Planktonic Foraminiferal Biostratigraphy and Carbon Isotope Records of Oceanic Anoxic Event 2 in Southern Tethyan Himalaya

Oceanic Anoxic Event 2 (OAE2) at the Cenomanian/Turonian boundary (~93.9 Ma) represents a major environmental perturbation, caused by an addition of large amounts of carbon into the ocean-atmosphere system. It is characterized by a positive carbon isotopic excursion (CIE) and is associated with increased organic carbon burial in some areas. Previous studies on planktonic foraminiferal biostratigraphy suggest that the OAE2 spans from the upper Rotalipora cushmani zone, through Whiteinella archaeocretacea zone, and to the lower Helvetoglobotruncana helvetica zone. However, some recent studies propose that the OAE2 ends within W. archaeocretacea zone. To further examine planktonic foraminiferal evolution during the OAE2, we investigate a new semi-pelagic, marl/mudstone section at Guru (southern Tibet), covering the upper Cenomanian – middle Turonian interval. Planktonic foraminiferal biostratigraphy, carbon isotopes (δ¹³Ccarb and δ¹³Corg) are analyzed. We find that the OAE2 at Guru covers R. cushmani and W. archaeocretacea two biozones, consistent with some recent studies. Positive CIEs in δ¹³Ccarb and δ¹³Corg are not identical, with larger amplitude in δ¹³Corg (~2.5 ‰) than δ¹³Ccarb (~1.5 ‰). Some key bioevents in the section are recognized, including the Highest Occurrence of R. cushmani and Thalmanninella greenhornensis as well as the Lowest Occurrence of H. helvetica. Comparisons between carbon isotope records and biostratigraphy from regional and global data implies that some bioevents during the OAE2 show a varying degree of diachroneity.