Iridium anomalies of late Eocene age at Massignano (Italy), and ODP Site 689B (Maud Rise, Antarctic)

Iridium anomalies of Late Eocene age are here reported in sequences from northern Italy and Weddell Sea, Antarctic. We have analyzed 270 samples of marly limestones from the Eocene/Oligocene boundary stratotype section at Massignano, Italy, ranging from 35.6 to 34.5 Ma (NP 18 Zone to NP19/20 Zone). We discovered a distinct Ir peak of 156 ± 19 parts per trillion (ppt) in the lower calcareous nannofossil NP19/20 Zone, corresponding to the mid-lower part of foraminiferal P16 Zone, and the top of magnetic Chron 16n2. The age of this Ir anomaly derived from several dated volcanic ashes found in the same section is 35.7 ± 0.4 Ma. A minor Ir anomaly also occurs within Chron 16r1 (∼35.55 Ma). Two small Ir anomalies were previously found in the lower part of Chron 13r (∼34.8 and 34.6 Ma) in the Contessa section near Gubbio (about 80 km west of Massignano), but were not found in equivalent stratigraphic intervals in this section. A total of 410 samples of Late Eocene age analyzed in ODP 689B (Maud Rise, Weddell Sea, Antarctic), also reveal a distinct Ir peak (156 ± 10 ppt). This occurs in sediments of normal remanent magnetization that can be interpreted either as Chron 16n1 (∼35.5 Ma), or as Chron 15n (∼35.0 Ma). This stratigraphic ambiguity results from scarcity of useful microfossil datums in this interval. Moreover, the last occurrence of Isthmolithus recurvus and the first occurrence of Globigerinatheka index, which are the only two datums defining a biostratigraphic zonation for the Late Eocene in ODP 689B, are apparently diachronous with the Massignano stratotype section. The principal Ir anomaly in ODP 689B seems 0.2 Ma, or 0.5 Ma younger than that of the Massignano section, depending on which alternative magnetostratigraphic interpretation is employed. The apparent diachroneity of microfossil datums and a lack of magnetostratigraphic data do not provide adequate stratigraphic resolution for correlation of Ir anomalies in Massignano or ODP 689B sections with those found in several Late Eocene sequences in the equatorial Indian and Pacific, and the Caribbean-Gulf of Mexico region. Nevertheless, multiple Ir anomalies as documented in this work, and differences in chemical composition among Late Eocene north American tektites, microtektites, and crystal-bearing spherules, the latter often associated with an iridium anomaly, suggest a sequence of closely spaced extraterrestrial impacts occurred during a million year period between 35.7 and 34.7 Ma.