Paleoenvironmental perturbation across the Cenomanian/Turonian boundary of the Kopet-Dagh Basin (NE Iran), inferred from geochemical anomalies and benthic foraminiferal assemblages

A Cenomanian/Turonian boundary succession in the northeastern Tethyan realm, Kopet-Dagh Basin in NE Iran, was studied to examine the effects of biological productivity, atmospheric CO2 concentration, and sea water temperature on the benthic foraminiferal assemblages. Our δ13Ccarb and δ13Corg chemostratigraphy of the Gharesu section reveals three positive peaks that can be correlated with carbon isotope peaks in the CTB reference sections (Eastbourne, England; Pont d'Issole, France; Rock Canyon, USA). Two intervals of suspected high sea surface temperature were distinguished according to pCO2 maxima (Δ13C; difference between δ13Ccarb and δ13Corg), low δ18O values, and high TOC bearing deposits. These intervals are followed by falling pCO2 and cooling caused by enhanced burial of organic matter into the sediments. Patterns of changes in the benthic foraminiferal assemblages are correlated with these climatic changes. Warm intervals are characterized by low diversity of benthic foraminifera, dominance of agglutinated forms, and high abundance of infaunal morphogroups or opportunistic epifauna due to intensified weathering, high primary productivity, and oxygen deficiency in the bottom water. The cold intervals coincide with higher diversity of benthic foraminifera and dominance of calcareous benthic taxa.