Cenomanian (upper Cretaceous) palynology and chemostratigraphy : dinoflagellate cysts as indicators of palaeoenvironmental and sea-level change
A high-resolution (sub-metre) quantitative study of the palynology (dominantly organic-walled dinoflagellate cysts, i.e. dinocysts) and host-rock geochemistry (major-element and stable isotope) is presented for seven Cenomanian sections from NW Europe (Culver Cliff and Folkestone, UK, Vergons and Pont d'Issole, SE France, and Wunstorf, Ratssteinbruch and Gröbern, northern Germany). All sections contained diverse dinocyst assemblages, including 158 dinocyst and acritarch species. Ten informal species/subspecies are recorded, and four transferrals are detailed. A revised palynological processing methodology is presented following a critical review of processing techniques. Correlation of the sections has been carried out using a combination of macrofossil (ammonite) and microfossil (foraminifera), dinocyst, litho-, and chemo- (isotope and elemental) stratigraphy. An improved dina cyst biostratigraphy is presented for the Cenomanian, and events have been correlated between the North Boreal, South Boreal and Tethyan realms. Palaeoenvironmental reconstructions have focused on the Albian/Cenomanian boundary, the mid-Cenomanian and the Cenomanian/Turonian boundary intervals. A significant transgression occurred in Albian/Cenomanian boundary times, though this is represented by a widespread hiatus in many areas. At Vergons, the transgression is represented by a sharp decline in the absolute and relative abundance of Surculosphaeridium longifurcatum over the boundary, and a gradual increase in the abundance of Epelidosphaeridia spinosa. An increase in the abundance of Palaeohystrichophora infusorioides is recorded during the late Early to early Middle Cenomanian in the Anglo-Paris, Vocontian and Lower Saxony basins, and it is suggested that this represents a widespread increase in the supply of nutrients to shelfal areas. In the early Middle Cenomanian, significant regressive and transgressive phases occurred.Typical dinocyst assemblages are dominated by P. tnfusorioides and Spiniferites ramosus ramosus, reflecting eutrophic conditions of the middle shelf. A unique dinocyst assemblage dominated by Oligosphaeridium complex is recorded at Wunstorf, and is thought to reflect the replacement of a mesotrophic to eutrophic middle shelf water mass, by oligotrophic oceanic water mass. The most significant and widespread change to the dinocyst assemblage occurs at the Cenomanian/Turonian boundary interval. Dinocyst absolute abundances record a large decrease, driven by fall in P. infusorioides, which dominates the Middle and Upper Cenomanian in most areas. The reduction in the proportion and abundance of P. infusorioides has been recorded in several basins, indicating the significance of the event. It is suggested that a widespread decrease in the supply of nutrients, associated with rising sea-levels and stagnation of the water column, was the primary cause of the observed changes to the dinocyst assemblages through the Cenomanian/Turonian boundary interval. Sea-level rise and water column stagnation were likely to have been the combined consequence of a lack of strong insolation variability, increased ocean crust production and exceptionally high global temperatures. The primary control on dinocyst assemblages through the Cenomanian are suggested to be sea-level, relative water depth and nutrient availability. Fuzzy cluster analysis of dinocyst assemblage data is a useful tool for reconstructing relative water depth fluctuations. Dinocyst absolute abundance data is believed to primarily reflect the degree of dinoflagellate productivity in the surface-water. Carbonate corrected absolute abundance values up to 10,000 dpg indicate relatively low productivity, with nutrients supplied by terrestrial input and water-column mixing. Peaks in dinocyst absolute abundance reflect periods of transgression or regression, where nutrient supply to the surface-water is increased by erosion of land-locked nutrients and/or by reworking of marine sediments. In areas of upwelling, dinocyst absolute abundances can attain values of over 100,000 dpg indicating very high dinoflagellate productivity. The absolute abundance of P. infusorioides is considered to be a more useful indication of upwelling, compared to the P/G ratio that is based on relative abundance data.