DISSOCIATION OF OCEANIC METHANE HYDRATE AS A CAUSE OF THE CARBON-ISOTOPE EXCURSION AT THE END OF THE PALEOCENE

Isotopic records across the ''Latest Paleocene Thermal Maximum'' (LPTM ) indicate that bottom water temperature increased by more than 4 degr ees C during a brief time interval (<10(4) years) of the latest Paleoc ene (similar to 55.6 Ma). There also was a coeval -2 to -3 parts per t housand excursion in the delta(13)C of the ocean/atmosphere inorganic carbon reservoir. Given the large mass of this reservoir, a rapid delt a(13)C shift of this magnitude is difficult to explain within the cont ext of conventional hypotheses for changing the mean carbon isotope co mposition of the ocean and atmosphere. However, a direct consequence o f warming bottom water temperature from 11 to 15 degrees C over 10(4) years would be a significant change in sediment thermal gradients and dissociation of oceanic CH4 hydrate at locations with intermediate wat er depths. In terms of the present-day oceanic CH4 hydrate reservoir, thermal dissociation of oceanic CH4 hydrate during the LPTM could have released greater than 1.1 to 2.1 x 10(18) g of carbon with a delta(13 )C of approximately -60 parts per thousand. The release and subsequent oxidation of this amount of carbon is sufficient to explain a -2 to - 3 parts per thousand excursion in delta(13)C across the LPTM. Fate of CH4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the LPTM.