Ice core records of δ18Oatm respond to abrupt climate change during the last glacial period (Rapid changes and teleconnections)

• Jacob Morgan (University of California, San Diego)

Room: Centre de Congrès Le Régent We present measurements of the isotopic composition of O2 (δ18Oatm) at sub-centennial resolution from the South Pole and WAIS Divide ice cores. Millennial-scale changes in ice core records of δ18Oatm are often interpreted as reflecting global signals related to changes in tropical precipitation and the spatial distribution of photosynthesis during Heinrich events. However, the unprecedented sub-centennial resolution of the South Pole and WAIS Divide ice cores allows us to investigate for the first time the response of δ18Oatm to climate changes during Dansgaard-Oeschger events between 68 and 10 kyr BP. During Heinrich stadials, when the North Atlantic region is cold, δ18Oatm increases as tropical rain belts and terrestrial oxygen production shift to the south. Here, we show for the first time that δ18Oatm also increases during non-Heinrich stadials and decreases during warm, interstadial periods, likely also due to changes in the position and or intensity of tropical precipitation.

Rapid atmospheric CO2 rise during Heinrich Stadials (Rapid changes and teleconnections)

• Kathleen Wendt (Oregon State University)

Room: Centre de Congrès Le Régent The recent discovery of centennial-scale rises in atmospheric carbon dioxide (CO2) during Heinrich Stadials (HS) sheds new light on the role of CO2 during rapid climate shifts (Bauska et al. 2021). Resolving the precise timing, rate, and magnitude of each CO2 rise, however, remains a challenge. This study examines CO2 variability at sub-centennial resolution during HS 2-5. We sampled at 0.2 to 0.5 m intervals along select depths from the West Antarctic Ice Sheet Divide ice core (WDC), which translates to 7 to 30 years between each sample. Each sampled depth was replicated to within 1ppm. Due to the site’s high accumulation rate (presently 20 cm/yr) and the excellent chronologic constraints for the WDC, CO2 variations can now be documented down to decadal timescales. Preliminary results suggest a near 20 ppm rise of CO2 over the span of a century from 39.58 to 39.47 kyrs before present (ka) on the WD2014 chronology, with the fastest rate currently resolved to nearly 5 ppm in a single decade. Our results reveal that the rise in atmospheric CO2 during HS 4 is similar in rate and magnitude to the CO2 pulses observed during the last deglaciation (Marcott et al. 2014). Rapid yet lower magnitude CO2 pluses during HS 5 (7 ppm between 48.59 and 48.31 ka) and HS 2 (7 ppm between 24.10 and 24.07 ka) have also been resolved. Atmospheric CO2 pulses during HS 2, 4 and 5 are synchronous with a distinct pulse in methane (CH4) recorded in WDC, which is interpreted as a rapid reorganization of the global carbon cycle. Little variation in atmospheric CO2 is observed during HS 3, which is consistent with minimal change in WDC CH4 at this time. While the exact forcings of abrupt CO2 changes remain unclear, our findings point to a mechanism operating at sub-centennial timescales that is ultimately linked to Northern Hemisphere climate shifts. Bauska et al. (2021) Abrupt changes in the global carbon cycle during the last glacial period. Nature Geoscience 14 91-96. Marcott et al. (2014) Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature 514 616–619.

Climatic impact of volcanic eruptions during the last glacial period inferred from Greenland and Antarctic ice cores (Rapid changes and teleconnections)

• Johannes Lohmann (Niels Bohr Institute, University of Copenhagen)

Room: Centre de Congrès Le Régent Ice core and tree ring records, as well as climate models suggest that volcanism plays a major role in generating the climate variability observed in the Common Era. Since the state of the climate in the Anthropocene already exerts significant stress on society, we are increasingly vulnerable to unpredictable perturbations. To be prepared, the impact of large future eruptions needs to be constrained better. Satellite observations only exist for few major eruptions, which are not large compared to eruptions that will eventually occur over time spans of a hundred years or more. To learn more about eruptions with return periods of several hundred years, we analyzed volcanic records from ice cores of the last glacial period and compared them with high-resolution paleoclimatic ice core records. Temperature proxies from Antarctica and Greenland show a multiannual cooling around the time of large eruptions due to the effect of stratospheric sulfate aerosols. While the glacial signal is significantly degraded over time, its average amplitude is comparable to the largest eruptions of the Common Era. Eruptions with even larger cooling exist, but their precise amplitude is uncertain. This is because the average cooling signal is smaller than the background proxy variability, and only the largest eruptions exceed the latter. The total variability may be dominated by stratigraphic and glaciological noise. The volcanic cooling is hemispherically asymmetric, and cold stadial periods are associated with a larger Greenland cooling signal. This suggests that the volcanic impact depends on the background climate state. Furthermore, non-linearities in the climate system may lead to large-scale instabilities (tipping points), where even small perturbations such as volcanic eruptions can lead to an abrupt transition to a different climate state. To this end, we investigated a potential connection of abrupt climate changes (Dansgaard-Oeschger events) in the last glacial and large volcanic eruptions.

Reconstruction of the pre-industrial to industrial changes of carbonaceous aerosols using ice core archives (Pollution records)

• Thomas Singer (PSI - Paul Scherrer Institut)

Room: Centre de Congrès Le Régent Atmospheric aerosols play a pivotal role in the Earth’s system by changing cloud features, affecting air quality and directly impacting the Earth’s radiative budget through the scattering and the absorption of the incoming solar radiation. However, the impact of aerosols on climate remains poorly constrained, leading to considerable uncertainties in predicting the climate sensitivity to greenhouse gases. A large fraction of these uncertainties is due to our deficient knowledge of the composition and magnitude of natural emissions before 1750. Ice sheets and glaciers are valuable natural archives that contain information about the history of the Earth’s atmosphere and that can fill this knowledge gap. Contrarily to polar ice cores, ice cores from high altitude and mid-latitude glaciers, due to their geographical position, preserve information about emissions of anthropogenic aerosols. In this study we provide ice-core based long-term records of mass concentration of the water-insoluble (WIOC) and water-soluble (WSOC) fraction of organic aerosols as well as elemental carbon (EC) from the pre-industrial to the industrial period from Colle Gnifetti glacier in Switzerland. We use the powerful tool of radiocarbon (14C) analysis to distinguish the contribution of natural (biogenic) and anthropogenic (fossil) sources to the individual fractions. The total concentration of carbonaceous aerosols (sum of WIOC, WSOC and EC) was three times the pre-industrial background at the end of the 20th century. Total organic aerosol concentrations (WIOC and WSOC) show a sharp increase around 1940, but largely of non-fossil origin, suggesting increased emissions of precursor gases and enhanced formation of secondary organic aerosols.

