Basaltic explosive volcanism, but no comet impact, at the Paleocene–Eocene boundary: high-resolution chemical and isotopic records from Egypt, Spain and Denmark

Abstract

In the search for a triggering mechanism for the Paleocene–Eocene (P–E) boundary event, 55 Ma, centimeter-resolution chemical (e.g., Ir, Os, Pt) and isotope (e.g., 187Os/ 188Os, 3He/ 4He, 87Sr/ 86Sr) records across this boundary have been established for six uplifted marine sections in Egypt, Spain and Denmark. The sections studied represent some of the stratigraphically most complete records across the onset of the carbon isotopic excursion (CIE) and associated benthic foraminifera extinctions that mark the Paleocene–Eocene boundary. High-sensitivity analyses failed to uncover evidence of extraterrestrial element or isotope enrichments in the six sections, refuting the hypothesis of a major comet impact at the boundary. Preliminary searches for other impact-indicative features, such as spherules or shocked quartz, also gave negative results. In the Danish section studied, three basaltic Ir-rich ash layers occur at the Paleocene–Eocene boundary, but no similar ashes were found in Egypt or Spain. The three ashes represent the earliest known manifestation of an unusual 1 to 2 million year long phase of explosive basaltic volcanism in the Færø-Greenland region. This volcanism is synchronous with major flood basalt effusions in East Greenland and is associated with prominent paleogeographic changes in the high-latitude North Atlantic region. Discharge of mantle-derived Os to seawater during this volcanism may explain a small decrease in 187Os/ 188Os ratio at the Paleocene–Eocene boundary in the Zumaya section in Spain. The environmental perturbations at the Paleocene–Eocene boundary appear to have been triggered by basaltic volcanism, but any model for the detailed causal relation remains speculative.