A volcanically induced climate warming and floral change preceded the onset of OAE1a (Early Cretaceous)

The Aptian Oceanic Anoxic Event 1a (OAE1a) is preceded by a prominent negative C-isotope excursion (NCIE) attributed to major volcanism on the Ontong Java plateau that is supposed to lead to a pCO2 increase and a climate change. Lower Aptian sporomorph assemblages in two pelagic sections from the southern Tethys margin (N-Italy) were analysed in order to test if the postulated climate changes affected the terrestrial vegetation. At the base of the NCIE the sporomorph assemblages comprise abundant bisaccate pollen reflecting a warm-temperate climate. Several tens of kiloyears (ka) after the start of the NCIE decreasing bisaccate pollen and increasing Classopollis spp. and Araucariacites spp. suggest the beginning of a long-term temperature rise. Palynomorphs indicate that maximum temperatures were reached several tens of ka after the end of the NCIE and the onset of OAE1a. The highest temperatures coincide with arid conditions, which could reflect a northward shift of the hot-arid Northern Gondwana floral province as a result of an increasing pCO2. Over 200 ka after the onset of OAE1a reduced volcanic activity and/or increased black shale deposition allowed for a drawdown of most of the excess CO2 and a southward shift of floral belts.