The palaeoceanographic crisis of the Early Aptian (OAE 1a) in the Vocontian Basin (SE France)

The Vocontian Basin (SE France) which presents lower Aptian expanded successions characterized by major lithological changes, is particularly suitable to determine palaeoenvironmental changes occurring across the OAE la. A multidisciplinary study (sedimentology, CaCO3, TOC, carbon and oxygen stable isotopes, micro palaeontology, cyclostratigraphy) was carried out in the Notre-Dame-de-Rosans section in order to establish a detailed chronological framework of these changes. The OAE la corresponds to carbon isotope segments C3-C6, and it lasted for 1.2 Myr. A first drop in the carbonate production occurs 500 kyr before the OAE la (upper part of the carbon isotope segment C2), and could result from both onset of Urgonian carbonate platform demise and associated reducing export of platform-derived sediments into the basin, and nannoconid crisis. A second drastic drop (crash) in the carbonate production is recorded within the lower part of the OAE la (latter part of segment C3 to C5) and is explained by a strong dissolution. This study then shows that the onset of the major carbonate crisis, that occurs before the OAE la, could be due to both rise in sea-level and in marine surface water fertility, whereas its acme that occurs within the OAE la, could be related to CO2-induced ocean acidification. Black shales of the Niveau Goguel occur in the upper part of the OAE la and represent the lower part of segment C6. Surface-waters primary producers are principally represented by cyanobacteria, whereas nannofossil primary productivity is reduced, and deep-water anoxia prevailed during the deposition of the Niveau Goguel. The last 300 kyr of the OAE la are characterized by a partial recovery of both nanno-fossil primary productivity and pelagic carbonate production, which sharply increase just after the end of the event. This study also shows that organic-rich layers associated to the OAE la are diachronous in the Tethyan realm.