Browsing by Author "Severinghaus, JP"

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    14-CO in glacial ice from Law Dome, Antarctica as a tracer of changes in atmospheric OH abundance from 1870 AD to present
    (Australian Nuclear Science and Technology Organisation, 2021-11-15) Smith, AM; Neff, PD; Petrenko, VV; Etheridge, DM; Crosier, EM; Hmiel, B; Thornton, DP; Jong, LM; Beaudette, R; Harth, CM; Langenfelds, RL; Mitrevski, B; Curran, MAJ; Buizert, C; Murray, LT; Trudinger, CM; Dyonisius, MN; Ng, J; Severinghaus, JP; Weiss, RF
    Hydroxyl, OH, is the main tropospheric oxidant and determines the lifetime of methane and most other trace gases in the atmosphere, thereby controlling the amount of greenhouse warming produced by these gases. Changes in OH concentration ([OH]) in response to large changes in reactive trace gas emissions (which may occur in the future) are uncertain. Measurements of 14C containing carbon monoxide (14CO) and other tracers such as methyl chloroform over the last ≈25 years have been successfully used to monitor changes in average [OH], but there are no observational constraints on [OH] further back in time. Reconstructions of 14CO from ice cores could in principle provide such constraints but are complicated by in-situ production of 14CO by cosmic rays directly in the ice. Recent work in Antarctica and Greenland shows that this in-situ component would be relatively small and can be accurately corrected for at sites with very high snow accumulation rates. A joint US and Australian team sampled and measured firn air and ice at Law Dome, Antarctica (2018-19 season, site DE08-OH, 1.2 m a-1 ice-equivalent snow accumulation), to a maximum depth of 240 m. Trapped air was extracted from the ice using an onsite large-volume ice melting system. Preliminary comparisons of methane measured in the samples to existing ice core records and atmospheric measurements suggest ice core air sample ages spanning from the 1870s to the early 2000s. Firn-air samples from the snow surface to 81 m depth capture air from the early 2000s to present. Analyses of [CO] and halocarbons in the samples show a relatively low and stable procedural CO blank and demonstrate that the samples are unaffected by ambient air inclusion. 14CO analyses in these firn and ice core air samples have been successfully completed. Corrections for in-situ 14CO production, validated against direct atmospheric measurements for the more recent samples, have allowed us to develop a preliminary 14CO history. This history will be interpreted with the aid of the GEOS-Chem chemistry-transport model to place the first observational constraints on the variability of Southern Hemisphere [OH] since ≈1870 AD. © The Authors
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    14CH4 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, JP
    The 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)
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    Characterization of in situ cosmogenic 14CO production, retention and loss in firn and shallow ice at summit, Greenland
    (Copernicus Publications, 2024-07-25) Hmiel, B; Petrenko, VV; Buizert, C; Smith, AM; Dyonisius, MN; Place, PF; Yang, B; Hua, Q; Beaudette, R; Severinghaus, JP; Harth, CM; Weiss, RF; Davidge, L; Diaz, M; Pacicco, M; Menking, JA; Kalk, M; Faïn, X; Adolph, A; Vimont, I; Murray, LT
    Measurements of carbon-14-containing carbon monoxide (14CO) in glacial ice are useful for studies of the past oxidative capacity of the atmosphere as well as for reconstructing the past cosmic ray flux. The 14CO abundance in glacial ice represents the combination of trapped atmospheric 14CO and in situ cosmogenic 14CO. The systematics of in situ cosmogenic 14CO production and retention in ice are not fully quantified, posing an obstacle to interpretation of ice core 14CO measurements. Here we provide the first comprehensive characterization of 14CO at an ice accumulation site (Summit, Greenland), including measurements in the ice grains of the firn matrix, firn air and bubbly ice below the firn zone. The results are interpreted with the aid of a firn gas transport model into which we implemented in situ cosmogenic 14C. We find that almost all (≈ 99.5 %) of in situ 14CO that is produced in the ice grains in firn is very rapidly (in <1 year) lost to the open porosity and from there mostly vented to the atmosphere. The timescale of this rapid loss is consistent with what is expected from gas diffusion through ice. The small fraction of in situ 14CO that initially stays in the ice grains continues to slowly leak out to the open porosity at a rate of ≈ 0.6 % yr−1. Below the firn zone we observe an increase in 14CO content with depth that is due to in situ 14CO production by deep-penetrating muons, confirming recent estimates of 14CO production rates in ice via the muon mechanisms and allowing for narrowing constraints on these production rates. © Author(s) 2024. This work is distributed under the Creative Commons Attribution 4.0 License.