Black Carbon and Dust Deposition on South Cascade Glacier, Washington State, USA, Spanning 1840-2014: A Reconstruction of Fossil Fuel and Forest Fire Emissions, and Associated Snow Albedo Impacts (Pollution records)

• Susan Kaspari (Central Washington University)

Room: Centre de Congrès Le Régent Light absorbing particles (LAPs) include black carbon (BC) and mineral dust and are of interest due to their positive radiative forcing and contribution to albedo reductions and snow and glacier melt. This study documents historic BC and dust deposition as well as their effect on albedo on South Cascade Glacier (SCG) in Washington State (USA) through the analysis of a 158-m (139.5-m water equivalent [w.e.]) ice core extracted in 1994 and spanning the period 1840–1991, and three shallow ice cores collected in 2014. Peak BC deposition occurred between 1940 and 1960, when median BC concentrations were 16 times higher than background, likely dominated by domestic coal and forest fire emissions. Post 1960 BC concentrations decrease, followed by an increase from 1977 to 1991 due to melt consolidation and higher emissions. BC concentrations in the 2014 shallow cores remain elevated above background levels, and C14 analysis indicates that ~70% of the 2014 BC is from non-fossil fuels sources. Differences between the SCG record and BC emission inventories, as well as ice core records from other regions, highlight regional differences in the timing of anthropogenic and biomass BC emissions. Dust deposition on SCG is dominated by local sources and is variable throughout the record. Albedo reductions from LAP are dominated by dust deposition, except during high BC deposition events from forest fires and during 1940–1960 when BC and dust similarly contribute to albedo reductions.

Industrial Lead Pollution in Modern Central Greenland Ice (Pollution records)

• Sophia Wensman (Desert Research Institute)

Room: Centre de Congrès Le Régent Emissions from mid-latitude industrial activities (e.g., mining, smelting, coal combustion) result in long-range atmospheric transport of lead (Pb) to the Arctic. While previous measurements of elemental concentrations and Pb isotopic ratios in ice and sediments have been used to suggest potential sources of toxic heavy metal pollution in these regions, high resolution Pb isotope records are largely unavailable due to the low Pb concentrations found in Arctic ice. Recent advancements in low-level measurements of lead (Pb) by high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) (e.g., Smith et al., 2019; Wensman et al., 2022), provide means for obtaining precise, high resolution Pb isotopic records. Here we present and discuss approximately annual high-resolution records of Pb isotopes for 1759–2008 measured in two central Greenland ice cores (Summit_2010 (Wensman et al., 2022) and NASAU); the only high-resolution Pb isotopic records in Greenland ice to include the First Industrial Revolution (1760–1840). Historical records of industrial activities coupled with Pb isotopic signatures for regional ores and coals suggest Pb pollution prior to the mid-19th century was dominated by emissions from the British Isles following technological advancements, which revolutionized coal mining. Rapid increases in Pb levels and decreases in 206Pb/207Pb ratios in the mid-19th century coincided with expansion of coal consumption in Europe and North America. Significant influence of 20th century smelting of Australian Broken Hill Pb ores in Europe resulted in a less radiogenic Pb isotope signature. We also highlight the impact of air-quality legislation on Pb isotopic ratios, demonstrating the reduced influence of United States leaded gasoline emissions following the Clean Air Act and phase-out of leaded gasoline. The research also highlights the rising influence of long-range transport from Asian Pb emission sources, in the context of declining North American and European Pb emissions. References Smith, K. E., Weis, D., Amini, M., Shiel, A. E., Lai, V. W. M., & Gordon, K. (2019). Honey as a biomonitor for a changing world. Nature Sustainability, 2(3), 223-232. Wensman, S.W., Shiel, A.E., McConnell, J.R. (2022). Lead isotopic fingerprinting of 250-years of Industrial Era pollution in Greenland Ice. Anthropocene.

Why is δ13CH4 so closely following atmospheric CO2 during glacial climate conditions? (Progress in proxy development and interpretation)

• Jochen Schmitt (University of Bern)

Room: Centre de Congrès Le Régent Reconstructions of atmospheric methane mixing ratios have been available for decades. The large dynamic range, 350 to 800 ppb, and the abruptness of these CH4 changes suggest that CH4 is a powerful parameter to constrain past climate conditions, e.g. infer the extent of wetlands and their geographic location. The current view suggests low latitude wetlands as the dominant sources during cold and warm climate conditions. Stable isotopes of CH4 offer more insight into the underlying budget changes for past CH4 concentration changes. Especially δD of CH4 and 14CH4 successfully ruled out some scenarios (clathrate hydrates destabilization) and constrained the geologic and biomass burning sources. In contrast, the carbon isotopic signature of CH4 (δ13CH4) provided many surprises. It revealed that it is not just the mixing of isotopically distinct methane sources controlling the δ13CH4 record but that changes in the isotopic signatures of the sources themselves are essential. A remarkable feature of the δ13CH4 record of the last 450 kyr is its close resemblance to the CO2 record. For glacial periods the correlation between CO2 and δ13CH4 is very tight (r2 = 0.9), and there is no apparent phase lag between the two parameters. Both observations suggest a fast response of the d13C composition of the plant material ultimately used by methanogens for CH4 production on the CO2. However, other explanations are equally possible, e.g. both CO2 and d13CH4 could be modulated by a third factor, like the position of wind belts in the southern hemisphere that drives CO2 and d13CH4 via geographic shifts in rainfall. The regional differences in δ13CH4 within the low latitudes are presumably caused by their relative abundance of functional plant types (C3 vs C4 plants). However, the relative dominance of C4 vs C3 plants is not constant over time but can be modulated by climate and CO2. A recent paper by Yamamoto et al. 2022 exploits this CO2 – C4 vs C3 relation using δ13C of plant tissue (leaf wax) representative of the northeastern part of the monsoon-controlled Indian subcontinent to reconstruct atmospheric CO2 over the last 1.5 Mio years. The dynamic range in δ13C of the Yamamoto leaf wax record is ~ 9‰, comparable to our δ13CH4 record, demonstrating that a δ13C change of the plant tissue is a potential player in the observed δ13CH4 dynamic. Here we will examine the shared similarities and differences between CO2 and the δ13C records of CH4 and the Yamamoto’s leaf wax record to understand the driving factors behind it.

