Isotopic variation in fossils and matrix of the Cretaceous Red Chalk at Speeton and South Ferriby, Yorkshire, England

Abstract

SUMMARY Macro- and micro-fossils (belemnites, planktonic and benthonic foraminifera) and coccolithic matrix from the middle Cretaceous Red Chalk of Yorkshire, UK, have been examined using petrographic, isotopic, and chemical methods to investigate the conditions within which they were formed or altered. Only the belemnites ( Neohibilites sp.) attain the petrographic and geochemical criteria for being well preserved. In contrast, the foraminifera and matrix reveal evidence of diagenetic alteration. Mean belemnite δ 18 O values are −0.97‰ (PDB) from South Ferriby and −1.28‰ (PDB) from Speeton. Assuming a sea water SMOW value of −1.0‰, mean palaeotemperatures calculated from the oxygen isotopic compositions are 15.9°C and 16.8°C respectively. The range of isotopic values provided by the belemnites in this study suggests that the isotopic values obtained from early studies of the Chalk, that used only petrographic and/or hand specimen descriptions to screen for diagenetic alteration, could largely represent unaltered values. The palaeotemperatures interpreted from the belemnites are comparable to recent estimates of middle Cretaceous temperature from general circulation model predictions.