Browsing by Author "Schaefer, H"
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- Item14CH4 measurements in Greenland ice: investigating last glacial termination CH4 sources(American Association for the Advancement of Science (AAAS), 2009-04-24) Petrenko, VV; Smith, AM; Brook, EJ; Lowe, DC; Riedel, K; Brailsford, G; Hua, Q; Schaefer, H; Reeh, N; Weiss, RF; Etheridge, DM; Severinghaus, JPThe cause of a large increase of atmospheric methane concentration during the Younger Dryas-Preboreal abrupt climatic transition (~11,600 years ago) has been the subject of much debate. The carbon-14 (14C) content of methane (14CH4) should distinguish between wetland and clathrate contributions to this increase. We present measurements of 14CH4 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured 14CH4 values were higher than predicted under any scenario. Sample 14CH4 appears to be elevated by direct cosmogenic 14C production in ice. 14C of CO was measured to better understand this process and correct the sample 14CH4. Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas-Preboreal CH4 rise. © 2009, American Association for the Advancement of Science (AAAS)
- ItemHigh-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, HMeasurements 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.
- ItemHigh-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, AMeasurements 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
- ItemIce core measurements of 14CH4 constrain the sources of atmospheric methane increase during abrupt warming events of the last deglaciation(ADS, 2015-12-01) Petrenko, VV; Severinghaus, JP; Smith, AM; Riedel, K; Brook, EJ; Schaefer, H; Baggenstos, D; Harth, CM; Hua, Q; Dyonisius, MN; Buizert, C; Schilt, A; Faïn, X; Mitchell, L; Bauska, TK; Orsi, AJ; Weiss, RFThawing permafrost and marine methane hydrate destabilization in the Arctic and elsewhere have been proposed as large sources of methane to the atmosphere in the future warming world. To evaluate this hypothesis it is useful to ask whether such methane releases happened during past warming events. The two major abrupt warming events of the last deglaciation, Oldest Dryas - Bølling (OD-B, ≈ 14,500 years ago) and Younger Dryas - Preboreal (YD-PB; ≈11,600 years ago), were associated with large (up to 50%) increases in atmospheric methane (CH4) concentrations. The sources of these large warming-driven CH4 increases remain incompletely understood, with possible contributions from tropical and boreal wetlands, thawing permafrost as well as marine CH4 hydrates. We present a record of 14C of paleoatmospheric CH4 over the YD-PB transition from ancient ice outcropping at Taylor Glacier, Antarctica. 14C can unambiguously identify CH4 emissions from old, 14C-depleted sources, such as permafrost and CH4 hydrates. The only prior study of paleoatmospheric 14CH4 (from Greenland ice) suggested that wetlands were the main driver of the YD-PB CH4 increase, but the results were weakened by an unexpected and poorly understood 14CH4 component from in situ cosmogenic production directly in near-surface ice. In this new study, we have been able to accurately characterize and correct for the cosmogenic 14CH4 component. All samples from before, during and after the abrupt warming and associated CH4 increase yielded 14CH4 values that are consistent with 14C of atmospheric CO2 at that time, indicating a purely contemporaneous methane source. These measurements rule out the possibility of large CH4 releases to the atmosphere from methane hydrates or old permafrost carbon in response to the large and rapid YD-PB warming. To the extent that the characteristics of the YD-PB warming are comparable to those of the current anthropogenic warming, our measurements suggest that large future atmospheric methane increases from old carbon sources in the Arctic are unlikely. Instead, our measurements indicate that global wetlands will likely respond to the warming with increased methane emissions. Analysis and interpretation of 14CH4 for the abrupt OD - B transition is in progress and these results will also be presented. © AGU
- ItemIce core measurements of 14CH4 show no evidence of methane release from methane hydrates or old permafrost carbon during a large warming event 11,600 years ago(European Geosciences Union, 2015-04-12) Petrenko, VV; Severinghaus, JP; Smith, AM; Riedel, K; Brook, EJ; Schaefer, H; Battenstos, D; Harth, CM; Hua, Q; Buizert, C; Schilt, A; Faïn, X; Mitchell, L; Bauska, TK; Orsi, AJThawing permafrost and marine methane hydrate destabilization in the Arctic and elsewhere have been proposed as large sources of methane to the atmosphere in the future warming world. To evaluate this hypothesis it is useful to ask whether such methane releases happened during past warming events. The two major abrupt warming events of the last deglaciation, Oldest Dryas - Bølling (OD-B, ≈ 14,500 years ago) and Younger Dryas - Preboreal (YD-PB; ≈11,600 years ago), were associated with large (up to 50%) increases in atmospheric methane (CH4) concentrations. The sources of these large warming-driven CH4 increases remain incompletely understood, with possible contributions from tropical and boreal wetlands, thawing permafrost as well as marine CH4 hydrates. We present new measurements of 14C of paleoatmospheric CH4 over the YD-PB transition from ancient ice outcropping at Taylor Glacier, Antarctica. 14C can unambiguously identify CH4 emissions from "old carbon" sources, such as permafrost and CH4 hydrates. The only prior study of paleoatmospheric 14CH4 (from Greenland ice) suggested that wetlands were the main driver of the YD-PB CH4 increase, but the results were weakened by an unexpected and poorly understood 14CH4 component from in situ cosmogenic production directly in near-surface ice. In this new study, we have been able to accurately characterize and correct for the cosmogenic 14CH4 component. All samples from before, during and after the abrupt warming and associated CH4 increase yielded 14CH4 values that are consistent with 14C of atmospheric CO2 at that time, indicating a purely contemporaneous methane source. These new measurements rule out the possibility of large CH4 releases to the atmosphere from methane hydrates or old permafrost carbon in response to the large and rapid YD-PB warming. To the extent that the characteristics of the YD-PB warming are comparable to those of the current anthropogenic warming, our measurements suggest that large future atmospheric methane increases from old carbon sources in the Arctic are unlikely. Instead, our measurements indicate that global wetlands will likely respond to the warming with increased methane emissions. © Author(s) 2015
- ItemIce core measurements of 14CH4 show no evidence of methane release to atmosphere from methane hydrates during a large warming event 11,600 years ago(Antarctic Climate and Ecosystems Cooperative Research Centre, 2016-03-07) Petrenko, VV; Severinghaus, JP; Smith, AM; Riedel, K; Brook, EJ; Schaefer, H; Baggenstos, D; Harth, CM; Hua, Q; Buizert, C; Schift, A; Faïn, X; Mitchell, L; Bauska, TK; Orsi, AJ; Weiss, RFThawing permafrost and marine methane hydrate destabilization have been proposed as large sources of methane to the atmosphere in response to both past and future warming. We present measurements of 14C of paleoatmospheric CH4 over the Younger Dryas – Preboreal (YD – PB) abrupt warming event (≈11,600 years ago) from ancient ice outcropping at Taylor Glacier, Antarctica. The YD – PB event was associated with a ≈ 50% increase in atmospheric CH4 concentrations. 14C can unambiguously identify CH4 emissions from “old carbon” sources, such as permafrost and CH4 hydrates. The only prior study of paleoatmospheric 14CH4 (from Greenland ice) suggested that wetlands were the main driver of the YD - PB CH4 increase, but the results were weakened by an unexpected and poorly understood 14CH4 component from in situ cosmogenic production directly in nearsurface ice. In this new study, we have been able to accurately characterize and correct for the cosmogenic 14CH4 component. All samples from before, during and after the abrupt warming and associated CH4 increase yielded 14CH4 values that are consistent with 14C of atmospheric CO2 at that time, indicating a purely contemporaneous methane source. These new measurements rule out the possibility of large CH4 releases to the atmosphere from methane hydrates or old permafrost carbon in response to the large and rapid YD - PB warming, and confirm that wetlands were the main driver of the CH4 increase.