Summer snow surface water isotope variability driven by vapor-snow exchange (Progress in proxy development and interpretation)

• Sonja Wahl (University of Bergen & CRYOS - EPFL, Lausanne)

Room: Centre de Congrès Le Régent Observed variability in summer surface snow isotopic composition (∂18O, ∂D) cannot solely be explained by precipitation events. This variability, however, influences the overall summer isotope signal that is archived in ice cores. It is important to identify and quantify the impact of all signal formation processes contributing to the snow isotopic composition to ensure an optimal interpretation of ice core isotope records. Through a combination of laboratory experiments and in-situ observations from the Greenland Ice Sheet the evidence of isotopic fractionation during snow sublimation has been demonstrated. However, the impact of post-depositional processes influencing the surface snow isotope signal before it gets buried remains unquantified to-date. Here we show that the continuous exchange of humidity between the atmosphere and the snow surface through sublimation and deposition is a driver for the summer surface isotope variability. By comparing modeled against in-situ observed surface isotope variability, we demonstrate that fractionation-including vapor-snow exchange can explain 52% of the ∂D day-to-day variability and 35% of the variability observed in snow ∂18O. Further, we document substantial isotopic enrichment in the uppermost centimeters of snow induced by sublimation which suggests a vapor-exchange-induced warm-bias in the buried summer isotope signal. Our results lead to an improved process understanding and necessitate the implementation of fractionation during the sublimation process in isotope-enabled climate models.

Isotopic composition along the atmosphere-precipitation-snow continuum in the Dome C region, East Antarctica (Progress in proxy development and interpretation)

• Inès Ollivier (University of Bergen)

Room: Centre de Congrès Le Régent The ability to infer past temperatures from ice core records has in the past relied on the assumption that after precipitation, the stable water isotopic composition of the snow surface layer is not modified before being buried deeper into the snowpack and transformed in ice. However, in extremely dry environments, such as the East Antarctic plateau, the precipitation is so sparse that the surface is exposed to the atmosphere for some time before burial. Several processes have been recently identified as impacting the snow isotopic composition after snowfall, also called post-depositional processes: (1) exchanges with the atmosphere (i.e. sublimation/condensation cycles), (2) wind effects (i.e. redistribution and pumping) and (3) exchanges with the firn below (i.e. metamorphism and diffusion). Here we present the results of the analysis of in-situ observations of the isotopic composition along the precipitation-atmosphere-snow continuum in the Dome C region, located on the high East Antarctic Plateau. Together with meteorological data, this dataset gives insight on how the isotopic composition varies in the upper layer of the snowpack and which surface processes are at play on different time scales. We also use a surface snow model which includes vapor-snow exchanges and diffusion in the snow to disentangle and quantify the impact of each physical process on the observed isotopic composition of the snow surface.

Nitrate isotope variability on the East Antarctic plateau: Results from the EAIIST (East Antarctic International Ice Sheet Traverse) project (Progress in proxy development and interpretation)

• Pete Akers (Trinity College Dublin)

Room: Centre de Congrès Le Régent Nitrate is one of the most abundant ions in Antarctic ice, but interpreting its concentration and isotopic variability in ice cores is difficult due to substantial photolytic mass loss after its atmospheric deposition onto the snow surface. To improve our understanding of Antarctic nitrate dynamics, particularly in extreme environments, nitrate sampling was a priority for the French-Italian-Australian EAIIST project’s Antarctic land vehicle traverse in austral summer 2019/2020. Over three months, the EAIIST team traveled from the Adélie coast to the remote interior Megadunes site (650 km south of Dome C) by way of Concordia station before returning back to the initial coastal starting point. This 3600 km transect covered a wide range of Antarctic environments from wet and windy coastal regions to the dry and relatively calm interior plateau, with the Megadunes site offering a particularly unusual landscape of wind glazed areas, rolling dunes with spatially variable accumulation rates, and visible surface cracking. During EAIIST, ~ 0.5–1kg of snow was collected twice a day from both the snow surface and at 1-m depth to be later melted and processed for nitrate isotopic analysis. Additionally, 1-m deep snow pits were incrementally sampled in 2–10 cm intervals at seven sites to precisely observe how the nitrate profile changed with depth. These 262 total samples show a clear spatial relationship between nitrate and snow accumulation rate, with more post-depositional changes (i.e., lower nitrate concentration, higher nitrogen isotopes, lower oxygen isotopes) observed in the nitrate at drier sites. Samples from the Megadunes region, however, suggest that this relationship becomes more complex at sites with very low accumulation rates due to wind-pumped mobilization of near-surface nitrate into deeper firn. This holds important context for interpreting nitrate in deep ice cores, as glacial period conditions at coring sites may have been similar to the modern environments that we observed at the Megadunes.

