CL1.2.5

The Southern Hemisphere Westerly Winds play a critical role in the global climate system by modulating the upwelling and the transfer of heat and carbon between the atmosphere and the ocean. Since observations started, the core of the westerly wind belt has increased in strength and has contracted towards Antarctica. It has been proposed that these deviations are among the main drivers of the observed widespread warming in West Antarctica, threatening the stability of ice shelves, and ultimately contributing to global sea level rise.

 Over the last decades, it has been widely believed these atmospheric changes have occurred in response to recently increased greenhouse gas concentrations and ozone depletion. However, the lack of long-term wind records in the Southern Hemisphere mid-latitudes hinders our ability to assess the wider context of the recently observed changes. This lack of a clear consistent timing limits our understanding of the causes of westerly wind changes and the roles they have played in driving recent environmental changes in Antarctica. Addressing these questions is crucial for future climate predictions.

 In this work, we present records of diatoms preserved in ice cores retrieved from the southern Antarctic Peninsula and the Ellsworth Land region. The diatom abundance and species assemblages from these ice cores prove to represent the regional variability in wind strength and circulation patterns that influence the onshore northerly winds. We use this novel proxy to produce an annual reconstruction of winds in the Pacific sector of the Southern Hemisphere Westerly Wind belt over the last 140 years. This wind reconstruction allows exploring the link between the recent increase in wind strength, greenhouse gases and ozone depletion in the atmosphere

How to cite: Tetzner, D., Thomas, E., and Allen, C.: Diatoms in Ice Cores, a novel proxy for reconstructing past wind variability in the Pacific sector of the Southern Hemisphere Westerly Wind belt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10952, https://doi.org/10.5194/egusphere-egu22-10952, 2022.

Large volcanic eruptions occurring in the last glacial period can be detected by their accompanying sulfuric acid deposition in continuous ice cores. Here we employ continuous sulfate and sulfur records from three Greenland and three Antarctic ice cores to estimate the emission strength, the frequency and the climatic forcing of large volcanic eruptions that occurred during the second half of the last glacial period and the early Holocene, 60-9 ka years before AD 2000 (b2k). Over most of the investigated interval the ice cores are synchronized making it possible to distinguish large eruptions with a global sulfate distribution from eruptions detectable in one hemisphere only. Due to limited data resolution and large variability in the sulfate background signal, particularly in the Greenland glacial climate, we only list Greenland sulfate depositions larger than 20 kg km^-2 and Antarctic sulfate depositions larger than 10 kg km^-2. With those restrictions, we identify 1113 volcanic eruptions in Greenland and 740 eruptions in Antarctica within the 51 ka period - where the sulfate deposition of 85 eruptions is found at both poles (bipolar eruptions). Based on the ratio of Greenland and Antarctic sulfate deposition, we estimate the latitudinal band of the bipolar eruptions and assess their approximate climatic forcing based on established methods. Twenty-five of the identified bipolar eruptions are larger than any volcanic eruption occurring in the last 2500 years and 69 eruptions are estimated to have larger sulfur emission strengths than the Tambora, Indonesia eruption (1815 AD). Throughout the investigated period, the frequency of volcanic eruptions is rather constant and comparable to that of recent times. During the deglacial period (16-9 ka b2k), however, there is a notable increase in the frequency of volcanic events recorded in Greenland and an obvious increase in the fraction of very large eruptions. For Antarctica, the deglacial period cannot be distinguished from other periods. This confirms the suggestion that the isostatic unloading of the Northern Hemisphere (NH) ice sheets may be related to the enhanced NH volcanic activity. Our ice-core based volcanic sulfate records provide the atmospheric sulfate burden and estimates of climate forcing for further research on climate impact and understanding the mechanism of the Earth system.

