During the early Toarcian (Early Jurassic, ~183 Ma) global climate and environmental change profoundly impacted biogeochemical cycles and marine ecosystems culminating in the early Toarcian Oceanic Anoxic Event (T-OAE). The T-OAE exhibited a global temperature rise and carbon cycle perturbation that manifested in a stepwise negative Toarcian carbon isotope excursion. While the response of marine invertebrates towards early Toarcian environmental change is well documented, that of marine primary producers is only poorly constrained and is mainly inferred from changes in algae groups producing hard parts, such as calcareous nannoplankton and organic-walled dinoflagellates. Both biotic groups, however, represent only a small part of the marine phytoplankton, the majority of which lack parts with preservation potential in the geosphere. In order to obtain a holistic view on the response of marine primary producers towards the early Toarcian environmental perturbation, we here conducted a high-resolution study based on the combined application of molecular fossils (biomarker) and calcareous nannofossils.

Upon the climax of the T-OAE our data indicate that the marine phytoplankton community structure was dominated by eukaryotic algae, which experienced at least five distinct paleoecological events. Each of these was characterized by a demise of red algae groups and shallow- and deep-dwelling calcareous nannoplankton, and a contemporaneous proliferation of opportunistic green algae groups. An increase of poorly developed coccoliths (resembling protococcoliths) during each phytoplankton event attests to periodically harsh environmental conditions in surface waters. Each phytoplankton event was followed by a recovery of pre-event algal communities, indicative of a cyclicity in unstable and fluctuating environmental conditions. During the onset of the T-OAE, phytoplankton events align with negative δ¹³C shifts, suggesting that changes in the global carbon cycle and the climate system impacted on marine primary producers. Phytoplankton events, particularly during the onset of the T-OAE, were accompanied by recurrent freshening of surface waters during warming periods, suggesting these were the most important ecological drivers.

Cyclicity in phytoplankton events followed the short eccentricity (100 kyr) orbital frequency, emphasizing the role of orbital forcing in early Toarcian environmental change and ecosystem disturbances. Ecosystem change and extinction were not driven by a single event, but resulted from multiple disturbances that occurred superimposed onto long-term environmental change and culminated in the early Toarcian ecosystem crisis.