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!