How to cite: Lin, J., Svensson, A., S. Hvidberg, C., Lohmann, J., Kristiansen, S., Dahl-Jensen, D., Peder Steffensen, J., Olander Rasmussen, S., Cook, E., Astrid Kjær, H., M. Vinther, B., Fischer, H., Stocker, T., Sigl, M., Bigler, M., Severi, M., Traversi, R., and Mulvaney, R.: Magnitude, frequency and climate forcing of global volcanism during the last glacial period as seen in Greenland and Antarctic ice cores (60-9 ka), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3916, https://doi.org/10.5194/egusphere-egu22-3916, 2022.

Ice cores are powerful archives for reconstructing volcanism and developing tephrochronological frameworks, as they can preserve both the soluble, i.e. aerosols, and non-soluble, i.e. tephra, products of volcanic eruptions. In addition, and particularly over Holocene timescales, high-precision annually resolved chronologies have been developed for these records and permit ages to be assigned to eruptions. The identification of tephra in ice cores in direct association with chemical indicators of volcanism, such as sulphate, can significantly enhance volcanic reconstructions as tephra can be linked to an eruptive source. Such source attributions can provide information on the location of the eruptions, the magnitude of aerosol emissions at the source and help assess any climatic impact. In addition, they can aid the reconstruction of volcanic histories and the assessment of future hazard risk.  

The tephra record for the interior of East Antarctica over the last 5,500 years is potentially underexploited as a prior focus on visible horizons and exploring the deep ice cores that cover longer time spans has resulted in only one horizon, dated to ~3.5 ka BP, being identified in these records. Here we discuss ongoing tephrochronological investigations of two ice-cores, B53 and B54, retrieved from the interior of the East Antarctic Plateau. High-resolution, sub-annual chemical records have been measured from both cores using a continuous melter system. These data were used to develop a sampling strategy to identify cryptotephra horizons with ice-core sections containing coeval peaks in fine insoluble particles and non-sea-salt sulphur targeted and >50 events were directly sampled. This approach recently has been used to identify cryptotephras in both Greenland and Antarctic ice cores. When glass tephra shards were identified thin sections were created and individual glass shards were geochemically analysed using electron-probe microanalysis to help identify their volcanic source and permit correlations between records.

Thus far, more than 10 cryptotephra horizons have been identified and linked to regional sources such as the South Sandwich and South Shetland Islands and the ~3.5 ka BP event has been traced in both cores as a visible layer. More detailed investigations are being conducted on samples from specific volcanic signals of interest that may derive from eruptions of ultra-distal volcanic sources. Such eruptions could have deposited very small glass tephra shards over Antarctica, which poses significant analytical challenges and necessitates the use of innovative approaches for tephra identification and geochemical analysis.

How to cite: Abbott, P., McConnell, J., Chellman, N., Kipfstuhl, S., Hörhold, M., Freitag, J., Plunkett, G., and Sigl, M.: Mid to Late Holocene East Antarctic ice-core tephrochronology: Implications for reconstructing volcanic eruptions and their impacts over the last 5,500 years, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7471, https://doi.org/10.5194/egusphere-egu22-7471, 2022.

Wildfires have an important role in affecting the Earth’s radiative balance. Biomass burning aerosols can scatter or absorb the incoming solar radiation, alter the ice and snow albedo and act as cloud condensation nuclei. Overall, their net contribution to the Earth’s radiative forcing is negative, however this estimate has large uncertainties. To better assess the impact of wildfires on climate (and vice versa), it is crucial to reconstruct their past regional and temporal variability on decadal and centennial timescales. Ice cores are excellent archives to perform such palaeofire reconstructions. Previous studies have reconstructed the occurrence of wildfires in ice cores using both inorganic (ammonium, potassium and black carbon) and organic proxies (levoglucosan, vanillic acid and p-hydroxybenzoic acid). However, a more comprehensive view that involved a broader suite of wildfire proxies was missing. Here, we present a new SPE-UHPLC-HRMS method for the determination of five organic biomass burning tracers (syringic acid, vanillic acid, vanillin, syringaldehyde and p-hydroxybenzoic acid) and pinic acid, as biogenic emission proxy, in ice core samples. This method showed average recoveries of 76% (58-88% range), excellent inter-day reproducibility, no significant matrix effects and fast analysis time (13 min per sample). Comparing the published concentration ranges of the selected species from different ice core regions (i.e. Alps, Greenland, Kamchatka, China and Svalbard Archipelago) with the procedural detection limits of this new methodology, we conclude that four of the six targeted compounds can be successfully detected in real ice and snow samples. Only for vanillin and syringaldehyde, no ice-core measurements have been reported in the scientific literature so far. The method development also involved the evaluation of common laboratory practices such as the melting and refreezing of ice samples before the analysis. We found that the melting and refreezing of the samples resulted in a mass loss for the majority of the investigated compounds, which was more evident at lower concentrations. We hypothesize that the reason of this phenomenon is the adsorption of the compounds on the walls of the glass vials used for this study. In light of this, we propose alternative sample storage strategies that can also be extended for the analysis of other compounds.

The method was successfully tested on nine ice core samples from the Colle Gnifetti (European Alps) and it will be applied on ice cores from the Alps and the Russian Altai, contributing to the better understanding of wildfire temporal evolution and their relations with climate.

How to cite: Burgay, F., Salionov, D., Huber, C., Singer, T., Ungeheuer, F., Eichler, A., Vogel, A., Bjelic, S., and Schwikowski, M.: Development and application of a novel UHPLC-HRMS method for the analysis of organic wildfire tracers in ice cores., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5470, https://doi.org/10.5194/egusphere-egu22-5470, 2022.

There is intense interest in the future stability of the West Antarctic Ice Sheet (WAIS).  Models range widely in their predictions and in the physics they include.  Because the timescales for ice sheets are long, our best hope of constraining the solutions is to look at the past behaviour of WAIS. The last interglacial (LIG) is a particularly important time because Antarctic temperature was higher than present and some models predict the complete loss of WAIS and of the large ice shelves adjacent to it.

Within the WACSWAIN (WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial) project, in 2019 we retrieved a 651 metre ice core to the bed of Skytrain Ice Rise. This ice rise is adjacent to the Ronne Ice shelf and the WAIS, but is expected to have maintained an independent ice flow because of the protection afforded by the Ellsworth Mountains.  The ice core has been processed and analysed continuously for a range of analytes, including water isotopes, methane and major chemistry.

In this presentation we will first describe the dating of the ice core achieved in the top half of the ice column by annual layer counting supplemented by fixed horizons, and deeper down by ice flow modelling supplemented by tie points from chemistry, ¹⁰Be, as well as atmospheric CH₄ and δ¹⁸O. The core is continuous through the last glacial period, and most of the last interglacial. Discontinuities occur near the base, in the ice at the older end of the LIG, so that although older ice may be  present, we can only interpret the core to 125 ka.

Overall, the ice core record shows the clearly recognisable pattern of all the Antarctic Isotopic Maxima seen in East Antarctic ice cores over the last glacial cycles. In the early part of the Holocene, we see a very interesting pattern representing thinning of the ice rise and retreat of the Ronne Ice shelf.  This allows us to add reliable dates to the history of ice retreat in the early Holocene.

In the LIG, the record of marine ions in the ice suggest that the Ronne Ice Shelf was present at least from 125 ka onwards. This rules out occurrence of some of the more extreme retreats of WAIS that would have led to seaways between the Weddell, Amundsen and Ross Seas.  We see somewhat higher water isotope ratios in the LIG than the Holocene, possibly consistent with some drawdown of WAIS in sectors other than the Weddell region.

How to cite: Wolff, E. and Hoffmann, H. and the WACSWAIN science team plus collaborating scientists: Climate of the last 125 kyr at Skytrain Ice Rise, Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3584, https://doi.org/10.5194/egusphere-egu22-3584, 2022.

