The Early Aptian migration of planktonic foraminifera to NW Europe: the onset of the mid-Cretaceous plankton revolution in the Boreal Realm

Occurrences of planktonic foraminifera in marine sediments of Early Aptian age (Early Cretaceous) are described from northwest Germany. The distribution patterns are discussed with respect to their abundance and diversity. The data reflect the palaeoceanographic change from a restricted marginal sea with a stratified water column in earliest Aptian times to a hemipelagic, open oceanic, well-oxygenated water setting in the late Early Aptian. The benthic foraminiferal assemblages changed from agglutinated-dominated associations in the earliest Aptian towards calcareous-dominated in the late Early Aptian. Small hedbergellids appear in the Early Aptian Fischschiefer. They are rare within this and the overlying Dark Clays and common within the Hedbergella Marl. Some horizons within the Fischschiefer and Dark Clays yield the planktonic foraminifera generaClaviblowiella , Blowiella and Globigerinelloides. These three genera indicate the first short-term Tethyan influxes. Occurrences of Leupoldina and Lilliputianella, typical Tethyan genera, within theHedbergella Marl support further warm-water influxes. The first appearance of Leupoldina cabri allows correlation with the planktonic foraminiferal zonation of the Tethys. Within the Rethmar section the Blowiella blowi Zone includes the Fischschiefer, the Dark Clays and the base of the Hedbergella Marl. The subsequent L. cabri Zone includes the upper part of the Hedbergella Marl.