New Measurements of 14C Provide Constraints on Sources of a Large Atmospheric Methane Increase During the Younger Dryas – Preboreal Abrupt Warming Event

Friday, 19 December 2014
Vasilii V Petrenko1, Jeffrey P Severinghaus2, Andrew Smith3, Katja Riedel4, Edward Brook5, Hinrich Schaefer4, Daniel Baggenstos2, Christina M Harth2, Quan Hua3, Christo Buizert5, Adrian Schilt6, Xavier Fain7, Logan Mitchell5,8, Thomas K Bauska5, Anais J Orsi2,9 and Ray F Weiss2, (1)University of Rochester, Earth and Environmental Sciences, Rochester, NY, United States, (2)University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA, United States, (3)Australian Nuclear Science and Technology Organisation (ANSTO), Kirrawee DC, NSW, Australia, (4)National Institute of Water and Atmospheric Research Ltd (NIWA), Wellington, New Zealand, (5)Oregon State University, College of Earth, Ocean and Atmospheric Sciences, Corvallis, OR, United States, (6)University of Bern, Physics Institute, Bern, Switzerland, (7)Univ. Grenoble Alpes / CNRS, Laboratoire de Glaciologie et Géophysique de l’Environnement (LGGE), Grenoble, France, (8)University of Utah, Department of Atmospheric Sciences, Salt Lake City, UT, United States, (9)CEA CNRS UVSQ, Lab Sci Climat & Environm, Gif Sur Yvette, France
Thawing permafrost and marine methane hydrate destabilization have been proposed as large sources of methane to the atmosphere in the future warming world. To evaluate this hypothesis it is useful to ask whether such methane releases happened during past warming events. The two major abrupt warming events of the last deglaciation, Oldest Dryas – Bølling (OD–B) and Younger Dryas – Preboreal (YD-PB), were associated with large (up to 50%) increases in atmospheric methane (CH4) concentrations. The sources of these large warming-driven CH4 increases remain incompletely understood, with possible contributions from tropical and boreal wetlands, thawing permafrost as well as marine CH4 hydrates. We present new measurements of 14C of paleoatmospheric CH4 over the YD-PB transition from ancient ice outcropping at Taylor Glacier, Antarctica. 14C can unambiguously identify CH4 emissions from “old carbon” sources, such as permafrost and CH4 hydrates. The only prior study of paleoatmospheric 14CH4 (from Greenland ice) suggested that wetlands were the main driver of the YD-PB CH4 increase, but the results were weakened by an unexpected and poorly understood 14CH4 component from in situ cosmogenic production directly in near-surface ice. In this new study, we have been able to accurately characterize and correct for the cosmogenic 14CH4 component. Preliminary analysis of the results indicates that ≈10% of the overall CH4 source to the atmosphere during the nearly-constant climate of the YD was attributable to 14C-free sources. This 14C-free source fraction increased slightly over the YD-PB transition, however, wetlands were nonetheless the main driver of the CH4 increase. Final analysis and interpretation of the 14CH4 data are currently in progress.