Impurities in polar ice play, among others, a crucial role as a proxy for the paleoclimate while at the same time impacting the internal deformation of ice on the micro-scale. In particular, solid and dissolved impurities can impact grain growth through Zener pinning or the drag of grain boundaries. Recent studies on natural ice from Antarctica and Greenland highlight the need for a multi-method approach to determine the differences in the localisation and chemistry of solid and dissolved impurities comprehensively, in order to ultimately gain a more holistic understanding. Here we report on a recent pilot investigation pursuing the direct integration of complimentary methods: microstructure-mapping, Cryo-Raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with 2D impurity imaging. While LA-ICP-MS enables the fast mapping of cm-size areas with lateral resolution in the order of tens of μm, Raman spectroscopy is more suited to identify the mineralogical composition of individual solid inclusions at the single μm scale. We analysed samples from the Holocene and Last Glacial from the Northeast Greenland Ice Core Project (EGRIP) and the North Greenland Eemian Ice Drilling (NEEM) ice cores. We find that the general localisation patterns of impurities (e.g., low vs. high concentration) are similar for both methods. Furthermore, both methods show (clusters of) inclusions in the grain interior. These findings display that a holistic approach is needed to truly decipher the localisation of impurities in the ice microstructure. Combining the advantages of both methods gives a good overview of the localisation of impurities, both solid and dissolved, on the micro-scale. Localisation patterns are related to the chemistry of the analysed impurities displaying the need for high-resolution methods. For example, Na is strongly located in the grain boundaries, Al is preferentially located within the ice grains and Mg can be located in both regimes. We analyse the role of inclusions in relation to 1) their chemistry and 2) their proximity to grain boundaries. Our approach of 2D impurity imaging in concert with established techniques, such as microstructure mapping and Raman spectroscopy, provides a detailed insight into the impurity distribution throughout a broad range of depths in an ice core. We demonstrate the potential of such an approach to carefully investigate the evolution of impurity localisation in ice cores, with special significance to ice deformation processes and the preservation of the climatic record.

How to cite: Stoll, N., Bohleber, P., Hörhold, M., Erhardt, T., Eichler, J., Roman, M., Delmonte, B., Barbante, C., and Weikusat, I.: Integrating Raman spectroscopy and LA-ICP-MS 2D imaging to decipher the localisation and chemistry of impurities on the micro-scale in Greenland ice: Consistencies and open question, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5035, https://doi.org/10.5194/egusphere-egu22-5035, 2022.

Some modelling studies and sea level reconstructions suggest the loss of the West Antarctic Ice Sheet (WAIS) during the Last Interglacial (LIG) about ~120’000 ago, but direct evidence for a collapse of the WAIS is lacking. The WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial (WACSWAIN) project aims at providing direct evidence allowing for a comprehensive assessment of whether or not the WAIS collapsed during the LIG. One of the expected consequences of such massive ice mass loss is the change of the elevation of land masses in close proximity of the WAIS due to isostatic adjustments. This process, together with changes in ice sheet thickness, may have altered the elevation of Skytrain Ice Rise above sea level on the order of 200 m. Such major changes in the elevation should be imprinted in the Total Air Content (TAC) based on simple barometric considerations. Here we present a new experimental setup of a high-accuracy, high-precision TAC measurement system constructed at the British Antarctic Survey. This setup is dedicated to and optimised for the measurement of TAC and is based on a vacuum extraction principle. The air is extracted from the ice by melting the sample by thermal radiation and the released air is dried and directly expanded into a 30-litre expansion chamber. State-of-the-art pressure gauges and thorough temperature control allow for an accuracy of 0.2% with a real ice reproducibility of 0.2% to 0.4% for 100 g and 30 g samples, respectively. Here, we discuss the performance of this new TAC system and present first TAC data from the Holocene section of the Skytrain Ice Core, Antarctica.

