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Galactic cosmic rays (GCRs) interact with matter in the atmosphere and at the surface of the Earth to produce a range of cosmogenic nuclides. Measurements of cosmogenic nuclides produced in surface rocks have been used to study past land ice extent as well as to estimate erosion rates. Because the GCR flux reaching the Earth is modulated by magnetic fields (solar and Earth's), records of cosmogenic nuclides produced in the atmosphere have also been used for studies of past solar activity. Studies utilizing cosmogenic nuclides assume that the GCR flux is constant in time, but this assumption may be uncertain by 30 % or more. Here we propose that measurements of 14 C of carbon monoxide (14 CO) in ice cores at low-accumulation sites can be used as a proxy for variations in GCR flux on timescales of several thousand years. At low-accumulation ice core sites, 14 CO in ice below the firn zone originates almost entirely from in situ cosmogenic production by deep-penetrating secondary cosmic ray muons. The flux of such muons is almost insensitive to solar and geomagnetic variations and depends only on the primary GCR flux intensity. We use an empirically constrained model of in situ cosmogenic 14 CO production in ice in combination with a statistical analysis to explore the sensitivity of ice core 14 CO measurements at Dome C, Antarctica, to variations in the GCR flux over the past ≈ 7000 years. We find that Dome C 14 CO measurements would be able to detect a linear change of 6 % over 7 ka, a step increase of 6 % at 3.5 ka or a transient 100-year spike of 190 % at 3.5 ka at the 3 σ significance level. The ice core 14 CO proxy therefore appears promising for the purpose of providing a high-precision test of the assumption of GCR flux constancy over the Holocene. [ABSTRACT FROM AUTHOR]

Measurements of carbon-14-containing carbon monoxide (14 CO) 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 14 CO abundance in glacial ice represents the combination of trapped atmospheric 14 CO and in situ cosmogenic 14 CO. The systematics of in situ cosmogenic 14 CO production and retention in ice are not fully quantified, posing an obstacle to interpretation of ice core 14 CO measurements. Here we provide the first comprehensive characterization of 14 CO 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 14 C. We find that almost all (≈ 99.5 %) of in situ 14 CO 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 14 CO 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 14 CO content with depth that is due to in situ 14 CO production by deep-penetrating muons, confirming recent estimates of 14 CO production rates in ice via the muon mechanisms and allowing for narrowing constraints on these production rates. [ABSTRACT FROM AUTHOR]

Global chemistry-climate models (CCMs) play an important role in assessing the climate and air pollution implications of aerosols and chemically reactive gases. Evaluating these models under past conditions and constraining historical sources and sinks necessitates reliable records of atmospheric mixing ratios spanning preindustrial times. Such precious records were recently obtained for carbon monoxide (CO) documenting for the first time the evolution of this reactive compound over the industrial era. In this study, we compare the simulated atmospheric surface CO mixing ratios ([CO]) from two different sets of CCMs and emissions in the frame of CMIP5 and of CMIP6 (Coupled Model Intercomparison Project Phases 5 and 6) with recent bipolar ice archive reconstructions for the period spanning 1850 to present. We analyze how historical (1850-2014) [CO] outputs from 16 (Atmospheric Chemistry and Climate Model Intercomparison Project) models and 6 AerChemMIP (Aerosol Chemistry Model Intercomparison Project) models over Greenland and Antarctica are able to capture both absolute values and trends recorded in multi-site ice archives. While most models underestimate [CO] at high northern latitudes, a reduction in this bias is observed from ACCMIP to AerChemMIP exercises. Over the 1980-2010 CE period, trends in ice archive and firn air observations and AerChemMIP outputs align remarkably well at high northern and southern latitudes, indicating improved quantification of CO anthropogenic emissions and the main CO sink (OH oxidation) compared to ACCMIP. From 1850 to 1980 CE, AerChemMIP models and observations consistently show increasing [CO] in both the Northern Hemisphere (NH) and Southern Hemisphere (SH), suggesting a robust understanding of the CO budget evolution. However, a divergence in the [CO] growth rate emerges in NH between models and observations over the 1920-1975 CE period, attributed to uncertainties in CO emission factors (EF), particularly EF for RCO (Residential, Commercial and Others) and transportation sectors, although we cannot totally rule out that the CO record based on Greenland ice archives may be biased high by CO chemical production processes occurring in the ice prior the measurements (i.e., in situ CO production). In the Southern Hemisphere, AerChemMIP models simulate an increase in atmospheric [CO] from 1850 to 1980 CE closely reproducing the observations (22±10 ppb and 13±7 ppb, respectively). Such agreement supports CMIP6 biomass burning CO emission inventories which do not reveal a peak in CO emissions in the late 19th century. Furthermore, both SH models and observations reveal an accelerated growth rate in [CO] during 1945-1980 CE relative to 1980-1945 CE, likely linked to increased anthropogenic transportation emissions. [ABSTRACT FROM AUTHOR]

