The contribution of geologic emissions, thawing permafrost and methane hydrates to the global methane budget – perspective from ice core records

Studies of methane (CH4) mole fraction and isotopes from trapped air in ice cores provide a long-term perspective on the natural CH4 budget. Among the CH4 isotopes, 14CH4 is unique in providing a definitive top-down constraint on the total fossil CH4 emissions from old carbon reservoirs (marine hydrates, permafrost, natural geologic seeps). We present new measurements of 14CH4 throughout most of the Last Deglaciation (≈15-8ka). Our 14CH4 data show that 14C-depleted CH4 sources (marine hydrates, geologic seeps and old permafrost) were not significant contributors to the deglacial CH4 rise. As the relatively large deglacial global warming (≈4oC, with warming further amplified at high latitudes) did not trigger CH4 emissions from old carbon reservoirs, such emissions in response to future warming also appear unlikely. Our results also strengthen the suggestion from an earlier study (Petrenko et al. 2017) that natural geologic emissions of CH4 are much lower (less than 15 Tg CH4 yr-1, 95% confidence) than recent bottom-up estimates (54-60 Tg CH4 yr-1) (Etiope 2015; Cias et al. 2013) and that, by extension, estimates of present-day total anthropogenic fossil CH4 emissions are likely too low.

Keywords

Geologic deposits, Thawing, Permafrost, Gas hydrates, Drill cores, Ice, Records management, Carbon 14, Seeps

Citation

Dyonisius, M., Petrenko, V. V., Smith, A. M., Beck, J., Schmitt, J., Menking, J., Shackleton, S., Hmiel, B., Vimont, I., Hua, Q., Yang., B., Seth, B., Bock, M., Beaudette, R., Harth, C. M., Baggenstos, D., Bauska, T. K., Rhodes, R., Brook, E. J., Fischer, H., Severinghaus, J. P., & Weiss, R. F. (2018). The contribution of geologic emissions, thawing permafrost and methane hydrates to the global methane budget – perspective from ice core records. Paper prestented to the AGU Fall Meeting, Washington, D. C., 10 to 14 December 2018. Retrieved from: https://agu.confex.com/agu/fm18/meetingapp.cgi/Paper/448986