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.