Redox- and bio-productivity changes in the Trans-Saharan epicontinental seaway during and after the Cenomanian-Turonian anoxic event (OAE2): insights from stable isotopes and trace metals

Redox- and bio-productivity changes in the Trans-Saharan epicontinental seaway during and after the Cenomanian-Turonian anoxic event (OAE2): insights from stable isotopes and trace metals

Usman Abubakar ^1,2*, Simon V. Hohl¹, Sebastian Viehmann³, Stefan Weyer³, Musa Bappah Usman²

¹State Key Laboratory of Marine Geology, Tongji University, Shanghai, P.R China

²Department of Geology, Gombe State University, Gombe, Nigeria

³Department of Mineralogy, Leibniz University Hannover, Hannover, Germany

*uabubakar2002@tongji.edu.cn

The Cenomanian-Turonian boundary (~ 94 Ma) marked a presumably global ocean anoxic event (OAE2), resulting in the widespread deposition of black shales, a positive carbon isotope excursion, biotic turnover, and significant changes in global climate. While increased volcanic activity is often linked to enhanced nutrient supply into the ocean and increasing primary productivity and O2 consumption during their decomposition, recent studies have revealed contradictory redox conditions ranging from anoxic to oxic from the open ocean to epicontinental seas. We present the first integrated geochemical data from the Ashaka quarry in one of the basins flooded by the Trans-Saharan epicontinental seaway: the Upper Benue Trough, Nigeria. The data include δ¹³Corg and δ²³⁸U, total organic carbon (TOC), and redox-sensitive and bio-essential trace metal concentrations. We aim to determine the possible location of OAE2 within this strata and reconstruct local variations in redox and bio-productivity systematics. δ¹³Corg displays a positive excursion of ~2 ‰ (-25.5 ‰ to -23.5 ‰) at the base of the section, indicating the occurrence of an OAE. However, this event coincides with relatively low TOC values (0.3-1.2 wt.%), showing regionally low burial rates at a potentially increasing influx of terrigenous organic matter, as evidenced by increasing C/N ratios from 4.4 to 10.3. Enrichment factors of redox-sensitive trace metals (U_EF and Mo_EF) exhibit depletion, enrichment, and subsequent depletion at the beginning, middle, and end of the OAE2 within the stratigraphy, respectively. In contrast, enrichment factors of bio-essential trace metals (Cd_EF and Zn_EF) consistently show a depletion throughout the event and display a low ratio of micronutrients to macronutrients (Cd/P and Zn/P). These patterns correspond with δ²³⁸U (-0.46 to -0.32 ‰) varying around the value of modern seawater (-0.4 ‰), suggesting fluctuations from oxic to sub-oxic redox conditions and a reduced element shuttle at possibly suppressed paleo-productivity. After the OAE2, the middle part of the Ashaka section records primarily oxic conditions supported by very low TOC and δ²³⁸U values similar to the modern ocean. The top of the section exhibits highly depleted redox-sensitive metals and high enrichment of bio-essential metals, indicating a rebound to fully open marine conditions characterized by high productivity, upwelling, and well-oxygenation concurrent with a slight positive shift in δ²³⁸U (-0.37 to -0.29).

This study demonstrates partly oxygenated conditions during the OAE2 in the epicontinental Trans-Sahara Seaway, correlating the Ashaka quarry section with strata deposited in the epicontinental Western Interior Seaway and several shallow marine environments of the Tethys Sea, bringing this new OAE2 interval into the global context for the first time.