Paleoceanographic change associated with the Early Aptian Oceanic Anoxic Event in the western Tethys

The Cretaceous oceanic anoxic events (OAEs) provide a good opportunity to study the biogeochemical pathways and feedbacks linked to environmental change. The early Aptian OAE, labelled OAE 1a, corresponds to one of the most studied anoxic events within the Cretaceous. This event is characterized by a positive excursion in δ13C, preceded by a pronounced negative spike. Here, we propose to improve our understanding of palaeoceanographic change leading to this event and test the proposed models by investigating phosphorus (P) and redox-sensitive trace-element (RSTE) contents in sections through lower Aptian sediments along a basin-shelf transect in the western Tethys. We complement our geochemical analysis by the analysis of organic matter contents. We selected three representative sections: Gorgo a Cerbara (central Italy) in the Umbria Marche basin, Glaise l’Ermitage (SE France) located in the Vocontian Trough and Cassis/La Bédoule (SE France) located along the Provencal platform. The general trend in P accumulation shows an increase at the onset of the early Aptian event followed by a rapid decrease. This suggests an increase in nutrient input, whereas the return to lower values through the first part of the anoxic event may be related to a weakened capacity to retain P in the sedimentary reservoir due to bottom-water oxygen depletion. This general pattern is contrasted by the data of Gorgo a Cerbara, which also show P-enrichments at the top of the Livello Selli. We compared these enrichments to the total organic carbon (TOC) values. The shales with the maximum TOC values correspond also to those with the highest P content. We calculated Corg:Ptot ratios and observed that the highest values correspond to the top of the Selli level. This is interpreted as a reflection of the decreased capacity of storing and preserving phosphorus in oxygen-depleted sediments. RSTE show similar behaviour in the basinal settings. In the section of Gorgo a Cerbara, the data for U, V, Mo, Co, As show a low background level, constrasted by different maxima in concentrations near the top of the Selli level. In the Glaise section, a weak increase is observed just after the negative spike in δ13C whereas in the Cassis/La Bédoule section, no significant enrichments have been observed in sediments equivalent to the Selli level. The different behaviour of the RSTE in the studied sections may be related to the palaeogeographic setting of the studied sections. These data seem to indicate anoxic conditions in the basin. In shallower-water environments, conditions may have been less reducing. Moreover, in Gorgo a Cerbara, three distinct enrichments have been observed. This seems to indicate fluctuations in the intensity of water-column anoxia during the shift in δ13C. Our results show that the expression of the OAE 1a is different following the palaeogeographic setting. The stratigraphic evolution of P contents suggests an increase in nutrient input at the onset of the anoxic event, just after the negative spike in δ13C. RSTE and high C:P values may indicate anoxia conditions in the deep environment characterized by several anoxic phases with intermittent return to less oxygen-depleted conditions.