Ostracod diversity and sea-level changes in the Late Cretaceous of southern England

Abstract

The available data of ostracod ranges for the Cenomanian, Campanian and Maastrichtian stages of the Late Cretaceous of the northern part of the Anglo-Paris Basin were examined and combined with new data from the Turonian, Santonian and Coniacian stages. A new cumulative species diversity curve is presented for the Ostracoda of the Late Cretaceous of Britain. The results obtained challenge the method of chronoecologic charts to determine sea-level from diversity. When a more complete data set is applied, and compared with published sea-level curves, the result is the inverse of that previously predicted by employing chronoecologic charts. A model is presented of changing sea-levels in S.E. England from the Cenomanian through to the Santonian, which integrates the new diversity data with published sea-level changes and curves of stable isotopes of oxygen and carbon. In the earliest Cenomanian, low diversity is associated with a deeper water depositional environment and warmer temperatures. The mid-Cenomanian diversity maximum corresponds to a regressive trough and cooler water. Over the Cenomanian–Turonian boundary interval the diversity minimum is correlated with global sea-level and temperature maxima. The proportion of ostracods possessing eye tubercles falls to a minimum over this period. After the diversity crash, the Cenomanian fauna was replaced by the new Turonian fauna; east–west migrations into the Anglo-Paris Basin were facilitated by the sea-level rise overcoming marginal basin highs. The pattern seen in the mid-Cenomanian is also present at the Turonian–Coniacian boundary interval; that of high diversity corresponding with a regressive trough on a long-term regressive trend with cooling conditions. The model for this northern part of the Anglo-Paris Basin then associates high diversity with regressive cooler conditions, and low diversity with deeper and warmer water.