Evidence for annual records of phytoplankton productivity in the Kimmeridge Clay Formation coccolith stone bands (Upper Jurassic, Dorset, UK)

The Upper Jurassic Kimmeridge Clay Formation is a Corg-rich sequence of mudrocks that was deposited in a dysoxic basin, which served, unusually, to preserve primary coccolithophorid proportions. Strikingly contrasting within this dark-coloured formation are the white coccolith stone bands, which contain pristinely preserved coccolithophorid assemblages, preserving potentially primary coccolithophorid abundances, with minimal diagenetic alteration. methodology described herein, scanning electron microscopy (SEM) observation of μm-scale laminae within selected stone bands has revealed five distinctive types of ‘microlamina’, Type 1 being barren clay, Type 2 comprising coccoliths associated with varying amounts of clay or amorphous organic matter, Type 3 being purely coccolithic, Type 4 being of putative microbial origin, and Type 5 containing frequent calcispheres. These are all described and illustrated here. Illustrated sequences of these are discussed with reference to an hypothetical model sequence, based on modern records, representing a Kimmeridgian year of coccolithophorid productivity. Annual signals are thus interpreted. on the observations presented here, and including summaries of the palynomorph records from the stone bands, we comment on watznaueriacean palaeoecology, and particularly the interpretation of Watznaueria fossacincta/barnesiae as an opportunistic, growth-maximising taxon.