Extending the fabric from the EGRIP ice core in space with geophysical methods and modelling (Ice dynamics, ice sheet instability and geophysics)

• Olaf Eisen (AWI)
• Tamara Gerber (University of Copenhagen)

Room: Centre de Congrès Le Régent Anisotropic crystal fabrics in ice sheets develop as a consequence of deformation and hence record information of past ice flow. Simultaneously, the fabric affects the present-day bulk mechanical properties of glacier ice because the susceptibility of ice crystals to deformation is highly anisotropic. This is particularly relevant in dynamic areas such as fast-flowing glaciers and ice streams, where the formation of strong fabrics might play a critical role in facilitating ice flow. Anisotropy is ignored in most state-of-the-art ice sheet models, and while its importance has long been recognized, accounting for fabric evolution and its impact on the ice viscosity has only recently become feasible. Both the application of such models to ice streams and their verification through in-situ observations are still rare. Ice cores provide direct and detailed information on the crystal fabric, but the logistical cost, technical challenges, particularly in fast-flowing ice and shear margins, difficulty in reconstructing the absolute orientation of the core, and their limitation of being a point measurement, make ice cores impractical for a spatially extensive evaluation of the fabric type. Indirect geophysical methods applied from or above the ice surface create the link between the small scale of laboratory experiments and ice–core observations to the large-scale coverage required for ice flow models and the complete understanding of ice stream dynamics. Here, we present a comprehensive analysis of the distribution of the ice fabric in the upstream part of the North-East Greenland Ice Stream (NEGIS). Our results are based on a combination of methods applied to extensive airborne and ground-based radar surveys, ice- and firn-core observations, and numerical ice-flow modelling. They show that in the onset region of NEGIS and around the EGRIP ice core drilling site, the fabric is horizontally strongly anisotropic, forming a horizontal girdle perpendicular to the ice flow, while the horizontal anisotropy reduces quickly over distances of less than five ice thicknesses outside of the ice stream’s shear margins. Downstream of the drill site, the fabric develops into a more vertically symmetric configuration on a time scale of around 2 ka, the first observation of this kind. Our study shows how ice-core based fabric observations, geophysical surveys and ice-flow modelling complement each other to obtain a more comprehensive picture of the spatially strongly varying fabric.

Ice core and stratigraphic constraints on modelling dynamic Antarctic outlet systems (Ice dynamics, ice sheet instability and geophysics)

• Johannes Carl Roderick Sutter (Universität Bern)

Room: Centre de Congrès Le Régent Model reconstruction of past ice dynamic changes are essential for our understanding of future ice sheet responses to climate change. However, paleo ice sheet model studies are poorly constrained as spatiotemporal coverage of proxy reconstructions are sparse. Previously, we showed, that it is possible to identify or exclude past ice sheet instabilities by using the isotopic record and age structure of a deep ice core in vicinity to dynamic outlet sectors as a constraint for flow parameterizations in an ice sheet model. Here, we highlight key Antarctic ice sheet domains in which deep ice cores in concert with radar observations of the ice sheet’s stratigraphy hold great potential to provide an even more rigid observational tuning target for ice flow models. In some of these regions dated deep ice cores are already available, often including coverage of internal reflection horizons potentially connecting the ice core age structure with faster flowing outlet sectors. In other regions either an ice core providing age constraints or radar observations are not yet available. We discuss the potential of ice core/stratigraphically calibrated ice flow modelling of dynamic Antarctic drainage systems. Furthermore, we present first model estimates of the age structure in these regions and identify promising sites for future ice coring expeditions or ice penetrating radar missions.

New isotopic records from the TALDICE ice core reveal past events of East Antarctic ice sheet retreat (Ice dynamics, ice sheet instability and geophysics)

• Barbara Stenni (Department of Environmental Sciences, Informatics and Statistics, Ca’ Foscari University)

Room: Centre de Congrès Le Régent The response of the East Antarctic Ice Sheet to past intervals of oceanic and atmospheric warming is still not well constrained but critical for understanding both past and future sea-level change. The ice sheet in the Wilkes Subglacial Basin, which is characterized by a reverse-sloping bed, appears to have undergone thinning and ice discharge events during recent decades, but its past dynamics are still under debate. The aim of our study is to investigate past ice margin retreat of the Wilkes Subglacial Basin ice sheet during late Pleistocene interglacials with the help of new high-resolution isotopic records from the TALDICE ice core. The δ18Oice signal spanning the late Pleisotocene interglacials MIS 7.5 and 9.3 reveal that those periods are characterized by a unique double-peak feature, previously observed for MIS 5.5 (Masson-Delmotte et al., 2011), that is not seen in other Antarctic ice cores. Through an hypothesis testing approach and a combination of glaciological evidence, an offshore Wilkes Subglacial Basin sediment core record (Wilson et al., 2018) and GRISLI ice sheet modelling experiments (Quiquet et al., 2018), we provide an interpretation of this peculiar record. Our results indicate that the interglacial double-peak δ18Oice signal could reflect Talos Dome site elevation decrease during the late stages of interglacials due to Wilkes Subglacial Basin retreat events. These changes coincided with warmer Southern Ocean temperatures and elevated global mean sea level, confirming the sensitivity of the Wilkes Subglacial Basin ice sheet to ocean warming and its potential role in sea-level change.

New dating experiments 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 of air trapped in the ice (Time scales and methods for ice dating)

• Marie Bouchet (CNRS - LSCE)

To understand the phasing of external forcing (greenhouse gases, orbital parameters) and past climate change, well-dated paleoclimatic archives are required. Ice cores are unique archives because they provide a direct record of greenhouse gas concentration over the last 8 climatic cycles. But to date ice cores, we need to construct two separate chronologies: one for the ice and one for the younger air trapped in the ice. 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) using the following method. A sedimentation model is used to calculate background age scales from an initial scenario for variations of three parameters: accumulation of snow at the surface, ice thinning and Lock-In-Depth (LID). A Bayesian tool then improves the age scales with respect to chronological observations (dating constraints, stratigraphic links between cores, tephra layers…) and accordingly adjusts the evolution of the three parameters mentioned above. The AICC2012 chronology is associated with an uncertainty of 6 kyrs, mainly due to uncertainties related to the orbital tuning. Since the construction of AICC2012, many new data have been obtained and it is the right period to produce an updated coherent chronology which could be further extended to other ice cores. Here, we present a first step toward the construction of the next coherent ice core chronology by including new constraints in the Paleochrono model from recent high-resolution data on the EDC ice core covering the last 800 kyrs: 1) air isotopes (δ18Oatm, δO2/N2, 40Ar, 81Kr) and total air content (TAC) used as dating constraints, 2) δ15N used to reconstruct the background scenario for LID. In addition, the East Asian speleothem δ18Ocalcite signal is used as an alternative synchronisation target for the δ18Oatm (Extier et al. 2018) and constraints resulting from volcanic synchronisation undertaken between Greenland and Antarctica are also considered (Svensson et al. 2020). This new dating experiment on EDC ice core aims to lower the uncertainty of the current chronology while providing a critical look on former hypotheses considered to establish AICC2012.

