Browsing by Author "Etheridge, DM"
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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
- Item14CH4 measurements in Greenland ice: investigating last glacial termination CH4 sources(American Association for the Advancement of Science (AAAS), 2009-04-24) Petrenko, VV; Smith, AM; Brook, EJ; Lowe, DC; Riedel, K; Brailsford, G; Hua, Q; Schaefer, H; Reeh, N; Weiss, RF; Etheridge, DM; Severinghaus, JPThe cause of a large increase of atmospheric methane concentration during the Younger Dryas-Preboreal abrupt climatic transition (~11,600 years ago) has been the subject of much debate. The carbon-14 (14C) content of methane (14CH4) should distinguish between wetland and clathrate contributions to this increase. We present measurements of 14CH4 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured 14CH4 values were higher than predicted under any scenario. Sample 14CH4 appears to be elevated by direct cosmogenic 14C production in ice. 14C of CO was measured to better understand this process and correct the sample 14CH4. Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas-Preboreal CH4 rise. © 2009, American Association for the Advancement of Science (AAAS)
- ItemThe 14CO2 bomb pulse in firn air at Aurora Basin, East Antarctica(Australian Partnerships in Ice Core Sciences (APICS) Workshop, 2016-03-07) Thornton, DP; Etheridge, DM; Trudinger, CM; Rubino, M; Smith, AM; Curran, MAJ; Vance, TR; Chappellaz, JThe 14C isotope of CO2 produced in the atmosphere by nuclear weapons testing in the 1960’s is incorporated in air in open pores of firn before close-off in bubbles in Antarctic ice. The rapid growth and subsequent decline provides a unique test for the smoothing of atmospheric CO2 signals due to firn diffusion and bubble close off, and the level of smoothing quantifies the time resolution with which trace gas histories can be reconstructed from ice cores. The presence of a ‘bomb pulse’ in the record also permits accurate dating of CO2 and other gases in air. Aurora Basin North (ABN) will contribute new and valuable 2000-year atmospheric records from this data sparse region of inland East Antarctica. ABN has an annual snow accumulation up to 150 kgm-2 year-1, a low mean annual temperature and high elevation. Firn air samples were collected from ABN during December 2013 in stainless-steel canisters and cylinders and 0.5L glass flasks, from varying depths covering the whole firn column at the ABN site. Extraction of CO2 from ABN samples has been performed at the CSIRO ICELAB and transferred to ANSTO to derive the 14C activity of CO2 in ABN firn air. As expected, results suggest the age spread at ABN is wider than sites with higher accumulation, such as Law Dome. Firn modelling is also planned and the 14C results will be used as inputs for the modelling to help determine (with other gas measurements) the age and age spread of air in firn and ice at ABN.
- Item7Be and 10Be concentrations in recent firn and ice at Law Dome, Antarctica(Elsevier, 2000-10-01) Smith, AM; Fink, D; Child, DP; Levchenko, VA; Morgan, VI; Curran, MAJ; Etheridge, DM; Elliott, GOver the past three years, the Australian National Tandem for Applied Research (ANTARES) AMS facility at ANSTO has been expanding its sample preparation and measurement capability, particularly for 10Be, 26Al and 36Cl. During this time, ANSTO has continued its collaboration with the AAD and CSIRO Atmospheric Research on the measurement of cosmogenic isotopes from Law Dome, Antarctica. This research program has been supported by the construction of a dedicated geochemistry laboratory for the processing of ice and rock samples for the preparation of AMS targets. Here we present our first results for 10Be concentrations measured in ice cores from three sites at Law Dome and describe the sample processing protocol and aspects of the AMS measurement procedure. These sites are characterised by an eightfold difference in accumulation rate with a common precipitation source. In combination with an established ice chronology, this has enabled some preliminary findings concerning the relationship between the snow accumulation rate and the measured 10Be concentration for Law Dome during recent times. Additionally, we present 7Be and 10Be/7Be measurements made for a few surface snow samples from Law Dome and Australia. © 2000 Elsevier Science B.V.
