期刊
EARTH AND PLANETARY SCIENCE LETTERS
卷 365, 期 -, 页码 190-197出版社
ELSEVIER SCIENCE BV
DOI: 10.1016/j.epsl.2013.01.032
关键词
radiocarbon; ice core; cosmogenic; firn; methane; carbon monoxide
资金
- NSF OPP [0806450]
- NOAA Climate and Global Change Postdoctoral Fellowships
- New Zealand Ministry for Science and Innovation [C01X0703]
- Directorate For Geosciences
- Office of Polar Programs (OPP) [1204084, 0839031] Funding Source: National Science Foundation
- Office of Polar Programs (OPP)
- Directorate For Geosciences [0806450] Funding Source: National Science Foundation
- Office Of The Director
- Office Of Internatl Science &Engineering [968391] Funding Source: National Science Foundation
Measurements of radiocarbon (C-14) in carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO) from glacial ice are potentially useful for absolute dating of ice cores, studies of the past atmospheric CH4 budget and for reconstructing the past cosmic ray flux and solar activity. Interpretation of C-14 signals in ice is complicated by the fact that the two major C-14 components-trapped atmospheric and in situ cosmogenic-are present in a combined form, as well as by a very limited understanding of the in situ component. This study measured (CH4)-C-14 and (CO)-C-14 content in glacial firn with unprecedented precision to advance understanding of the in situ C-14 component. (CH4)-C-14 and (CO)-C-14 were melt-extracted on site at Summit, Greenland from three very large (similar to 1000 kg each) replicate samples of firn that spanned a depth range of 3.6-5.6 m. Non-cosmogenic C-14 contributions were carefully characterized through simulated extractions and a suite of supporting measurements. In situ cosmogenic (CO)-C-14 was quantified to better than +/- 0.6 molecules g(-1) ice, improving on the precision of the best prior ice (CO)-C-14 measurements by an order of magnitude. The (CO)-C-14 measurements indicate that most (>99%) of the in situ cosmogenic C-14 is rapidly lost from shallow Summit firn to the atmosphere. Despite this rapid C-14 loss, our measurements successfully quantified (CH4)-C-14 in the retained fraction of cosmogenic C-14 (to +/- 0.01 molecules g(-1) ice or better), and demonstrate for the first time that a significant amount of (CH4)-C-14 is produced by cosmic rays in natural ice. This conclusion increases the confidence in the results of an earlier study that used measurements of (CH4)-C-14 in glacial ice to show that wetlands were the likely main driver of the large and rapid atmospheric CH4 increase approximately 1 1.6 kyr ago. (C) 2013 Elsevier B.V. All rights reserved.
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