Mid to Late Holocene East Antarctic ice-core tephrochronology: Implications for reconstructing volcanic eruptions and their impacts over the last 5,500 years (Time scales and methods for ice dating)

• Peter Abbott (University of Bern)

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 prior research has focussed on visible horizons and deep ice cores that cover longer time spans. Thus, only one horizon, dated to ~3.5 ka BP (the Vostok Tephra), has been 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 identify and sample > 50 potential cryptotephra horizons from ice containing coeval peaks in fine insoluble particles and non-sea-salt sulphur. 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 15 cryptotephra horizons have been identified, geochemically characterised and linked to regional sources such as the South Sandwich and South Shetland Islands. One cryptotephra derives from North Victoria Land, Antarctica and can be linked to the Rittmann Tephra (1252 CE), significantly increasing the known distribution of this event. In addition, the ~3.5 ka Vostok Tephra 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.

Synchronization of ice-core and speleothem records around Heinrich Event 2 using cosmogenic radionuclides (Time scales and methods for ice dating)

• Giulia Sinnl (Niels Bohr Institute)

Cosmogenic radionuclides are an excellent synchronization tool thanks to their globally synchronous production and modulation by changes of the helio- and geomagnetic fields. The cosmogenic isotopes of $^{10}$Be are recorded in ice cores, while $^{14}$C isotopes are recorded in speleothems; hence, these isotopes can be used to align chronologies from distant regions. Here, new measurements of $^{10}$Be in ice cores from Greenland (NorthGRIP; resolution ~10 years/sample) and Antarctica (WDC; resolution ~67 years/sample) allowed us to synchronize the two ice-core chronologies GICC05 and WD2014, aligning similar radionuclide production features mostly concentrated around 22 ka b2k. Moreover, the Hulu Cave speleothem U/Th chronology was synchronized to the ice-core datasets using carbon-cycle modelling and probabilistic wiggle matching. The triangle of chronologies (GICC05, WD2014, and Hulu Cave U/Th) had not yet been tied together in the period from 20 to 25 ka b2k. We find our results confirming the previous GICC05-Hulu synchronization by Adolphi et al. (2018). Both ice-core chronologies present centennial offsets with respect to the U/Th timescale, both towards younger ages. In the case of GICC05, we investigated where the offset originated, finding an answer in the many unusually wide layers recorded during Greenland Stadial (GS) 2. Our results indicate the almost synchronous occurrence of important signatures in the oxygen isotopes and dust records of the archives, providing new insights on the sequence of events around Heinrich Event 2 within GS 3. The new relative timing of the proxy data in this period could open new pathways for a better understanding of bipolar-seesaw-like mechanisms around Heinrich Events, as well as the influence of these on the Asian climate. References: Adolphi, F., Bronk Ramsey, C., Erhardt, T., Edwards, R. L., Cheng, H., Turney, C. S., ... & Muscheler, R.: Connecting the Greenland ice-core and U∕ Th timescales via cosmogenic radionuclides: testing the synchroneity of Dansgaard–Oeschger events. Clim. Past, 14, 1755–1781, 2018

High resolution Laser-Ablation ICP-MS chemical impurity analysis of Skytrain Ice Core (Antarctica) (Progress in proxy development and interpretation)

• Helene Hoffmann (University of Cambridge)

Polar ice cores are invaluable environmental archives, directly recording atmospheric conditions of the past. Investigations of periods of fast climatic changes and periods slightly warmer than present day climate, such as the Last Interglacial (LIG) 115-130 ka before present (BP), are of particular interest. The analysis of fast changes in the impurity signal, as well as of very old ice close to bedrock where the internal layers are highly thinned, both require a measurement depth resolution on the order of a millimetre or even less. This is far beyond the capabilities of conventional continuous flow analysis (CFA) systems. To achieve this high depth resolution, we have set up a system to perform high-resolution laser-ablation inductively coupled plasma – mass spectrometry (LA-ICP-MS) and a cryocell stage. This method was applied to selected segments of an ice core recently drilled to bedrock at Skytrain Ice Rise in the framework of the WACSWAIN (WArm Climate Stability of the West Antarctic ice sheet in the last Interglacial) project. The main objective of this project is to obtain unique information on the state of the Filchner-Ronne ice shelf during the LIG. Sections of 80 cm of ice from five different depth intervals covering time frames from Late Holocene to the LIG were analysed via LA-ICP-MS and compared for their overall impurity content as well their signal variability. Here we focus on the analysis of the most important marine and terrestrial markers: sodium, magnesium, calcium and aluminium. The high resolution (mm to sub-mm scale) LA-ICP-MS data is compared to low-resolution (cm scale) chemistry data from CFA of the Skytrain ice core performed on adjacent ice samples from the same depth. This comparison aims to evaluate the capabilities of the method in terms of improving depth resolution and detection of annual variability. We statistically evaluate the horizontal and vertical variability of the LA-ICP-MS signal across the ice core and the representativity of the LA-ICP-MS signal for an overall impurity content for different depth levels in the core. Finally, we investigate the potential of the method for resolving annual layers and fast changing climate signals within the selected core sections, especially the late LIG (about 115 - 120 ka BP).