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    Constraining the evolution of the fossil component of the global methane budget since the pre-industrial using 14C measurements in firn air and ice cores
    (American Geophysical Union, 2018-12-13) Hmiel, B; Dyonisius, MN; Petrenko, VV; Buizert, C; Smith, AM; Place, PF; Etheridge, DM; Harth, CM; Beaudette, R; Hua, Q; Yang, B; Vimont, I; Brook, EJ; Weiss, RF; Severinghaus, JP
    Radiocarbon of atmospheric methane (14CH4) is much less studied than radiocarbon of atmospheric carbon dioxide (14CO2) yet has potential to serve as an unambiguous indicator of the balance between fossil and contemporaneous sources of this important greenhouse gas. Few measurements of atmospheric 14CH4 exist before the late 20th century. We present measurements of past atmospheric 14CH4 in firn air and ice at Summit, Greenland. These data provide a record of atmospheric 14CH4 from 2013 back to ~1750 CE. Results have been corrected for a small amount of cosmogenic in-situ production of 14CH4 within the ice crystal lattice. A firn gas transport model was used to simulate the transport of gases through the porous firn column and into fully closed ice, and an inverse model reconstructed the firn air and ice 14CH4 data into an atmospheric history. Our results from the mid-late 20th century agree with the only previously published measurements of 14CH4 from firn air (at Law Dome, Antarctica). Pre-industrial 14CH4 samples agree with the INTCAL13 14CO2 history within uncertainties, indicating that natural geologic methane emissions are very low and have been commonly overestimated in the global methane budget. From ~1880 to ~1950 CE, the atmospheric 14CH4 activity decreased via the Suess effect, indicating a 14 ± 2% fossil CH4 source in the mid 1900’s. After mid-century, despite increasing anthropogenic fossil CH4 emissions, the 14CH4 activity began increasing due to atmospheric nuclear bomb testing and direct 14CH4 emissions from nuclear power plants.
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    Constraining the sources of the CH4 increase during the Oldest Dryas-Bølling abrupt warming event using 14CH4 measurements from Taylor Glacier, Antarctica
    (Antarctic Climate and Ecosystems Cooperative Research Centre, 2016-03-07) Dyonisius, MN; Petrenko, VV; Smith, AM; Hmiel, B; Hua, Q; Harth, CM; Baggenstos, D; Bauska, TK; Bock, M; Beck, J; Seth, B; Beaudette, R; Schmitt, J; Palardy, A; Brook, EJ; Weiss, RF; Fischer, H; Severinghaus, JP
    Methane (CH4) is an important greenhouse gas with both natural and anthropogenic sources. Understanding how the natural CH4 budget has changed in response to changing climate in the past can provide insights on the sensitivity of the natural CH4 emissions to the current anthropogenic warming. Low latitude wetlands are the largest natural source of CH¬4 to the atmosphere. It has been proposed, however, that in the future warming world emissions from marine CH4 clathrates and Arctic permafrost might increase significantly. CH4 isotopes from ice cores in Greenland and Antarctica have been used to constrain the past CH¬4 budget. 14CH4 is unique in its ability to unambiguously distinguish between “old” CH4 sources (e.g. marine clathrate, geologic sources, old permafrost) and “modern” CH4 sources (e.g. tropical and boreal wetlands). We have successfully collected six large volume (~1000 kg) samples of ancient ice from Taylor Glacier, Antarctica that span the Oldest Dryas – Bølling (OD-BO) CH4 transition (~14.5ka). The OD-BO is the first large abrupt CH4 increase following the Last Glacial Maximum, with atmospheric CH4 increasing by ≈30% in the span of ≈ 200 years. All samples have recently been successfully measured for 14CH4, δ13C-CH4, and δD-CH4. 14CH4 measurements of accompanying procedural blanks show that effects from extraneous carbon addition during processing are small. Results are currently undergoing corrections for in-situ cosmogenic 14C based on 14CO measurements in the same samples. We will present the corrected 14CH4 results and preliminary interpretation with regard to causes of the OD-BO CH4 increase.