Galactic cosmic rays (GCRs) interact with matter in the atmosphere and at the surface of the Earth to produce a range of cosmogenic nuclides. Measurements of cosmogenic nuclides produced in surface rocks have been used to study past land ice extent as well as to estimate erosion rates. Because the GCR flux reaching the Earth is modulated by magnetic fields (solar and Earth's), records of cosmogenic nuclides produced in the atmosphere have also been used for studies of past solar activity. Studies utilizing cosmogenic nuclides assume that the GCR flux is constant in time, but this assumption may be uncertain by 30% or more. Here we propose that measurements of 14C of carbon monoxide (14CO) in ice cores at low-accumulation sites can be used as a proxy for variations in GCR flux on timescales of several thousand years. At low-accumulation ice core sites, 14CO in ice below the firn zone originates almost entirely from in situ cosmogenic production by deep-penetrating secondary cosmic ray muons. The flux of such muons is insensitive to solar and geomagnetic variations, and depends only on the primary GCR flux intensity. We use an empirically-constrained model of in situ cosmogenic 14CO production in ice in combination with a statistical analysis to explore the sensitivity of ice core 14CO measurements at Dome C, Antarctica to variations in the GCR flux over the past ≈7000 years. We find that Dome C 14CO measurements would be able to detect a linear change of 4%, a step increase of 4% or a transient 100-year spike of 250% at the 3 σ significance level. The ice core 14CO proxy therefore appears promising for the purpose of providing a high-precision test of the assumption of GCR flux constancy over the Holocene. [ABSTRACT FROM AUTHOR]

Historical atmospheric H 2 levels were reconstructed using firn air measurements from two sites in Greenland (NEEM and Summit) and two sites in Antarctica (South Pole and Megadunes). A joint reconstruction based on the two Antarctic sites yields H 2 levels monotonically increasing from about 330 ppb in 1900 to 550 ppb in the late 1990s, leveling off thereafter. These results are similar to individual reconstructions published previously (Patterson et al., 2020, 2021). Interpretation of the Greenland firn air measurements is complicated by challenges in modeling enrichment induced by pore close-off at these sites. We used observations of neon enrichment at NEEM and Summit to tune the parameterization of enrichment induced by pore close-off in our firn air model. The joint reconstruction from the Greenland data shows H 2 levels rising 30 % between 1950 and the late 1980s, reaching a maximum of 530 ppb. After 1990, reconstructed atmospheric H 2 levels over Greenland are roughly constant, with a small decline of 3 % over the next 25 years. The reconstruction shows good agreement with the available flask measurements of H 2 at high northern latitudes. [ABSTRACT FROM AUTHOR]

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. 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 time scale 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% per year. 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 production rates in ice via the muon mechanisms and allowing for narrowing constraints on these production rates. [ABSTRACT FROM AUTHOR]

