Browsing by Author "Hmiel, B"
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- Item14-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, RFHydroxyl, 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
- ItemCharacterization of in situ cosmogenic 14CO in glacial ice and applications of ice core 14CO as a tracer(American Geophysical Union (AGU), 2022-12-15) Petrenko, VV; Hmiel, B; Dyonisius, MN; Smith, AM; Neff, PD; BenZvi, SCarbon-14 (14C) is included in glacial ice via trapping of air and in situ cosmogenic production. In the carbon monoxide phase (14CO), ice core 14C has two promising applications. First, the trapped atmospheric component of 14CO has the potential to serve as a tracer of past hydroxyl radical (OH) abundance and variability. Second, the in situ cosmogenic component can in principle be used to reconstruct variations in the past flux of galactic cosmic rays. A detailed understanding of the in situ cosmogenic 14CO production and retention in ice is needed to disentangle the trapped atmospheric and in situ cosmogenic components in measurements of ice core 14CO. We will present the most recent interpretations of ice core 14CO measurements from Taylor Glacier, Antarctica and Summit, Greenland. Taylor Glacier is an ablation site with easily accessible ancient (>50 kyr) ice at the surface that allows for the determination of in situ cosmogenic 14CO production rates in the absence of a trapped atmospheric component. Summit is a traditional ice coring site that allows for the examination of how well in situ cosmogenic 14CO is retained in the firn. The results form the basis for the interpretation of new measurements from Law Dome, Antarctica, which are aimed at reconstructing paleoatmospheric 14CO. The results also support the feasibility of using 14CO measurements at a low-accumulation site such as Dome C, Antarctica to study past variations in the galactic cosmic ray flux.
- ItemCharacterization 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, LTMeasurements 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.
- ItemConstraining 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, JPRadiocarbon 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.
- ItemConstraining 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, JPMethane (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.
- ItemThe 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, RFStudies 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.
- ItemIce 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, JPConcentrations 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.
- ItemInsights 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, JPIn 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.
- ItemLaw 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, JPMethane 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
- ItemLaw 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, JPMethane 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.
- ItemObtaining a history of the flux of cosmic rays using in situ cosmogenic 14C trapped in polar ice(International Union of Pure and Applied Physics (IUPAP), 2019-07-24) BenZvi, S; Petrenko, VV; Hmiel, B; Dyonisius, MN; Smith, AM; Yang, B; Hua, QCarbon-14 (14C) is produced in the atmosphere when neutrons from cosmic-ray air showers are captured by 14N nuclei. Atmospheric 14C becomes trapped in air bubbles in polar ice as compacted snow (firn) transforms into ice. 14C is also produced in situ in ice grains by penetrating cosmic-ray neutrons and muons. Recent ice core measurements indicate that in the 14CO phase, the 14C is dominated by the in situ cosmogenic component at most ice coring sites. Thus, it should be possible to use ice-bound 14CO to reconstruct the historical flux of cosmic rays at Earth, without the transport and deposition uncertainties associated with 10Be or the carbon cycle uncertainties affecting atmospheric 14CO2. The measurements will be sensitive to the cosmic-ray flux above the energy range most affected by solar modulation. We present estimates of the expected sensitivity of 14CO in ice cores to the historical flux of Galactic cosmic rays, based on recent studies of 14CO in polar ice. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0)
