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°C during a brief time interval (<10⁴ years) of the latest Paleocene (∼55.6 Ma). There also was a coeval −2 to −3‰ excursion in the δ¹³C of the ocean/atmosphere inorganic carbon reservoir. Given the large mass of this reservoir, a rapid δ¹³C shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere. However, a direct consequence of warming bottom water temperature from 11 to 15°C over 10⁴ years would be a significant change in sediment thermal gradients and dissociation of oceanic CH₄ hydrate at locations with intermediate water 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 × 10¹⁸ g of carbon with a δ¹³C of approximately −60‰. The release and subsequent oxidation of this amount of carbon is sufficient to explain a −2 to −3‰ excursion in δ¹³C 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.