ISOTOPIC VARIATION IN FOSSILS AND MATRIX OF THE CRETACEOUS RED CHALK AT SPEETON AND SOUTH FERRIBY, YORKSHIRE, ENGLAND

Macro- and micro-fossils (belemnites, planktonic and benthonic foramin ifera) and coccolithic matrix from the middle Cretaceous Red Chalk of Yorkshire, UK, have been examined using petrographic, isotopic, and ch emical methods to investigate the conditions within which they were fo rmed or altered. Only the belemnites (Neohibilites sp.) attain the pet rographic and geochemical criteria for being well preserved. Zn contra st, the foraminifera and matrix reveal evidence of diagenetic alterati on. Mean belemnite delta(18)O values are -0.97% (PDB) from South Ferri by and 1.28% (PDB) from Speeton. Assuming a sea water SMOW value of -1 .0%, mean palaeotemperatures calculated from the oxygen isotopic compo sitions are 15.9 degrees C and 16.8 degrees C respectively. The range of isotopic values provided by the belemnites in this study suggests t hat the isotopic values obtained from early studies of the Chalk, that used only petrographic and/or hand specimen descriptions to screen fo r diagenetic alteration, could largely represent unaltered values. The palaeotemperatures interpreted from the belemnites are comparable to recent estimates of middle Cretaceous temperature from general circula tion model predictions.