Alessandro Montanari, 1990. "Geochronology of the terminal Eocene impacts; An update", Global Catastrophes in Earth History; An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality, Virgil L. Sharpton, Peter D. Ward
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Detailed work on planktonic foraminiferal and nannofossil biostratigraphy, magnetostratigraphy, chemostratigraphy, and geochronologic calibration of the pelagic sequence of the Northern Apennines has been done by a number of researchers in recent years. This work provides the means to locate, in the geochronologic time scale, biological and physical events that occurred in the critical period across the Eocene/Oligocene boundary with unprecedented precision and accuracy. In particular, this tight geochronological calibration confirms previous estimates of the age of two closely spaced impact events that occurred about 34.2 to 34.5 Ma, at the beginning of polarity zone 13R, corresponding to the upper part of NP19/20 nannofossil range zone and the middle of P16 (G. cerroazulensis Zone). An older microspherule layer associated with an Ir anomaly reported in 1987 by Keller and co-workers occurs in the top part of the G. semiinvoluta Zone (top NP18, upper P15), corresponding to the upper part of magnetozone 16N. According to the geochronologically calibrated biostratigraphic and magnetostratigraphic Italian sections, the age of this older impact is 36 to 36.5 Ma. The timing of the late Eocene impacts correlates well with a period of accelerated marine biotic crisis that characterized the uppermost part of the Eocene. The tight interdisciplinary stratigraphic calibration of the Italian pelagic sequence provides a practical stratigraphic standard for further studies of the high-resolution event stratigraphy of mass extinctions and large extraterrestrial impacts.