How to cite: Nehrbass-Ahles, C., King, A., Hoffmann, H., Grieman, M., Rowell, I., Humby, J., Miller, S., Thomas, E., Bauska, T., Schmitt, J., Mulvaney, R., and Wolff, E.: A high-accuracy Total Air Content setup: System performance and first results from Skytrain Ice Rise, Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9210, https://doi.org/10.5194/egusphere-egu22-9210, 2022.

Few ice cores from the Amundsen and Bellingshausen sectors of the West Antarctic Ice Sheet (WAIS) extend back in time further than a few hundred years. The WAIS is believed to be susceptible to collapse as a result of anthropogenic climate change. Modelling studies and palaeoclimatic evidence have suggested at least partial WAIS collapse and resulting sea level rise during previous warm periods, therefore understanding the stability of the WAIS during warm periods is important. The WACSWAIN project successfully drilled a 651 m ice core on Skytrain Ice Rise, adjacent to the Ronne Ice Shelf, in 2018, some data from which are now being published. The second WACSWAIN drilling project took place in 2020 on Sherman Island in the Abbott Ice Shelf, where the British Antarctic Survey’s (BAS) Rapid Access Isotope Drill (RAID) was deployed. The team drilled a 323 m deep borehole, with discrete samples of ice chippings collected from the entire depth range of the drilled ice. The samples were analysed for water isotopes and major ion content at BAS from 2020-2022. Validation of the RAID-ice data is confirmed through comparison with a shallow core drilled on Sherman Island during the same field campaign. Using annual layer counting of chemical records and volcanic horizon identification, an age scale for the record of 1724 discrete samples is presented. The Sherman Island ice record extends back to at least 1000 years before present, providing the oldest ice-derived continuous palaeoclimate record for the coastal Amundsen-Bellingshausen sector to date. An estimation of accumulation history at the site is presented. A comparison of the longer chemical and water isotope records with other regional ice cores provides insights into the spatial variability of change over recent centuries. Climate trends in the region of the Amundsen sea glaciers (including Thwaites) - considered the most vulnerable to future warming - are investigated.

How to cite: Rowell, I., Mulvaney, R., Wolff, E., Pryer, H., Tetzner, D., Thomas, L., Rix, J., and Martin, C.: 1000 years of climate history from a coastal West Antarctic ice core site, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5914, https://doi.org/10.5194/egusphere-egu22-5914, 2022.

We present new measurements of methane (CH4) and carbon dioxide (CO2) in the Skytrain ice core, with gas ages dated around 1610AD. The aim of these measurements is to improve our understanding of why there is a significant difference between measured CO2 at that time in current ice core records.

A pronounced feature of the Law Dome record (accumulation 60 cm ice eq. yr; gas age distribution 8 years,) is a rapid decrease in CO2 of ~10 ppm over 50 years with a distinct minimum at 1610. The cause of this decrease is much debated, with complex carbon cycle feedbacks required in explanation. However, other ice cores do not show the same event. The West Antarctic Ice Sheet (WAIS) divide record (accumulation 22 cm ice eq. yr; gas age distribution 19 years) shows a steadier decline in CO2 of approximately 6 ppm over the same period, with the record also ~2-3 ppm higher than Law Dome throughout 900-1800 CE. A follow-up study using the Dronning Maud Land (DML) ice core (accumulation 7 cm ice eq. yr; gas age distribution 65 years) attempted to prove which core showed the real atmospheric signal, but results were inconclusive due to the wide gas-age distribution of the record. While Skytrain (accumulation 14 cm ice eq. yr) does not match the accumulation rate of Law Dome, we present these new, high-resolution gas measurements over the period to work towards answering the following questions: (1) if the Law Dome record is correct, what caused this amplitude of CO2 change over a short timescale? (2) Does one of the records suffer from contamination? (3) Is our understanding of gas smoothing processes in these ice cores inaccurate? We will then use these measurements, from a well-validated ‘needle-crusher’ CO2 device at the ice core labs at Oregon State University, USA, to validate a new semi-continuous ice-grating device (for which we present a preliminary outline) at the new ice core gas analysis lab at the British Antarctic Survey, UK.

