni ammonite
Zone).
In the Lusitanian Basin, West Portugal, Exton (1979) reported the Metacopina extinction at
the end of the Pliensbachian in the Zambujal section (also known as Rabaçal). For this
same section, a more recent study (Boomer et al., 1998) assigns this event to the
lowermost part of the Lower Toarcian (Tenuicostatum=Polymorphum ammonite
Zone).
Based on high-resolution stratigraphic analysis, including ammonite biostratigraphy and
isotopic geochemistry, of the aforementioned locality and also of the Peniche reference
section (GSSP candidate for the base of the Toarcian), the extinction of Metacopina in
Portugal is more accurately constrained in this work. In fact, the detailed study of
the ostracod fauna performed in these two sectors of the basin, clearly shows, in
both sections, that the last Metacopina occurs at the top of the Polymorphum Zone.
Over almost the whole of this Zone, several species of the genera Ogmoconcha
and Ogmoconchella (the most important among the represented Metacopina) are
recorded.
A total of 51 outcrop marl samples were collected in the Lower Toarcian (24 samples in
Rabaçal, 10 from Polymorphum Zone, 14 from Levisoni Zone; 27 samples in Peniche, 11
from Polymorphum Zone, 16 from Levisoni Zone). In both sections the ostracod assemblages
are more or less similar and most of the taxa (42 in Rabaçal; 38 in Peniche) have been
described throughout northwestern Europe. In Polymorphum Zone the assemblages are
abundant and diversified, dominated by Metacopina (at least 9 species in Peniche, 11
in Rabaçal); Liasina lanceolata, Ptychobairdia hahni, Isobythocypris tatei and
Ektyphocythere knitteri are also well represented. In the Levisoni Zone, the abundance and
diversity are very low. A few of the previous taxa continue (e. g. Liasina lanceolata
and Isobythocypris tatei), but most of them disappear and are replaced by new
genera such as Bairdiacypris (very abundant in Rabaçal) and Cytherella (only in
Peniche).
Therefore, in both sections, the last occurrence of Metacopina is safely assigned to the
top of the Polymorphum Zone. Moreover, this extinction is recorded immediately
before the C-isotope negative excursion (Duarte et al., 2007; Hesselbo et al., 2007)
associated with the early Toarcian oceanic anoxic event. In the western Iberian
margin, this geochemical global event is dated from the earliest base of the Levisoni
Zone.
Boomer, I., Ainsworth, N. R. & Exton, J. (1998) – A re-examination of the Pliensbachian and
Toarcian Ostracoda of Zambujal, west-central Portugal. Journal of Micropalaeontology, 17,
(1): 1-14.
Boomer, I., Lord, A. & Crasquin, S. (2008) – The extinction of the Metacopina (Ostracoda).
Senckenbergiana lethaea, 88 (1): 47-53.
Duarte, L. V., Oliveira, L. C. & Rodrigues, R. (2007) - Carbon isotopes as a sequence
stratigraphic tool: examples from the Lower and Middle Toarcian marly limestones of
Portugal. Boletín Geológico y Minero, 118 (1): 3-17.
Exton, J. (1979) – Pliensbachian and Toarcian Microfaunas of Zambujal, Portugal:
Systematic Paleontology. Geological Paper, Carleton University, Ottawa, 79: 1-104.
Hesselbo, S., Jenkyns, H., Duarte, L. V. & Oliveira, L. C. (2007) – Carbon-isotope record of
the Early Jurassic (Toarcian) Oceanic Anoxic Event from fossil wood and marine
carbonate (Lusitanian Basin, Portugal). Earth and Planetary Science Letters, 253:
455-470.
Lord, A. R. (1982) – Metacopine ostracods in the Lower Jurassic. In: Banner, F. T. & Lord,
A. R. (Eds.). Aspects of Micropalaeontology, Allen & Unwin, London: 262-277. |
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