Quantitative estimates of calcareous nannofossil changes across the Plenus Marls (latest Cenomanian), Dover, England: implications for the generation of the Cenomanian-Turonian Boundary Event

Abstract

Quantitative data on absolute abundance and diversity of nannofossils, insoluble residues, δ 13C and δ 18O from the top Abbots Cliff Chalk, through the Plenus Marls and into the Melbourn Rock at Dover, England suggest, contrary to most previous interpretations, that: (1) The Cenomanian-Turonian Boundary Event was initiated by a sharp fall in sea level which produced the sub-plenus erosion surface, followed by a gradual recovery over 100 000 years to a higher level in the latest Cenomanian, as shown by declining rates of accumulation of terrigenous clastics through the Plenus Marls. (2) Productivity, as reflected by nannofossil abundance and diversity, percentage of Watznaueria barnesae (a measure of dissolution) and by accumulation rates of total carbonate and intact nannofossils, declined through the Plenus Marls and recovered weakly in the Melbourn Rock. None of these trends, nor the previously reported decline in dinocyst abundance (Jarvis et al., 1988b), correlates with lithology and they reflect primary, not post-burial, effects. (3) in the Plenus Marls and basal Melbourn Rock δ 13C and δ 18O show clear, rhythmic fluctuations in phase with lithology which reflect Milankovitch control, superimposed on their major excursions. We suggest that the major δ 13C excursion results from increased rates of burial of organic carbon which sequestered nutrients and caused the decline in productivity observed at Dover, δ 18C values are consistent with a decline in sea water temperature through beds 4-8 of the Plenus Marls.