A new method for robust multilinear modelling of fluorescent organic matter applied to Antarctic firn from Patriot Hills, West Antarctica (Progress in proxy development and interpretation)

• Matthew RP Harris (School of Geography, Geology and the Environment, Keele University)

Analysis of fluorescent organic matter (FOM) is increasingly applied to ice cores as a sensitive means of detecting ice-bound biomarkers, with these signatures showing great potential as proxies for marine productivity and sea ice changes in the Southern Ocean. A range of instrumentation, including in-situ borehole fluorometers, core scanners such as the Berkeley Fluorescence Spectrometer, and benchtop spectrofluorometers, have been applied in search of FOM within ancient ice. All share a key limitation, impeding species-specific analysis – fluorescing substances (fluorophores) in ice layers will produce overlapping signals that are extremely difficult to deconvolute. Multi-band fluorescence measurements in the form of excitation-emission-matrices (EEMs) have the advantage of encapsulating the entire UV-Vis fluorescence response of an ice core meltwater sample. In this case, multilinear modelling techniques such as Parallel Factor Analysis (PARAFAC) appear to offer a solution to the mixed fluorescence problem by decomposing overlapping signatures into individual fluorophores. However, a key issue in PARAFAC modelling of EEM data is the lack of any tool to quantify per-sample and per-component model fit. In essence, the ‘best’ model is selected using whole-model metrics, then resolved fluorophores are reported and interpreted without accompanying per-sample information on the effectiveness of the model. We present, discuss, and apply a tool that quantifies the per-sample and per-component fit of PARAFAC models of ice core fluorescence data. Using contiguous measurements of FOM in Antarctic firn from Patriot Hills in the Ellsworth Mountains, West Antarctica, we demonstrate that intra-model per-sample similarity analysis allows for superior positive identification of ice-bound fluorophores. Subsequently, the technique provides a clear visualisation of where precisely in a contiguous modelled fluorophore record the model fails to adequately represent the underlying fluorescence data. Quantifying fluorescence model variability in this manner will be vital for developing proxy relationships and interrogating the processes (both external and in-situ) that control the presence and intensity of ice-bound fluorophores.

Determining mineralogy, size, and concentration of individual aeolian particles entrapped in Antarctic ice over the last Glacial-Interglacial cycle by single particle inductively coupled plasma mass spectrometry (Progress in proxy development and interpretation)

• Madeleine Lomax-Vogt (The Ohio State University)

There is virtually no data on nanoparticles or microparticles less than 0.5 µm entrapped in ancient Antarctic ice. Until now, size distributions of aeolian dust particles have been measured over a glacial-interglacial time scale using Coulter Counter and Abakus (that only measure particles larger than about 0.5 µm and provide no chemical information). To date, information on the elemental chemical composition of particles in ice cores has come almost entirely from bulk measurement by ICP-MS following acid digestion, making it challenging to differentiate whether the bulk composition is primarily linked to larger micro-particles or many smaller nanoparticles unobserved by other analysis methods. Electron microscopy with Energy Dispersive X-ray Spectrometry can measure the size and elemental chemical composition of individual micro- and nano-particles but typically requires many hours to measure enough particles to obtain a statistically significant sample. We used Single Particle Inductively Coupled Plasma Time of Flight Mass Spectrometry (spICP-TOFMS) to measure (in a few minutes) a complete elemental mass spectrum for each of *thousands of individual aeolian dust particles* entrapped in a horizontal ice core from the Taylor Glacier (East Antarctica) including the last glacial-interglacial transition (9,000-44,000 years BP). This allowed us to calculate particle number concentration from the number of detected particles and to crudely estimate nano- and micro-particle size from the total mass of detected elements and estimated density. The capabilities and limitations of spICP-TOFMS will be discussed. The elemental composition of individual aeolian particles obtained by spICP-TOFMS is essential to infer their mineralogy by comparing ratios of detected elements in each individual particle to those in minerals with known chemical composition.Ultimately, identifying minerals in aeolian dust particles will improve simulations of the radiative forcing of Earth’s past climate by incorporating the optical properties of different minerals into global climate models. Furthermore, understanding past particle mineral composition, number concentration, and size distribution (especially for particles < 0.5 µm in diameter) is a critical piece in reconstructing environmental changes at the source of emission (tracing volcanic emissions and crustal sources from different continents) over the glacial-interglacial timescale.

Variable atmospheric iron solubility over the past 54,000 years and possible implications for atmosphere-ocean carbon cycling (Progress in proxy development and interpretation)

• Aaron Chesler (School of Earth and Climate Sciences and the Climate Change Institute, University of Maine)

Deposition of aerosol Fe in the Southern Ocean during the last glacial period (LGP) has been linked to the coupled atmosphere-ocean carbon cycle via changes in the efficiency of the biological pump. Most Antarctic ice core studies that have explored this relationship either a) use the assumption that Fe concentrations measured at pH < 2 reflect biologically-relevant Fe (i.e., phytoplankton-accessible Fe is a constant proportion of acid-reactive Fe concentrations), or b) directly estimate biologically-relevant Fe using oceanographically defined, weak acid-leach approaches (e.g., pH = 4.5 – 5.2). However, changes in modern Antarctic Fe solubility percentage (biologically relevant Fe/total Fe * 100; Fe%) appear to be coupled to atmospheric conditions (i.e., relative humidity, photoreduction) and source region minerology, indicating that the assumption of constant fractional proportions are misleading. Here we present the first record of biologically-relevant (buffer to pH 5 using ammonia acetic acid) and total (HF-HNO3 digestion) Fe concentrations and Fe% records spanning 50 – 6 ka from the South Pole ice core (SPICEcore; SPC14). We use a combination of high- and low-resolution samples (866 high-resolution samples; ~245 years per sample; and 41 low-resolution samples; ~490 years per sample in the LGP, respectively), and compare the fractional Fe concentrations with corresponding particle concentration, size, and shape, as well as SPC14 δ18O and insolation variability. High resolution biologically-relevant Fe reaches a maximum concentration of 1.73 µg L-1 at ~17.5 ka while maximum total Fe concentration is 81.8 µg L-1 at ~25.9 ka. Both records are significantly related to particle concentration (biologically relevant, r = 0.7, p < 0.01; total, r = 0.85, p < 0.01). However, while both fractional concentrations increase in the LGP, Fe% values reach a minimum value of 0.53% at ~25.6 ka and maximum value of 14.3% at 7.1 ka. Fe% has a significant negative correlation with particle concentration (r = -0.70, p < 0.01) and a weak relationship with particle size (r = 0.28, p = 0.05). Fe% has a positive correlation to SPC14 δ18O (r = 0.73, p < 0.01) and a negative log scale correlation with Fe total concentrations (r = -0.83, p < 0.01). SPC14 Fe% record has a negative correlation to insolation at 45°S and 90°S (r = -0.65, p < 0.01; r = -0.68, p < 0.01, respectively). We explore the idea that the proportion of biologically relevant Fe may be controlled by a combination of atmospheric water conditions (i.e., relative humidity) and activation of local dust source regions in Antarctica. Our findings suggest that direct measurements of biologically relevant Fe concentrations are needed to assess past relationships between aerosol deposition and potential ocean ecosystem impacts.

