Measurements of 14C in ancient ice from Taylor Glacier, Anta

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TitleMeasurements of 14C in ancient ice from Taylor Glacier, Antarctica constrain in situ cosmogenic 14CH4 and 14CO production ratesAbstractCarbon-14 (14C) is incorporated into glacial ice by trapping of atmospheric gases as well as direct near-surface in situ cosmogenic production. 14C of trapped methane (14CH4) is a powerful tracer for past CH4 emissions from “old” carbon sources such as permafrost and marine CH4 clathrates. 14C in trapped carbon dioxide (14CO2) can be used for absolute dating of ice cores. In situ produced cosmogenic 14C in carbon monoxide (14CO) can potentially be used to reconstruct the past cosmic ray flux and past solar activity. Unfortunately, the trapped atmospheric and in situ cosmogenic components of 14C in glacial ice are difficult to disentangle and a thorough understanding of the in situ cosmogenic component is needed in order to extract useful information from ice core 14C. We analyzed very large (≈1000 kg) ice samples in the 2.26–19.53 m depth range from the ablation zone of Taylor Glacier, Antarctica, to study in situ cosmogenic production of 14CH4 and 14CO. All sampled ice is >50 ka in age, allowing for the assumption that most of the measured 14C originates from recent in situ cosmogenic production as ancient ice is brought to the surface via ablation. Our results place the first constraints on cosmogenic 14CH4 production rates and improve on prior estimates of 14CO production rates in ice. We find a constant 14CH4/14CO production ratio (0.0076 ± 0.0003) for samples deeper than 3 m, which allows the use of 14CO for correcting the 14CH4 signals for the in situ cosmogenic component. Our results also provide the first unambiguous confirmation of 14C production by fast muons in a natural setting (ice or rock) and suggest that the 14C production rates in ice commonly used in the literature may be too high. Copyright 2016 Elsevier Ltd.AcknowledgementsThis work was supported by US NSF Awards 0839031 (Severinghaus), 0838936 (Brook), 1245659 (Petrenko), the NOAA Climate and Global Change Postdoctoral Fellowship (Petrenko, Buizert), the Packard Fellowship for Science and Engineering (Petrenko), the Marsden Fund Council from New Zealand Government funding administered by the Royal Society of New Zealand (Schaefer), and by the ANSTO Cosmogenic Climate Archives of the Southern Hemisphere project (Smith). Further support came from NIWA under Climate and Atmosphere Research Programme CAAC1504 (2014/15 SCI). We thank the US Antarctic Program for field support, Philip Place for ice core [CO] analyses, Rowena Moss for d13CO analyses and Andrew Dickson for helpful discussions. The manuscript was improved by constructive reviews from Nat Lifton and an anonymous reviewer, and subsequent helpful discussions with Nat Lifton.Funding Details0838936; 0839031, NSF; 1245659, NSF

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1st AuthorPetrenko, V.AuthorPetrenko, V.Severinghaus, J.Schaefer, H.Smith, A.Kuhl, T.Baggenstos, D.Hua, Q.Brook, E.Rose, P.Kulin, R.Bauska, T.Harth, C.Buizert, C.Orsi, A.Emanuele, G.Lee, J.Brailsford, G.Keeling, R.Weiss, R.Year2016JournalGeochimica et Cosmochimica ActaVolume177Pages62-77DOI10.1016/j.gca.2016.01.004URLhttps://www.scopus.com/inward/recor.....e82b63ad852d782c45fdb14c2Keywordscarbon dioxidecarbon isotopecarbon monoxideglacier dynamicsmeasurement methodmethane, AntarcticaEast AntarcticaTaylor Glacier, rank3

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TypeArticleCitationPetrenko, V., Severinghaus, J., Schaefer, H., Smith, A., Kuhl, T., Baggenstos, D., Hua, Q., Brook, E., Rose, P., Kulin, R., Bauska, T., Harth, C., Buizert, C., Orsi, A., Emanuele, G., Lee, J., Brailsford, G., Keeling, R. and Weiss, R. (2016). Measurements of 14C in ancient ice from Taylor Glacier, Antarctica constrain in situ cosmogenic 14CH4 and 14CO production rates. Geochimica et Cosmochimica Acta, 177: 62-77 IdentifierPetrenko2016Relevancerank3