Ichnological insights into deoxygenation across the Cenomanian–Turonian Boundary Oceanic Anoxic Event 2 in the northern extent of Western Interior Seaway (west‐central Alberta)

ABSTRACT In‐depth ichnological and sedimentological analyses of the Cenomanian–Turonian boundary Oceanic Anoxic Event (OAE2) from the Western Interior Seaway of west‐central Alberta reveal a persistent physico‐chemically stressed setting. The interval is characterised by a dominantly diminutive and diminished ichnological assemblage, with familiar ichnotaxa (e.g. Phycosiphon , Chondrites , Nereites , Planolites , Teichichnus , Cylindrichnus and Palaeophycus ), fugichnia and navichnia ethological groups, and evidence of meiofaunal reworking and two other indistinct biodeformational fabrics. Grouping trace fossils into four ichnoguilds (the fugichnia ichnoguild, the Planolites ichnoguild, the Phycosiphon ichnoguild and the Chondrites ichnoguild) reveals fluctuating oxygen, salinity and sedimentation stress over time. Integrated analysis of ichnological characteristics, ichnofacies and ichnoguilds reveals five different bottom water oxygen scenarios, including: (i) poorly oxygenated, (ii) severely dysoxic, (iii) extremely dysoxic, (iv) episodic storm‐injected oxygen and (v) anoxia. Comparing trace fossil data with carbon isotope data, where the OAE2 is defined as a positive excursion, reveals oxygen trends over the event. Oxygenation was found to be highest below the OAE2 and declined rapidly over the Cenomanian–Turonian boundary. The Cenomanian–Turonian boundary itself is characterised by low and constantly fluctuating depositional dissolved oxygen, but never persistent anoxia. Counterintuitively, the most deoxygenated interval occurs after the termination of the OAE2 event. The delayed onset of lowest oxygen levels may be due to peak transgression occurring post‐OAE2 and slowed northward migration of warm oxygen‐poor Tethyan waters.