The Lower Cretaceous Munk Marl Bed in the central North Sea area consists of alternating laminae of calcareous nannofossils and clayey material. The lamination was most likely caused by seasonal variation in the production rate of calcareous nannofossils. Calcareous nannofossil deposition reflects short blooms. Many blooms were totally dominated by a single species, and at least nine species occasionally monopolized the flora. The production rate and mode of deposition is comparable to that of the modern Black Sea. The Munk Marl Bed contains a diverse marine calcareous nannofossil flora comprising 48 species. The species composition exhibits changes that show similarities to the seasonal fluctuations in the modern Atlantic Ocean.