HR: 1330h
AN: A52A-0766    [PDF]
TI: Using Methane $^{14}$C to Determine the Origins of the Rapid Methane Rise at the End of the Younger Dryas 11,600 Years Ago: Increased Wetland Production Only or a Contribution From Methane Hydrates? A Progress Report
AU: * Petrenko, V V
EM: vpetrenko@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA 92093 United States
AU: Severinghaus, J
EM: jseveringhaus@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA 92093 United States
AU: Brook, E
EM: brook@vancouver.wsu.edu
AF: Washington State University, 14204 NE Salmon Creek Ave, Vancouver, WA 98686 United States
AU: Reeh, N
EM: nr@emi.dtu.dk
AF: Arctic Technology Centre, Technical University of Denmark, Building 204., Kgs. Lyngby, DK-2800 Denmark
AU: Lowe, D
EM: d.lowe@niwa.co.nz
AF: National Institute of Water and Atmospheric Research Ltd, PO Box 14901, Kilbirnie, 301 Evans Bay Parade, Wellington, 1 New Zealand
AU: Smith, A
EM: ams@ansto.gov.au
AF: Australian Nuclear Science and Technology Organisation (ANSTO), PMB 1, Menai, NSW 2234 Australia
AU: Etheridge, D
EM: David.Etheridge@csiro.au
AF: Commonwealth Scientific and Industrial Research Organisation (CSIRO), Atmospheric Research, PMB 1, Aspendale, Vic. 3195 Australia
AU: Weiss, R
EM: rfweiss@ucsd.edu
AF: Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA 92093 United States
AB: The global atmospheric methane concentration rose from about 500 parts per billion (ppb) to about 750 ppb over a period of just 150 years at the termination of the Younger Dryas cold interval 11,600 years ago, as indicated by Greenland ice core records. The start of this rapid methane increase was synchronous with an even more rapid climate warming -- ice core nitrogen and argon isotope records indicate that temperatures in central Greenland rose about 10$\deg$C over just a few decades. Current hypotheses attribute this methane rise to increased bacterial methane production in wetlands, and/or to the dissociation of large quantities of methane hydrates. Here we describe the progress of a project whose aim is to quantify the role of methane hydrates in this observed methane rise by measuring the $^{14}$C of methane derived from air bubbles in glacial ice. Methane hydrates are too old to contain measurable $^{14}$C, whereas wetland-derived methane has a $\Delta$$^{14}$C nearly identical to that of atmospheric CO$_{2}$ at the time of production. One $^{14}$C measurement requires about 2 cubic meters of ice. We have located a site on the western margin of the Greenland ice sheet where sufficient amounts of uncontaminated ancient ice are available at the surface. We measured $\delta$$^{18}$O of ice and $\delta$$^{15}$N of N$_{2}$, $\delta$$^{18}$O of O$_{2}$ and methane concentration in the trapped air along horizontal sample profiles collected in 2001 and 2002. Data from these profiles show a remarkable similarity to the GISP2 isotopic records, and uniquely identify the end-of-Younger-Dryas transition in the ice at our sampling site. Our measurements of methane concentration in samples collected in 2003 have allowed us to successfully re-establish the location of the end-of-Younger-Dryas transition, and measurements of $\delta$$^{18}$O of ice, $\delta$$^{15}$N of N$_{2}$ and $\delta$$^{18}$O of O$_{2}$ on these samples are in progress. During the summer of 2003, we collected two 200-L air samples from approximately 5 tons of ice for methane $^{14}$C analysis. Field methane measurements indicate that the age of one sample is confined to about 0-200 years immediately prior to the end-of-Younger-Dryas methane transition. The age of the second sample is confined to about 0-200 years immediately following the transition. Laboratory methane concentration measurements on these air samples (526 ppb for the pre-transition sample, and 735 ppb for the post-transition sample) are consistent with GISP2 methane values for these ages. CFC-12 measurements on these samples, as well as methane concentration measurements on our blanks show that our samples do not contain a significant amount of contamination and are suitable for methane $^{14}$C analysis, which is currently in progress.
DE: 0322 Constituent sources and sinks
DE: 0400 Biogeosciences
DE: 1610 Atmosphere (0315, 0325)
DE: 1615 Biogeochemical processes (4805)
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting