First Estimates for Jurassic Seawater Temperatures Based on Oxygen Isotope Analyses of Calcitic Fossils from Central Iran: Evidence for a Major Plate Tectonic Shift?

Stable oxygen isotope values of calcitic marine fossils (delta O-18(shell)) were used to calculate the first absolute seawater temperature estimates for the Jurassic of Central Iran. Results indicate a significant long-term warming in the study area through the Middle to Late Jurassic of around 6 degrees C or more (0.3 to 0.4 degrees C per million years). Following recent methodological advances in temperature reconstructions, the delta O-18(shell) values of bivalves translate into water temperatures slightly warmer than in today's oceans at comparable latitudes with averages of 26.6 degrees C in the Middle Jurassic and 32.1 degrees C in the Late Jurassic. Belemnites would indicate still warmer temperatures, but these values might be less reliable and partly overestimated due to potentially altered specimens. The recorded long-term warming can be explained by a concurrent major plate tectonic shift of Central Iran from temperate to tropical latitudes. The gradual influence of this southward drift is also seen by changes in sedimentation and ecosystem composition. This way, the stable oxygen isotope record of Central Iran is further evidence for the strong impact of plate tectonics on regional long-term climate patterns in the Middle and Late Jurassic.