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

International audienceThe Cenomanian-Turonian boundary (~94 Ma) was marked by an episode of widespread organic carbon buriallargely 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 22m interval called NiveauThomel, the local sedimentary expression of the OAE 2, characterized by levels of laminated dark shales withmarine-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-waterreoxygenation associated to the Plenus Cold Event. Oscillations in redox conditions during the OAE 2 are evidencedby variations in the degree of pyritisation of the sediments reflecting suboxic to anoxic conditions associatedwith organic-rich intervals. These variations are additionally highlighted by enrichments in redoxsensitivetrace elements (Mo, V, Zn) in sediments containing at least 1.5 wt% TOC, which may reflect a thresholdbetween 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 deoxygenatedconditions could have contributed to sustain the primary productivity in the basin, promoting furtherO2 consumption. A large negative excursion in the sulfur isotopic composition of pyrite (δ34Spy) with minimumvalues down to −40‰ suggests that microbial sulfate reduction was non sulfate-limited and was characterizedby pyrite precipitation in an open system with regard to the marine sulfate reservoir. This feature is probablylinked to a higher sulfate availability within the Vocontian Basin due to important Large Igneous Provinceoutgassing at the onset of the OAE 2