Unravelling the base Bathonian-Callovian boundary event: New insights from the SE France Basin

Global perturbations to the Jurassic environment have been extensively documented over the last decades, notably those that were associated with major events such as mass extinctions, global environmental crisis and hyperthermal events that often mark stage boundaries. However, much less attention has been given to background times and to lesser palaeoenvironmental and palaeoclimatic events. As such, the long-term secular evolution in the Jurassic carbon cycle and palaeoenvironmental response, that provide the broader context, is very poorly understood. One such understudied time interval is the Bathonian-Callovian transition (Middle Jurassic). Although at first it may appear as a relatively stable time, there is evidence for biotic turnover, climate warming, dramatic sea-level rise, and carbon cycle disturbance. A positive excursion in the carbon isotope ratio (CIE) has been documented in Europe and Greenland, but the current state of records does not yet allow determination of the exact causes and nature of the isotope shift and the environmental changes across the Bathonian-Callovian boundary. As for other Mesozoic positive CIEs, it has been proposed that increased burial of organic matter due to elevated primary productivity might have generated the positive shift. The lack of continuous well-dated records with robust stable carbon isotope chemostratigraphy and environmental context complicate the attribution of a Global Boundary Stratotype Section and Point for the Bathonian-Callovian.

This study aims to tackle this lack of empirical data by providing a multi-proxy dataset combining sedimentological observations, nannofossil biostratigraphy, mineralogical and geochemical analyses. The Ravin des Vas section in SE France has been selected because it encompasses the Bathonian-Callovian boundary and comprises an extended 100 m-thick homogeneous succession of dark marls, with little lithological change, which is an advantage for geochemical analysis. Organic carbon isotope analysis reveals a pronounced positive excursion at the base of the Callovian, associated with an increase in organic carbon content. This excursion can be correlated to other coeval sites, confirming the likely global nature of this carbon cycle perturbation. Whole-rock and clay mineralogy will be used to determine the climatic and weathering conditions, and phosphorus content will inform on the nutrient availability. Altogether, the new dataset from France will give new insights into our understanding of the palaeoclimatic and palaeoenvironmental conditions across the Bathonian-Callovian boundary, and hence provide a reference framework for further studies on this yet poorly known time interval.