Carbon and oxygen stable isotopes in Chalky limestone, and its relation to phytoplanktons bloom: an example from Ab-Deraz Formation in Kopet-Dagh Basin

The Upper Cretaceous carbonate sequence of Ab-Deraz Formation is well exposed in the Kopet- Dagh basin. This Formation comprises marly limestone and three chalky limestone units. Chalky limestone of Ab-Deraz Formation is mainly composed of calcareous dinoflagellate (pithonellid calcispher)، cocolithofore and planktonic foraminifera. Carbon and oxygen stable isotopes analysis of the third Chalky limestone unit in Hammam-Ghale section is the aim of this study to examine relation between phytoplanktones bloom and the stable isotopes variations. The carbon isotope data varies between +0. 241‰ and +2. 491‰ (PDB) and oxygen isotope between -5. 008‰ and -3. 081‰ (PDB). Both carbon and oxygen isotopes data show abrupt change of 2‰ at the beginning of the chalky limestone formation. Carbon isotope variations of the studied samples represent oceanic water condition due to carbon cycle changes. These changes may occur by various factors such as organic carbon production، and greenhouse gases (CO2) exchange between atmosphere and seawater through photosynthesis of algae. Oxygen isotopes data (δ18O) of the studied samples also indicate the role of photosyntetic algae in isotopic fractionation of oxygen during calcification of their carbonate cycts and shells. The consistent isotopic variations of carbon and oxygen with lithological changes are due to high presence of dinoflagellates and cocolithofores in the Chalky limestone.