Historical hemispheric atmospheric H2 levels since 1930 were reconstructed using the UCI_2 firn air model and firn air measurements from three sites in Greenland: (NEEM, Summit, and Tunu) and two sites in Antarctica (South Pole and Megadunes). A joint reconstruction based on the two Antarctic sites yields H2 levels monotonically increasing from about 350 ppb in 1900 to 550 ppb in the late 1990's, levelling off thereafter. These results are similar to individual reconstructions published previously (Patterson et al., 2020; 2021). Reconstruction of the Greenland data is complicated by a systematic bias between Tunu and the other sites. The Tunu reconstruction shows substantially lower historical H2 levels than the other two sites, a difference we attribute to possible bias in the calibration of the Tunu measurements. All three reconstructions show a late 20th century maximum in H2 levels over Greenland. A joint reconstruction of the Greenland data shows H2 levels rising 40% from 1930-1990, reaching a maximum of 550 ppb. After 1990, reconstructed atmospheric H2 decrease by 6% over the next 20 years. The reconstruction deviates by at most 4% from the few available surface air measurements of atmospheric H2 levels over Greenland from 1998-2004. However, the longer instrumental records from sampling sites outside of Greenland show a more rapid decrease and stabilization after 1990 compared to the reconstruction. We explore the possibility that this difference is an artefact caused by the firn air model underestimating pore close-off induced enrichment, evidenced by a mismatch between measured and modelled Ne in firn air. We developed new parameterizations which more accurately capture pore close-off induced enrichment at the Greenland sites. Incorporating those 30 parameterizations into the UCI_2 model yields reconstructions with lower H2 levels throughout the mid-late 20th century and more stable H2 levels during the 1990's, in better agreement with the flask measurements. [ABSTRACT FROM AUTHOR]

Cosmic rays entering the Earth's atmosphere produce showers of secondary particles such as protons, neutrons, and muons. The interaction of these particles with oxygen-16 (16O) in minerals such as ice and quartz can produce carbon-14 (14C). 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 ka) from the Taylor Glacier, an ablation site in Antarctica, in combination with a 2D ice flow model to better constrain the compound-specific rates of 14C production by muons and the partitioning of in situ 14C between CO 2 , CO, and CH4. Our measurements show that 33.7 % (±11.4% ; 95 % confidence interval) of the produced cosmogenic 14C forms 14CO and 66.1 % (±11.5% ; 95 % confidence interval) of the produced cosmogenic 14C forms 14CO2. 14CH4 represents a very small fraction (<0.3%) of the total. Assuming that the majority of in situ muogenic 14C in ice forms 14CO2 , 14CO , and 14CH4 , we also calculated muogenic 14C production rates that are lower 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, when compared to values determined in quartz from laboratory studies (Heisinger et al., 2002a, b) and in a natural setting (Lupker et al., 2015). This apparent discrepancy in muogenic 14C production rates in ice and quartz currently lacks a good explanation and requires further investigation. [ABSTRACT FROM AUTHOR]

Carbon monoxide (CO) is a regulated pollutant and one of the key components determining the oxidizing capacity of the atmosphere. Obtaining a reliable record of atmospheric CO mixing ratios ([CO]) since preindustrial times is necessary to evaluate climate–chemistry models under conditions different from today and to constrain past CO sources. We present high-resolution measurements of CO mixing ratios from ice cores drilled at five different sites on the Greenland ice sheet that experience a range of snow accumulation rates, mean surface temperatures, and different chemical compositions. An optical-feedback cavity-enhanced absorption spectrometer (OF-CEAS) was coupled with continuous melter systems and operated during four analytical campaigns conducted between 2013 and 2019. Overall, continuous flow analysis (CFA) of CO was carried out on over 700 m of ice. The CFA-based CO measurements exhibit excellent external precision (ranging from 3.3 to 6.6 ppbv , 1 σ) and achieve consistently low blanks (ranging from 4.1±1.2 to 12.6±4.4 ppbv), enabling paleoatmospheric interpretations. However, the five CO records all exhibit variability that is too large and rapid to reflect past atmospheric mixing ratio changes. Complementary tests conducted on discrete ice samples demonstrate that these variations are not artifacts of the analytical method (i.e., production of CO from organics in the ice during melting) but are very likely related to in situ CO production within the ice before analysis. Evaluation of the signal resolution and co-investigation of high-resolution records of CO and total organic carbon (TOC) suggest that past atmospheric CO variations can be extracted from the records' baselines with accumulation rates higher than 20 cm w.e.yr-1 (water equivalent per year). Consistent baseline CO records from four Greenland sites are combined to produce a multisite average ice core reconstruction of past atmospheric CO for the Northern Hemisphere high latitudes, covering the period from 1700 to 1957 CE. Such a reconstruction should be taken as an upper bound of past atmospheric CO abundance. From 1700 to 1875 CE, the record reveals stable or slightly increasing values in the 100–115 ppbv range. From 1875 to 1957 CE, the record indicates a monotonic increase from 114±4 to 147±6 ppbv. The ice core multisite CO record exhibits an excellent overlap with the atmospheric CO record from Greenland firn air which spans the 1950–2010 CE time period. The combined ice core and firn air CO history, spanning 1700–2010 CE, provides useful constraints for future model studies of atmospheric changes since the preindustrial period. [ABSTRACT FROM AUTHOR]

