The 4–8 m segment of the Global Stratotype Section and Point (GSSP) for the Eocene-Oligocene boundary at Massignano contains a well studied and documented impactoclastic layer with anomalous concentrations of extraterrestrial iridium, shocked quartz, Ni-rich spinels, impact spherules, and a prominent 3He anomaly. This layer, located at 5.65 m and dated at 35.7 ± 0.4 Ma (Late Eocene), appears to be coeval with the Chesapeake Bay or the Popigai impact structure. We have investigated planktonic foraminifers from this interval with the aim of assessing the effects that the impact may have had on the environment and this group of organisms.

Interpretation of data suggests that the impact had no abrupt, dramatic effects on planktonic foraminifers. However, since it occurred in a stressed environment characterized by gradual and progressive cooling, it enhanced climatic deterioration and variations in water mass structure. The cooling produced shallowing of the thermocline depth and drastic reduction in abundance of shallow- and warm-water species. The short-term warming episode in the lower part of Zone P16 between 5.8 m and 5.9 m may be due to a short-term-impact-induced greenhouse episode and seems to be global in extent.

Finally, the relatively long duration of the environmental perturbation that followed the impact event, which culminated with the Early Oligocene sharp temperature drop, suggests that a feedback mechanism sustained the initial impact-induced changes.

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