A semi-quantitative ichnological approach to relative oxygen reconstruction: A case study over the Cenomanian-Turonian Oceanic Anoxic Event 2 in west-Central Alberta, Canada

An exceptionally well-preserved organic-rich mudstone core from the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) interval in the Canadian Western Interior Seaway offers an excellent opportunity for comprehensive ichnological analysis of paleo‑oxygen fluctuations. Vertical trends in burrow size and diversity (i.e., Size-Diversity Index - SDI) have previously served as a proxy for relative oxygenation in low-oxygen stratigraphic settings. We note two factors that greatly impact oxygenation interpretations of SDI: (1) high bioturbation intensities or reworking dominantly by small (meiofaunal) animals lead to a large underestimation of ichnofossil diversity; and (2) paleoenvironmental stressors other than lowered oxygen levels (e.g., fluctuating salinity, reduced food availability, and rapid sedimentation). A semi-quantitative relative oxygen equation is proposed to account for these complexities and better proxy bottom water oxygenation over the interval. The equation combines diversity counts, burrow sizes, and bioturbation intensity (all indicators of high oxygenation), scaled with total organic carbon measurements (the presence of which is interpreted as an indicator of low oxygenation, corroborated by carbon isotope measurements over the interval). Integrating ichnological data with total organic carbon measurements accounts for the diminution and low diversity that may stem from a multitude of physicochemical factors. The results suggest that the bottom water oxygen fluctuated continuously and only very rarely, if at all, reached persistent anoxia. The proposed equation offers a repeatable approach to integrate ichnological and TOC data, providing a better ichnological-oxygen proxy for organic-rich mudstones. • Common ichnological oxygen proxies (burrow size/diversity) have inherent problems. • Ichnology (size-diversity index + BI) divided by TOC yields a relative oxygen curve. • Oxygenation over the OAE2 was unstable, fluctuating between low-oxic and dysoxic. • Deoxygenation was most intense after the termination of OAE2.