Thermal evolution of Tethyan surface waters during the Middle‐Late Jurassic: Evidence from δ18O values of marine fish teeth

Abstract

Oxygen isotope compositions of phosphate from vertebrate tooth enamel were measured to determine the evolution of tropical sea surface (<∼200 m depth) temperatures in the western Tethys during the Middle‐Late Jurassic. On the basis of a high‐resolution stratigraphic framework with a 1 Myr time resolution, vertebrate teeth were sampled on Aalenian to Portlandian isochrons over the Anglo‐Paris Basin. Asteracanthus sharks and Pycnodontidae teleosteans, identified as sea surface dwellers, have enamel with δ18O values that range from 18.5 to 22.3‰. Thermal variations of tropical surface waters, with amplitudes of a few degrees per few million years, suggest that Middle to Late Jurassic climates were quite variable. Assuming a seawater δ18O value of 0‰ for surface tropical waters in the absence of polar ice caps, temperatures increased from 25 to 29°C from the mid‐Bajocian to mid‐Bathonian. During the middle to late Bathonian, a strong geographic zonation in isotopic compositions is observed between the eastern and western parts of the basin. High δ18O values of fish tooth enamel (up to 22.3‰) could reflect the arrival of a cold water current from the Arctic during the opening of the North Sea rift. An apparent large drop of temperatures from 28 to 21°C is identified at the Callovian‐Oxfordian boundary over no more than ≈2–3 Myr. This cooling is compatible with previous paleobotanical and geochemical studies and can be precisely correlated with the migration of boreal ammonites into the Tethyan domain. Because isotopic sea surface temperatures are probably too low to be compatible with tropical climatic conditions, the δ18O value of seawater could have been >0‰ owing to limited growth of continental ice during the early middle Oxfordian. The resulting sea level fall is estimated to be at least 50 m and is compatible with a global regression stage. The middle Oxfordian thermal minimum is followed by a new warming stage of 3–4°C from the middle to the late Oxfordian.