The response of marine phytoplankton and sedimentary organic matter to the early Toarcian (Lower Jurassic) oceanic anoxic event in northern England

Abstract

Early Toarcian organic-rich sediments, reflecting the Lower Jurassic oceanic anoxic event, were investigated in the Brown Moor Borehole, North Yorkshire (northern England). Integrated micropalaeontological (calcareous nannofossils and dinoflagellate cysts) and geochemical (rock-eval pyrolysis) analyses reveal a sequence of changes mainly driven by palaeoecological shifts. These changes mainly involve the composition, source and preservation rate of sedimentary organic matter as well as algal population dynamics. A sequence of successive disappearances of individual species during the early Toarcian ( Dactylioceras semicelatum and Harpoceras exaratum Ammonite Subzones) affect the composition of the phytoplankton assemblages which become gradually dominated by opportunistic taxa. This range top sequence is followed by the temporary disappearance of both calcareous nannofossils and dinoflagellate cysts (the disappearance event), which are replaced by Tasmanites. An increase in type II kerogen marks this interval. These modifications were related to a gradual water stratification and to the development of a stable pycnocline. These factors controlled the development of an oxygen-minimum zone within the water column and the distribution of nutrients in the surface waters. The disappearance event is coincident with the maximum extent of the oxygen minimum zone. A repopulation event and an increase in type III kerogen mark the end of the phytoplankton crisis in the upper part of the H. exaratum Ammonite Subzone. Preadapted survivors (e.g. Biscutum dubium, Biscutum finchii and Crepidolithus crassus) mainly characterise the repopulated calcareous nannofossil assemblages, whereas a significant turnover affected the dinoflagellate cyst community. The re-established phytoplankton populations are a response to the gradual restoration of mixed marine waters, characterised by meso-eutrophic conditions at the surface and by sustained nutrient regeneration. The integrated calcareous nannofossil and dinoflagellate cyst biostratigraphy provided a reliable biochronological framework for the Brown Moor Borehole succession, allowing correlation with lower Toarcian anoxic strata in central Italy. The integrated micropalaeontological approach indicates some diachroneity of Lower Jurassic Ammonite zones in the Boreal and Tethyan realms.