Early–Middle Jurassic Carbon Isotope Record from the Central High Atlas (Morocco): Implications for Oceanic Anoxic Events and Paleoenvironmental Change

Reconstructing the Early–Middle Jurassic paleoenvironmental and paleoclimatic evolution of the western Tethys margin is crucial for refining regional stratigraphic correlations and understanding the impact of global oceanic anoxic events (OAEs) in North African basins. This study examines the Jurassic succession in the Tazzouguert area of the Central High Atlas in Morocco through integrated stratigraphic and geochemical analyses. Three stratigraphic sections were precisely measured at Haddou Village (PM401), Mellaha Village (PM403), and Toulal Village (PM404), totaling 2,592.8 meters in thickness. Carbonate samples were systematically collected for carbon and oxygen isotope analyses and combined with detailed lithological observations. The isotopic curve spans the Hettangian–Bathonian interval and identifies nine negative carbon isotope excursions (NE1–NE9). NE4 occurs at the boundary between Pliensbachian and Toarcian, showing a decrease in δ¹³C values from 0.94‰ to −0.14‰, which aligns with the global Pliensbachian–Toarcian Event linked to climatic warming and sea-level rise. NE5, situated in the lower Toarcian, represents the most significant excursion (δ¹³C from 0.21‰ to −5.55‰) and marks the regional expression of the Toarcian Oceanic Anoxic Event. The other excursions record successive paleoenvironmental disruptions throughout the Early–Middle Jurassic: NE1–NE3 indicate middle Hettangian, Sinemurian-Pliensbachian transition and middle Pliensbachian environmental instability associated with lithofacies changes; NE6–NE7 reflect late Toarcian to Aalenian disturbances; and NE8–NE9 relate to Bajocian–Bathonian environmental shifts linked to reef limestone development and bioclastic deposition respectively, followed by a gradual positive δ¹³C trend signaling environmental recovery. Lithological features, including carbonate facies changes, fluctuations in bioclastic abundance, and sedimentary structures, further support the paleoenvironmental signals captured in the isotopic data. This study establishes a high-resolution carbon isotope stratigraphic framework for the Early–Middle Jurassic of the Central High Atlas and offers new insights for correlating regional successions with global Jurassic climatic and oceanographic events along the western Tethys margin.