Paper No. 192-3
Presentation Time: 8:40 AM
DISTAL IMPACT EJECTA AT PALEOCENE-EOCENE BOUNDARY SECTIONS ON THE ATLANTIC MARGIN (Invited Presentation)
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.