- ItemAtmospheric CO2 and d13C-CO2 reconstruction of the little ice age from antarctic ice cores(Copernicus Publications, 2015-04-12) Rubino, M; Etheridge, DM; Trudinger, CM; Allison, CE; Rayner, PJ; Mulvaney, R; Steele, LP; Langenfelds, RL; Sturges, WT; Curran, MAJ; Smith, AMThe decrease of atmospheric CO2 concentration recorded in Antarctic ice around 1600 AD is one of the most significant atmospheric changes to have occurred during the last millennia, before the onset of the industrial period.Together with the temperature decrease, the CO2 drop has been used to derive the sensitivity of carbon stores to climate. However, the cause of it is still under debate because models are not yet able to reproduce either its magnitude, or its timing. Here we present new measurements of the CO2 concentration decrease recorded in an ice core from a medium accumulation rate site in Antarctica (DML). We show that the new record is compatible(differences <2 ppm) with the CO2 record from the high accumulation rate DSS site on Law Dome (East Antarctica), when the different age distributions are taken into account. We have also measured the d13C-CO2 change in DML ice, filling a gap around 1600 AD in the DSS d13C record. We use a double deconvolution of the CO2 and d13C records together to provide quantitative evidence that the CO2 decrease was caused by a change in the net flux to the terrestrial biosphere. Finally, we provide a new interpretation of a published record showing increasing atmospheric carbonyl sulphide during the CO2 decrease, suggesting that cooler LIA climate affected terrestrial biospheric fluxes. Altogether our findings support the hypothesis that reduced soil heterotrophic respiration is likely to have given the most significant contribution to the LIA CO2 decrease implying a positive CO2-climate feedback. © 2015, Authors.
- ItemBuilding a future on knowledge from the past: what paleo-science can reveal about climate change and its potential impacts in Australia(Commonwealth Scientific and Industrial Research Organisation, 2005-06) Harle, KJ; Etheridge, DM; Whetton, P; Jones, R; Hennessy, K; Goodwin, ID; Brooke, BP; van Ommen, TD; Barbetti, M; Barrows, TT; Chappell, J; De Deckker, P; Fink, D; Gagan, MK; Haberle, SG; Heijnis, H; Henderson-Sellers, A; Hesse, PP; Hope, GS; Kershaw, P; Nicholls, NIn Australia, high quality instrumental climate records only extend back to the late 19th century and therefore only provide us with a brief snapshot of our climate, its mean state and its short-term variability. Palaeo-records extend our knowledge of climate back beyond the instrumental record, providing us with the means of testing and improving our understanding of the nature and impacts of climate change and variability in Australia. There is a vast body of palaeo-records available for the Australian region (including Antarctica), ranging from continuous records of sub-decadal up to millennial scale (such as those derived from tree rings, speleothems, corals, ice cores, and lake and marine sediments) through to discontinuous records representing key periods in time (such as coastal deposits, palaeo-channels, glacial deposits and dunes). These records provide a large array of evidence of past atmospheric, terrestrial and marine environments and their varying interactions through time. There are a number of key ways in which this evidence can, in turn, be used to constrain uncertainties about climate change and its potential impacts in Australia.
- ItemCentennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us?(European Geosciences Union, 2007-05-02) Lassey, KR; Etheridge, DM; Lowe, DC; Smith, AM; Ferretti, DFLittle is known about how the methane source inventory and sinks have evolved over recent centuries. New and detailed records of methane mixing ratio and isotopic composition ((CH4)-C-12, (CH4)-C-13 and (CH4)-C-14) from analyses of air trapped in polar ice and firn can enhance this knowledge. We use existing bottom-up constructions of the source history, including "EDGAR"- based constructions, as inputs to a model of the evolving global budget for methane and for its carbon isotope composition through the 20th century. By matching such budgets to atmospheric data, we examine the constraints imposed by isotope information on those budget evolutions. Reconciling both (CH4)-C-12 and (CH4)-C-13 budgets with EDGAR-based source histories requires a combination of: a greater proportion of emissions from biomass burning and/or of fossil methane than EDGAR constructions suggest; a greater contribution from natural such emissions than is commonly supposed; and/or a significant role for active chlorine or other highly-fractionating tropospheric sink as has been independently proposed. Examining a companion budget evolution for (CH4)-C-14 exposes uncertainties in inferring the fossil-methane source from atmospheric (CH4)-C-14 data. Specifically, methane evolution during the nuclear era is sensitive to the cycling dynamics of "bomb C-14" ( originating from atmospheric weapons tests) through the biosphere. In addition, since ca. 1970, direct production and release of (CH4)-C-14 from nuclear-power facilities is influential but poorly quantified. Atmospheric (CH4)-C-14 determinations in the nuclear era have the potential to better characterize both biospheric carbon cycling, from photosynthesis to methane synthesis, and the nuclear-power source. © Author(s) 2007