How to cite: King, A., Bauska, T., Brook, E., Kalk, M., Strawson, I., Epifanio, J., Hoffman, H., and Wolff, E.: The Little Ice Age CO2 drop:  Natural, Anthropogenic or Artefact?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2075, https://doi.org/10.5194/egusphere-egu22-2075, 2022.

Common methods to liberate the air bubbles trapped in ice cores include melting (wet extraction) and mechanical destruction (dry extraction) of ice under vacuum. Wet extraction is commonly used for CH₄ analyses; however, a recent study (Lee et al., 2020) showed that the presence of liquid water can introduce extraneous (non-paleoatmospheric) CH₄ from the reaction between the meltwater and impurities. Measurements of CH₄ using dry extraction methods had previously been done (e.g., Ferretti et al., 2005); however, a laboratory intercomparison study showed that dry extraction system generally have higher blanks (Sowers et al., 1997) for CH₄. It is hypothesized that friction between components (either metal with metal or metal with ice) can be a source of contamination (e.g., Nicewonger et al., 2016; Sowers et al., 1997). A third method to liberate air trapped in ice core bubbles is sublimation under vacuum (e.g., Wilson and Donahue, 1989; Schmitt et al., 2011). The sublimation method guarantees complete gas extraction from both the air bubbles and the ice matrix / clathrates (Bereiter et al., 2015), and should be free of problems associated with wet extraction. Here we present a new sublimation system that aims to sublimate ~250g of ice and extract ~25 mL STP of air for measuring CH₄ isotopes in ice with high impurities. We use three high powered (500W each) infrared lamps that emit peak radiation at wavelength of 1500 nm, which coincides with a local maxima in ice absorption spectra (Warren and Brandt, 2008). To ensure even sublimation, the infrared lamps are attached to a custom-made mounting bracket that is rotated around the glass vessel. This system is coupled to a previously built Gas Chromatography Isotope Ratio Mass Spectrometry (GC-IRMS) system (Sperlich et al., 2013) to purify and analyze δ¹³CH₄ isotopes. At the time of writing, the GC-IRMS system is able to achieve 0.06 ‰ precision on standard air injections with contemporary CH₄ mole fraction (1850 nmol/mol). Further testing to characterize the performance of the system using blank ice and test ice of known mole fraction and isotope values is underway.

How to cite: Dyonisius, M., Döring, M., and Blunier, T.: ​Development of ice sublimation device for analyses of methane isotopes in ice with high impurities, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7056, https://doi.org/10.5194/egusphere-egu22-7056, 2022.

To understand the causal relationship between forcing (orbital parameters, greenhouse gas concentration…) and the climate change, dating climate archives is crucial. Ice cores are unique archives because they provide a direct record of greenhouse gas concentration. However, dating ice cores is particular since they require two chronologies: one for the ice and one for the younger air trapped in bubbles inside the core. The coherent AICC2012 chronology was established for five ice cores: EPICA Dome C (EDC), EPICA Dronning Maud Land (EDML), North Greenland Ice core Project (NGRIP), Vostok (VK) and TALos Dome Ice CorE (TALDICE). A sedimentation model was used to reconstruct past variations of three parameters: accumulation of snow at surface, ice layer thinning in depth and Lock-In-Depth (LID), the depth where air is trapped. Ice and gas ages along the core are estimated from these parameters. Then, a Bayesian tool optimised the age scale by constraining the chronology to respect chronological observations (orbital tuning, stratigraphic links between cores, tephra layers…) and by fitting the three parameters to background scenarios (accumulation deduced from ice isotopes, LID from δ¹⁵N, …). The AICC2012 chronology is associated with an uncertainty which arises up to 6 kyrs due to the discontinuity of the ice core composition records and to the poor knowledge when it comes to choose an optimised target for orbital tuning.

