Browsing by Author "Keeling, RF"
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- ItemInitial results of an intercomparison of AMS-based atmospheric 14CO2 measurements(Cambridge University Press, 2016-02-16) Miller, J; Lehman, SJ; Wolak, C; Turnbull, J; Dunn, G; Graven, H; Keeling, RF; Meijer, HAJ; Aerts-Bijma, AT; Palstra, SWL; Smith, AM; Allison, CE; Southon, J; Xu, XM; Nakazawa, T; Aoki, S; Nakamura, T; Guilderson, TP; LaFranchi, B; Mukai, M; Terao, Y; Uchida, M; Kondo, MThis article presents results from the first 3 rounds of an international intercomparison of measurements of Δ14CO2 in liter-scale samples of whole air by groups using accelerator mass spectrometry (AMS). The ultimate goal of the intercomparison is to allow the merging of Δ14CO2 data from different groups, with the confidence that differences in the data are geophysical gradients and not artifacts of calibration. Eight groups have participated in at least 1 round of the intercomparison, which has so far included 3 rounds of air distribution between 2007 and 2010. The comparison is intended to be ongoing, so that: a) the community obtains a regular assessment of differences between laboratories; and b) individual laboratories can begin to assess the long-term repeatability of their measurements of the same source air. Air used in the intercomparison was compressed into 2 high-pressure cylinders in 2005 and 2006 at Niwot Ridge, Colorado (USA), with one of the tanks "spiked" with fossil CO2, so that the 2 tanks span the range of Δ14CO2 typically encountered when measuring air from both remote background locations and polluted urban ones. Three groups show interlaboratory comparability within 1‰ for ambient level Δ14CO2. For high CO2/low Δ14CO2 air, 4 laboratories showed comparability within 2‰. This approaches the goals set out by the World Meteorological Organization (WMO) CO2 Measurements Experts Group in 2005. One important observation is that single-sample precisions typically reported by the AMS community cannot always explain the observed differences within and between laboratories. This emphasizes the need to use long-term repeatability as a metric for measurement precision, especially in the context of long-term atmospheric monitoring. © 2013 by the Arizona Board of Regents on behalf of the University of Arizona.
- ItemMeasurements of 14C in ancient ice from Taylor Glacier, Antarctica constrain in situ cosmogenic 14CH4 and 14CO production rates(Elsevier, 2016-03-15) Petrenko, VV; Severinghaus, JP; Schaefer, H; Smith, AM; Kuhl, TW; Baggenstos, D; Hua, Q; Brook, EJ; Rose, P; Kulin, R; Bauska, TK; Harth, CM; Buizert, C; Orsi, AJ; Emanuele, G; Lee, JE; Brailsford, G; Keeling, RF; Weiss, RFCarbon-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. © 2016, Elsevier Ltd.
- ItemNIWA’s δ13C-CO2 measurement programme: twenty years of monitoring in New Zealand and Antarctica, including the performance assessment of an in-situ analyser at Baring Head(American Geophysical Union, 2018-12-13) Moss, RC; Brailsford, GW; Martin, R; Nankivell, C; Nicol, S; Trans, PP; Mikaloff-Fletcher, SE; Michel, S; Keeling, RF; Werczynski, S; Gorjan, P; Sperlich, PNIWA is monitoring atmospheric trace gas species at multiple locations in New Zealand and Antarctica. NIWA’s main monitoring sites include i) Baring Head (BHD), a coastal site at the Southern tip of New Zealand’s North Island, ii) Lauder (LAU), an inland site in the central South Island of New Zealand and iii) Arrival Heights (ARH), an observatory on Ross Island in McMurdo Sound in Antarctica. Stable carbon isotopes in atmospheric carbon dioxide (δ13C-CO2) are measured at all three sites and represent a tracer to constrain CO2 fluxes. NIWA’s δ13C-CO2 measurements started in 1997 at BHD and ARH, while it commenced in 2009 at LAU. At all three sites, air is sampled in flasks during specific meteorological conditions, i.e. during Southerly events at BHD to sample Southern Ocean background air, or during mid-afternoon at LAU when the atmospheric boundary layer is well mixed. All flasks are analysed for δ13C-CO2 at the main gaslab in Wellington, using the same Gas Chromatography coupled Isotope Ratio Mass Spectrometry (GC-IRMS) system, ensuring optimal internal data consistency. Our instrument comprises a purpose-built GC unit and a commercial IRMS (MAT 252, Thermo Fisher, Bremen, Germany). In the last 20 years, the δ13C-CO2 monitoring programme has generated 1,708 measurements, with about 1182 from BHD, 158 from ARH, 368 from LAU. BHD is also sampled for δ13C-CO2 analysis within the NOAA and the Scripps flask networks, providing continuous intercomparison time series. In collaboration with the Australian Nuclear Science and Technology Organisation (ANSTO), we monitor Radon at BHD since 2015. Radon indicates if the sampled air has been in contact with terrestrial air masses, highlighting the potential for contamination with CO2 from terrestrial sources, which would impact on δ13C-CO2 observations. We also collaborate with Thermo Fisher (Bremen, Germany) and deployed Delta Ray – an in-situ analyser for δ13C and δ18O in atmospheric CO2 – at BHD. We compare the Delta Ray time series of δ13C and δ18O to δ13C-CO2 measurements in co-located flask samples as well as to continuous CO2 mole fractions, Radon and meteorological data from BHD.