- ItemInstruments and methods: a novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon(International Glaciological Society, 2008-03) Petrenko, VV; Severinghaus, JP; Brook, EJ; Muhle, J; Headly, M; Harth, CM; Schaefer, H; Reeh, N; Weiss, RF; Lowe, DC; Smith, AMWe present techniques for obtaining large (similar to 100 L STP) samples of ancient air for analysis of C-14 of methane ((CH4)-C-14) and other trace constituents. Paleoatmospheric (CH4)-C-14 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 ([CH4]) 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 N-15 of N-2, delta O-18 of O-2, delta O-18(ice) and [CH4] measurements. The [CH4] blank of the melt extractions was <4 ppb. Measurements of delta O-18 and delta N-15 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 [CH4]. Corrected [CH4] is elevated over expected values by up to 132 ppb for most samples, suggesting some in situ CH4 production in ice at this site. © 2008, International Glaciological Society
- 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, R; 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.
- ItemMeasurements of carbon-14 of methane in Greenland ice: investigating methane sources during the Last Glacial Termination(American Geophysical Union (AGU), 2008-12-15) Petrenko, VV; Smith, AM; Severinghaus, JP; Brook, EJ; Lowe, DC; Riedel, K; Brailsford, G; Hua, Q; Reeh, N; Schaefer, H; Weiss, RF; Etheridge, DMWe present the first measurements of 14C of methane (14CH4) in ancient glacial ice. 14CH4 should distinguish unambiguously between wetland and fossil (clathrate or other geologic CH4) contributions to abrupt atmospheric CH4 increases observed at times of rapid warming in Greenland ice cores. 1000-kg-sized ice samples, dating to the Younger Dryas - Preboreal (around 11,600 yr BP) and Oldest Dryas - Bølling (around 14,700 yr BP) abrupt climatic transitions, were obtained from an ablation site in West Greenland. Measured 14CH4 values (28 - 35 pMC) were higher than predicted under any scenario based on sample age. Sample 14CH4 appears to be elevated by in- situ CH4 production in the ice for some samples as well as by a second process that is likely direct cosmogenic production of 14CH4 molecules in the ice. 14C of CO and CO2 was measured to better understand these processes and corrections were applied to sample 14CH4. Although the corrected results have substantial uncertainties, they suggest that wetland sources were responsible for the majority of the Younger Dryas - Preboreal CH4 rise. The uncertainties in the corrected results for the Oldest Dryas - Bølling transition are too large to draw conclusions about 14CH4 changes during that transition. © 2008 American Geophysical Union
- ItemMethane from the east Siberian Arctic Shelf(American Association for the Advancement of Science (AAAS), 2010-09-03) Petrenko, VV; Etheridge, DM; Weiss, RF; Brook, EJ; Schaefer, H; Severinghaus, JP; Smith, AM; Lowe, DC; Hua, Q; Riedel, K
- ItemMinimal geological methane emissions during the Younger Dryas–Preboreal abrupt warming event(Springer Nature, 2017-08-24) Petrenko, VV; Smith, AM; Schaefer, H; Riedel, K; Brook, EJ; Baggenstos, D; Harth, CM; Hua, Q; Buizert, C; Schilt, A; Faïn, X; Mitchell, L; Bauska, TK; Orsi, AJ; Weiss, RF; Severinghaus, JPMethane (CH4) is a powerful greenhouse gas and plays a key part in global atmospheric chemistry. Natural geological emissions (fossil methane vented naturally from marine and terrestrial seeps and mud volcanoes) are thought to contribute around 52 teragrams of methane per year to the global methane source, about 10 per cent of the total, but both bottom-up methods (measuring emissions)1 and top-down approaches (measuring atmospheric mole fractions and isotopes)2 for constraining these geological emissions have been associated with large uncertainties. Here we use ice core measurements to quantify the absolute amount of radiocarbon-containing methane (14CH4) in the past atmosphere and show that geological methane emissions were no higher than 15.4 teragrams per year (95 per cent confidence), averaged over the abrupt warming event that occurred between the Younger Dryas and Preboreal intervals, approximately 11,600 years ago. Assuming that past geological methane emissions were no lower than today3,4, our results indicate that current estimates of today’s natural geological methane emissions (about 52 teragrams per year)1,2 are too high and, by extension, that current estimates of anthropogenic fossil methane emissions2 are too low. Our results also improve on and confirm earlier findings5,6,7 that the rapid increase of about 50 per cent in mole fraction of atmospheric methane at the Younger Dryas–Preboreal event was driven by contemporaneous methane from sources such as wetlands; our findings constrain the contribution from old carbon reservoirs (marine methane hydrates8, permafrost9 and methane trapped under ice10) to 19 per cent or less (95 per cent confidence). To the extent that the characteristics of the most recent deglaciation and the Younger Dryas–Preboreal warming are comparable to those of the current anthropogenic warming, our measurements suggest that large future atmospheric releases of methane from old carbon sources are unlikely to occur. © 2017 Macmillan Publishers Limited, part of Springer Nature.
