Late Jurassic (Kimmeridgian–Tithonian) stable isotopes ( δ18O, δ13C) and Mg/Ca ratios: New palaeoclimate data from Helmsdale, northeast Scotland

Abstract

The data presented here provide the first detailed stable isotope ( δ 18O, δ 13C) and geochemical (Mg/Ca) investigation of Kimmeridgian–Tithonian belemnites from the Helmsdale Coast, Scotland, UK. Oxygen and carbon stable isotope values from well preserved specimens range from − 2.8 to + 0.3‰ and from − 2.3 to + 2.8‰ respectively. The oxygen isotope data are consistent with palaeotemperatures of up to 24 °C in the Early Kimmeridgian cymodoce Zone and down to 11 °C in the Mid Tithonian rotunda–fittoni Zones. These estimates are strongly supported by the Mg/Ca data, which also indicate a cooling episode (and very similar palaeotemperatures, 11–22 °C) at this time. The cooling event is associated with an approximately 5‰ decline in δ 13C ratios. This shift towards more negative δ 13C values has also been recorded in the Tethys and on the Russian Platform, confirming that this event was of global origin. Such globally synchronous records can provide a valuable tool for stratigraphic correlation. The Helmsdale δ 13C data are therefore combined here with other published belemnite isotope data from the Jurassic of Scotland (from the Callovian–Kimmeridgian of the Isle of Skye and the Toarcian–Aalenian of Raasay). The resultant belemnite δ 13C curve provides a detailed record of the Toarcian–Tithonian British Boreal Realm that can be compared with coeval records to investigate regional differences in Jurassic carbon isotope stratigraphy.