The role and contribution of eustasy as a triggering force of the onset of Oceanic Anoxic Events: A case study of the early Aptian OAE1a in the Tethyan margin of Tunisia

Oceanic Anoxic Events (OAEs) are short-lived global carbon cycle perturbations characterized by significant changes in the chemistry, temperature and circulation of the oceans of the world. The forcing mechanisms behind these events remain a subject of discussion, especially for the Jurassic and Cretaceous periods. Typically, these events are thought to be triggered by a contemporaneous emplacement of a large igneous province (LIP) associated with a significant release of CO₂ into the atmosphere and leading to rapid global warming. In turn, induced sharp climatic change promotes an acceleration of the hydrological cycle which intensifies continental weathering, increases input of nutrients into the oceans and drives elevated rates of primary productivity. Organic productivity increase causes high oxygen demand leading to anoxia accompanied by enhanced carbon burial.

The primary carbon source of the Aptian OAE1a, (~120 Ma, D. forbesi, D. deshayesi and D. furcata ammonite zones or G. blowi and L. cabri planktonic foraminiferal zones) is suggested to be the Greater Ontong Java Plateau (G-OJP) which peaks of activity are interpreted to be coeval to specific phases of the global crisis, such as a global biocalcification crisis. It is also known that the GOJP initial emplacement preceded the onset of OAE1a. What remains unclear is therefore what other processes could have triggered the OAE1a and what is the exact timing of its onset. OAEs commonly relate to periods of world scale major marine transgression (eustatic maximum flooding) as a consequence of global warming, potential deglaciation and/or addition of massive volcanogenic rock volume into the ocean. Nevertheless, the relationship of the onset of OAEs with the maximum flooding and the exact position of their time interval with respect to the global eustatic sea-level curve are still ambiguous.

This study focuses on the Aptian of the Tethyan margin in Tunisia, which sediment record covers the OAE1a and aims at understanding the exact mechanisms and their interplay in relation to the initiation and onset of this event, with a particular focus on the eustatic factor. The study methods include sedimentology, biostratigraphy, sequence stratigraphy and chemostratigraphy integrated with petroleum exploration well data coupled with published datasets and studies. Emphasis will be given to the reconstruction of the accommodation space with the aid of paleo-bathymetry estimations based on facies sedimentology and benthic foraminifera. At a later stage, the results will be correlated with data from SE Spain and Northern Europe (Greensand facies). One of the main goals of this project is to provide a global assessment and overview of the positioning of the OAE1a in relation to the early Aptian maximum floodings and understand in depth the critical factors, confluences, as well as thresholds that were necessary to be exceeded to trigger this event in a qualitatively and quantitatively manner.

The project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant Agreement No 860383 (https://www.s2s-future.eu/).