251 Phanerozoic Earth System Shifts in the Marine Sedimentary Record

This session explores Earth System shifts documented as perturbations to the Phanerozoic geochemical, lithological, and biogeochemical marine records, leading to climate shifts, anoxia, ocean acidification, and extinction events.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 303AB

Ocean Anoxic Event 2 in the Equatorial Atlantic Trans-Saharan Epicontinental Seaway: Implications for Redox and Carbon dynamics.

Paper number: 251-11
Program: Technical Sessions
Time: 10:35 AM – 10:50 AM

Authors

Abstract

During the mid-Cretaceous greenhouse, which was characterized by major disruptions in the carbon cycle including Oceanic Anoxic Event 2 (OAE 2, ~94 Ma), epicontinental seaways played a key role in promoting global carbon burial and enhancing marine productivity. These shallow, often restricted basins provide critical windows into carbon cycling and redox fluctuations associated with OAE 2. Although widely distributed, most efforts have concentrated on the open ocean North Atlantic and the Tethyan realms. Consequently, attempts to constrain the global carbon budget associated with OAE 2 remain incomplete, partly due to limited data from the equatorial Atlantic and contemporaneous epicontinental seaways. This study presents an integrated geochemical and sedimentological transect capturing the expression of OAE 2 across the Benue Trough in the Trans-Saharan seaway, an equatorial, transitional basin along the proto-Atlantic margin of West Africa. Using organic carbon isotopes (δ¹³Corg), trace metal enrichments, total organic carbon (TOC), and osmium isotope (¹⁸⁷Os/¹⁸⁸Os), we reconstruct redox conditions, productivity patterns, and basin connectivity across the lower, middle and upper Benue Trough. The δ¹³Corg data revealed up to +3‰ positive excursion, characteristic of OAE 2 global records. Trace metals indicate dominantly suboxic bottom-water conditions, interrupted by pulses of anoxic to euxinic conditions as evidenced by enrichment of molybdenum. TOC across the Benue Trough increases consistently from north (upper, proximal) to south (lower, distal), suggesting enhanced preservation toward the deeper parts of the basin. Across the trough, the northern sections transition from fossiliferous carbonates to dark grey laminated shales, while the southern sections consist entirely of laminated organic-rich shales, reflecting more sustained anoxic conditions and a deepening basin profile. Initial ¹⁸⁷Os/¹⁸⁸Os values show a pronounced unradiogenic shift coincident with the onset of the δ¹³Corg excursion, signaling volcanic input likely linked to one or more large igneous province (LIP) emplacements as potential trigger for the onset of OAE 2 in the Benue Trough. Together, these findings support a weakly restricted epicontinental basin model for the Benue Trough during OAE 2, highlighting its role as a significant carbon sink during this time.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414450

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