The middle-late Aalenian event: A precursor of the Mesozoic Marine Revolution

Abstract

The Aalenian was a time marked by profound environmental and carbon cycle changes. Still, the scarcity of detailed studies hinders a better understanding of the triggering mechanisms and the larger-scale context of Lower to Middle Jurassic environmental perturbations. This study provides an unprecedented high-resolution biostratigraphically well-constrained carbon isotope record (δ13Corg and δ13Ccarb) for the upper Toarcian–lower Bajocian interval of two expanded limestone-marl alternation successions in France (French Subalpine Basin) and Chile (Andean Basin). The comparison with available records from the Tethyan and Boreal domains highlights that medium-term carbon isotope fluctuations are reproducible across different palaeoceanographic settings from both hemispheres, providing for the first time compelling evidence for recurrent perturbations of the global carbon cycle during the Aalenian. Combined with a review of geological events, climate modes, abundance and diversity of major fossil groups, and trophic conditions inferred from the calcareous nannofossil record, our study fills the gap in our understanding of global environmental changes in a so-far poorly documented Middle Jurassic stage, intercalated between the early Toarcian Oceanic Anoxic Event and the early Bajocian carbonate crisis. Importantly, this compilation indicates that the Aalenian was a pivotal time interval of environmental perturbations, likely triggering the Mesozoic Marine Revolution.