Elevated sea surface temperature and enhanced primary productivity during Ocean Anoxic Event 1d in the eastern Tethys: Calcareous nannofossil evidence from southern Tibet, China

Mid-Cretaceous oceanic anoxic events (OAEs) are characterized by major disturbances to the global carbon cycle and are considered ancient analogs for anthropogenic global warming. Among these events, OAE 1d, occurring around the Albian-Cenomanian boundary, remains understudied in the eastern Tethys region, leaving a critical palaeogeographic gap that hampers a global understanding of its impacts. In this paper, we present a detailed record of calcareous nannofossil assemblages from the lower to middle part of the Lengqingre Formation in the Chaqiela section of Gamba, southern Tibet, China. The study interval contains abundant, moderately to well preserved calcareous nannofossil assemblages, with Watznaueria barnesiae, Discorhabdus ignotus, Biscutum constans, and Zeugrhabdotus erectus as the dominant species constituting >50% of the assemblages. These nannofossil assemblages allow the study interval to be constrained to Upper Cretaceous (UC) Biozones UC0 through UC2, with the Albian-Cenomanian boundary located in the lower part of the Lengqingre Formation, and the middle part assigned to Cenomanian in the study area. As such, the position of OAE 1d in this section is accurately determined. Calculated nutrient and temperature indices based on nannofossil species with preferred ecologies reveal significant palaeoceanographic changes across the OAE 1d. These changes show increased abundance of warm-water taxa (e.g., Rhagodiscus asper and Zeugrhabdotus diplogrammus) and high-productivity taxa (e.g., D. ignotus, B. constans, and Z. erectus), suggesting increased sea surface temperature and enhanced productivity during OAE 1d in the southwestern shelf sea of the eastern Tethys Ocean.