Since AICC2012, many new data have been obtained to improve the ice core chronology and it is the right period to produce an updated coherent chronology which could also be extended to other ice cores. Here, we present a first step toward the construction of the next coherent ice core chronology by including new dating constraints from recent data on the EDC ice core: 1) air isotopes (δ¹⁸O_atm , δO₂ /N₂) and air content used as orbital dating constraints, 2) the δ¹⁵N signal used to estimate the background scenario for LID. In addition, we make use of the East Asian stalagmite δ¹⁸O_calcite signal as an alternative synchronisation target for the δ¹⁸O_atm (Extier et al. 2018).

This new dating experiment on EDC ice core aims to lower uncertainty of the chronology while providing a critical look on former hypotheses considered to establish AICC2012. For example, δ¹⁵N record was discontinuous at the time and it has been reconstructed based on its correlation with δD. Now that we have a continuous δ¹⁵N signal, we can evaluate the relevance of this reconstruction. Following this work, we will use new tie point constraints resulting from volcanic synchronisation which has recently been undertaken between Greenland and Antarctica (Svensson et al. 2020) and the ice cores Dome Fuji and WAIS Divide will be further studied to be included in the chronology.

How to cite: Bouchet, M., Grisart, A., Landais, A., Parrenin, F., Prié, F., Raynaud, D., Lipenkov, V. Y., Capron, E., Legrain, E., Extier, T., and Svensson, A.: New dating experiment on EPICA Dome C (EDC) ice core over the last 800 kyrs using the Bayesian tool Paleochrono and new records of elemental and isotopic composition in the air trapped in the EDC ice core., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5166, https://doi.org/10.5194/egusphere-egu22-5166, 2022.

Global biosphere primary productivity via photosynthesis is the largest carbon uptake flux from the atmosphere. The Earth biosphere currently absorbs near half of the carbon emitted by anthropogenic activities (Friedlingstein et al., 2020). Therefore, for a better projection of future carbon cycle, it is important to understand how the global biosphere would respond to abrupt climate changes which occurred in the past. The last glacial period is punctuated by a number of rapid shifts between relatively cold (stadial) and warm (interstadial) stages named Dansgaard-Oeschger events (DO). Some stadials are also associated with abrupt, massive iceberg discharge event and are called Heinrich Stadials (HSs). The high-resolution CO₂ reconstruction from polar ice cores demonstrated millennial-scale CO₂ variations over HS-DO events (Bauska et al., 2021). The gradual rising of CO₂ over HS has been attributed to ventilation changes in Southern Ocean (Gottschalk et al., 2016; Menviel et al., 2018) and/or reduced biological uptake (Ahn et al., 2012; Gottschalk et al., 2016; Schmittner and Lund, 2015). However, the role of the global biosphere is not well understood because of difficulties in estimating global biosphere productivity from local reconstructions based on indirect tracers.

To address this, here we use the triple isotopic composition of air oxygen (¹⁷Δ = ln(δ¹⁷O+1) - λ_ref·ln(δ¹⁸O+1), λ_ref = 0.516), which is a biogeochemical tracer of global biosphere primary productivity (Luz et al., 1999). We measured ¹⁷Δ of trapped air in the NEEM ice core over 36 to 42 ka interval, covering HS4 and DO8 events. The new NEEM ¹⁷Δ data show no significant change over HS4, while CO₂ records from multiple ice cores indicate near ~20 ppm increase (e.g., Ahn and Brook, 2014; Bauska et al., 2021). By using the box models describing ¹⁷Δ systematics between biosphere-troposphere-stratosphere (e.g., Landais et al., 2007; Blunier et al., 2012), our preliminary results suggest that global biosphere productivity increases during HS4. This result is inconsistent with previous estimates based on ice-core records of non-sea-salt Na and Ca (Fischer et al., 2007), and the marine sediment core opal flux record (Gottschalk et al., 2016), both indicating a reduction of Southern Ocean biological productivity. More ¹⁷Δ samples remain to be measured up to the General Assembly 2022 and we hope to have a clearer picture by then.

