The Paleocene^Eocene thermal maximum (PETM) ca. 55 Ma is characterized by a remarkable negative excursion in the stable carbon isotope composition of carbonate and organic matter deposited in terrestrial, shallow marine and deep marine environments [1^3]. In well-dated benthic foraminiferal isotope records, N13C steps down by 32.5 to 33.0x within 20 kyr, and then gradually returns to near initial values over V120^220 kyr [4,5]. The magnitude, timing, shape, and global nature of this N13C excursion strongly suggest a massive injection of 13C-depleted carbon into the ocean^atmosphere system [6]. To date, thermal dissociation of marine gas hydrates and subsequent release of sea£oor CH4 has been widely cited as the most plausible explanation for the carbon addition [7]. In a recent contribution [8], Kent et al. challenge the idea of a massive CH4 release during the PETM, suggesting instead that a comet impact provided much of the excess 13C-depleted carbon. Their primary grounds for this alternative hypothesis are unusual magnetic properties in relatively thick (V5^8 m) clay-rich sequences deposited on the outer shelf of the New Jersey continental margin during the initial phase of the PETM. The magnetic properties apparently signify an anomalous enrichment of single-domain magnetite, which the authors suggest represents extraterrestrial ejecta. They bolster this assertion by noting that high Ir contents have been found in several sections at the very start of the PETM [9,10]. The ‘methane hydrate dissociation hypothesis’ carries some profound conceptual problems [11] and should not be accepted blindly with available information. Consequently, it is appropriate to entertain multiple working hypotheses for the extraordinary biogeochemical perturbations that occurred during the PETM. However, it is equally important to stress the fundamental problems with any alternative hypothesis, in this case a comet impact. The primary di⁄culty with invoking a comet (or any extraterrestrial object) as an explanation for global environmental change at the PETM is that there is no supporting evidence, except perhaps the unusual magnetic signatures in their sediment cores. As sprinkled in various parts of the paper by Kent et al. [8] but worth iterating collectively: 1. There is no crater ; 2. The remarkable fossil turnovers, including a

We hypothesize that the rapid onset of the carbon isotope excursion (CIE) at the Paleocene/Eocene boundary (∼55 Ma) may have resulted from the accretion of a significant amount of 12C-enriched carbon from the impact of a ∼10 km comet, an event that would also trigger greenhouse warming leading to the Paleocene/Eocene thermal maximum and, possibly, thermal dissociation of seafloor methane hydrate. Indirect evidence of an impact is the unusual abundance of magnetic nanoparticles in kaolinite-rich shelf sediments that closely coincide with the onset and nadir of the CIE at three drill sites on the Atlantic Coastal Plain. After considering various alternative mechanisms that could have produced the magnetic nanoparticle assemblage and by analogy with the reported detection of iron-rich nanophase material at the Cretaceous/Tertiary boundary, we suggest that the CIE occurrence was derived from an impact plume condensate. The sudden increase in kaolinite is thus thought to represent the redeposition on the marine shelf of a rapidly weathered impact ejecta dust blanket. Published reports of a small but significant iridium anomaly at or close to the Paleocene/Eocene boundary provide supportive evidence for an impact.