Biogeochemical evolution and organic carbon deposition on the Northwestern European Shelf during the Toarcian Ocean Anoxic Event

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) represents a well-known episode of organic-rich deposition, which is accompanied by a substantial negative carbon-isotope excursion (CIE). Underpinning the relationships between the carbon-cycle perturbation, ocean anoxia, primary productivity feedbacks and the enrichment of sedimentary organic carbon remains a major challenge. Here, we present high-resolution geochemical, palynological and organofacies data from three lower Toarcian successions from the NW European shelf, spanning the T-OAE. Chronostratigraphic calibration of the successions is achieved through organic carbon isotope (δ 13C) records. Iron-speciation and major and trace-element data indicate that bottom-waters were euxinic and intermittently anoxic-ferruginous prior to, throughout and beyond the CIE. In terms of organofacies and palynological composition, the CIE-interval is dominated by dense clusters of amorphous organic matter containing abundant small spherical prasinophyte cysts (Halosphaeropsis liassica). The peak CIE is bracketed by a major increase in abundance of prasinophycean vegetative cysts (Tasmanites and Pleurozonaria spp.). On the basis of their modern physiology, this suggests shoaling of the chemocline into the photic zone. The combined proxy data suggest a scenario in which anoxygenic photosynthetic productivity proliferated in nutrient-rich, anoxic to seasonally euxinic surface-waters of the stratified NW-European shelf during the CIE. A return to oxic-marine conditions is recorded by the recurrence of organic-walled dinoflagellate cysts in accordance with enhanced water column mixing post CIE. This is concurrent with a gradual termination of strongly stratified conditions across the NW-European shelf.