Cenomanian palaeotemperatures derived from the oxygen isotopic composition of brachiopods and belemnites: evaluation of Cretaceous palaeotemperature proxies
Sixty low-magnesium-calcite samples from Cenomanian articulate brachiopods, belemnites and oysters from the epicontinental shelf sea of Europe were geochemically and microscopically studied in order to evaluate their preservation and potential as carriers of palaeoenvironmental information. The sampled localities in northern Spain, Germany and southern England cover the transition between subtropical (25–30°N) and temperate (33–38°N) climates. Mean Cenomanian salinity-adjusted palaeotemperatures of diagenetically unaltered terebratulid and rhynchonellid brachiopods are 25.5±1.3 °C in Spain and 19.4±1.9 °C in southern England. The resulting low-to-mid-latitude meridional temperature gradient of ~0.7 °C per degree latitude is similar to the gradient today and suggests modern heat transport values for Cenomanian low latitudes. Oxygen isotopic composition of unaltered specimens of the Upper Cenomanian belemnite Praeactinocamax plenus (mean: 0.2‰) is enriched in 18O by 1‰ in comparison to brachiopod shells. Correspondingly, Upper Cenomanian palaeotemperatures derived from belemnite calcite (~13.0±1.3 °C) underestimate modelled mid-latitude sea surface temperatures by ~6 °C. Since P. plenus occurs as Boreal pulse fauna during a short interval in the Late Cenomanian, its heavy oxygen isotopic composition can be attributed either to migration from a cooler and/or deeper water mass or to unknown vital effects.