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 (δ¹⁸O_sw) 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 δ¹⁸O_sw trends across the Callovian–Kimmeridgian in the Hebrides Basin. The δ¹⁸O_sw varied significantly in the Hebrides Basin throughout this interval, possibly as a result of changing currents through the Laurasian seaway. Trends in temperature and δ¹⁸O_sw are compared to published palaeoceanographic studies to shed light on changing palaeoceanography in the Tethyan and Boreal realms throughout the Middle–Late Jurassic.