Geology: A blast of gas in the latest Paleocene: Simulating first-order effects of massive dissociation of oceanic methane hydrate

Carbonate and organic matter deposited during the latest Paleocene thermal maximum is characterized by a remarkable –2.5‰ excursion in δ13C that occurred over ~104 yr and returned to near initial values in an exponential pattern over ~2 × 105 yr. It has been hypothesized that this excursion signifies transfer of 1.4 to 2.8 × 1018 g of CH4 from oceanic hydrates to the combined ocean-atmosphere inorganic carbon reservoir. A scenario with 1.12 × 1018 g of CH4 is numerically simulated here within the framework of the present-day global carbon cycle to test the plausibility of the hypothesis. We find that (1) the δ13C of the deep ocean, shallow ocean, and atmosphere decreases by –2.3‰ over 104 yr and returns to initial values in an exponential pattern over ~2 × 105 yr; (2) the depth of the lysocline shoals by up to 400 m over 104 yr, and this rise is most pronounced in one ocean region; and (3) global surface temperature increases by ~2 °C over 104 yr and returns to initial values over ~2 × 106 yr. The first effect is quantitatively consistent with the geologic record; the latter two effects are qualitatively consistent with observations. Thus, significant CH4 release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum. This conclusion is of broad interest because the flux of CH4 invoked during the maximum is of similar magnitude to that released to the atmosphere from present-day anthropogenic CH4 sources. *Present address: Department of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia. Figure 1. High-resolution carbon isotope records in benthic foraminifera at Ocean Drilling Program (ODP) Site 690 on Maud Rise in South Atlantic (Kennett and Stott, 1991), and Site 865 on Allison Guyot in Equatorial Pacific (Bralower et al., 1995). Both records have been placed on common depth scale; 0.0 m placed at δ13C minimum. Entire δ13C excursion (rapid decrease and return to near initial values) occurs within 2 m at both sites.This stratigraphy constrains duration of excursion to within 2 × 105 yr (Kennett and Stott, 1991; Bralower et al., 1995). on January 2, 2011 geology.gsapubs.org Downloaded from