Browsing by Author "Franz, P"

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    High-precision C-14 measurements demonstrate production of in situ cosmogenic (CH4)-C-14 and rapid loss of in situ cosmogenic (CO)-C-14 in shallow Greenland firn
    (Elsevier Science BV., 2013-03-01) Petrenko, VV; Severinghaus, JP; Smith, AM; Riedel, K; Baggenstos, D; Harth, CM; Orsi, AJ; Hua, Q; Franz, P; Takeshita, Y; Brailsford, G; Weiss, RF; Buizert, C; Dickson, A; Schaefer, H
    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. © 2013, Elsevier Ltd.
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    High-precision measurements of 14C in ice cores: results and future prospects
    (American Geophysical Union (AGU), 2012-12-03) Petrenko, VV; Severinghaus, JP; Smith, AM; Schaefer, H; Riedel, K; Brook, EJ; Buizert, C; Baggenstos, D; Harth, CM; Hua, Q; Orsi, AJ; Bauska, TK; Schilt, A; Mitchell, L; Faïn, X; Takeshita, Y; Lee, JE; Brailsford, G; Franz, P; Weiss, RF; Dickson, A
    Measurements of 14C 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 14C signals in ice is complicated by the fact that there is a poorly-understood in situ cosmogenic component in addition to the trapped atmospheric component. A new analytical system allowed 14C of CH4 in glacial ice to be measured for the first time and improved measurement precision for 14C of CO in ice by an order of magnitude over prior work. Measurements of 14C of CH4 in ablating Greenland ice suggested that wetlands were the likely main driver of the Younger Dryas - Preboreal rapid atmospheric CH4 rise ≈ 11,600 yr ago, but interpretation was complicated by what appeared to be an unexpected significant in situ cosmogenic 14CH4 component. Subsequent measurements in shallow firn at Greenland Summit and in 50-kyr-old ablating ice at Taylor Glacier, Antarctica ice definitively confirmed in situ cosmogenic 14CH4 production in glacial ice. The Taylor Glacier measurements also precisely quantified the in situ 14CH4 / 14CO ratio for muogenic 14C production (0.0078 ± 0.0001). The observed constancy of this ratio demonstrated that 14C of CO can be used to quantify the cosmogenic 14CH4 content, allowing for accurate reconstructions of the absolute paleo-atmospheric 14C of CH4 from glacial ice. Measurements in Greenland shallow firn clearly demonstrated that almost all in situ cosmogenic 14C is rapidly lost from the shallow firn to the atmosphere. This implies that 14C of CO2 at most ice core sites is dominated by the atmospheric component and, with a 14CO-based correction for the cosmogenic component, can likely be used for absolute dating of ice. Even given the rapid in-situ cosmogenic 14C loss in the firn, 14C of CO is still expected to be dominated by the cosmogenic component and is a promising tracer for past cosmic ray flux. © AGU 2012