Marine microplankton and calcareous nannofossil palaeoecology of the Toarcian stratotype

Abstract

In the Bifrons to Aalensis Ammonite Zones of the Vrines section (Toarcian stratotype), woody phytoclasts, nonsaccate pollen grains, and marine assemblages (dinoflagellate cysts, acritarchs, and foraminiferal linings) dominate the palynofacies. The dinoflagellate cyst assemblage is cosmopolitan with minor Boreal influences, characterized by relatively high quantities of Micrhystridium, Baltisphaeridium, Mendicodinium spinosum, Nannoceratopsis, and the Parvocysta suite, dominating in turn the marine assemblages. Marine assemblage compositions, both dinoflagellate cysts and acritarchs, and calcareous nannofossil abundances are different in marl and limestone lithotypes of the Vrines section. Calcareous nannofossils are generally more abundant in marls than in limestones, they display however a cyclic pattern of semiquantitative abundances in phase with lithological cycles. Although a diagenetic overprint cannot be completely excluded to explain such a difference, it seems likely that these dissimilarities are in part primary, the results of variations in terms of proximality–distality, and climatic fluctuations. A mean duration of 117.6 Kyr per marl–limestone alternation, and the stacking of four marl–limestone alternations for 470.6 Kyr, suggest a control by the Earth's two orbital eccentricity cycles. It is likely that the palaeoenvironmental conditions, which influenced the formation of marl–limestone alternations, also controlled the variations in marine phytoplankton assemblages.