Deglaciations are characterized by relatively fast and near-synchronous changes in ice sheet volume, ocean temperature, and atmospheric greenhouse gas concentrations, but glacial inception occurs more gradually. Understanding the evolution of ice sheet, ocean, and atmosphere conditions from interglacial to glacial maximum provides insight into the interplay of these components of the climate system. Using noble gas measurements in ancient ice samples, we reconstruct mean ocean temperature (MOT) from 74 to 59.7 ka, covering the Marine Isotope Stage (MIS) 5a–4 boundary, MIS 4, and part of the MIS 4–3 transition. Comparing this MOT reconstruction to previously published MOT reconstructions from the last and penultimate deglaciation, we find that the majority of the last interglacial–glacial ocean cooling must have occurred within MIS 5. MOT reached equally cold conditions in MIS 4 as in MIS 2 (-2.7 ± 0.3 ∘ C relative to the Holocene, -0.1 ± 0.3 ∘ C relative to MIS 2). Using a carbon cycle model to quantify the CO 2 solubility pump, we show that ocean cooling can explain most of the CO 2 drawdown (32 ± 4 of 40 ppm) across MIS 5. Comparing MOT to contemporaneous records of benthic δ18 O, we find that ocean cooling can also explain the majority of the δ18 O increase across MIS 5 (0.7 ‰ of 1.3 ‰). The timing of ocean warming and cooling in the record and the comparison to coeval Antarctic isotope data suggest an intimate link between ocean heat content, Southern Hemisphere high-latitude climate, and ocean circulation on orbital and millennial timescales. [ABSTRACT FROM AUTHOR]

Carbon monoxide (CO) is a regulated pollutant and one of the key components determining the oxidizing capacity of the atmosphere. Obtaining a reliable record of atmospheric CO mixing ratios since pre-industrial times is necessary to evaluate climate-chemistry models in conditions different from today and to constrain past CO sources. We present high-resolution measurements of CO mixing ratios from ice cores drilled at five different sites on the Greenland ice sheet which experience a range of snow accumulation rates, mean surface temperatures, and different chemical compositions. An optical-feedback cavity-enhanced absorption spectrometer (OF-CEAS) was coupled to continuous melter systems and operated during four analytical campaigns conducted between 2013 and 2019. Overall, continuous flow analyses (CFA) of CO were carried out on over 700 m of ice. The CFA-based CO measurements exhibit excellent external precision (ranging 3.3-6.6 ppbv, 1sigma), and achieve consistently low blanks (ranging from 4.1+/-1.2 to 12.6+/-4.4 ppbv), enabling paleo-atmospheric interpretations. However the five CO records all exhibit variability too large and rapid to reflect past atmospheric mixing ratio changes. Complementary tests conducted on discrete ice samples demonstrate that these variations are not artifacts of the analytical method (i.e., production of CO from organics in the ice during melting), but very likely are related to in situ CO production within the ice before analysis. Evaluation of signal resolution and co-investigation of high-resolution records of CO and TOC show that past atmospheric CO variations can be extracted from the records' baselines at four sites with accumulation rates higher than 20 cm water equivalent per year (weq yr-1). However, such baselines should be taken as upper bounds of past atmospheric CO burden. Baseline CO records from four sites are combined to produce a multisite average ice core reconstruction of past atmospheric CO for the Northern Hemisphere high latitudes, covering the period from 1700 to 1957 CE. From 1700 to 1875 CE, the record reveals stable or slightly increasing values in the 100-115 ppbv range. From 1875 to 1957 CE, the record indicates a monotonic increase from 114+/-4 ppbv to 147+/-6 ppbv. The ice-core multisite CO record exhibits an excellent overlap with the atmospheric CO record from Greenland firn air which spans the 1950-2010 time period. The combined ice-core and firn air CO history, spanning 1700-2010 CE provides useful constraints for future model studies of atmospheric changes since the preindustrial period. [ABSTRACT FROM AUTHOR]

