Chemostratigraphy of Albian deposits in northern Tunisia: Identification of early Albian Oceanic Anoxic Event in the southern Tethys

Abstract

An integrated analysis of multiple geochemical proxies, including δ¹³C_org, total organic carbon (TOC), trace elements, and chemical gamma ray, coupled with a microfacies study and an age calibration using planktonic foraminiferal markers, was conducted at the Ain Asker section in northern Tunisia. This study provides significant insights into the depositional and paleoenvironmental dynamics of the mid-Cretaceous.

The carbon isotopic excursions observed at the Ain Asker section reflect a notable increase in the burial rate of marine organic carbon, specifically confined to the middle part of the Microhedbergella rischi biozone. TOC values recorded from the F2-ASK unit reach up to 2.3% combined with a δ¹³C_org values reaching -24.49‰, suggesting substantial accumulation of decayed marine organic matter. Although full anoxic conditions were not prevalent during deposition, evidence of oxygen-depleted bottom waters is clearly supported by the distribution patterns of redox-sensitive trace elements, such as vanadium and nickel. These findings underscore the role of partial oxygen restriction in facilitating organic matter preservation.

The enhanced burial of organic carbon is attributed to increased nutrient influx, likely driven by global sea-level transgressions and heightened surface productivity. These processes created favorable conditions for organic matter production and preservation. The integration of these sedimentological and geochemical findings with comparative records from Europe (France and Italy), China, and Mexico enables reliable correlation with the Paquier black shale level, a hallmark of OAE1b.

The Ain Asker section, through its detailed geochemical and isotopic signatures, emerges as a critical reference section for unraveling mid-Cretaceous anoxia dynamics within the southern Tethyan Realm. Its study enhances our understanding of the interplay between global paleoceanographic events and local depositional processes, shedding light on the mechanisms of organic carbon burial during a pivotal period in Earth’s history.