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The late Cenomanian carbon isotope excursion can be shown to be synchronous over most areas of the globe where micropalaeontological data are adequate. The sedimentological variations seen in these different areas are thought to be due to palaeobathymetry. The relationship between this 'anoxic' event and the major faunal change in the late Cenomanian is described.In a recent review of 'anoxic events ', Funnell (1987) has questioned (1) how anoxic, anaerobic or dysaerobic such events were, and ( 2 ) how localized or widespread were their effects. Funnell (1987) does, however, indicate that the 'event' in the late Cenomanian-early Turonian is the most likely candidate for a widespread, if not global, distribution. Recent work on the detailed microbiostratigraphy of the associated sediments, together with a greater appreciation of the ecological niches affected by the changes in water chemistry at this time lead us to conclude that we have a widespread synchronous event. Unfortunately the progressive build-up of the event affected differing palaeobathymetric levels at different times and this has led to some confusion over the age of the organic-rich sediments so produced.

The late Cenomanian sea-level rise was coupled with an expansion of the oxygen-minimum zone. The position of its upper surface may be discerned from the sequential effects on the benthonic and planktonic biota.

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