How to cite: Yang, J.-W., Landais, A., Blunier, T., Duchamp-Alphonse, S., and Prié, F.: Preliminary results of global biosphere productivity reconstruction over Heinrich Stadial 4 from the triple isotopic composition of air oxygen trapped in NEEM ice core, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4287, https://doi.org/10.5194/egusphere-egu22-4287, 2022.

Paleoclimate reconstructions from ice core records can be hampered due to the lack of a reliable chronology, especially when the stratigraphy is disturbed and conventional dating methods cannot be readily applied. The noble-gas radioisotopes ⁸¹Kr and ³⁹Ar can in these cases provide robust constraints as they yield absolute, radiometric ages. ⁸¹Kr (half-life 229 ka) covers the time span of 30-1300 ka, which is particularly relevant for polar ice cores, whereas ³⁹Ar (half-life 268 a) with a dating range of 50-1800 a is suitable for alpine glaciers. For a long time the use of ⁸¹Kr and ³⁹Ar for dating of ice samples was impeded by the lack of a detection technique that can measure its extremely small abundance at a reasonable sample size.

We present ⁸¹Kr and ³⁹Ar dating of Antarctic and Tibetan ice cores with the detection method Atom Trap Trace Analysis (ATTA), using 5-10 kg of ice for ⁸¹Kr and 2-5 kg for ³⁹Ar. Recent studies in Antarctica include ⁸¹Kr dating in ice cores from the Larsen Blue ice area, Talos Dome and Epica Dome C. Moreover, we have used ³⁹Ar for dating an ice core from central Tibet covering the past 1500 years, in order to validate a previous timescale based on layer counting. The  studies demonstrate how ⁸¹Kr and ³⁹Ar can provide age constraints and complement other methods in developing an ice core chronology. As the sample size requirement for ⁸¹Kr and ³⁹Ar analysis still hinders its use in ice cores, developments on the ATTA systems are in progress to further decrease the sample size and increase the dating precision. Here, we present our latest advances towards ⁸¹Kr and ³⁹Ar dating with ~ 1 kg of ice.

[1] Z.-T. Lu, et al. (2014) Tracer applications of noble gas radionuclides in the geosciences, Earth-Science Reviews 138, 196-214

[2] C. Buizert et al. (2014), Radiometric ⁸¹Kr dating identifies 120,000-year-old ice at Taylor Glacier, Antarctica Proceedings of the National Academy of Sciences, 111, 6876

[3] L. Tian et al. (2019), ⁸¹Kr Dating at the Guliya Ice Cap, Tibetan Plateau, Geophysical Research Letters 46, 6636–6643

[4] Crotti I, et al. (2021) An extension of the TALDICE ice core age scale reaching back to MIS 10.1. Quaternary Science Reviews 266:107078

[5] Lee, G., et al. (2021) Chronostratigraphy of blue ice at the Larsen Glacier in Northern Victoria Land, East Antarctica, The Cryosphere Discuss. [in review]

How to cite: Ritterbusch, F., Crotti, I., Dong, X.-Z., Fourré, E., Gu, J.-Q., Jacob, R., Jiang, W., Landais, A., Lu, Z.-T., Orsi, A., Prié, F., Shao, L., Tian, L., Tong, A.-M., Yang, G.-M., and Wang, J.: Towards 81Kr and 39Ar dating with 1 kg of ice, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9013, https://doi.org/10.5194/egusphere-egu22-9013, 2022.