Oscillating redox conditions in the Vocontian Basin (SE France) during Oceanic Anoxic Event 2 (OAE 2)
Résumé
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 22m 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 redoxsensitive
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.