2003 Articles

Reply to a comment on ‘‘A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion’’ by G.R. Dickens and J.M. Francis

Kent, Dennis V.; Cramer, B. S.; Lanci, L.; Wang, D.; Wright, J. D.; Van der Voo, R.

Contrary to Dickens and Francis’s claim that we ‘challenge the idea of a massive CH4 re- lease during the PETM (Paleocene/Eocene thermal maximum)’, our consideration of an extraterrestrial carbon contribution to the carbon isotope excursion (CIE) is specifically limited to the initial and most rapid decrease in N13C, which accounts for less than half of the full magnitude of the CIE. Thermal dissociation in response to the warm- ing at the PETM is explicitly allowed in our hypothesis, as reiterated in our conclusions that the impact ‘may have triggered a more gradual thermal dissociation of seafloor methane hydrates’. We directly challenge only that portion of the hydrate dissociation hypothesis that relies on gradual warming intrinsic to Earth’s climate system as the triggering mechanism. Such a mechanism is not consistent with the documented essentially synchronous and instantaneous warming and decrease in N13C values at the onset of the event and is also at odds with the occurrence of the CIE during an interval of low amplitude orbital forcing of climate. Instead, we postulate a comet impact as an explanation for the rapid onset of the event.