Global redox and bio-productivity changes during the oceanic anoxic event 2 (OAE2): Insights from combined U-C isotopes of the Trans-Saharan epicontinental Seaway

Abstract

The Cenomanian-Turonian boundary marks a global ocean anoxic event (OAE2), leading to the widespread deposition of black-shales due to enhanced primary productivity and O2 consumption. However, recent studies predict contradictory redox conditions from the open ocean to the epicontinental seas. This study combined existing C and U isotopic compositions of marine OAE2-bearing sediments to understand global redox-variations better and introduce the previously overlooked Trans-Saharan Epicontinental Seaway. We examined the first integrated geochemical dataset from the Ashaka section, Nigeria, including δ¹³C_org, δ²³⁸U, TOC, redox-sensitive and bio-essential trace metal concentrations in authigenic sediments. We propose the potential location of the OAE2 and reconstruct local variations in redox and bio-productivity in the Trans-Saharan Seaway. The chemo-stratigraphic onset of the OAE2 is marked by a globally occurring positive δ¹³C_org excursion (−25.5 to –23.5‰). However, unlike many OAE2 sections (e.g., Tarfaya Basin and Demerara Rise in the N.-Atlantic), exhibiting high TOC, the Trans-Saharan Seaway records low TOC, comparable to the Western Interior Seaway and the Paleo-Pacific Ocean. These differences indicate poor regional preservation, contrasting with organic-rich, marine-dominated preservation in the deeper Tarfaya Basin and N. Atlantic. Micro-to-macronutrient ratios in the Trans-Saharan Seaway were low during the OAE2, suggesting suppressed productivity akin to the Western Interior Seaway and differing from the high-productivity regimes in the Tarfaya Basin and Tethys Sea. Furthermore, redox conditions highlight regional contrasts: predominantly oxic-suboxic conditions in the Trans-Saharan and Western Interior Seaway and Paleo-Pacific Ocean vs. anoxic in the Gubbio section and Demerara Rise. Despite partially oxygenated conditions, a negative δ²³⁸U_sw shift in the Trans-Saharan Seaway (−1.6‰) mirrors the Demerara Rise, Eastbourne, Western Interior Seaway, and Morelos Formation. However, the magnitude varies globally, with epicontinental seaways recording the largest shifts. These inconsistencies suggest that the extent of ocean anoxia undulates across basins calling for a cautious interpretation of U isotopes as a global redox proxy.