Nannoplankton response indicates intra-OAE2 cooling (Plenus Cold Event) in southeastern Tethys (Qiangdong, Tibet)

The Cretaceous Oceanic Anoxic Event 2 (OAE2, ~93.9 Ma) was a period of rapid global environmental change and one of the warmest intervals in the Phanerozoic. Despite its global significance, we still know little about the effects of this greenhouse event from the shallow marine shelf environments of the Southern Hemisphere. Here, we present a paleoenvironmental reconstruction from the eastern Tethys Ocean based on calcareous nannofossil paleoecological records from an OAE2 section (Qiangdong) in southern Tibet. Our nannofossil temperature index indicates onset of warming ca. 75 kyrs before the OAE2, peaking in the early OAE, but followed soon (~60 kyr after the OAE onset) by a phase of climatic instability. This cool interval correlates well with the Plenus Cold Event (PCE), previously documented in the Northern Hemisphere (e.g., western Europe). The PCE cooling is followed by further warming in the later OAE, continuing into the post-OAE2 early Turonian interval consistent with peak warmth of the Cretaceous thermal maximum (KTM). A calcareous nannofossil productivity index reveals dynamic surface water productivity trends with maximum values during the PCE cool interval indicating that sequestration of CO2 through elevated marine primary productivity was likely an important feedback during this carbon cycle perturbation event.