- ItemOld carbon reservoirs were not important in the deglacial methane budget(AAAS, 2020-02-21) Dyonisius, MN; Petrenko, VV; Smith, AM; Hua, Q; Yang, B; Schmitt, J; Beck, J; Seth, B; Bock, M; Hmiel, B; Vimont, I; Menking, JA; Shackleton, SA; Baggenstos, D; Bauska, TK; Rhodes, RH; Sperlich, P; Beaudette, R; Harth, CM; Kalk, M; Brook, EJ; Fischer, H; Severinghaus, JP; Weiss, RFPermafrost and methane hydrates are large, climate-sensitive old carbon reservoirs that have the potential to emit large quantities of methane, a potent greenhouse gas, as the Earth continues to warm. We present ice core isotopic measurements of methane (Δ14C, δ13C, and δD) from the last deglaciation, which is a partial analog for modern warming. Our results show that methane emissions from old carbon reservoirs in response to deglacial warming were small (<19 teragrams of methane per year, 95% confidence interval) and argue against similar methane emissions in response to future warming. Our results also indicate that methane emissions from biomass burning in the pre-Industrial Holocene were 22 to 56 teragrams of methane per year (95% confidence interval), which is comparable to today. Copyright © 2020 The Authors
- ItemThe potential of 14CO in glacial ice as a tracer for past cosmic ray flux and atmospheric hydroxyl radical abundance(Antarctic Climate and Ecosystems Cooperative Research Centre, 2016-03-07) Petrenko, VV; Hmiel, B; Neff, PD; Smith, AM; Buizert, C; Etheridge, DM; Dyonisius, MNThe amount of 14C-containing carbon monoxide (14CO) in glacial ice is determined by trapping of atmospheric 14CO into air bubbles in the ice and in situ cosmogenic production of 14CO in relatively shallow ice and firn. Earlier studies of 14CO in ice cores showed large disagreements with regard to rates of in situ cosmogenic production as well as with regard to whether 14CO produced in the firn layer is well retained or largely escapes to the atmosphere via the interconnected pore space. We have reviewed previously published work that included 14CO measurements in ice or firn air, and compared with our more recent high-precision measurements on very large ice and firn samples. The available evidence suggests that very little in situ cosmogenic 14CO is retained in the diffusive part of the firn (the upper ≈ 40 – 100m). In situ cosmogenic 14CO production rates below the firn diffusive zone are non-negligible, with production due to deeper-penetrating muons. At sites with low snow accumulation rates, the in situ cosmogenic 14CO component is expected to be larger than the trapped atmospheric component. This potentially allows to use ice core 14CO measurements from such sites to improve our understanding of past cosmic ray flux variations. In contrast, at sites with very high accumulation rates, trapped atmospheric 14CO is expected to be dominant over the in situ cosmogenic component. This potentially allows 14CO records from such sites to be used for reconstructions of past atmospheric hydroxyl radical (OH) variations.
- ItemThe potential of using in situ cosmogenic 14CO in ice cores at Dome C to examine the assumption of a constant galactic cosmic ray flux(American Geophysical Union (AGU), 2021-12-17) Petrenko, VV; BenZvi, S; Smith, AM; Dyonisus, MN; Hmiel, B; Neft, PD; Buizert, C; Severinghaus, JPCosmogenic nuclides produced in the Earth’s atmosphere and at the surface are powerful proxies for important climate processes and drivers. Records of atmospherically-produced 14C and 10Be have been used to reconstruct past solar activity and solar irradiance. 10Be, 14C, 26Al and other nuclides produced in surface rock are widely used in studies of past ice dynamics and extent. All these studies generally assume that the galactic cosmic ray (GCR) flux at Earth is constant in time. However, the available geochemical evidence for GCR flux constancy is complicated by processes that are not fully constrained. As a result, the assumption of a constant GCR flux may be uncertain by 30% or more. Cosmic rays also produce 14C in situ in glacial ice and firn; this 14C then reacts rapidly to form mainly 14CO and 14CO2. Almost all of the 14C produced in the firn layer is lost to the atmosphere via gas diffusion, and in situ 14C only starts to accumulate in the deepest firn (≈95 m at Dome C) where gas exchange with the atmosphere effectively stops. At this depth, all of the in situ 14C production is via interactions with deep-penetrating muons. Such muons are generated by high-energy primary GCRs that are unaffected by geomagnetic and solar modulation. Further, at sites with low snow accumulation such as Dome C, in situ 14CO strongly dominates over trapped atmospheric 14CO in the ice. As a result, 14CO in ice at Dome C would provide a record of the past GCR flux that is virtually free of confounding factors and should allow to constrain any past flux variations to within ≈ 10%. This presentation will provide a brief overview of results from recent studies of in situ cosmogenic 14CO in Greenland and Antarctica, as well as predictions for Dome C under a range of different GCR flux scenarios.
