Oxygen and carbon isotope analyses of several species of size-controlled planktonic and benthic foraminifera from the Plenus Marls of Dover (Kent, England) show a clear separation in oxygen and carbon isotope ratios consistent with different habitats; the analyses indicate the existence of vertical gradients of temperature and 13C in dissolved CO2 in the Chalk Sea. Whereas sequential ontogenetic stages of planktonic foraminifera do not show systematic variations in δ18O or δ13C values (implying no systematic change in depth habitat during the growth of individual species), species of the genera Hedbergella and Dicarinella are interpreted to have lived nearer to the surface than Rotalipora. Because vertical 13C gradients in the Chalk Sea remained stable during the late Cenomanian, the oxygen-minimum zone is unlikely to have impinged on this area.

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