KFA Juelich, Institut fuer Chemie und Dynamik der Geosphaere, Federal Republic of Germany

The relation between sea-level changes, plankton productivity, and evolution, as well as the occurrence of anoxic sediments, provides an interesting avenue of paleooceanographic research. In this context, we examined mid-Cretaceous radiolarian faunas, carbonate isotopic and organic matter type data of Deep Sea Drilling Project and Ocean Drilling Program cores from the North Atlantic, and samples from outcrops of the western Tethys from central and northern Italy. Former studies indicate that an expansion of the oxygen minimum zone caused plankton extinctions at the Cenomanian-Turonian boundary. An expanded oxygen minimum zone would destroy deeper habitats of planktic foraminifera, causing the extinction of deeper dwelling forms. Although this model is well established for the Cenomanian-Turonian boundary, not much is known about the causes of extinctions and radiations during the entire mid-Cretaceous (Aptian-Turonian). We demonstrate that the dimension of the oxygen minimum zone, which depends on the relative sea level and the corresponding nutrient supply, causes the complex pattern of evolution and radiation of planktic protozoa and the sedimentation of black shales in the mid-Cretaceous. This new depositional model allows correlation of micropaleontologic data and different types of black shales in the pelagic realm within a sequence stratigraphic framework.