The ostracod response to Cenomanian seal evel, food supplyand

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					     The ostracod response to Cenomanian seal evel, food supply and
  oxygenation changes in the Tarfaya upwelling region, southern Morocco



                         Holger Gebhardt and Irene Zorn


                          Geologische Bundesanstalt, Wien




Many thanks to Wolfgang Kuhnt and Ann Holbourn (Kiel), Hiroshi Kawamura (Sapporo),
   Sadat Kolonic (Bremen), Prof. Chellai (Marrakesh) and A. El Albani (Poitiers) for
technical support, logging, sampling, geochemical data, sedimentological interpretation
                                     etc. etc. etc.....
                    last tempestites




non-marine facies
1, 4, 7: Brachycythere sapucariensis

2, 5:   Brachycythere ekpo

3:      Brachycythere angulata

6:      Brachycythere cf. oguni

8, 11: Nigeroloxochoncha sp.

9, 12: Xestoleberis sp.

10:     Cytherelloidea cf.melleguensis

13, 16: Veenia sp. 2

14:     Cythereis sp.

15, 18: Veenia cf. rahhali

17:     Veenia sp. 1

 Scale bars: 0.1 mm
1, 2, 4: Reticulicosta tarfayaensis

3, 5-9: Veenia rotunda


  Scale bars: 0.1 mm
Questions:

Do Cretaceous ostracods have a value for the paleoenvironmental interpretation
of upwelling areas?

Can indicator species or assemblages be identified?

Which paleoenvironmental parameters do they indicate?


Problems:

There are no modern analogues.

Calibrations and transfer functions do not extist (yet).


Possible solutions:

Comparison of assemblage counts with other, i.e., better known proxies or taxa.

Application of multivariate statistics.
Conclusions:

-Assamblages of upwelling areas may be characterised by low diversities
 (sometimes monospecific).

- Assemblages at Tarfaya represent a depth gradient, Axis 1 of CA

-Successive predominance of Veenia, Brachycythere, Reticulicosta and Nigeroloxoconcha
 Indicates the progressive deepening of the environment

- Ostracods in general show highest frequencies with
  - moderate food supply and sufficient oxygenation
  - high food supply and high oxygenation.

- Brachycythere is more sensitive to oxygen deficiency than other genera.

- Reticulicosta and Cytherelloidea seem to avoid food pulses but may survive better than
  others at the transition to oxygen deficiency.


Therefore:

Yes, ostracods may be useful environmental indicators in Cretaceous upwelling areas!

				
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posted:8/11/2010
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