Late Jurassic temperatures for the southern Tethyan margin based on belemnites δ18O from the Ouarsenis Mountains, northwestern Algeria

Abstract

This study presents the first record of stable carbon and oxygen isotopes of well-preserved belemnites and bulk sediment from the Upper Jurassic pelagic carbonates of the Ouarsenis Mountains (northwestern Algeria), which were deposited at the southern margin of the Tethys ocean. Cathodoluminescence and scanning electron microscopy was used to eliminate potentially diagenetically altered specimens (brachiopods, aptychi, and some belemnites). Ultimately, 55 belemnite specimens were used to reconstruct water temperatures for the Middle Oxfordian to the Early Tithonian. Their δ18Ocarb values ranged from −0.35‰ to −3.88‰, suggesting warm and mostly stable temperature conditions during the Late Jurassic. Using a δ18Osea value of 0‰ for the tropical palaeolatitude of 15–20°N, the δ18Ocarb values of the well-preserved belemnite rostra translate into average temperatures of 27.8 °C in the Middle Oxfordian (Gregoryceras transversarium Zone), 27.0 to 27.5 °C in the Late Oxfordian (Epipeltoceras bimmamatum and Idoceras planula zones, respectively), 26.2 °C in the Early Kimmeridgian, and 27.3 °C in the Early Tithonian (Hybonoticeras hybonotum Zone).