Late Eocene impact ejecta in Italy

Author(s)

Christian Koeberl, Alessandro Montanari, Toni Schulz, Jonas Tusch, Florence Mougel, Frederic Moynier

Abstract

The late Eocene was marked by multiple impact events, possibly related to a comet or asteroid shower. Marine sediments worldwide contain evidence for at least two closely spaced impactoclastic layers. The upper layer might be correlated with the North American tektite-strewn field (with the 85-km-diameter Chesapeake Bay impact structure [USA] as its source crater), although this is debated, whereas the lower, microkrystite layer (with clinopyroxene [cpx]-bearing spherules) was most likely derived from the 100-km-diameter Popigai impact crater (Russia). The Eocene-Oligocene global stratotype section and point is located at Massignano, Italy, and below the boundary, in the late Eocene, at the 5.61 m level, shocked quartz and pancake-shaped smectite spherules that contain (Ni- and Cr-rich) magnesioferrite spinel crystals are found. These are associated with a positive Ir anomaly in deposits with the same age as the Popigai-derived cpx spherule layer. This layer is overlain by another Ir-rich layer, likely due to another large impact event. From a large amount of “pancake-bearing” rock, we isolated a few hundred milligrams of this spinel-rich material. The tungsten isotopic composition of this material shows more or less a terrestrial composition. However, the spinel-rich materials have excess

⁵⁴Cr values (expressed as e

⁵⁴Cr, which is the per ten thousand deviation of the

⁵⁴Cr/

⁵²Cr ratio from a terrestrial standard) of around –0.4 to –0.5 e

⁵⁴Cr, which distinctly point to an ordinary chondritic impactor. This result supports the asteroid impact interpretation but not the comet impact hypothesis.

Organisation(s)

Department of Lithospheric Research

External organisation(s)

Osservatorio Geologico di Coldigioco, Naturhistorisches Museum Wien (NHM), Universität zu Köln, Université de Paris, Universidad Nacional Autonoma de Mexico

Volume

542

Pages

347-354

No. of pages

8

DOI

https://doi.org/10.1130/2019.2542(18)

Publication date

2019

Peer reviewed

Yes

Austrian Fields of Science 2012

105105 Geochemistry

ASJC Scopus subject areas

Geology

Sustainable Development Goals

SDG 14 - Life Below Water

Portal url

https://ucrisportal.univie.ac.at/en/publications/da7a03a3-6f7f-4c74-b193-04345672514b