Article: Isotopic ratios and Wealden environments

Isotopic methods of assessing Wealden palaeoenvironments are reviewed. Emphasis is placed on the importance of using only primary, skeletal, untransported carbonates. Recrystallization and cementation generally reduce the 13C/12C and 18O/16O ratios of Wealden carbonates. Hence, while those with high 13C/12C are likely to be marine, those with low 13C/12C cannot be assigned to a depositional environment unless known to be primary.Ratios from aragonitic shells of one species each of Cassiope, Neomiodon, and Filosina? indicate 'marine' conditions. Less certainly, marine conditions may be indicated by calcitic shells of pelecypods (one species each of Lio-strea and Neomiodon), gastropods (one species of Paraglauconia), and ostracods (two species of Cypridea and one of Mantelliana). Forms not assignable to 'marine' or 'fresh' water are three species of Cypridea, one species each of Theriosynoecum, Damonella, Filosina, Unio, Viviparus, and a corbulid, and two species of Equisetites. The new data support the transgressive models for the major Wealden clay formations.Interpretation of transitional water temperatures from 18O/16O ratios is unreliable. Nevertheless, values from primary aragonites in the Hastings Beds (Neomiodon) and Weald Clay (Filosina, Cassiope) lie in the range of modern marine molluscs from warm temperate waters. This is consistent with current opinion based on other palaeontological evidence.