Important uncertainties remain in our understanding of the spatial and temporal variability of atmospheric hydroxyl radical concentration ([OH]). Carbon-14-containing carbon monoxide (14CO) is a useful tracer that can help in the characterization of [OH] variability. Prior measurements of atmospheric 14CO concentration ([ 14CO ] are limited in both their spatial and temporal extent, partly due to the very large air sample volumes that have been required for measurements (500–1000 L at standard temperature and pressure, L STP) and the difficulty and expense associated with the collection, shipment, and processing of such samples. Here we present a new method that reduces the air sample volume requirement to ≈90 L STP while allowing for [ 14CO ] measurement uncertainties that are on par with or better than prior work (≈3 % or better, 1 σ). The method also for the first time includes accurate characterization of the overall procedural [ 14CO ] blank associated with individual samples, which is a key improvement over prior atmospheric 14CO work. The method was used to make measurements of [ 14CO ] at the NOAA Mauna Loa Observatory, Hawaii, USA, between November 2017 and November 2018. The measurements show the expected [ 14CO ] seasonal cycle (lowest in summer) and are in good agreement with prior [ 14CO ] results from another low-latitude site in the Northern Hemisphere. The lowest overall [ 14CO ] uncertainties (2.1 %, 1 σ) are achieved for samples that are directly accompanied by procedural blanks and whose mass is increased to ≈50 µgC (micrograms of carbon) prior to the 14C measurement via dilution with a high-CO 14C -depleted gas. [ABSTRACT FROM AUTHOR]

Deglaciations are characterized by relatively fast and near-synchronous changes in ice sheet volume, ocean temperature, and atmospheric greenhouse gas concentrations, but glacial inceptions occur more gradually. Understanding the evolution of ice sheet, ocean, and atmospheric conditions from interglacial to glacial maximum provides important insight into the interplay of these components of our climate system. Using noble gas measurements in ancient ice samples, we reconstruct mean ocean temperature (MOT) from 74 to 59.5 ka BP, covering the Marine Isotope Stage (MIS) 5-4 boundary, MIS 4, and part of the MIS 4-3 transition. Comparing this MOT reconstruction to previously published MOT reconstructions from the last glacial cycle, we find that the majority of interglacial-glacial ocean cooling occurred across MIS 5, and MOT reached full glacial levels by MIS 4 (-2.7±0.3°C relative to the Holocene). Comparing MOT to contemporaneous records of CO2 and benthic ??18O, we find that ocean cooling and the solubility pump can explain most of the CO2 drawdown and increase in ??18O across MIS 5. The timing of ocean warming and cooling in our record indicates that millennial scale climate variability plays a crucial role in setting mean ocean temperature during this interval, as seen during other periods, such as the last deglaciation. [ABSTRACT FROM AUTHOR]

