Intrinsic processes control late Turonian calcareous dinoflagellate cyst assemblages – A case study from the Sussex chalk (England)

Abstract

Changes in late Turonian calcareous dinoflagellate cyst assemblages (c. 6400 specimens from 45 samples of the 27 m-thick Bridgewick section; Sussex, southern England) were plotted against four sedimentary sequences to test its response to eurybatic sea level changes. Only seven taxa occur, and Pithonella sphaerica, P. ovalis and P. spiralis dominate the flora (c. 98%). Pithonella cardiiformis, P. discoidea, N. circumperforata and Pirumella sp. are rare. The composition is identical to that of Salzgitter-Salder (NW Germany). The ratio P. sphaerica/P. ovalis (Ps/Po ratio) shows positive peaks during eurybatic sea level lows (max. 1.6) and negative peaks during maximum flooding (min. 0.2). The cumulative c-dinocysts composition, CCC (P. sphaerica, P. ovalis, P. spiralis, others), shows a proportional decrease of P. sphaerica and P. ovalis during eurybatic sea level falls and predominance of P. ovalis during sea level high. The species richness curve shows lowest species numbers in the context of peak transgression. Higher species numbers are associated with lower eurybatic sea levels. In conclusion, the Ps/Po ratio, the CCC and species richness in conjunction aid the recognition of eurybatic sea level changes. Due to the remote position of the working area on the well-oxygenated, drowned Turonian shelf, we exclude variations of external nutrient input, salinity, temperature and oxygen changes as a possible trigger for the observed floral changes. Instead, we suggest that intrinsic mechanisms drive the floral dynamics. In the sense of the intermediate disturbance hypotheses, the progressive increase of P. sphaerica, P. spiralis and other during progressive eurybatic sea level fall is triggered by higher disturbance frequencies (episodic water turbulence, resuspension events, increased internal cycling rate of Corg); vice versa, maximum flooding is characterized by lower disturbance frequencies (decrease of episodic turbulences, less internal cycling) and the progressive predominance of the optimal competitor, which is P. ovalis. The presence of seafloor topography (intra-shelf depressions and swells) could have created downslope currents where parautochthonous assemblages would be expected and could explain the offset of depositional dynamics at the seafloor leading to changes in the floral assemblages.