Tracking ocean circulation and palaeotemperature in Maastrichtian seas using C and O isotopes in belemnite rostra fossils

Abstract

The Cretaceous period 145-66 million years ago is often cited as a textbook example of a greenhouse climate with high levels of atmospheric carbon dioxide ([CO2]atm) A wealth of palaeoclimate proxy data from the fossil record and studies in isotope geochemistry had previously shown that ocean temperatures were much higher across all latitudes. However, recent findings highlight that great climate variability exists in the late Cretaceous, characterized by global carbon cycle perturbations, ocean anoxic events and falling CO2 concentrations. Although δ18O of planktonic and benthic foraminifera calcite have traditionally been used for palaeotemperature reconstructions during this period, results are often marred by diagenetic alteration of their calcite. Additionally, temporal gaps in temperature data exist for the late Cretaceous mid-latitude North Atlantic Ocean. In this project, we analyze carbon, oxygen isotopes and trace element ratios in late Cretaceous belemnite rostra from the Boreal Chalk Sea (BCS) to reconstruct sea surface palaeotemperatures, carbon burial and ocean circulation. Mg/Ca and δ18O results suggest that temperatures decreased by ~3°C from the Coniacian to the early Maastrichtian while δ13C trends were interpreted as more reflective of sea level changes than carbon burial. Reconstructed δ18Osw showed the possibility of an influx of isotopically lighter, higher latitude water into the BCS from the Arctic Ocean as sea levels rose from the Coniacian to the mid-Campanian before falling from the late Campanian to early Maastrichtian. Hence, both falling CO2 levels and changes in ocean circulation likely contributed to the cooling of the late Cretaceous climate.