Atmospheric carbon monoxide (CO) is a key player in global atmospheric chemistry and a regulated pollutant in urban areas. Oxidation of volatile organic compounds (VOCs) is an important component of the global CO budget and has also been hypothesized to contribute substantially to the summertime urban CO budget. In principle, stable isotopic analysis of CO could constrain the magnitude of this source. However, the isotopic signature of VOC-produced CO has not been well quantified, especially for the oxygen isotopes. We performed measurements of CO stable isotopes on air samples from two sites around Indianapolis, US, over three summers to investigate the isotopic signature of VOC-produced CO. One of the sites is located upwind of the city, allowing us to quantitatively remove the background air signal and isolate the urban CO enhancements. as well as the isotopic signature of these enhancements. In addition, we use measurements of Δ14CO2 in combination with the CO:CO2 emission ratio from fossil fuels to constrain the fossil-fuel-derived CO and thereby isolate the VOC-derived component of the CO enhancement. Combining these measurements and analyses, we are able to determine the carbon and oxygen isotopic signatures of CO derived from VOC oxidation as -32.8‰±0.5‰ and 3.6‰±1.2‰ , respectively. Additionally, we analyzed CO stable isotopes for 1 year at Beech Island, South Carolina, US, a site thought to have large VOC-derived contributions to the summertime CO budget. The Beech Island results are consistent with isotopic signatures of VOC-derived CO determined from the Indianapolis data. This study represents the first direct determination of the isotopic signatures of VOC-derived CO and will allow for improved use of isotopes in constraining the global and regional CO budgets. [ABSTRACT FROM AUTHOR]

New ice cores retrieved from the Taylor Glacier (Antarctica) blue ice area contain ice and air spanning the Marine Isotope Stage (MIS) 5–4 transition, a period of global cooling and ice sheet expansion. We determine chronologies for the ice and air bubbles in the new ice cores by visually matching variations in gas- and ice-phase tracers to preexisting ice core records. The chronologies reveal an ice age–gas age difference (Δ age) approaching 10 ka during MIS 4, implying very low snow accumulation in the Taylor Glacier accumulation zone. A revised chronology for the analogous section of the Taylor Dome ice core (84 to 55 ka), located to the south of the Taylor Glacier accumulation zone, shows that Δ age did not exceed 3 ka. The difference in Δ age between the two records during MIS 4 is similar in magnitude but opposite in direction to what is observed at the Last Glacial Maximum. This relationship implies that a spatial gradient in snow accumulation existed across the Taylor Dome region during MIS 4 that was oriented in the opposite direction of the accumulation gradient during the Last Glacial Maximum. [ABSTRACT FROM AUTHOR]

Atmospheric carbon monoxide (CO) is a regulated pollutant in urban centers. Oxidation of volatile organic compounds (VOCs) has been hypothesized to contribute substantially to the summertime urban CO budget. We performed measurements of CO stable isotopes on air samples from three sites in and around Indianapolis, USA over three summers to investigate the VOC contribution to urban CO. One of the sites is located upwind of the city, allowing us to quantitatively remove the background air signal and isolate the urban CO enhancements. The distinct isotopic signatures of CO produced from fossil fuel combustion and VOC oxidation allow us to separate contributions from these two sources. Our results provide the strongest empirical evidence to date of large contributions from VOC oxidation to the urban summertime CO source and show that this contribution varies in time and location between 0 and 58 %. We attribute the remainder of the Indianapolis summertime CO budget to fossil fuel combustion. We assess the reactivities of different VOCs and determine that biogenic sources are likely responsible for the majority of CO produced by VOC oxidation reactions within Indianapolis. [ABSTRACT FROM AUTHOR]

Methane (CH4) is a powerful greenhouse gas and plays a key part in global atmospheric chemistry. Natural geological emissions (fossil methane vented naturally from marine and terrestrial seeps and mud volcanoes) are thought to contribute around 52 teragrams of methane per year to the global methane source, about 10 per cent of the total, but both bottom-up methods (measuring emissions) and top-down approaches (measuring atmospheric mole fractions and isotopes) for constraining these geological emissions have been associated with large uncertainties. Here we use ice core measurements to quantify the absolute amount of radiocarbon-containing methane (14CH4) in the past atmosphere and show that geological methane emissions were no higher than 15.4 teragrams per year (95 per cent confidence), averaged over the abrupt warming event that occurred between the Younger Dryas and Preboreal intervals, approximately 11,600 years ago. Assuming that past geological methane emissions were no lower than today, our results indicate that current estimates of today's natural geological methane emissions (about 52 teragrams per year) are too high and, by extension, that current estimates of anthropogenic fossil methane emissions are too low. Our results also improve on and confirm earlier findings that the rapid increase of about 50 per cent in mole fraction of atmospheric methane at the Younger Dryas-Preboreal event was driven by contemporaneous methane from sources such as wetlands; our findings constrain the contribution from old carbon reservoirs (marine methane hydrates, permafrost and methane trapped under ice) to 19 per cent or less (95 per cent confidence). To the extent that the characteristics of the most recent deglaciation and the Younger Dryas-Preboreal warming are comparable to those of the current anthropogenic warming, our measurements suggest that large future atmospheric releases of methane from old carbon sources are unlikely to occur. [ABSTRACT FROM AUTHOR]