- ItemCorrigendum to "Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland'' published in Atmos. Chem. Phys., 12, 4259–-4277, 2012(Copernicus Publications, 2014-04-09) Buizert, C; Martinerie, P; Petrenko, VV; Severinghaus, JP; Trudinger, CM; Witrant, E; Rosen, JL; Orsi, AJ; Rubino, M; Etheridge, DM; Steele, LP; Hogan, C; Laube, JC; Sturges, WT; Levchenko, VA; Smith, AM; Levin, I; Conway, TJ; Dlugokencky, EJ; Lang, PM; Kawamura, K; Jenk, TM; White, JWC; Sowers, T; Schwander, J; Blunier, TIt was kindly pointed out to us by M. Battle that Eq. (2) on p. 4263 contains a typo, and should instead be [X]corr(z) = [X]meas(z) ΔMδgrav(z)/1000 + 1 , (2) where [X]corr ([X]meas) is the gravity-corrected (measured) mixing ratio of gas species X, 1M = MX − Mair is the molar mass difference between gas X and air, and grav(z) is the gravitational fractionation per unit mass difference at depth z. All calculations in the study were done correctly, following Eq. (2) as given here. Furthermore, the present-day 1age value for NEEM is incorrect in the original manuscript, and underestimates Δage by 6 years. The correct value is 188+3 −9 yr. In our original, incorrect calculation we used the ice age in years before 2000 CE (b2k), while we should have used the ice age relative to the surface ice age. In the updated 1age calculation we use the ice age found by annual layer counting of the shallow NEEM 2011 S1 core (Sigl et al., 2013). The NEEM chronology published in Rasmussen et al. (2013) uses the correct, updated Δage estimate. Both errors addressed in this corrigendum affect neither the discussion nor the main conclusions of the original publication. © Author(s) 2014.
- ItemGas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland(Copernicus Publications, 2012-05-14) Buizert, C; Martinerie, P; Petrenko, VV; Severinghaus, JP; Trudinger, CM; Witrant, E; Rosen, JL; Orsi, AJ; Rubino, M; Etheridge, DM; Steele, LP; Hogan, C; Laube, JC; Sturges, WT; Levchenko, VA; Smith, AM; Levin, I; Conway, TJ; Dlugokencky, EJ; Lang, PM; Kawamura, K; Jenk, TM; White, JWC; Sowers, T; Schwander, J; Blunier, TAir was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1σ Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (Δage) at the firn-ice transition is calculated to be 182+3−9 yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records. © Author(s) 2012.
- ItemA global transport model comparison for methane: results for two Australian sites(International Union of Geodesy and Geophysics, 2011-07-06) Law, RM; Loh, ZM; Corbin, KD; Krummel, PB; Steele, LP; Fraser, PJ; Etheridge, DM; Zahorowski, WMethane (CH4) is an important greenhouse gas. Using atmospheric CH4 measurements to estimate CH4 emissions requires a good understanding of how CH4 is transported in the atmosphere. Hence, simulations of atmospheric CH4 concentration have been made with two atmospheric models, namely ACCESS and CCAM, as part of the Transport Model Intercomparison project, TransCom-CH4. The simulations ran for the period 1990-2008 and used six different sets of surface CH4 emissions, while the chemical CH4 sink was modelled using prescribed OH and stratospheric loss fields. Radon, sulphur hexafluoride and methyl chloroform tracers were also simulated. Model output has been analysed for two Australian sites with in-situ CH4 measurements: Cape Grim, Tasmania (AGAGE in-situ data) and the CO2CRC Otway project, Victoria. Cape Grim is a coastal site, observing periods of clean (baseline) air from the Southern Ocean and periods of non-baseline air, influenced by emissions from South Eastern Australia including Melbourne. Otway is a rural location, 4 km from the coast, where the land use is predominantly dairy farming, resulting in a large local CH4 signal from enteric fermentation (diurnal amplitudes up to 250 ppb). Nevertheless, during well mixed periods, measured CH4 concentrations at Otway may be similar to the baseline CH4 concentrations measured at Cape Grim or represent broader regional South Eastern Australian emissions. Preliminary findings indicate that CH4 at Otway and non-baseline CH4 at Cape Grim are sensitive to the choice of wetland emissions. There is also some indication that Melbourne emissions may be underestimated in these simulations.
