Unravelling Middle to Late Jurassic palaeoceanographic and palaeoclimatic signals in the Hebrides Basin using belemnite clumped isotope thermometry

Clumped isotope based temperature estimates from exceptionally well-preserved belemnites from Staffin Bay (Isle of Skye, Scotland) reveal that seawater temperatures throughout the Middle-Late Jurassic were significantly warmer than previously reconstructed by conventional oxygen isotope thermometry. We demonstrate here that this underestimation by oxygen isotope thermometry was likely due to a) using the incorrect calcite thermometry equation for belemnite temperature reconstructions and b) by incorrectly estimating the seawater δ18O (δ18Osw) for the Hebrides Basin. Our data suggests that the fractionation factor for oxygen isotopes in belemnites from seawater was closer to that of slow-growing abiogenic calcites than that of other marine calcifying organisms. Our clumped isotope temperatures are used to reconstruct δ18Osw trends across the Callovian–Kimmeridgian in the Hebrides Basin. The δ18Osw varied significantly in the Hebrides Basin throughout this interval, possibly as a result of changing currents through the Laurasian seaway. Trends in temperature and δ18Osw are compared to published palaeoceanographic studies to shed light on changing palaeoceanography in the Tethyan and Boreal realms throughout the Middle–Late Jurassic. •Scottish Jurassic seawater were temperatures much warmer than previously thought.•δ18Osw varied significantly throughout Mid-Late Jurassic in the Hebrides Basin.•Changes in current drove the observed temperature changes in the Hebrides Basin.•Belemnites likely precipitate their calcite in equilibrium with seawater.•Belemnite calcite exhibits a different 18O fractionation factor to other biogenic calcites.