Old ice for paleo-environmental studies, traditionally accessed through deep core drilling on domes and ridges on the large ice sheets, can also be retrieved at the surface from ice sheet margins and blue ice areas. The practically unlimited amount of ice available at these sites satisfies a need in the community for studies of trace components requiring large sample volumes. For margin sites to be useful as ancient ice archives, the ice stratigraphy needs to be understood and age models need to be established. We present measurements of trapped gases in ice from Taylor Glacier, Antarctica, to date the ice and assess the completeness of the stratigraphic section. Using δ18O of O2 and methane concentrations, we unambiguously identify ice from the last glacial cycle, covering every climate interval from the early Holocene to the penultimate interglacial. A high-resolution transect reveals the last deglaciation and the Last Glacial Maximum (LGM) in detail. We observe large-scale deformation in the form of folding, but individual stratigraphic layers do not appear to have undergone irregular thinning. Rather, it appears that the entire LGM-deglaciation sequence has been transported from the interior of the ice sheet to the surface of Taylor Glacier relatively undisturbed. We present an age model that builds the foundation for gas studies on Taylor Glacier. A comparison with the Taylor Dome ice core confirms that the section we studied on Taylor Glacier is better suited for paleo-climate reconstructions of the LGM due to higher accumulation rates. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is an important greenhouse gas and ozone-depleting substance that has anthropogenic as well as natural marine and terrestrial sources. The tropospheric N2O concentrations have varied substantially in the past in concert with changing climate on glacial-interglacial and millennial timescales. It is not well understood, however, how N2O emissions from marine and terrestrial sources change in response to varying environmental conditions. The distinct isotopic compositions of marine and terrestrial N2O sources can help disentangle the relative changes in marine and terrestrial N2O emissions during past climate variations. Here we present N2O concentration and isotopic data for the last deglaciation, from 16,000 to 10,000 years before present, retrieved from air bubbles trapped in polar ice at Taylor Glacier, Antarctica. With the help of our data and a box model of the N2O cycle, we find a 30 per cent increase in total N2O emissions from the late glacial to the interglacial, with terrestrial and marine emissions contributing equally to the overall increase and generally evolving in parallel over the last deglaciation, even though there is no a priori connection between the drivers of the two sources. However, we find that terrestrial emissions dominated on centennial timescales, consistent with a state-of-the-art dynamic global vegetation and land surface process model that suggests that during the last deglaciation emission changes were strongly influenced by temperature and precipitation patterns over land surfaces. The results improve our understanding of the drivers of natural N2O emissions and are consistent with the idea that natural N2O emissions will probably increase in response to anthropogenic warming. [ABSTRACT FROM AUTHOR]

We present successful 81Kr-Kr radiometric dating of ancient polar ice. Krypton was extracted from the air bubbles in four ~350-kg polar ice samples from Taylor Glacier in the McMurdo Dry Valleys, Antarctica, and dated using Atom Trap Trace Analysis (ATTA). The 81Kr radiometric ages agree with independent age estimates obtained from stratigraphic dating techniques with a mean absolute age offset of 6 ± 2.5 ka. Our experimental methods and sampling strategy are validated by (i) 85Kr and 39Ar analyses that show the samples to be free of modern air contamination and (ii) air content measurements that show the ice did not experience gas loss. We estimate the error in the 81Kr ages due to past geomagnetic variability to be below 3 ka. We show that ice from the previous interglacial period (Marine Isotope Stage 5e, 130-115 ka before present) can be found in abundance near the surface of Taylor Glacier. Our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available, greatly enhancing their scientific value as archives of old ice and meteorites. At present, ATTA 81Kr analysis requires a 40-80-kg ice sample; as sample requirements continue to decrease, 81Kr dating of ice cores is a future possibility. [ABSTRACT FROM AUTHOR]