- 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.
- ItemIn search of in-situ radiocarbon in Law Dome ice and firn(Elsevier, 2000-10-01) Smith, AM; Levchenko, VA; Etheridge, DM; Lowe, DC; Hua, Q; Trudinger, CM; Zoppi, U; Elcheikh, AResults of AMS radiocarbon measurements on CO and CO2 separated from firn air directly pumped from the ice sheet, and on CO2 separated from air extracted from ice cores by a dry grating technique, are presented. The firn air samples and ice cores used in this study were collected from the region of Law Dome, Antarctica. No evidence of in-situ 14CO2 was found in the firn air samples or the ice core air samples from one site although a slight enhancement of 14CO above expected polar atmospheric concentrations was observed for some firn air samples. A clear in-situ 14CO2 signal for ice pre-dating the radiocarbon bomb pulse was found, however, in air samples extracted from an ice core from a second site. We compare these results and propose an hypothesis to explain this apparent contradiction. The degree to which in-situ 14C is released from the ice crystals during trapping and bubble formation is considered and discussed. The selectivity of the dry grating technique for the extraction of trapped atmospheric gases from ice cores is also discussed and compared with other methods. © 2000 Elsevier Science B.V
- 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.
- ItemLow atmospheric CO2 levels during the Little Ice Age due to cooling-induced terrestrial uptake(Springer Nature, 2016-07-25) Rubino, M; Etheridge, DM; Trudinger, CM; Allison, CE; Rayner, PJ; Enting, I; Mulvaney, R; Steele, LP; Langenfelds, RL; Sturges, WT; Curran, MAJ; Smith, AMLow atmospheric carbon dioxide (CO2) concentration1 during the Little Ice Age has been used to derive the global carbon cycle sensitivity to temperature2. Recent evidence3 confirms earlier indications4 that the low CO2 was caused by increased terrestrial carbon storage. It remains unknown whether the terrestrial biosphere responded to temperature variations, or there was vegetation re-growth on abandoned farmland5. Here we present a global numerical simulation of atmospheric carbonyl sulfide concentrations in the pre-industrial period. Carbonyl sulfide concentration is linked to changes in gross primary production6 and shows a positive anomaly7 during the Little Ice Age. We show that a decrease in gross primary production and a larger decrease in ecosystem respiration is the most likely explanation for the decrease in atmospheric CO2 and increase in atmospheric carbonyl sulfide concentrations. Therefore, temperature change, not vegetation re-growth, was the main cause of the increased terrestrial carbon storage. We address the inconsistency between ice-core CO2 records from different sites8 measuring CO2 and δ13CO2 in ice from Dronning Maud Land (Antarctica). Our interpretation allows us to derive the temperature sensitivity of pre-industrial CO2 fluxes for the terrestrial biosphere (γL = −10 to −90 Pg C K−1), implying a positive climate feedback and providing a benchmark to reduce model uncertainties. © 2016, Nature Publishing Group.
- ItemMeasurements of carbon-14 of methane in Greenland ice: investigating methane sources during the Last Glacial Termination(American Geophysical Union (AGU), 2008-12-15) Petrenko, VV; Smith, AM; Severinghaus, JP; Brook, EJ; Lowe, DC; Riedel, K; Brailsford, G; Hua, Q; Reeh, N; Schaefer, H; Weiss, RF; Etheridge, DMWe present the first measurements of 14C of methane (14CH4) in ancient glacial ice. 14CH4 should distinguish unambiguously between wetland and fossil (clathrate or other geologic CH4) contributions to abrupt atmospheric CH4 increases observed at times of rapid warming in Greenland ice cores. 1000-kg-sized ice samples, dating to the Younger Dryas - Preboreal (around 11,600 yr BP) and Oldest Dryas - Bølling (around 14,700 yr BP) abrupt climatic transitions, were obtained from an ablation site in West Greenland. Measured 14CH4 values (28 - 35 pMC) were higher than predicted under any scenario based on sample age. Sample 14CH4 appears to be elevated by in- situ CH4 production in the ice for some samples as well as by a second process that is likely direct cosmogenic production of 14CH4 molecules in the ice. 14C of CO and CO2 was measured to better understand these processes and corrections were applied to sample 14CH4. Although the corrected results have substantial uncertainties, they suggest that wetland sources were responsible for the majority of the Younger Dryas - Preboreal CH4 rise. The uncertainties in the corrected results for the Oldest Dryas - Bølling transition are too large to draw conclusions about 14CH4 changes during that transition. © 2008 American Geophysical Union
- ItemMeasurements of the 14CO2 bomb pulse in firn and ice at Law Dome, Antarctica(Elsevier, 1997-03-02) Levchenko, VA; Etheridge, DM; Francey, RJ; Trudinger, CM; Tuniz, C; Lawson, EM; Smith, AM; Jacobsen, GE; Hua, Q; Hotchkis, MAC; Fink, D; Morgan, VI; Head, J14CO2 produced in the atmosphere by nuclear weapons testing in the 1960's is now incorporated in the air bubbles of Antarctic ice. The high atmospheric radiocarbon growth rates through the period of tests and subsequent decline provide a unique and independent test for the smoothing of atmospheric CO2 signals due to firn diffusion and bubble close off. The level of smoothing quantifies the time resolution with which atmospheric trace gas histories can be reconstructed from ice cores. In this paper, the methodologies for the preparation and AMS measurements of ice core and firn 14CO2 from high accumulation sites at Law Dome are detailed. The results are compared with predictions of a numerical model incorporating firn air diffusion and bubble close-off. The sample sizes, precision of measurements and sources of contamination are discussed for both firn and ice samples. © 1997 Elsevier Science B.V.