The Beyond EPICA – oldest ice core drilling operation (Advances in drilling engineering and borehole observations)

• Frank Wilhelms (Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung; Geoscience Center – University of Göttingen, Göttingen, Germany)

Beyond EPICA – oldest ice core drilling group (presented by Frank Wilhelms) O. Alemany, S. Banfi,, C. Barbante, G. Bianchi Fasani, G. Boeckmann, B. Broy, D. Dahl-Jensen, R. Duphil, H. Fischer, S. Bo Hansen, M. Hüther, I. Koldtoft, R. Mulvaney, A. Patoir, L. Piard, T. Popp, P. Possenti, K. Poupon, J. Rix, B. Seth, J. Vaele, C. Venier, C. Wesche, J. Westhoff, F. Wilhelms Beyond EPICA – oldest ice core is a joint response of twelve partners from ten European nations towards the IPICS oldest ice challenge with further support by a Horizon 2020 European Commission Research and Innovation Action. An extensive pre-site survey, suggesting an age of about one-and-a-half million years and a resolution of at least about ten thousand years per metre, identified a drill site at Little Dome C (LDC, 75.29917°S, 122.44516°E) 40 km south-west of the French-Italian Concordia station in accord with the objectives set out for an oldest ice drilling site. During the 2019/20 field season we started the construction of a drilling camp at LDC and built a drill shelter and housing tents. During the 2021/22 field season the infrastructure was complemented by the installation of the drill area, a 120 m casing into the pilot hole and the core storage area. For the up-coming seasons, we plan to drill to bedrock (2800 m) and to replicate the lowermost section, which is older than 700 kyr. The harsh environmental conditions due to high altitude and low night time temperatures limit the daily operable time and the length of the field season. Besides the established EGRIP drill system, we will deploy newly built components with up to 4.5 m core barrel length, overworked details and new electronics, with up to about triple the power by rising the electric tension to 750 V in a shielded cable and communication over the cable with wireline modems at about 30 kBaud data rate. For the up to 13.8 m long drill, we extended the tower and opened the roof of the drill tent. The new components are compatible with the existing EGRIP equipment, which incorporates optimized redundancy and increases the reliability. We will report on the installation of the drilling camp at LDC and the planned implementation of the drilling operation to bedrock during the upcoming field seasons.

Restarting EastGRIP (Advances in drilling engineering and borehole observations)

• Grant Boeckmann (University of Copenhagen)

The objective of the East Greenland Ice-core Project (EastGRIP) is to retrieve a continuous ice core from Northeast Greenland Ice Stream. The core will reveal new and fundamental information about the ice stream that will be used to further our understanding of how ice streams will contribute to future sea-level change. EastGRIP camp opened in 2015 and has been operational every summer consecutively from 2015 to 2019. The impacts of the COVID19 pandemic forced the station to be unmanned for 2 years, 2020 and 2021. The duration of the camp closure has added new considerations and challenges that need to be mitigated before drilling can continue. This talk will present the specific considerations, mitigations, and results from the field. The list also includes challenges that are not related to the delay in field work but existed at the end of drilling in 2019. Anticipated challenges/mitigations include but are not limited to: **Challenge.........................................................Mitigration** Drill and Science Trence Closure.....................Contingency Time Built into Field Plan Shifting of Drill Equipment..............................Time dedicated to Reposition Winch & Tower High Core Break Tension..................................New Winch Motor & Controller. Shift in Personnel/Loss of Knowledge..............Lean on Int. Collaboration and Sharing Knowledge Drill Cable Thinning.........................................Replace Cable Inclination Correction (not replicate)...............Sensors & Springs

Research on the key parameters of structure and morphology of drill bit for ice core drilling in polar ice sheet (Advances in drilling engineering and borehole observations)

• Xinyu Lv (Institute of Polar Science and Engineering, Jilin University)

Ice core drilling in polar ice sheet is an important technical means to study the process of global climate change, and find paleontological life forms, as well as explore the formation and evolution of ice sheets and the geological evolution of glacial continents. For a long time, various drilling methods have been developed for ice drilling. Until now, some kinds of mechanical drill have been widely used in a variety of polar ice core drilling, such as auger drills, cable-suspended electro-mechanical drills, and so on. The core is formed with cutting the ice in rotation by annular drill head. The working process of ice core drilling is composed with three interrelated steps: ice sheet cutting, ice chips cleaning and ice core obtaining. The structure and morphology of drill bit are the key factors that influence the success of rapid ice core drilling, for example, they have a great impact on the rate of penetration, cutting torque and power consumption but also straightly affect the ice core surface quality, particle size, ice chip shape and quantity. Therefore, according to the urgent need of rapid drilling and coring in polar ice, this study has been made of the key parameters of structure and morphology of drill bit under different working conditions (i.e. depth of ice and temperature, cutting pitch, load on bit, rotation speed, etc.). During the research, the stress state and characteristic of ice and cutters were firstly analyzed. After a test-bed was established, the experiment of ice cutting had been carried out by different drill bit under different ice temperature, drilling pressure and turning speed. Then those influence laws were gained, and the mechanism of ice cutting was revealed. Finally, the optimal structure and morphology of drill bit and cutters were determined under the different working conditions.

