Late Cretaceous carbon- and oxygen isotope stratigraphy, nannofossil events and paleoclimate fluctuations in the Haţeg area (SW Romania)
Abstract This study presents carbon and oxygen isotope fluctuations identified in marine red marlstones and claystones of the Fizesti Formation from the SE Haţeg area (Southern Carpathians, Romania). Biostratigraphy based on calcareous nannoplankton allow the identification of nannofossil standard zones and subzones CC16–CC17 up to CC20, and respectively UC11c up to UC15b, indicating that the investigated deposits are latest Coniacian–late Early Campanian in age. Throughout the studied succession, δ13C values fluctuate between 1.4‰ and 2.65‰, while δ18O values are between − 3‰ and − 4‰. In the middle Santonian (within the UC12 calcareous nannofossil zone), we found a slight positive carbon isotope excursion, with values increasing by 0.3‰, up to a value of 2.44‰, which we assume to represent the regional expression of the Horseshoe Bay Event, originally recognized in the English Chalk. In the uppermost Santonian, just below the first occurrence of the crinoid Marsupites testudinarius (within the UC13 calcareous nannofossil zone), we recorded a δ13C increase, from 2.38‰ up to 2.63‰, correlative with the Hawks Brow Event, firstly described in UK. The most significant δ13C increase identified by us, up to 2.63‰, is coincident with the last occurrence of the crinoid Marsupites testudinarius (= the Santonian–Campanian boundary) and represents the regional expression of the worldwide distributed Santonian/Campanian Boundary Event. The Santonian–Campanian boundary is associated with a slight positive excursion for δ18O. In the lower part of the Campanian stage, δ13C values progressively decrease to around 1.5‰. A decrease of the water surface temperature in the region is suggested for the lower part of the Santonian, an assumption based on the significant shift to less negative values of δ18O and on the mixed (Tethyan and Boreal) character of the identified nannofloras. From the Santonian–Campanian boundary interval upwards, the temperatures were rising, leading to an arid and warm climate in the region. This climate mode continued probably into the Maastrichtian, when the Haţeg area became part of an island situated in the Northern Tethys Realm.