In a global scale, the Cenomanian-Turonian boundary event (OAE2) has been characterized by its warm climate interval accompanied by a major sea level transgression. However, chemo- and biostratigraphy studies in the Kopet-Dagh basin (NE Iran) indicate two intervals of low sea surface temperature in the uppermost of Rotalipora cushmani and Whiteinella archaeocretacea biozones, according to pCO2 (∆13C; difference between δ13Ccarb and δ13Corg) minima and high δ18O values. These cold intervals began shortly after episodes of warm sea surface temperature and enhanced burial of organic matter (high TOC contents) and have been attributed to a drop in atmospheric pCO2 level which in turn was caused by enhanced carbon sequestration by black shale formation. The first cooling interval at onset of OAE2 is recorded throughout the world and called “Plenus cold event” in Europe and “benthic oxic zone” in the proto north Atlantic and Western Interior Seaway of North America and coincides with positive shift of carbon isotope (δ13C) values of carbonate and organic matter. Also a common feature to the two organic-rich sediments intervals (warm episodes) in the Kopet-Dagh basin is the low δ15N values that falls around 0‰ and implies enhanced nitrogen fixation process as a consequence of enhanced productivity and expanded oxygen minimum zone.

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