GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 192-3
Presentation Time: 8:40 AM


SCHALLER, Morgan F.1, FUNG, Megan K.1, WRIGHT, James D.2, KATZ, Miriam E.1 and KENT, Dennis V.2, (1)Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, 110 8th St, Troy, NY 12180, (2)Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854,

A rapid global warming event ~56 million years ago at the Paleocene-Eocene (P-E) boundary (the Paleocene-Eocene Thermal Maximum, PETM) was accompanied by a negative carbon isotope excursion (CIE) observed globally. We report the discovery of silicate glass spherules in a discrete stratigraphic layer from several marine P-E boundary sections on the Atlantic Margin. The spherules are found in the onset of the CIE that defines the P-E boundary at each site. They average 275 mm in diameter, and have rotational and splash form morphologies, surficial microcraters, and are translucent colorless to brown, green and black. Energy dispersive x-ray spectroscopy (EDS) from grain mounts and polished sections of representative spherules show that they have related major oxide chemistries of up to ~50% silica, with the remainder comprised of CaO, FeO, and Al2O3, which all vary in relative proportion with silica content. The chemistries of the spherules form a population that is distinct from impact ejecta from other major strewn fields, but shows more variability than is expected from volcanism. Field transmission infrared spectroscopy on a subset of spherules reveals water content <0.03%, much lower than volcanic glass spherules. They also contain inclusions of lechatelierite (a high temperature quartz glass), and quartz grain inclusions that show characteristic Raman spectra indicative of shock metamorphism: in particular relaxation of the spectral peak corresponding to SiO2 bond-bending vibration from 464 to 460 cm-1, consistent with observations from other Raman studies of quartz experimentally shocked to peak pressures of 25.8 GPa. The summation of these characteristics is not compatible with a volcanic origin for the spherules, but is consistent with features of melt-drop microtektites and microkrystites from other known impact strewn fields. We therefore interpret the P-E boundary spherules as a component of a distal impact ejecta layer, indicating that an extraterrestrial impact was coincident with the onset of the CIE.