Abstract: Measurements of radiocarbon (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 the two major 14C 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 14CH4 and 14CO content in glacial firn with unprecedented precision to advance understanding of the in situ 14C component. 14CH4 and 14CO were melt-extracted on site at Summit, Greenland from three very large (∼1000kg each) replicate samples of firn that spanned a depth range of 3.6–5.6m. Non-cosmogenic 14C contributions were carefully characterized through simulated extractions and a suite of supporting measurements. In situ cosmogenic 14CO was quantified to better than ±0.6moleculesg−1 ice, improving on the precision of the best prior ice 14CO measurements by an order of magnitude. The 14CO measurements indicate that most (>99%) of the in situ cosmogenic 14C is rapidly lost from shallow Summit firn to the atmosphere. Despite this rapid 14C loss, our measurements successfully quantified 14CH4 in the retained fraction of cosmogenic 14C (to ±0.01moleculesg−1 ice or better), and demonstrate for the first time that a significant amount of 14CH4 is produced by cosmic rays in natural ice. This conclusion increases the confidence in the results of an earlier study that used measurements of 14CH4 in glacial ice to show that wetlands were the likely main driver of the large and rapid atmospheric CH4 increase approximately 11.6kyr ago. [Copyright &y& Elsevier]

The article discusses a study which focuses on the ice stratigraphy at the Pȃtsoq ice margin in West Greenland. The study used geochemical parameters measured in ice matrix and air occlusions. The study found evidence for climatic fluctuations during the Holocene period which were similar to those found in deep ice cores from Greenland. The changes in surface appearance were detected during the transition between Holocene ice and ice from the last glacial period. The data obtained from gas records helped in understanding the stratigraphy and the three-dimensional structure of ice layers found at Pȃkitsoq.

Concentrations of atmospheric methane (CH4), a potent greenhouse gas, have more thandoubled since preindustrial times yet its contemporary budget is incompletely understood,with substantial discrepancies between global emission inventories and atmosphericobservations (Kirschke et al., 2013; Saunois et al., 2016). Radiomethane (14CH4) candistinguish between fossil emissions from geologic reservoirs (radiocarbon free) andcontemporaneous biogenic sources, although poorly constrained direct 14CH4 emissionsfrom 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 TgCH4/yr, (Saunois et al., 2016; Schwietzke et al., 2016)) are partitioned betweenanthropogenic sources (such as fossil fuel extraction and consumption) and naturalsources (such as geologic seeps); emission inventories suggest the latter accountsfor ∼50-60 Tg CH4/yr (Etiope, 2015; Etiope et al., 2008). Geologic emissionswere recently shown to be much smaller at the end of the Pleistocene ∼11,600years ago (Petrenko et al. 2017); However, this period is an imperfect analog for thepresent day due to the much larger terrestrial ice sheet cover, lowered sea level,and more extensive permafrost. We use preindustrial ice core measurements of14CH4 to show that natural fossil CH4 emissions to the atmosphere are ∼1.7 TgCH4/yr, with a maximum of 6.1 Tg CH4/yr (95% confidence limit), an order ofmagnitude smaller than estimates from global inventories. This result suggeststhat contemporary anthropogenic fossil emissions are likely underestimated by acorresponding amount (∼48-58 Tg CH4/yr, or ∼25-33% of current estimates). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We report atmospheric methane carbon isotope ratios (δ13CH4) from the Western Greenland ice margin spanning the Younger Dryas-to-Preboreal (YD-PB) transition. Over the recorded ∼800 years, δ13CH4 was around -46 per mit (‰); that is, ∼1‰ higher than in the modern atmosphere and ∼5.5‰ higher than would be expected from budgets without 13C-rich anthropogenic emissions. This requires higher natural 13C-rich emissions or stronger sink fractionation than conventionally assumed. Constant δ13CH4 during the rise in methane concentration at the YD-PB transition is consistent with additional emissions from tropical wetlands, or aerobic plant CH4 production, or with a multisource scenario. A marine clathrate source is unlikely. [ABSTRACT FROM AUTHOR]