Comparison of carbonate C and O stable isotope records across the Jurassic/Cretaceous boundary in the Tethyan and Boreal Realms
Abstrakt
Carbon and oxygen stable isotope records were compared for Jurassic/Cretaceous (J/K) boundary sections located in the Tethyan Realm (Brodno. Western Slovakia, and Puerto Escano, Southern Spain; bulk limestones), and the Boreal Realm (Nordvik Peninsula, Northern Siberia, belemnites).
Since a detailed biostratigraphic correlation of these Tethyan and Boreal sections is impossible due to different faunal assemblages, correlation of the isotope records was based on paleomagnetic data. This novel approach can improve our understanding of the synchroneity of individual isotope excursions in sections where detailed biostratigraphic correlation is impossible.
No significant excursions in either the carbon or oxygen isotope records to be used for future Boreal/Tethyan correlations were found around the J/K boundary (the upper Tithonian and lower Berriasian; magnetozones M20n to M18n) in the studied sections. At the Nordvik section, where a much longer section (middle Oxfordian-basal Boreal Berriasian) was documented, the transition from the middle Oxfordian to the Kimmeridgian and further to the Volgian is characterized by a decrease in belemnite delta(18)O values (from delta(18)O values up to +1.6 parts per thousand vs.
V-PDB in the Oxfordian to values between +0.3 and -0.8 parts per thousand in the late Volgian and earliest Boreal Berriasian). This trend, which has previously been reported from the Russian Platform and Tethyan Realm sections, corresponds either to gradual warming or a decrease in seawater delta(18)O.
Supposing that the oxygen isotope compositions of seawater in the Arctic/Boreal and Tethyan Realms were similar, then the differences between oxygen isotope datasets for these records indicate differences in temperature. The Boreal/Tethyan temperature difference of 7-9 degrees T in the middle and late Oxfordian decreases towards the J/K boundary, indicating a significant decrease in latitudinal climatic gradients during the Late Jurassic.