Abstract The leading hypothesis for the Toarcian oceanic anoxic event (T-OAE; c. 183 Ma) and the associated negative C-isotope excursion is the massive release of 12 C favouring greenhouse conditions and continental weathering. The nutrient delivery to shallow basins supported productivity and, because of O 2 consumption by organic-matter respiration, anoxia development. However, several studies have shown that calcareous nannoplankton experienced a decrease during the T-OAE. Nannofossil fluxes measured in the Llanbedr (Mochras Farm) borehole, Wales, UK, were the highest prior to the negative C-isotope excursion, along with high amounts of taxa indicative of nutrient-rich environments (Biscutaceae). Such conditions attest to high productivity. Fluxes show the lowest values in the core of the event, along with a size decrease of Schizosphaerella and a peak in Calyculaceae. The recovery of nannofossil fluxes and Schizosphaerella size occurred concomitant with the return of C-isotopes to more positive values. Concomitantly, deep dwellers ( Crepidolithus crassus ) dominated, indicating a recovery of the photic-zone productivity. These observations demonstrate that the cascade of environmental responses to the initial perturbation was more complex than previously considered. In spite of elevated nutrient delivery to epicontinental basins in the early Toarcian, carbonate and primary productions of nannoplankton were depressed in the core the T-OAE, probably because of prolonged thermohaline seawater stratification.

Abstract The organic-rich upper Lower Jurassic Da'anzhai Member (Ziliujing Formation) of the Sichuan Basin, China is the first stratigraphically well-constrained lacustrine succession associated with the Toarcian Oceanic Anoxic Event (T-OAE; c. 183 Ma). The expansion of the palaeo-Sichuan mega-lake, probably one of the most extensive freshwater systems to have existed on the planet, is marked by large-scale lacustrine organic productivity and carbon burial during the T-OAE, possibly owing to intensified hydrological cycling and nutrient supply. New molecular biomarker and organic petrographical analyses, combined with bulk organic and inorganic geochemical and palynological data, are presented here, providing insight into aquatic productivity, land-plant biodiversity and terrestrial ecosystem evolution in continental interiors during the T-OAE. We show that lacustrine algal growth during the T-OAE accounted for a significant organic-matter flux to the lakebed in the palaeo-Sichuan mega-lake. Lacustrine water-column stratification during the T-OAE facilitated the formation of dysoxic–anoxic conditions at the lake bottom, favouring organic-matter preservation and carbon sequestration into organic-rich black shales in the Sichuan Basin. We attribute the palaeo-Sichuan mega-lake expansion to enhanced hydrological cycling in a more vigorous monsoonal climate in the hinterland during the T-OAE greenhouse.