- ItemModern to Glacial Age subglacial meltwater drainage at Law Dome, coastal East Antarctica from topography, sediments and jökulhlaup observations(The Geological Society of London, 2017-07-12) Goodwin, ID; Roberts, JL; Etheridge, DM; Hellstrom, JC; Moy, AD; Ribo, M; Smith, AMRare jökulhlaup events, also known as subglacial lake outburst flood events, have been observed at the Law Dome ice margin and provide an insight into the physical characteristics of subglacial meltwater and drainage. The subglacial topography based on data from the BEDMAP2 and ICECAP projects, together with subsurface transects of the ice margin obtained using ground-penetrating radar, reveal several lakes and lake-like depressions and the drainage pathways of two jökulhlaup events. Oxygen isotope typing of the meltwater during the most recent (2014) jökulhlaup event, combined with ice margin stratigraphy, enable the identification of ice tunnel melt pathways that exploit the 30–90° dipping basal ice layering. The presence of subglacial meltwater beneath Law Dome during the Holocene to Glacial periods is confirmed by the dendritic drainage pattern in the subglacial morphology and extensive layers of basal regelation ice and subglacial carbonate precipitate deposits found within the Løken Moraines sediments. These subglacial carbonates, including ooid layers, formed from the mixing of glacial meltwater and seawater at 72 ka BP. The combined evidence indicates that the ocean discharge of subglacial meltwater may be variable and/or is periodically blocked by basal freezing events near the ice sheet terminus. © 2018 The Author(s). Published by The Geological Society of London.
- ItemNatural and anthropogenic changes in atmospheric greenhouse gases over the past 2 millennia(Australian Antarctic Division, 2013-06-24) Etheridge, DM; Rubino, M; Trudinger, CM; Allison, CE; Steele, LP; Thornton, DP; Vollmer, M; Krummel, PB; Smith, AM; Curran, MAJ; Sturgess, WTMillennial changes in atmospheric trace gas composition are best determined from air enclosed in ice sheets. Air extracted from the open pores in firn and the bubbles in ice is measured to derive the past concentrations and isotopic ratios of the long lived trace gases. The significant increases observed in CO2, CH4 and N2O since about 1750 and the more recent appearance of synthetic gases such as the CFCs in the atmosphere are a key feature of the anthropocene. The millennia preceding the anthropocene, the Late Pre-Industrial Holocene (LPIH), show evidence of natural changes in trace gases that can be used to constrain models and improve their ability to predict future changes under scenarios of anthropogenic emissions and climate change. Precise measurements and ice core air samples that are accurately dated and highly resolved in time are required to record the small and rapid trace gas signals of this period. The atmospheric composition records produced by CSIRO and collaborators using the Law Dome, Antarctica ice cores are widely used in models of climate, atmospheric chemistry and the carbon cycle over the anthropocene and the LPIH. Results from these studies have been influential in informing global policies, including the Montreal and Kyoto Protocols. We will present the recently revised trace gas records from Law Dome and new measurements of tracers from these and other ice sites that reveal the causes of atmospheric changes over the anthropocene and the LPIH.