Late Valanginian–Barremian (Early Cretaceous) palaeotemperatures inferred from belemnite stable isotope and Mg/Ca ratios from Bersek Quarry (Gerecse Mountains, Transdanubian Range, Hungary)

A high resolution stable isotope study of Upper Valanginian–Barremian (Early Cretaceous) belemnites from Bersek Quarry (Gerecse Mountains, Transdanubian Range, Hungary) is presented. Over 190 belemnite rostra (including Hibolithes subfusiformis, Duvalia dilatata and Conohibolites gladiiformis) have been analysed for oxygen and carbon isotopes and for trace element geochemistry. The obtained carbon isotope curve shows a long term decrease from ~ 1.2‰ in the Upper Valanginian to ~−0.5‰ in the Upper Hauterivian followed by more variable values in the Early Barremian. Superimposed on this trend are a number of possible shorter term peaks. This pattern broadly follows published carbon isotope curves for the same interval and is therefore thought to reflect a global rather than a regional signal. The oxygen isotopes show the most positive values in the uppermost Valanginian and become increasingly more negative through the Hauterivian into the Barremian. Such changes are interpreted as an increase in marine temperatures through the section. The Mg/Ca data paralleling the oxygen isotope trend confirms our temperature interpretation. The oxygen isotope ratios are generally more negative and therefore allow us to infer warmer temperatures, than those derived from belemnites from time equivalent sections in Germany and Speeton, UK, consistent with the more southerly latitudinal position of the Gerecse Mts. within the Tethys Ocean. The oxygen isotope and Mg/Ca data also reveal habitat differences for the different belemnite groups analysed. Vaunagites pistilliformis, “Belemnites” pistilliformis and Hibolithes typically have more negative oxygen isotope values than Pseudobelus and Duvalia and are therefore interpreted to have lived in warmer and/or shallower parts of the water column, consistent with previous interpretations.

Research highlights
► A high resolution Valanginian–Barremian stable isotope curve.
► A global rather than a regional signal.
► Warm subtropical temperatures.
► Isotope & Mg/Ca data reveal discrete habitats for morphologically distinct belemnites.