Early Cretaceous chalks from the North Sea giving evidence for global change

In northern Europe and elsewhere chalks are widespread throughout the Cenomanian – Maastrichtian interval. Less well known are, however, early Cretaceous chalks which have been encountered in the Central Graben of the North Sea. These chalks (Hauterivian, Barremian; Tuxen Formation) and the overlying mudstones of the Sola Formation (late Barremian to Aptian) have been studied by us (calcareous nannofossils, geochemistry) in order to better understand the palaeoceanographic setting of the Barremian – Aptian interval in the Boreal Realm. The Barremian chalks were generated by the blooming of endemic nannoconids under relative warm and arid conditions. Among calcareous nannofossils, important primary producers in Jurassic and Cretaceous oceans, nannoconids were carbonate rock-forming organisms. In the Tethys nannoconids went through a crisis during the late Barremian and early Aptian, culminating during the Oceanic Anoxic Event 1a. This decline and subsequent crisis of nannoconids is now seen in the North Sea cores, thereby recording this signal also from the Boreal Realm. The decline of nannoconids in the latest Barremian coincides with increased nutrient and clay input. The nannoconid decline, also detected at low latitudes, was associated with the Ontong Java Plateau emplacement. We conclude that nannoconids were rock forming also at high latitudes, under clear and oligotrophic waters. Their decline was related to increased continental runoff under reinforced greenhouse conditions.