Integrated stratigraphic and astrochronologic calibration of the Eocene-Oligocene transition in the Monte Cagnero section (northeastern Apennines, Italy): A potential parastratotype for the Massignano global stratotype section and point (GSSP)

We present the results of integrated biostratigraphic (planktonic foraminifera, calcareous nannofossils, and dinofl agellates), magnetostratigraphic, and cyclostratigraphic analyses of the lower part of Monte Cagnero section (Umbria-Marche Apennines of Italy), a continuous and complete succession of pelagic limestone and marls that provides the means for an accurate and precise astrochronologic calibration of the Eocene-Oligocene transition. This 38.5-m-thick section overlaps the Oligocene section, which, at meter level 188, contains the Rupelian-Chattian boundary corresponding to the O4-O5 planktonic foraminiferal zonal boundary within the upper half of magnetochron C10n. The Eocene-Oligocene boundary at Monte Cagnero, as defined by the last occurrence of hantkeninid planktonic foraminifers (E14-E15 zonal boundary), is found at meter level 114.1, in the upper part of calcareous nanno fossil zone CP16a, and very near the Aal-Gse dinocyst zonal boundary. Paleomagnetic analysis has identified all the magnetic reversals from the lower C13r to the lower C12n, precisely overlapping the base of the Oligocene magnetostratigraphic succession and placing the Eocene-Oligocene boundary in the upper part of C13r, in full agreement with the global stratotype section and point (GSSP) at Massignano. Spectral analysis of calcium carbonate data from bulk samples, collected at 5 cm intervals, indicates that orbital forcing of depositional cycles (i.e., limestone versus marl alternations) is dominant at frequencies corresponding to the theoretical astronomical curves of eccentricity, obliquity, and precessional cycles throughout the studied Eocene-Oligocene transition. Correlation with the astrochronologic time scale allows an age assignment of 33.95 Ma for the Eocene-Oligocene boundary, which is in close agreement with the astrochronologic age for the boundary in the GSSP of Massignano obtained in a similar study by R.E. Brown and colleagues in this volume. Thus, the Monte Cagnero section represents a candidate parastratotype for the Eocene-Oligocene GSSP of Massignano in the eventuality that the oxygen and carbon stable isotope shifts defining the oxygen isotope Oi-1 glaciation will be preferred over the last occurrence of hantkeninids as marker for the boundary, since, at Massignano, the beginning of this isotope shift is barely represented in the uppermost part of the exposed section. The excellent integrated stratigraphic framework of Monte Cagnero presented here will significantly facilitate further high-resolution isotope and paleoecological studies across the time of transition from a hothouse to icehouse Earth.