The Barremian and Aptian stepwise development of the 'Oceanic Anoxic Event 1a' (OAE 1a) crisis: Integrated benthic and planktic high-resolution palaeoecology along the Gorgo a Cerbara stratotype section (Umbria-Marche Basin, Italy)

The Barremian and Aptian were times of global plate reconfiguration and profound changes in the ocean-climate system, culminating in the worldwide deposition of lower Aptian black shale layers (OAE 1a). Based on high-resolution lithostratigraphic and micropaleontological analyses, precursor conditions and timing of the anoxia are here reconstructed along a 33.06. m thick section in the proposed Barremian/Aptian boundary GSSP stratotype at Gorgo a Cerbara (Umbria-Marche Basin, Italy).A non-uniform history of benthic foraminiferal diversification is interrupted by the Selli Level (= OAE 1a) and by three turnover points for both benthic and planktic organisms, each highlighted by prominent breaks in lithology and outcrop morphology. The first two points, at ~. 1.1. Myr and ~. 20-50. kyr prior to the onset of the OAE 1a, correspond respectively to the nannoconid (=. bathypelagic calcareous nannofossils) 'decline' and 'crisis' events, and separate: (a) pelagic limestone/chert-claystone lithological cycles of the uppermost Maiolica Formation, showing frequencies reminiscent of orbital eccentricity and containing an oligotrophic Barremian benthic and planktic assemblage, dominated by k-selected nannoconids and Rhizammina; (b) greenish-grey cherty marls of the lowermost Marne a Fucoidi Formation, associated to declining sedimentation rates and eutrophic early Aptian assemblages dominated by radiolarians and lituolid benthic foraminifera; and (c) anoxic radiolarites and shales of the carbonate-free Selli Level. High magnitude and frequency assemblage fluctuations occur between the nannoconid crisis (~. 25. cm below the lowermost OAE 1a black shale layer) and the Selli Level base, with suggestions of bottom-water acidification. At ~. 800. kyr after the OAE 1a end, a third turnover point in the basal reddish member of the Marne a Fucoidi highlights 'middle' Aptian assemblages characterized by a bloom of early macroperforate planktic foraminifera (. Hedbergella spp.) and appearances of new benthic species, marking a shift towards better oxygenation.OAE 1a oxygen and pelagic carbonate factory crises are here suggested to have been step-wise and linked to global warming conditions.