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    The contribution of geologic emissions, thawing permafrost and methane hydrates to the global methane budget – perspective from ice core records
    (American Geophysical Union, 2018-12-13) Dynonisius, MN; Petrenko, VV; Smith, AM; Beck, J; Schmitt, J; Menking, JA; Shackleton, SA; Hmiel, B; Vimont, I; Hua, Q; Yang, B; Seth, B; Bock, M; Beaudette, R; Harth, CM; Baggenstos, D; Bauska, TK; Rhodes, RH; Brook, EJ; Fischer, H; Severinghaus, JP; Weiss, RF
    Studies of methane (CH4) mole fraction and isotopes from trapped air in ice cores provide a long-term perspective on the natural CH4 budget. Among the CH4 isotopes, 14CH4 is unique in providing a definitive top-down constraint on the total fossil CH4 emissions from old carbon reservoirs (marine hydrates, permafrost, natural geologic seeps). We present new measurements of 14CH4 throughout most of the Last Deglaciation (≈15-8ka). Our 14CH4 data show that 14C-depleted CH4 sources (marine hydrates, geologic seeps and old permafrost) were not significant contributors to the deglacial CH4 rise. As the relatively large deglacial global warming (≈4oC, with warming further amplified at high latitudes) did not trigger CH4 emissions from old carbon reservoirs, such emissions in response to future warming also appear unlikely. Our results also strengthen the suggestion from an earlier study (Petrenko et al. 2017) that natural geologic emissions of CH4 are much lower (less than 15 Tg CH4 yr-1, 95% confidence) than recent bottom-up estimates (54-60 Tg CH4 yr-1) (Etiope 2015; Cias et al. 2013) and that, by extension, estimates of present-day total anthropogenic fossil CH4 emissions are likely too low.
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    Corrigendum to "Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland'' published in Atmos. Chem. Phys., 12, 4259–-4277, 2012
    (Copernicus Publications, 2014-04-09) Buizert, C; Martinerie, P; Petrenko, VV; Severinghaus, JP; Trudinger, CM; Witrant, E; Rosen, JL; Orsi, AJ; Rubino, M; Etheridge, DM; Steele, LP; Hogan, C; Laube, JC; Sturges, WT; Levchenko, VA; Smith, AM; Levin, I; Conway, TJ; Dlugokencky, EJ; Lang, PM; Kawamura, K; Jenk, TM; White, JWC; Sowers, T; Schwander, J; Blunier, T
    It was kindly pointed out to us by M. Battle that Eq. (2) on p. 4263 contains a typo, and should instead be [X]corr(z) = [X]meas(z) ΔMδgrav(z)/1000 + 1 , (2) where [X]corr ([X]meas) is the gravity-corrected (measured) mixing ratio of gas species X, 1M = MX − Mair is the molar mass difference between gas X and air, and grav(z) is the gravitational fractionation per unit mass difference at depth z. All calculations in the study were done correctly, following Eq. (2) as given here. Furthermore, the present-day 1age value for NEEM is incorrect in the original manuscript, and underestimates Δage by 6 years. The correct value is 188+3 −9 yr. In our original, incorrect calculation we used the ice age in years before 2000 CE (b2k), while we should have used the ice age relative to the surface ice age. In the updated 1age calculation we use the ice age found by annual layer counting of the shallow NEEM 2011 S1 core (Sigl et al., 2013). The NEEM chronology published in Rasmussen et al. (2013) uses the correct, updated Δage estimate. Both errors addressed in this corrigendum affect neither the discussion nor the main conclusions of the original publication. © Author(s) 2014.
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    Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland
    (Copernicus Publications, 2012-05-14) Buizert, C; Martinerie, P; Petrenko, VV; Severinghaus, JP; Trudinger, CM; Witrant, E; Rosen, JL; Orsi, AJ; Rubino, M; Etheridge, DM; Steele, LP; Hogan, C; Laube, JC; Sturges, WT; Levchenko, VA; Smith, AM; Levin, I; Conway, TJ; Dlugokencky, EJ; Lang, PM; Kawamura, K; Jenk, TM; White, JWC; Sowers, T; Schwander, J; Blunier, T
    Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1σ Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (Δage) at the firn-ice transition is calculated to be 182+3−9 yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records. © Author(s) 2012.
