An ichnological record of oxygen fluctuations over the Cenomanian-Turonian Oceanic Anoxic Event 2 from the Vocontian Basin, SE France

An interval through the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) from the Vocontian Basin in southeastern France was examined using integrated sedimentology and ichnology at both macro- and microscopic scales to elucidate subtle bottom water and pore water oxygen changes. This study provides the first comprehensive trace fossil record for the region, confirming a diverse assemblage of ichnogenera and revealing evidence of microscopic reworking even in mudstones previously considered unbioturbated. Relative dissolved oxygen changes were interpreted through temporal fluctuations in trace fossil attributes (diversity, burrow size, and ichnoguilds) to illuminate the dynamic redox history. The onset of the OAE2 marked a rapid decline from pre-event oxic conditions to sustained suboixa. Peak deoxygenation occurred just below the Cenomanian-Turonian Boundary, marked by oscillations between suboxic and temporarily anoxic conditions. Prior to peak deoxygenation, a significant reoxygenation event (the Plenus Cold Event) resulted in fluctuating oxic and dysoxic conditions. Post-OAE2 recovery was protracted and gradual, transitioning from dysoxic to weakly oxic bottom waters. Integrating these ichnological findings with existing geochemical and micropaleontological data corroborates the interpretation that OAE2 in this region was a dynamic event characterized by fluctuating redox conditions, rather than persistent anoxia. This work underscores the critical value of ichnological analysis in paleoredox studies for resolving subtle changes within the oxic-dysoxic-suboxic spectrum, demonstrating that the absence of macroscopic bioturbation is an unreliable proxy for anoxia.