Isotopic evidence for temperature variation during the early Cretaceous (late Ryazanian-mid-Hauterivian)

Oxygen and carbon isotopic compositions have been determined from the belem nite genera Acroteuthis and Hibolites sampled from the early Cretaceous (Ry azanian-Hauterivian) interval of the Speeton Clay Formation, Filey Bay, Eng land. The Speeton Clay Formation consists of a series of claystones and cal careous mudrocks deposited in an epicontinental sea. delta(18)O values from belemnites, which met petrographic and chemical criteria for well preserve d skeletal carbonate, indicate warm marine palaeotemperatures (c. 12-15 deg rees C) for much of the early Valanginian whilst cool temperatures (<9 degr ees C) are inferred for the earliest Hauterivian. During the remainder of t he Hauterivian, temperatures fluctuated considerably and rose to a maximum of 15.5 degrees C. Changes in kaolinite and smectite abundances, considered to reflect humid and arid phases of climate, correlate with warm and cool episodes. The palaeotemperature record, appears to contradict evidence from cephalopod faunas, which show a Tethyan influx during the inferred early H auterivian cool period. However, this was a transgressive phase and thus th e cephalopods could have been less sensitive to temperature than to water c olumn stability and to land barriers. A positive shift in the carbon isotop e profile obtained from the Speeton belemnites appears correlatable: with c arbon isotope profiles recorded from pelagic Tethyan successions, albeit wi th somewhat differing absolute values. The data support earlier models of c arbon isotope variation, in that positive excursions are associated with an inferred global rise in sea level.