The extraterrestrial impact evidence at the Palaeocene-Eocene boundary and sequence of environmental change on the continental shelf.

We have identified clear evidence of an extraterrestrial impact within the onset of the carbon isotope excursion (CIE) that defines the Palaeocene-Eocene (P-E) boundary hyperthermal event (approx. 56 Ma) from several sites on the eastern Atlantic Coastal Plain and offshore. We review and update the state of the evidence for an impact at the P-E boundary, including a K-Ar cooling age of the ejecta that is indistinguishable from the depositional age at the P-E, which establishes the ejecta horizon as an isochronous stratigraphic indicator at the P-E. Immediately above the ejecta peak at the base of the coastal plain Marlboro Clay unit, we identify a sharp increase in charcoal abundance coincident with the previously observed dramatic increase in magnetic nanoparticles of soil pyrogenic origin. We therefore revisit the observed sequence of events through the P-E boundary on the western Atlantic Coastal Plain, showing that an extraterrestrial impact led to wildfires, landscape denudation and deposition of the thick Marlboro Clay, whose base coincides with the spherule horizon and CIE onset. The Sr/Ca ratio of the spherules indicates that the carbon responsible for the onset may be vaporized CaCO3 target rock mixed with isotopically light carbon from the impactor or elsewhere. Crucially, we do not argue that the impact was responsible for the full manifestation of the CIE observed globally (onset to recovery approx. 170 kyr), rather that a rapid onset was triggered by the impact and followed by additional carbon from other processes such as the eruption of the North Atlantic Igneous Province. Such a scenario agrees well with recent modelling work, though it should be revisited more explicitly.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.

Impact ejecta at the Paleocene-Eocene boundary.

Extraterrestrial impacts have left a substantial imprint on the climate and evolutionary history of Earth. A rapid carbon cycle perturbation and global warming event about 56 million years ago at the Paleocene-Eocene (P-E) boundary (the Paleocene-Eocene Thermal Maximum) was accompanied by rapid expansions of mammals and terrestrial plants and extinctions of deep-sea benthic organisms. Here, we report the discovery of silicate glass spherules in a discrete stratigraphic layer from three marine P-E boundary sections on the Atlantic margin. Distinct characteristics identify the spherules as microtektites and microkrystites, indicating that an extraterrestrial impact occurred during the carbon isotope excursion at the P-E boundary.