Browsing by Author "Schmitt, J"
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- 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.
- 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.
- 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
- 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.
- 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
- ItemUsing 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, JRCosmic 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.