Browsing by Author "Neff, PD"

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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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    Characterization 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, S
    Carbon-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.
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    Dating Antarctic ice cores using high-temporal resolution black carbon records
    (Antarctic Climate and Ecosystems Cooperative Research Centre, 2016-03-07) Edwards, R; Vallelonga, P; McConnell, JR; Bertler, NAN; Curran, MAJ; Sigil, M; Fudge, TJ; Anschuetz, H; Neff, PD; Emanuelsson, D; Bisiaux, M; Goodwin, D; Smith, AM; Taylor, KC; Moy, AD; Fetieng, W; Ellis, A
    Black carbon aerosols (BC) emitted by fires in the Southern Hemisphere (SH) are transported to Antarctica and preserved in the Antarctic ice sheet. Recent efforts to develop ice core records of BC deposition to Antarctica show variability in BC over a broad range of time scales. The ~ monthly-resolution BC record from the WAIS divide deep ice core displayed strong seasonal variability in modern sections of the record consistent with the timing of SH biomass burning. The record was subsequently used as an annual layer dating proxy in conjunction with other chemical species. If the emissions and transport of BC to Antarctica are stable over long periods of time it may be useful as an annual layer proxy at sites other than WAIS. To date, a rigorous comparison of Antarctic ice core BC seasonality from different locations have not been conducted. Here we present a comparison of BC ice core data from the top sections of the WAIS divide deep core, the Roosevelt Island RICE core, and the Law Dome DSS1213 core. The RICE and Law Dome sites are separated from WAIS by large distances and experience different atmospheric circulation and climate regimes. A detailed description of the data uncertainties and its use in annual layer counting will be discussed.
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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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    The 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, MN
    The 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.
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    The 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, JP
    Cosmogenic 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
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    A 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, 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 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.
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    Towards 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, JR
    Radiocarbon 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.
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    Using atmospheric 14CO to provide additional constraints for global OH: results from a new approach and potential for future measurements
    (Copernicus GmbH, 2019-04-10) Petrenko, VV; Murray, LT; Crosier, EM; Colton, A; Hua, Q; Smith, AM; Yang, B; Kazemi, R; Neff, PD; Etheridge, DM; Usoskin, IG; Poluianov, S
    The 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 global abundance of 14CO that are not explained by variations in 14C production are mainly driven by variations in the global abundance of OH. Monitoring OH variability via methyl chloroform is becoming increasingly difficult as methyl chloroform abundance is continuing to decline. Measurements of atmospheric 14CO have previously been successfully used to infer OH variability. However, these measurements have only continued at one location (Baring Head, New Zealand), which is insufficient to infer global trends. We propose to restart global 14CO monitoring with the aim of providing an additional constraint on OH variability. A new analytical system for 14CO sampling and measurements has been developed, allowing for a ten-fold reduction in the required sample air volumes and simplified field logistics. The first 14CO measurements from Mauna Loa Observatory show good agreement with prior measurements in the same latitude band. Preliminary work with a state-of-the-art chemical transport model is exploring sensitivity of 14CO at potential sampling locations to changes in production rates and OH. This presentation will also provide an update on a project which aims to improve the understanding of long-term OH variability via reconstructing a 150-year history of atmospheric 14CO from ice cores at Law Dome, Antarctica. Sampling of the ice and on-site extractions of large volumes of ancient air were in progress during December 2018 – January 2019. © Author(s) 2019. CC Attribution 4.0 license
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    Using ice core measurements from Taylor Glacier, Antarctica to calibrate in situ cosmogenic 14C production rates by muons
    (Copernicus Publications, 2022-01-26) Dyonisius, MN; Petrenko, VV; Smith, AM; Hmiel, B; Neff, PD; Yang, B; Hua, Q; Schmitt, J; Shackleton, SA; Buizert, C; Place, PF; Menking, JA; Beaudette, R; Harth, CM; Kalk, M; Roop, H; Bereiter, B; Armanetti, C; Vimont, I; Michel, SE; Brook, EJ; Severinghaus, JP; Weiss, RF; McConnell, JR
    Cosmic rays entering the Earth’s atmosphere produce showers of secondary particles such as neutrons and muons. The interaction of these neutrons and muons with oxygen-16 (16O) in minerals such as ice and quartz can produce carbon-14 (14C). Analyses of in situ produced cosmogenic 14C in quartz are commonly used to investigate the Earth’s landscape evolution. In glacial ice, 14C is also incorporated through trapping of 14C-containing atmospheric gases (14CO2, 14CO, and 14CH4). Understanding the production rates of in situ cosmogenic 14C is important to deconvolve the in situ cosmogenic and atmospheric 14C signals in ice, both of which contain valuable paleoenvironmental information. Unfortunately, the in situ 14C production rates by muons (which are the dominant production mechanism at depths of > 6 m solid ice equivalent) are uncertain. In this study, we use measurements of in situ 14C in ancient ice (> 50 kilo-annum before present, ka BP) from the Taylor Glacier ablation site, Antarctica in combination with a 2D ice flow model to better constrain the rates of 14C production by muons. We find that the commonly used values for muogenic 14C production rates (Heisinger et al., 2002a, 2002b) in ice are too high 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. Our constraints on muogenic 14C production rates in ice allow for future measurements of 14C in ice cores to be used for other applications and imply that muogenic 14C production rates in quartz are overestimated as well. © Author(s) 2022.