Reconstruction of mid-Cenomanian orbitally forced palaeoenvironmental changes based on calcareous dinoflagellate cysts

Mid-Cenomanian, precession-controlled (21 ka) chalk-marl couplets of the Cap Blanc Nez section (Anglo-Paris Basin) have been studied with focus on the effects which Milankovitch cycles have had on the palaeoenvironment. In this paper, we present micropalaeontological and lithological proxies that enable the reconstruction of both the cycle architecture and the transformation of the orbitally forced signal into the sediment. A palaeoecological reconstruction based on changes in calcareous dinoflagellate cysts (c-dinocysts) assemblages was carried out, in which two characteristic ecological assemblages of c-dinocysts were identified. Gradual changes in absolute and relative abundance of the cyst species in these assemblages over several couplets depict a bundling pattern which is interpreted to reflect the modulation of the intensity of the precession cycle by the eccentricity cycle (100 ka). The stacking pattern in the natural gamma ray signal and the carbonate and TOC content has the same period and provides lithological support of the bundling. A shelf basin circulation model is proposed to explain the relation between orbitally forced climate change, its palaeoenvironmental consequences and the resulting sedimentary cyclicity. Variations in surface water circulation are reflected in the sediment by the chalk-marl couplets, the most distinctive couplets ocurring at the base and top of the bundles. While the chalks reflect well-mixed surface water conditions, the marls, particularly those at the bundle boundaries, can be interpreted as the sedimentary expression of stratified water masses. During deposition of these marls, reduced oceanic mixing due to low seasonality during strong precession maxima at the eccentricity maxima caused periods of water column stratification that in turn led to nutrient depletion and decreased productivity in the surface water masses. (C) 2002 Elsevier Science B.V. All rights reserved.