STABLE CARBON AND OXYGEN ISOTOPES OF SOME CARBONATE-FLUORAPATITES FROM CENOMANIAN AND TURONIAN SEQUENCES, BOHEMIAN CRETACEOUS BASIN, CZECH REPUBLIC

Abstract

Stable carbon and oxygen isotopes were analysed in the structural CO3 of carbonate-fluorapatite in phosphate nodules and coprolites present in distinctive horizons at the Cenomanian – Turonian boundary and in late Turonian sequence at the Bohemian Cretaceous Basin of the Czech Republic. The results indicate that the structural CO3 in the analysed carbonate-fluorapatites is depleted in δ13C relative to CO3 in carbonates of comparable age and all samples show negative δ18O values close to those recorded in mid-Cretaceous carbonate-CO3. The obtained δ13C values, however, are higher than those reported in apatite-CO3 of the late Cretaceous Tythyan upwelling-type phosphates. The δ13C results are interpreted to indicate that phosphogenesis have taken place in an environment rich in the light isotope of carbon, similar to that usually prevails in the organic-rich pore water below sedimentwater interface. The higher δ13C values in the studied apatites relative to their Tethyan analogues may reflect the signature of the well-documented δ13C excursion of the Oceanic Anoxic Event 2 at the Cenomanian-Turonian boundary. The δ18O values suggest paleotemperatures expressive of the mid-Cretaceous climatic warming and a lower paleosalinity than that of normal well-circulated seawater.