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2009
Petrenko, VV, Smith AM, Brook EJ, Lowe D, Riedel K, Brailsford G, Hua Q, Schaefer H, Reeh N, Weiss RF, Etheridge D, Severinghaus JP.  2009.  14C-CH4 Measurements in Greenland Ice: Investigating Last Glacial Termination CH4 Sources. Science. 324:506-508.   10.1126/science.1168909   AbstractWebsite

The cause of a large increase of atmospheric methane concentration during the Younger Dryas-Preboreal abrupt climatic transition (similar to 11,600 years ago) has been the subject of much debate. The carbon-14 (C-14) content of methane ((CH4)-C-14) should distinguish between wetland and clathrate contributions to this increase. We present measurements of (CH4)-C-14 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured (CH4)-C-14 values were higher than predicted under any scenario. Sample (CH4)-C-14 appears to be elevated by direct cosmogenic C-14 production in ice. C-14 of CO was measured to better understand this process and correct the sample (CH4)-C-14. Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas-Preboreal CH4 rise.

2008
Petrenko, VV, Smith AM, Brailsford G, Riedel K, Hua Q, Lowe D, Severinghaus JP, Levchenko V, Bromley T, Moss R, Muhle J, Brook EJ.  2008.  A new method for analyzing (14)C of methane in ancient air extracted from glacial ice. Radiocarbon. 50:53-73. AbstractWebsite

We present a new method developed for measuring radiocarbon of methane ((14)CH(4)) in ancient air samples extracted from glacial ice and dating 11,000-15,000 calendar years before present. The small size (similar to 20 mu g CH(4) carbon), low CH(4) concentrations ([CH(4)], 400-800 parts per billion [ppb]), high carbon monoxide concentrations ([CO]), and low (14)C activity of the samples created unusually high risks of contamination by extraneous carbon. Up to 2500 ppb CO in the air samples was quantitatively removed using the Sofnocat reagent. (14)C procedural blanks were greatly reduced through the construction of a new CH(4) conversion line utilizing platinized quartz wool for CH(4) combustion and the use of an ultra-high-purity iron catalyst for graphitization. The amount and (14)C activity of extraneous carbon added in the new CH(4) conversion line were determined to be 0.23 +/- 0.16 pg and 23.57 +/- 16.22 pMC, respectively. The amount of modern (100 pMC) carbon added during the graphitization step has been reduced to 0.03 mu g. The overall procedural blank for all stages of sample handling was 0.75 0.38 pMC for similar to 20-mu g, (14)C-free air samples with [CH(4)] of 500 ppb. Duration of the graphitization reactions for small (<25 mu g C) samples was greatly reduced and reaction yields improved through more efficient water vapor trapping and the use of a new iron catalyst with higher surface area. (14)C corrections for each step of sample handling have been determined. The resulting overall (14)CH(4) uncertainties for the ancient air samples are similar to 1.0 pMC.

1997
Severinghaus, JP, Keeling RF, Miller BR, Weiss RF, Deck B, Broecker WS.  1997.  Feasibility of using sand dunes as archives of old air. Journal of Geophysical Research-Atmospheres. 102:16783-16792.   10.1029/97jd00525   AbstractWebsite

Large unaltered samples of the atmosphere covering the past century would complement the history of atmospheric gases obtained from bubbles in ice cores, enabling measurement of geochemically important species such as O-2, (CH4)-C-14, and (CO)-C-14. Sand dunes are a porous media with interstitial air in diffusive contact with the atmosphere, somewhat analogous to the unconsolidated layer of firn atop glaciers. Recent studies have demonstrated the value of firn as an archive of old air [Battle et al., 1996; Bender et al., 1994a]. Unlike firn, sand dunes are incompressible and so remain permeable to greater depths and may extend the firn record into the past century. To evaluate the feasibility of using sand dunes as archives of old air, we drilled 60 m deep test holes in the Algodones Dunes, Imperial Valley, California. The main objective was to see if the air in a sand dune is as old as predicted by a diffusion model, or if the dune is rapidly flushed by advective pumping during windstorms and barometric pressure changes. We dated the air with chlorofluorocarbons and krypton-85, anthropogenic tracers whose atmospheric concentrations are known and have been increasing rapidly in the past half century. These tracer data match the pure diffusion model well, showing that advection in this dune is negligible compared to diffusion as a transport mechanism and that the mean age of the air at 61 m depth is similar to 10 years. Dunes therefore do contain old air. However, dunes appear to suffer from two serious drawbacks as archives. Microbial metabolism is evident in elevated CO2 and N2O and depressed CH4 and O-2 concentrations in this dune, corrupting the signals of interest in this and probably most dunes. Second, isotopic analyses of N-2 and O-2 from the dune show that fractionation of the gases occurs due to diffusion of water vapor, complicating the interpretation of the O-2 signal beyond the point of viability for an air archive. Sand dunes may be useful for relatively inert gases with large atmospheric concentration changes such as chlorofluorocarbons.