- ItemThe potential of using in situ cosmogenic 14CO in ice cores at Dome C to examine the assumption of a constant galactic cosmic ray flux(Australian Nuclear Science and Technology Organisation, 2021-11-17) Petrenko, VV; BenZvi, S; Smith, AM; Dyonisius, MN; Hmiel, B; Neff, PD; Buizert, C; Severinghaus, JPCosmogenic nuclides produced in the Earth’s atmosphere and at the surface are powerful proxies for important climate processes and drivers. Records of atmospherically-produced 14C and 10Be have been used to reconstruct past solar activity and solar irradiance. 10Be, 14C, 26Al and other nuclides produced in surface rock are widely used in studies of past ice dynamics and extent. All these studies generally assume that the galactic cosmic ray (GCR) flux at Earth is constant in time. However, the available geochemical evidence for GCR flux constancy is complicated by processes that are not fully constrained. As a result, the assumption of a constant GCR flux may be uncertain by 30% or more. Cosmic rays also produce 14C in situ in glacial ice and firn; this 14C then reacts rapidly to form mainly 14CO and 14CO2. Almost all of the 14C produced in the firn layer is lost to the atmosphere via gas diffusion, and in situ 14C only starts to accumulate in the deepest firn (≈95 m at Dome C) where gas exchange with the atmosphere effectively stops. At this depth, all of the in situ 14C production is via interactions with deep-penetrating muons. Such muons are generated by highenergy primary GCRs that are unaffected by geomagnetic and solar modulation. Further, at sites with low snow accumulation such as Dome C, in situ 14CO strongly dominates over trapped atmospheric 14CO in the ice. As a result, 14CO in ice at Dome C would provide a record of the past GCR flux that is virtually free of confounding factors and should allow to constrain any past flux variations to within ≈ 10%. This presentation will provide a brief overview of results from recent studies of in situ cosmogenic 14CO in Greenland and Antarctica, as well as predictions for Dome C under a range of different GCR flux scenarios. © The Authors
- ItemPreindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions(Springer Nature, 2020-02-19) Hmiel, B; Petrenko, VV; Dyonisius, MN; Buizert, C; Smith, AM; Place, PF; Harth, CM; Beaudette, R; Hua, Q; Yang, B; Vimont, I; Michel, SE; Severinghaus, JP; Etheridge, DM; Bromley, T; Schmitt, J; Faïn, X; Weiss, RF; Dlugokencky, EAtmospheric methane (CH4) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era1. Fossil fuel extraction and use are among the largest anthropogenic sources of CH4 emissions, but the precise magnitude of these contributions is a subject of debate2,3. Carbon-14 in CH4 (14CH4) can be used to distinguish between fossil (14C-free) CH4 emissions and contemporaneous biogenic sources; however, poorly constrained direct 14CH4 emissions from nuclear reactors have complicated this approach since the middle of the 20th century4,5. Moreover, the partitioning of total fossil CH4 emissions (presently 172 to 195 teragrams CH4 per year)2,3 between anthropogenic and natural geological sources (such as seeps and mud volcanoes) is under debate; emission inventories suggest that the latter account for about 40 to 60 teragrams CH4 per year6,7. Geological emissions were less than 15.4 teragrams CH4 per year at the end of the Pleistocene, about 11,600 years ago8, but that period is an imperfect analogue for present-day emissions owing to the large terrestrial ice sheet cover, lower sea level and extensive permafrost. Here we use preindustrial-era ice core 14CH4 measurements to show that natural geological CH4 emissions to the atmosphere were about 1.6 teragrams CH4 per year, with a maximum of 5.4 teragrams CH4 per year (95 per cent confidence limit)—an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH4 emissions are underestimated by about 38 to 58 teragrams CH4 per year, or about 25 to 40 per cent of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH4 budget, and will help to inform strategies for targeted emission reductions 9,10. © The Author(s), under exclusive licence to Springer Nature Limited 2020.
- ItemA preliminary record of changes in Southern Hemisphere atmospheric OH abundance from 14CO in glacial ice (Law Dome, Antarctica, 1870 AD to present)(American Geophysical Union (AGU), 2021-12-17) Neff, PD; Petrenko, VV; Etheridge, DM; Smith, AM; 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, RFHydroxyl, 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 at sites with very high snow accumulation rates can provide such constraints, as rapid snow burial limits in-situ production of 14CO by cosmic rays directly in the ice. 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 on-site 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.
