Sea-level changes and organic-walled phytoplankton response in a Late Albian epicontinental setting, Lower Saxony basin, NW Germany

Abstract

Samples from a 245 m long core from the Upper Albian of NW Germany have been analysed palynologically. In accordance with the concept of ‘sequence palynology’ of Prauss (1993), two completed cycles of 3rd order sea level change are inferred, superimposed by a 2nd order cycle. During the 3rd order cycles the response of both the ratio of terrigenous sporomorphs to marine palynomorphs ( t/ m index) and dinoflagellate diversity shows a gradual decrease within the ‘transgressive systems tracts’. The quantitative response of certain dinoflagellate taxa, however, is different within each 3rd order cycle. The abundance of skolochorate cysts increases strongly only during the earlier cycle. This difference is suggested to be a function of the 2nd order cycle, which is transgressive during most of the earlier 3rd order cycle but regressive throughout the later one. The layer at about 135 m is interpreted to represent the ‘maximum flooding surface’ of the 2nd order cycle. The succession in peak abundance of the dinoflagellate genera Oligosphaeridium→ Surculosphaeridium→ Pterodinium may characterise a ‘transgressive systems tract’ to ‘early highstand systems tract’ interval during a 3rd order cycle. However, the proportions of Surculosphaeridium respond much more distinctly to the 2nd order sea level changes, being highest during the corresponding ‘maximum flooding surface’. Higher proportions of Palaeoperidinium cretaceum may indicate enhanced primary productivity and/or estuarine circulation during transgressive and advanced ‘late highstand’ conditions. With respect to water mass characteristics within the photic zone, no clear dominance of Tethyan or Arctic waters is distinguishable, based on dinoflagellate taxa. A rather mixed water mass dominated by cosmopolitan and ‘Boreal’ taxa seems to be present throughout most of the section. The influence of warmer water masses, largely indicated by the proportion of skolochorate cysts, is enhanced during each transgressive interval of the 3rd order cycles, but it is less well developed within the ‘transgressive systems tract’ at the top of the section. It is suggested that a strong Tethyan influence, in terms of water mass introduction into the NW German basin, was predominant only during the transgressive pulse of the 2nd order cycle. Above 135 m the Tethyan or warm water influence is considered largely as a function of oceanic circulation rather than sea level rise, but is episodically enhanced during the relatively weak pulses of 3rd order transgressions. Certain dinoflagellate taxa, known from the Upper Albian Arctic region ( Vesperopsis mayii, Luxadinium primulum, L. propatulum, Luxadinium cf. L. primulum) are absent.