INTRODUCTION

The Cretaceous Period extended from 145 Ma to 65 Ma ago, and is believed to have differed from our present world in several major respects. First, the period has been characterized as a time of globally warm ‘greenhouse’ conditions, in which there were no polar ice caps. Although this is generally correct, there is evidence for considerable climatic change in both temperature and patterns of humidity. Secondly, Cretaceous deep ocean water was warm and saline and derived from low latitude areas of high evaporation, in contrast to the cold polar water which occupies the deep oceans at present. Thirdly, eustatic sea levels were very high, particularly during the Late Cretaceous, and the interiors of the major continents were covered with shallow seas.

Scientists have only recently provided good explanations for these differences. Although detailed interpretations vary, there is general agreement that high levels of atmospheric carbon dioxide (a well-known greenhouse gas), perhaps eight to ten times those at present, were responsible for elevated temperatures in the Cretaceous. The resulting climates were equable, with moderately high polar temperatures and a low pole to equator temperature gradient. As a consequence, high latitude sea water was neither sufficiently cold nor saline to sink and form bottom water. The source of high carbon dioxide levels has been attributed to outgassing during periods of exceptionally fast ocean floor spreading.