- ItemRadioactive and stable paleoatmospheric methane isotopes across the last deglaciation and early holocene from Taylor Glacier, Antarctica(American Geophysical Union, 2016-12-13) Dyonisius, MN; Petrenko, VV; Smith, AW; Hmiel, B; Vimont, I; Hua, Q; Yang, B; Menking, JA; Shackleton, SA; Rhodes, RH; Baggenstos, D; Bauska, TK; Bock, M; Beck, J; Seth, B; Harth, CM; Beaudette, R; Schmitt, J; Brook, EJ; Weiss, RF; Fischer, H; Severinghaus, JP; McConnel, JPMethane (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. Both radioactive and stable CH4 isotopes (Delta14C-CH4, delta13C-CH4, and deltaD-CH4) from ice cores in Greenland and Antarctica have been used to constrain the past CH4 budget. Among the CH4 isotopes, 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). During the 2013-2014 and 2014-2015 field seasons at Taylor Glacier, Antarctica, we have successfully extracted 12 large volume ice samples across the Last Deglaciation to early Holocene (20ka-8ka BP). All samples have been successfully measured for CH4 mole fraction ([CH4]), Delta14C-14CH4, delta13C-CH4, and deltaD-CH4. The [CH4], delta13C-CH4, and deltaD-CH4 measurements in our samples are consistent with existing delta13C-CH4, and deltaD-CH4 datasets from other deep cores, confirming the integrity of CH4 in Taylor Glacier ice. Preliminary 14CH4 results across the Oldest Dryas - Bølling (OD-BO) CH4 transition suggest that the 150 ppb [CH4] increase during the transition was caused by increased wetland emissions. Early Holocene and Last Glacial Maximum (LGM) 14C results are still undergoing corrections for in-situ cosmogenic 14C based on 14CO measurements in the same samples. We will present the corrected 14CH4 results from these samples and our preliminary interpretations with regard to the strength of old CH4 sources during the LGM and early Holocene. © 2016 American Geophysical Union
- ItemTowards 14C-dating of gases in ice cores – constraining the in situ cosmogenic 14C production rates by muons(Australian Nuclear Science and Technology Organisation, 2021-11-17) Dyonisius, MN; Petrenko, VV; Smith, AM; Hmiel, B; Neff, PD; Yang, B; Hua, Q; Place, PF; Menking, J; Shackleton, SA; Beaudette, R; Harth, CM; Kalk, M; Roop, H; Bereiter, B; Armanetti, C; Buizert, C; Schmitt, J; Brook, EJ; Severinghaus, JP; Weiss, RF; McConnell, JRRadiocarbon dating of glacial ice has been a longstanding goal in ice core science. In glacial ice, ¹⁴ C is incorporated mainly through trapping of ¹⁴ C-containing atmospheric gases (¹⁴ CO₂ , ¹⁴ CO, and ¹⁴ CH₄ ). However, ¹⁴ C in ice is also produced in situ, directly in the ice lattice from reactions with secondary cosmic rays. In situ ¹⁴ C in ice mostly accumulates after bubble close-off (generally at firn depths between 50-120 m) because almost all of the in situ produced ¹⁴ C in the firn column is lost to the atmosphere via diffusion. The in situ ¹⁴ C at corresponding close-off depths of most ice core sites is generally dominated by production from deep penetrating muons. Understanding the muogenic ¹⁴ C production rates is thus important to deconvolve the in situ cosmogenic and atmospheric ¹⁴ C signals in ice cores. In this study, we use measurements of ¹⁴ C in ancient ice (>50 kilo-annum before present, ka BP) from the Taylor Glacier ablation site, Antarctica to calibrate the muogenic ¹⁴ C production rates. We find that literature values are overestimated by factors of 5.7 (3.6-13.9, 95% confidence interval) and 3.7 (2.0-11.9 95% confidence interval) for negative muon capture and fast muon interactions respectively. Furthermore, the partitioning between the in situ ¹⁴ C species appears to be constant (¹⁴ CO:¹⁴ CO₂ ratio of 1:2, with small <0.2% contributions from ¹⁴ CH₄ ). Our results allow for future ice core ¹⁴ C studies to be potentially used for several applications, including absolute dating of gases and improving the ¹⁴ C calibration curve in periods where high-resolution tree ring data are not available.
- ItemUnderstanding the production and retention of in situ cosmogenic 14C in polar firn(AGU Fall Meeting, 12-16 Dec 2016, San Francisco, USA., 2016-12-01) Hmiel, B; Petrenko, VV; Dyonisius, MN; Smith, AM; Schmitt, J; Buizert, C; Place, PF; Harth, CM; Beaudette, R; Hua, Q; Yang, B; Vimont, I; Kalk, M; Weiss, RF; Severinghaus, JP; Brook, EJ; White, JWCRadiocarbon in CO2, CO and CH4 trapped in polar ice is of interest for dating of ice cores, studies of past solar activity and cosmic ray flux, as well as studies of the paleoatmospheric CH4 budget. The major difficulty with interpreting 14C measurements in ice cores stems from the fact that the measured 14C represents a combination of trapped paleoatmospheric 14C and 14C that is produced within the firn and ice lattice by secondary cosmic ray particles. This in situ cosmogenic 14C component in ice is at present poorly understood. Prior ice core 14C studies show conflicting results with regard to the retention of in situ cosmogenic 14C in polar firn and partitioning of this 14C among CO2, CO and CH4. Our study aims to comprehensively characterize the 14C of CO2, CO, and CH4 in both the air and the ice matrix throughout the firn column at Summit, Greenland. We will present preliminary measurements of 14C in Summit firn air and the firn matrix, along with initial interpretations with regard to in situ cosmogenic 14C retention. Preliminary results from firn air indicate a 14CO increase with depth in the lock-in zone resulting from in situ production by muons, as well as a lock-in zone 14CO2 bomb peak originating from nuclear testing in the late 1950s and early 1960s. A decrease in 14CH4 with depth is observed in the lock-in zone that is in agreement with observations of increasing atmospheric 14CH4 over the past several decades. We observe that only a small fraction of in-situ produced 14CO, 14CH4 and 14CO2 is retained in the firn matrix. Additionally, we describe progress in the development of a field-portable sublimation apparatus for extraction of CO2 from firn and ice for 14C measurements. © 2016 AGU