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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
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    Ice core and firn air 14CH4 measurements from preindustrial to present suggest that anthropogenic fossil CH4 emissions are underestimated
    (Copernicus GmbH, 2019-04-08) Hmiel, B; Petrenko, VV; Dyonisius, MN; Buizert, C; Smith, AM; Place, PF; Harth, CM; Beaudette, R; Hua, Q; Yang, B; Vimont, I; Schmitt, J; Etheridge, DM; Fain, X; Weiss, RF; Severinghaus, JP
    Concentrations of atmospheric methane (CH4), a potent greenhouse gas, have more than doubled since preindustrial times yet its contemporary budget is incompletely understood, with substantial discrepancies between global emission inventories and atmospheric observations (Kirschke et al., 2013; Saunois et al., 2016). Radiomethane (14CH4) can distinguish between fossil emissions from geologic reservoirs (radiocarbon free) and contemporaneous biogenic sources, although poorly constrained direct 14CH4 emissions from nuclear reactors complicate this interpretation in the modern era (Lassey et al., 2007; Zazzeri et al 2018). It has been debated how fossil emissions (172-195 Tg CH4/yr, (Saunois et al., 2016; Schwietzke et al., 2016)) are partitioned between anthropogenic sources (such as fossil fuel extraction and consumption) and natural sources (such as geologic seeps); emission inventories suggest the latter accounts for ~50-60 Tg CH4/yr (Etiope, 2015; Etiope et al., 2008). Geologic emissions were recently shown to be much smaller at the end of the Pleistocene ~11,600 years ago (Petrenko et al. 2017); However, this period is an imperfect analog for the present day due to the much larger terrestrial ice sheet cover, lowered sea level, and more extensive permafrost. We use preindustrial ice core measurements of 14CH4 to show that natural fossil CH4 emissions to the atmosphere are ~1.7 Tg CH4/yr, with a maximum of 6.1 Tg CH4/yr (95% confidence limit), an order of magnitude smaller than estimates from global inventories. This result suggests that contemporary anthropogenic fossil emissions are likely underestimated by a corresponding amount (~48-58 Tg CH4/yr, or ~25-33% of current estimates). © Author(s) 2019. CC Attribution 4.0 license.
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    Ice 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, RF
    Thawing 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
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    Ice 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, AJ
    Thawing 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
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    Ice 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, RF
    Thawing 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.
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    Insights on muonic production of radiocarbon (14C) from ablating and accumulating ice sheets: revised production rates and improved estimates of 14C retention in firn
    (American Geophysical Union (AGU), 2021-12-16) Hmiel, B; Dyonisius, MN; Petrenko, VV; Smith, AM; Buizert, C; Schmitt, J; Severinghaus, JP
    In situ cosmogenic Radiocarbon (14C) production from 16O occurs at Earth’s surface via three mechanisms: neutron-induced spallation, negative muon capture and fast muon interactions. The majority of in situ cosmogenic 14C investigations utilize the near-surface production in quartz for the determination of exposure ages where cosmogenic 14C production is dominated by spallation while near-surface muonic production represents a small correction factor in most analyses. In contrast, in situ cosmogenic 14C produced in the polar ice sheet lattice is dominated by the muonic mechanisms as a result of rapid burial from the surface in accumulation regions and extended exposure for centuries to millennia at depth before the samples are drilled and extracted from the ice sheet for analysis. Here we present two significant updates regarding the understanding of in situ cosmogenic 14C production in ice. First, measurements of ice >50ka ice 14C from Taylor Glacier are combined with an ice-flow model to find that the commonly used muogenic 14C production rates (Heisinger et al., 2002) are overestimated by factors of 5.7 (3.6-13.9, 95% CI) and 3.7 (2.0-11.9 95%CI) for negative muon capture and fast muon interactions respectively. Utilizing these revised production rates, 14C measurements of snow and ice are quantified in an ice accumulation region, finding only ~0.5% of in situ 14C is retained above the depth at which bubble closure occurs in the porous firn. Parameters are developed in a forward model to quantify the in situ cosmogenic component of accumulation zone ice core measurements and segregate them from the atmospheric component, thus expanding the utility of ice core 14C measurements for paleoclimatic reconstructions.
