Seasonal variation in boreal Early Cretaceous calcareous nannofossils

Abstract

The laminated Munk Marl Bed of Barremian, Early Cretaceous, age in the Central Trough of the North Sea consists of alternating laminae of calcareous nannofossils, 15–70 μm thick and clay laminae, 3–15 μm thick. Comparison with laminated marls from the deeper parts of the modern Black Sea suggests that the nannoplankton laminae in the Munk Marl record annual variations in the nannofossils production. The production rate of calcareous nannoplankton in the Munk Marl sea was comparable to that of modern seas. The species composition was determined on 64 bedding planes distributed on three samples, on the basis of SEM photographs of bedding planes. Forty bedding planes, comprising a total of 48 species, were examined in the lowermost sample. The species composition fluctuated strongly and 19 bedding planes were almost monopolized by a single species. At least nine different species occasionally attained complete dominance. Succession of different floras within a single lamina was demonstrated in the uppermost sample. Watznaueria barnesae and Micrantholithus obtusus characteristically dominated in the lower and upper part of the laminae, while the middle part was occupied by other species, for example Axopodorhabdus dietzmannii and Rhagodiscus asper. A nannofossil lamina represents the annual nannoplankton production. Succession within a lamina is, therefore, the result of seasonal variation in the species composition. The same succession could be repeated in several consecutive years, but often a certain pattern was suddenly interrupted giving way to a complete different one.