TWICE THE BANG: COMET IMPACT AND VOLCANISM JOINTLY CONTRIBUTED TO THE PALEOCENE–EOCENE THERMAL MAXIMUM

The Paleocene–Eocene thermal maximum (PETM) records a hyperthermal event that occurred ~56 Ma, that was associated with a biotic turnover, and is characterized in the sedimentary record by a negative excursion in 𝛿¹³C that is thought to have been caused by the emission of thousands of gigatons of isotopically light carbon into the atmosphere. Proposed sources of this carbon include: volcanism, release of thermogenic carbon during magmatic-sill intrusion, extraterrestrial-impact, release of marine gas hydrates and/or permafrost through melting and oxidation of marine sedimentary organic matter. Here, we argue for a contribution of isotopically light carbon from a comet impact in addition to volcanism based on high-resolution Os-isotope (¹⁸⁷Os/¹⁸⁸Os) stratigraphy, coupled with mercury (Hg) and PGE (Pt-Pd-Ir-Ru) analysis of two marine sections (Millville, NJ and Blake Nose, N. Atlantic) that record both proximal and open marine settings, respectively. The Os-isotope compositions shift to more unradiogenic values (~0.30) at the Paleocene–Eocene (P–E) boundary, and then rapidly return to pre-boundary values of ~0.35. Enrichment in Hg also occurs at the P–E boundary, but remains elevated upsection, however the PGE abundance does not change in this interval. The increase in Hg enrichment is interpreted to be driven by volcanism, which is consistent with the interpretation of the nonradiogenic Os-isotope excursions documented in other sections within the same 𝛿¹³C interval that are linked to the mafic North Atlantic Igneous Province (NAIP). In contrast, although we observe a similar scale Os isotope excursion, the excursion only occurs in strata bearing impact derived spherules (microtektites / microkrystites) and shocked quartz, that are yet to be observed in other sites. As such, the Os-isotope excursion observed could reflect nonradiogenic Os input from both NAIP and an impact body. The lack of any PGE enrichment suggests that the impact body is possibly a comet comprised predominantly of ice. Carbon modelling suggests that the comet body alone would have had insufficient carbon to cause the PETM carbon-cycle perturbation. Consequently, volcanism coupled with a comet impact as well as other proposed carbon sources might have jointly contributed to the carbon-cycle perturbation associated with the PETM event.