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    Instruments 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, AM
    We 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
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    Law Dome 14CH4 measurements confirm revised fossil methane emissions estimates
    (Australian Nuclear Science and Technology Organisation, 2021-11-17) Etheridge, DM; Petrenko, VA; Smith, AM; Neff, PD; Hmiel, B; Trudinger, CM; Crosier, EM; Thornton, DP; Langefelds, RL; Jong, LM; Harth, CM; Mitrevski, B; Buizert, C; Yang, B; Weiss, RF; Severinghaus, JP
    Methane is a powerful greenhouse gas and has significant roles in the chemistry of the atmosphere. Its global concentration has risen by 240% since 1750 AD. Atmospheric 14CH4 is an independent and potentially unambiguous tracer of fossil CH4 emissions from anthropogenic and natural geologic sources, however 14C from nuclear weapons tests and 14CH4 from nuclear power plants complicate its interpretation after the late 1950s. Measurements before then rely on air extracted from polar ice and firn. Hmiel et al. (Nature, 2020) measured 14CH4 in air extracted from firn and ice in Greenland and Antarctica and found that the natural global fossil CH4 source is very small (<6 Tg CH4 yr-1). This is inconsistent with bottom-up geological CH4 emissions estimates (40-60 Tg CH4 yr-1) and implies a significant upward revision of anthropogenic fossil source emissions, emphasising the need for further measurements. We present new 14CH4 measurements of air extracted from the high accumulation site DE08-OH on the Law Dome ice sheet in 2018/19, including firn air to 81 m depth and large ice samples combined from parallel ice cores to 240 m. Measurements of trace gases confirm that the samples were uncontaminated and only minor corrections are required for sample processing. The correction for cosmogenic in-situ production of 14CH4 is very small at DE08-OH due to its high accumulation rate and relatively low elevation. The new 14CH4 results compare closely with the previous measurements from the other sites. An atmospheric 14CH4 history is reconstructed from inverse modelling of the combined ice and firn data. The pre-industrial 14CH4 level is almost identical to that expected from contemporaneous biogenic sources, confirming very minor natural fossil CH4 emissions. 14CH4 decreases to a minimum in about 1940 as anthropogenic fossil methane is emitted followed by an increase during the nuclear era from 1950 to present. The record since the 1950s would allow the evolution of the anthropogenic fossil source to be quantified when improved nuclear 14CH4 emissions estimates become available. The larger emissions from anthropogenic fossil sources implied by this result highlight opportunities for methane emissions reductions. © The Authors
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    Law Dome 14CH4 measurements confirm revised fossil methane emissions estimates
    (American Geophysical Union (AGU), 2021-12-17) Etheridge, DM; Petrenko, VA; Smith, AM; Neff, PD; Hmiel, B; Trudinger, CM; Crosier, EM; Thornton, DP; Langenfelds, RL; Jong, LM; Harth, CM; Mitrevski, B; Buizert, C; Yang, B; Weiss, RF; Severinghaus, JP
    Methane is a powerful greenhouse gas and has significant roles in the chemistry of the atmosphere. Its global concentration has risen by 240% since 1750 AD. Atmospheric 14CH4 is an independent and potentially unambiguous tracer of fossil CH4 emissions from anthropogenic and natural geologic sources, however 14C from nuclear weapons tests and 14CH4 from nuclear power plants complicate its interpretation after the late 1950s. Measurements before then rely on air extracted from polar ice and firn. Hmiel et al. (Nature, 2020) measured 14CH4 in air extracted from firn and ice in Greenland and Antarctica and found that the natural global fossil CH4 source is very small (<6 Tg CH4 yr-1). This is inconsistent with bottom-up geological CH4 emissions estimates (40-60 Tg CH4 yr-1) and implies a significant upward revision of anthropogenic fossil source emissions, emphasising the need for further measurements. We present new 14CH4 measurements of air extracted from the high accumulation site DE08-OH on the Law Dome ice sheet in 2018/19, including firn air to 81 m depth and large ice samples combined from parallel ice cores to 240 m. Measurements of trace gases confirm that the samples were uncontaminated and only minor corrections are required for sample processing. The correction for cosmogenic in-situ production of 14CH4 is very small at DE08-OH due to its high accumulation rate and relatively low elevation. The new 14CH4 results compare closely with the previous measurements from the other sites. An atmospheric 14CH4 history is reconstructed from inverse modelling of the combined ice and firn data. The pre-industrial 14CH4 level is almost identical to that expected from contemporaneous biogenic sources, confirming very minor natural fossil CH4 emissions. 14CH4 decreases to a minimum in about 1940 as anthropogenic fossil methane is emitted followed by an increase during the nuclear era from 1950 to present. The record since the 1950s would allow the evolution of the anthropogenic fossil source to be quantified when improved nuclear 14CH4 emissions estimates become available. The larger emissions from anthropogenic fossil sources implied by this result highlight opportunities for methane emissions reductions.
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    Measurements 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, RF
    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. © 2016, Elsevier Ltd.
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    Measurements 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, DM
    We 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