| Summary: | We present a mechanism linking large impacts, such as Chicxulub, to signifi cant continental sedimentary slope failures and gas hydrate releases, and hence to carbon isotope excursions. Extensive continental margin failures and seabed sediment liquefaction at the Cretaceous-Tertiary boundary up to thousands of kilometers from Chicxulub have been linked to this impact in previous studies: here we analyze the implied seismic shaking and explore its effects in terms of gas hydrate release from failed continental margin sediments. Paleoseismic analysis of published studies of liquefaction and slope failure at the Cretaceous-Tertiary boundary in North America and adjacent regions suggests that, due to low seismic attenuation in plate interior rocks, there was a suffi cient seismic forcing to cause the observed widespread sediment liquefaction and failure along tens of thousands of kilometers of continental slope. The implied magnitude of the impact-related seismicity (equivalent to an earthquake with moment magnitude <11) is shown to be broadly consistent with the characteristics of the Chicxulub impact structure. An extended period of post-impact liquefaction and slope failure may account for the observed complexity of Cretaceous-Tertiary boundary sequences in Mexico and North America. We favor a seismic shaking model for the triggering of slope failure over previous models implicating impact generated tsunamis, because the shallow-water Chicxulub impact itself is now recognized as an ineffi cient tsunami source. We have calculated the potential storage of gas hydrates based on known environmental conditions during the Cretaceous. This suggests that slope failures caused by the Chicxulub impact could have released between 300 and 1300 GtC (best estimate ∼700 GtC) of methane from the destabilization of gas hydrates. This would produce a global carbon isotopic excursion of between -0.5% and -2% (best estimate ∼-1%). This compares well with the observed carbon isotopic excursion of -1% in the planktonic foraminifera records across the K-T boundary. This large release of methane may also account for the recently reconstructed very high atmospheric pCO2 levels after the Cretaceous-Tertiary boundary as our estimated gas hydrate releases could have increased atmospheric carbon dioxide by a maximum of 600-2300 ppm (best estimate ∼1200 ppm). This mechanism linking impacts to carbon isotope excursions may apply to other signifi cant excursions, such as that at the Paleocene-Eocene Thermal Maximum. The diffi culty in identifying impact craters means that many of the other abrupt carbon isotope excursions found in the geological record could be related to impacts and not to climatic changes. © 2005 Geological Society of America.
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