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Petrenko, VV, Severinghaus JP, Schaefer H, Smith AM, Kuhl T, Baggenstos D, Hua Q, Brook EJ, Rose P, Kulin R, Bauska T, Harth C, Buizert C, Orsi A, Emanuele G, Lee JE, Brailsford G, Keeling R, Weiss RF.  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.   10.1016/j.gca.2016.01.004   Abstract

Carbon-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.

Petrenko, VV, Etheridge DM, Weiss RF, Brook EJ, Schaefer H, Severinghaus JP, Smith AM, Lowe D, Hua QA, Riedel K.  2010.  Methane from the East Siberian Arctic Shelf. Science. 329:1146-1147.   10.1126/science.329.5996.1146-b   AbstractWebsite
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Petrenko, VV, Severinghaus JP, Brook EJ, Muhle J, Headly M, Harth CM, Schaefer H, Reeh N, Weiss RF, Lowe D, Smith AM.  2008.  A novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon. Journal of Glaciology. 54:233-244.   10.3189/002214308784886135   AbstractWebsite

We present techniques for obtaining large (similar to 100 L STP) samples of ancient air for analysis of (14)C of methane ((14)CH(4)) and other trace constituents. Paleoatmospheric (14)CH(4) measurements should constrain the fossil fraction of past methane budgets, as well as provide a definitive test of methane clathrate involvement in large and rapid methane concentration ([CH(4)]) increases that accompanied rapid warming events during the last deglaciation. Air dating to the Younger Dryas-Preboreal and Oldest Dryas-Bolling abrupt climatic transitions was obtained by melt extraction from old glacial ice outcropping at an ablation margin in West Greenland. The outcropping ice and occluded air were dated using a combination of delta(15)N of N(2), delta(18)O of O(2), delta(18)O(ice) and [CH(4)] measurements. The [CH(4)] blank of the melt extractions was <4 ppb. Measurements of delta(18)O and delta(15)N indicated no significant gas isotopic fractionation from handling. Measured Ar/N(2), CFC-11 and CFC-12 in the samples indicated no significant contamination from ambient air. Ar/N(2), Kr/Ar and Xe/Ar ratios in the samples were used to quantify effects of gas dissolution during the melt extractions and correct the sample [CH(4)]. Corrected [CH(4)] is elevated over expected values by up to 132 ppb for most samples, suggesting some in situ CH(4) production in ice at this site.

Petrenko, VV, Severinghaus JP, Smith AM, Riedel K, Baggenstos D, Harth C, Orsi A, Hua Q, Franz P, Takeshita Y, Brailsford GW, Weiss RF, Buizert C, Dickson A, Schaefer H.  2013.  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. Earth and Planetary Science Letters. 365:190-197.   10.1016/j.epsl.2013.01.032   AbstractWebsite

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.

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.

Petrenko, VV, Martinerie P, Novelli P, Etheridge DM, Levin I, Wang Z, Blunier T, Chappellaz J, Kaiser J, Lang P, Steele LP, Hammer S, Mak J, Langenfelds RL, Schwander J, Severinghaus JP, Witrant E, Petron G, Battle MO, Forster G, Sturges WT, Lamarque JF, Steffen K, White JWC.  2013.  A 60 yr record of atmospheric carbon monoxide reconstructed from Greenland firn air. Atmospheric Chemistry and Physics. 13:7567-7585.   10.5194/acp-13-7567-2013   AbstractWebsite

We present the first reconstruction of the Northern Hemisphere (NH) high latitude atmospheric carbon monoxide (CO) mole fraction from Greenland firn air. Firn air samples were collected at three deep ice core sites in Greenland (NGRIP in 2001, Summit in 2006 and NEEM in 2008). CO records from the three sites agree well with each other as well as with recent atmospheric measurements, indicating that CO is well preserved in the firn at these sites. CO atmospheric history was reconstructed back to the year 1950 from the measurements using a combination of two forward models of gas transport in firn and an inverse model. The reconstructed history suggests that Arctic CO in 1950 was 140-150 nmol mol(-1), which is higher than today's values. CO mole fractions rose by 10-15 nmol mol(-1) from 1950 to the 1970s and peaked in the 1970s or early 1980s, followed by a approximate to 30 nmol mol(-1) decline to today's levels. We compare the CO history with the atmospheric histories of methane, light hydrocarbons, molecular hydrogen, CO stable isotopes and hydroxyl radicals (OH), as well as with published CO emission inventories and results of a historical run from a chemistry-transport model. We find that the reconstructed Greenland CO history cannot be reconciled with available emission inventories unless unrealistically large changes in OH are assumed. We argue that the available CO emission inventories strongly underestimate historical NH emissions, and fail to capture the emission decline starting in the late 1970s, which was most likely due to reduced emissions from road transportation in North America and Europe.

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.

Petrenko, VV, Severinghaus JP, Brook EJ, Reeh N, Schaefer H.  2006.  Gas records from the West Greenland ice margin covering the Last Glacial Termination: a horizontal ice core. Quaternary Science Reviews. 25:865-875.   10.1016/j.quascirev.2005.09.005   AbstractWebsite

Certain sites along ice sheet margins provide an easily accessible and almost unlimited supply of ancient ice at the surface. Measurements of gases in trapped air from ice outcropping at Pakitsoq, West Greenland, demonstrate that ancient air is mostly well preserved. No alterations in delta O-18(atm) and delta N-15 of N-2 are apparent, and alterations in methane are found in only a few ice sections. Using measurements of these gases, we have unambiguously identified a stratigraphic section containing ice from the end of last glacial period as well as Bolling-Allerod, Younger Dryas and Preboreal intervals. Extensive sections of ice from the Holocene and most ages within the last glacial period are probably also present. Very accurate dating has been possible in the ice section containing the Younger Dryas-Preboreal abrupt climate transition signal. The ice at Pakitsoq is folded and non-uniformly thinned, with many cross-cutting bands of bubble-free ice and dust. The cross-cutting features are associated with anomalies in both the gas and the ice records. With careful sampling to avoid these, the ice at Pakitsoq is suitable for recovery of large-volume samples of the ancient atmosphere for analysis of trace constituents such as (CH4)-C-14. (c) 2005 Elsevier Ltd. All rights reserved.