Oscillating redox conditions in the Vocontian Basin (SE France) during Oceanic Anoxic Event 2 (OAE 2)

The Cenomanian-Turonian boundary (~94 Ma) was marked by an episode of widespread organic carbon burial largely associated with low oxygen concentrations in bottom oceanic waters, named Oceanic Anoxic Event 2 (OAE 2). In the Vocontian Basin (SE France), the Pont d'Issole section exposes a 22 m interval called Niveau Thomel, the local sedimentary expression of the OAE 2, characterized by levels of laminated dark shales with marine-derived organic matter enrichments (up to 2.5 wt%). An episodic disruption in total organic carbon (TOC) enrichment is observed in bioturbated limestones within the Niveau Thomel, reflecting bottom-water reoxygenation associated to the Plenus Cold Event. Oscillations in redox conditions during the OAE 2 are evidenced by variations in the degree of pyritisation of the sediments reflecting suboxic to anoxic conditions associated with organic-rich intervals. These variations are additionally highlighted by enrichments in redox-sensitive trace elements (Mo, V, Zn) in sediments containing at least 1.5 wt% TOC, which may reflect a threshold between suboxic and anoxic conditions at Pont d'Issole section. Low Mn and Fe concentrations in >0.3 wt% TOC samples mark a large O2 depletion in pore waters. Phosphorus released from the sediments under deoxygenated conditions could have contributed to sustain the primary productivity in the basin, promoting further O2 consumption. A large negative excursion in the sulfur isotopic composition of pyrite (δ34Spy) with minimum values down to −40‰ suggests that microbial sulfate reduction was non sulfate-limited and was characterized by pyrite precipitation in an open system with regard to the marine sulfate reservoir. This feature is probably linked to a higher sulfate availability within the Vocontian Basin due to important Large Igneous Province outgassing at the onset of the OAE 2. •RSTE reveal suboxic to anoxic conditions in the Vocontian Basin during OAE 2.•PCE is marked by a return to oxic conditions, related to better-ventilated waters.•Sulfide production in sediments has limited P burial sorption onto Fe-oxyhydroxides.•P regeneration in the water column may have sustained primary productivity.•Volcanic SO4 and CO2 input could have been the triggering factor for OAE 2.