Browsing by Author "Reeh, N"

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    14CH4 measurements in Greenland ice: investigating last glacial termination CH4 sources
    (American Association for the Advancement of Science (AAAS), 2009-04-24) Petrenko, VV; Smith, AM; Brook, EJ; Lowe, DC; Riedel, K; Brailsford, G; Hua, Q; Schaefer, H; Reeh, N; Weiss, RF; Etheridge, DM; Severinghaus, JP
    The cause of a large increase of atmospheric methane concentration during the Younger Dryas-Preboreal abrupt climatic transition (~11,600 years ago) has been the subject of much debate. The carbon-14 (14C) content of methane (14CH4) should distinguish between wetland and clathrate contributions to this increase. We present measurements of 14CH4 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured 14CH4 values were higher than predicted under any scenario. Sample 14CH4 appears to be elevated by direct cosmogenic 14C production in ice. 14C of CO was measured to better understand this process and correct the sample 14CH4. Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas-Preboreal CH4 rise. © 2009, American Association for the Advancement of Science (AAAS)
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    Instruments and methods: a novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon
    (International Glaciological Society, 2008-03) Petrenko, VV; Severinghaus, JP; Brook, EJ; Muhle, J; Headly, M; Harth, CM; Schaefer, H; Reeh, N; Weiss, RF; Lowe, DC; Smith, AM
    We present techniques for obtaining large (similar to 100 L STP) samples of ancient air for analysis of C-14 of methane ((CH4)-C-14) and other trace constituents. Paleoatmospheric (CH4)-C-14 measurements should constrain the fossil fraction of past methane budgets, as well as provide a definitive test of methane clathrate involvement in large and rapid methane concentration ([CH4]) increases that accompanied rapid warming events during the last deglaciation. Air dating to the Younger Dryas-Preboreal and Oldest Dryas-Bolling abrupt climatic transitions was obtained by melt extraction from old glacial ice outcropping at an ablation margin in West Greenland. The outcropping ice and occluded air were dated using a combination of delta N-15 of N-2, delta O-18 of O-2, delta O-18(ice) and [CH4] measurements. The [CH4] blank of the melt extractions was <4 ppb. Measurements of delta O-18 and delta N-15 indicated no significant gas isotopic fractionation from handling. Measured Ar/N-2, CFC-11 and CFC-12 in the samples indicated no significant contamination from ambient air. Ar/N-2, Kr/Ar and Xe/Ar ratios in the samples were used to quantify effects of gas dissolution during the melt extractions and correct the sample [CH4]. Corrected [CH4] is elevated over expected values by up to 132 ppb for most samples, suggesting some in situ CH4 production in ice at this site. © 2008, International Glaciological Society
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    Measurements of carbon-14 of methane in Greenland ice: investigating methane sources during the Last Glacial Termination
    (American Geophysical Union (AGU), 2008-12-15) Petrenko, VV; Smith, AM; Severinghaus, JP; Brook, EJ; Lowe, DC; Riedel, K; Brailsford, G; Hua, Q; Reeh, N; Schaefer, H; Weiss, RF; Etheridge, DM
    We present the first measurements of 14C of methane (14CH4) in ancient glacial ice. 14CH4 should distinguish unambiguously between wetland and fossil (clathrate or other geologic CH4) contributions to abrupt atmospheric CH4 increases observed at times of rapid warming in Greenland ice cores. 1000-kg-sized ice samples, dating to the Younger Dryas - Preboreal (around 11,600 yr BP) and Oldest Dryas - Bølling (around 14,700 yr BP) abrupt climatic transitions, were obtained from an ablation site in West Greenland. Measured 14CH4 values (28 - 35 pMC) were higher than predicted under any scenario based on sample age. Sample 14CH4 appears to be elevated by in- situ CH4 production in the ice for some samples as well as by a second process that is likely direct cosmogenic production of 14CH4 molecules in the ice. 14C of CO and CO2 was measured to better understand these processes and corrections were applied to sample 14CH4. Although the corrected results have substantial uncertainties, they suggest that wetland sources were responsible for the majority of the Younger Dryas - Preboreal CH4 rise. The uncertainties in the corrected results for the Oldest Dryas - Bølling transition are too large to draw conclusions about 14CH4 changes during that transition. © 2008 American Geophysical Union