- ItemNew measurements of 14C provide constraints on sources of a large atmospheric methane increase during the Younger Dryas-Preboreal Abrupt Warming Event(American Geophysical Union (AGU), 2014-12-19) Petrenko, VV; Severinghaus, JP; Smith, AM; Riedel, K; Brook, EJ; Schaefer, H; Baggenstos, D; Harth, CM; Hua, Q; Buizert, C; Schilt, A; Faïn, X; Mitchell, L; Bauska, TK; Orsi, AJ; Weiss, RFThawing permafrost and marine methane hydrate destabilization have been proposed as large sources of methane to the atmosphere in the future warming world. To evaluate this hypothesis it is useful to ask whether such methane releases happened during past warming events. The two major abrupt warming events of the last deglaciation, Oldest Dryas – Bølling (OD–B) and Younger Dryas – Preboreal (YD-PB), were associated with large (up to 50%) increases in atmospheric methane (CH4) concentrations. The sources of these large warming-driven CH4 increases remain incompletely understood, with possible contributions from tropical and boreal wetlands, thawing permafrost as well as marine CH4 hydrates. We present new measurements of 14C of paleoatmospheric CH4 over the YD-PB transition from ancient ice outcropping at Taylor Glacier, Antarctica. 14C can unambiguously identify CH4 emissions from “old carbon” sources, such as permafrost and CH4 hydrates. The only prior study of paleoatmospheric 14CH4 (from Greenland ice) suggested that wetlands were the main driver of the YD-PB CH4 increase, but the results were weakened by an unexpected and poorly understood 14CH4 component from in situ cosmogenic production directly in near-surface ice. In this new study, we have been able to accurately characterize and correct for the cosmogenic 14CH4 component. Preliminary analysis of the results indicates that ≈10% of the overall CH4 source to the atmosphere during the nearly-constant climate of the YD was attributable to 14C-free sources. This 14C-free source fraction increased slightly over the YD-PB transition, however, wetlands were nonetheless the main driver of the CH4 increase. Final analysis and interpretation of the 14CH4 data are currently in progress. © AGU 2014
- ItemUsing measurements of atmospheric 14CO in a global network to improve understanding of OH spatial and temporal variability(American Geophysical Union (AGU), 2022-12-16) Petrenko, VV; Crosier, EM; Smith, AM; Yang, B; Scholer, M; McCrea, K; Murray, LT; Colton, A; Thomas, B; Talamoa, G; Musick, R; Schaefer, H; Moss, RC; Spain, G; Yann, H; Hernandez, P; Blades, E; Chewitt-Lucas, RM; Kazemi, R; Stock, MPThe primary source of 14C-containing carbon monoxide (14CO) in the atmosphere is via 14C production from 14N by secondary cosmic rays, and the primary sink is removal by hydroxyl radicals (OH). Variations in the abundance of 14CO that are not explained by variations in 14CO production or transport are mainly driven by variations in the abundance of OH. Because of its relatively short atmospheric lifetime (≈2 months on average), 14CO abundance is sensitive to spatial and seasonal OH variability. 14CO measurements in a new global network were made for one calendar year in 2021. Simulations in the GEOS-Chem chemical transport model (CTM) indicate that our 14CO network has good sensitivity to variations in both regional and global OH. In this presentation, we will show the new measurements as well as the CTM results available to-date.