Consistent histories of anthropogenic Western European air pollution preserved in different Alpine ice cores (High-alpine ice cores)

• Anja Eichler (PSI - Paul Scherrer Institut)

Individual high-Alpine ice cores have been proven to contain a well-preserved history of past anthropogenic air pollution in Western Europe. The question, how representative one ice core is with respect to the reconstruction of atmospheric composition in a given source region, has not been addressed so far. Here, we present the first study systematically comparing long-term ice-core records (AD 1750-2015) of various anthropogenic related compounds, such as major inorganic aerosol species, black carbon (BC), and trace species obtained from four high-Alpine sites located in the French and Swiss Alps. We observe a consistent timing in anthropogenic changes of BC, Cd, F-, NH4+, NO3-, Pb, and exSO42- (non-dust, non-sea salt SO42-) at all investigated Alpine sites. This is related to common source regions of anthropogenic pollution impacting the four sites including Western European countries surrounding the Alps, i.e. Switzerland, France, Italy, Germany, and Spain. For individual compounds, the newly obtained Alpine ice core composites allow us to precisely time the anthropogenically caused onset of increased pollution levels. BC, exSO42-, Pb, and NH4+ concentrations exceeded pre-industrial levels (AD 1750-1850) already in the 1870s and 1880s, mainly related to emissions from coal combustion and agriculture, respectively. The observed maxima of BC, Cd, F-, Pb, and exSO42- concentrations in the 20th century and a significant decline afterwards, clearly reveal the efficiency of air pollution control measures such as desulphurisation of coal, the introduction of filters and scrubbers in power plants and metal smelters, and the ban of leaded gasoline improving the air quality in Western Europe. In contrast, the composite records of NO3- and NH4+ in the beginning of the 21th century are unprecedented in the context of the past 250 years, indicating that the introduced abatement measures to reduce these pollutants were still not sufficient to have a major effect on recent levels at high altitudes over Western Europe. Only four composite records (BC, F, Pb, exSO42-) of the seven investigated pollutants are in agreement with their modelled deposition or estimated emissions of their precursor species suggesting a large uncertainty in emission estimates. Our results demonstrate that individual ice-core records from different sites in the European Alps provide a spatial representative signal of anthropogenic pollution from Western European countries and are essential to constrain emission or deposition data of air pollutants in this region.

Retrieving paleoclimatic information from the isotopic records of the ice cores drilled on Mt Ortles, Eastern Alps, Italy (High-alpine ice cores)

• Giuliano Dreossi (Institute of Polar Sciences - National Research Council of Italy (ISP-CNR))

While ice cores in the Alps have been drilled for about 50 years, very few attempts to obtain temperature reconstructions from the isotopic records of Alpine cores have been successful. The proximity to a densely populated and developed area, as well as the presence of a dense network of meteorological stations which have been operating for over two centuries, make the Alpine glaciers a unique spot to obtain paleoenvironmental and paleoclimatic information through ice coring. In autumn 2011, four cores were extracted from the accumulation area (3859 m a.s.l.) of the Alto dell’Ortles glacier, in the Eastern Italian Alps: three were drilled down to bedrock to an approximate depth of 75 m. The glacier is currently transitioning from a cold to a temperate state: the first 30 m of firn are characterized by a temperature at the pressure melting point, while the underlying ice is still preserved in a cold state. Carbon-14 determination on Water Insoluble Organic Carbon (WIOC) supported a time scale extending back to about 7000 years before present; this is one of very few ice cores in the Alps covering such a long period. A novel approach was recently used to refine the dating of these cores (Gabrielli et al., CP, in discussion). Here we present a comparison between the delta18O records from core #1, #2 and #3 based on the revised chronology and an instrumental temperature series dating back to 1775 C.E., using different low-pass gaussian filters with increasing sigma values. The linear regression between temperature and isotopic data is characterized by an increasing R2 and an increasing slope when increasing the sigma of the low-pass gaussian filter. The overall agreement between temperature and 18O is robust, with few periods characterized by opposing trends.

Paleoclimate and morphological studies of semi-permanent ice patches from the Northern Rocky Mountains (High-alpine ice cores)

• Nathan Chellman (Desert Research Institute)

Glacial ice cores preserve detailed records of atmospheric aerosols and other chemical proxies that can be used to reconstruct past environmental change. In western North America, however, ice core records are rare and existing records from mountain glaciers span only recent centuries. Thus, paleoclimate reconstructions spanning the Holocene are limited to lake sediment core and speleothem records. High-elevation, semi-permanent ice patches provide the opportunity to develop accurately dated, ice-derived records from these alpine regions to develop a better understanding of Holocene climate variability. While these ice patches have been recognized as valuable archeological archives for decades, they remain relatively unexplored as paleoclimate archives. Here we investigate alpine ice patches as paleoclimate indicators using stable water isotope and ice accretion records developed from ice cores recovered from ice patches in northern Wyoming. Additionally, we examine the morphology and internal structure of these ice patches using detailed aerial imagery and ground penetrating radar. Two ~5.6-m deep ice cores, along with several shorter ice cores, were recovered from two ice patches on the Beartooth Plateau. The ice cores consisted of clean ice units intersected by organic-rich layers that were radiocarbon dated to develop accurate chronologies, with the oldest layers dating to 10,400 and 1,250 cal yr BP. Unlike alpine glaciers, the ice patches show no evidence of internal flow, thus the oldest, deepest ice is preserved. Comparisons to nearby lake sediment and speleothem records suggest that these ice patch water isotope and ice accretion records document Holocene wintertime climate, including a sustained era of relatively mild winter conditions centered at 4,100 cal yr BP followed by an era of cooler and wetter conditions. High-resolution, drone-based aerial imagery and elevation data, combined with surface-based ground penetrating radar surveys, were used to map the ice patch extent and thickness as well as understand the spatial variability of early- and late-season snow cover. These datasets indicate that the ice patches are up to 12 m thick and covered by over 10 m of seasonal snow. Temperature measurements from a thermistor string placed down one of the 5.6 m boreholes will be used to better understand the process of ice accretion and the role of seasonal snow in insulating the ice patches from summer warmth.

Antarctic Peninsula ice core captures Larsen ice shelf surface melt (New ice archives)

• Diana Vladimirova (British Antarctic Survey)

The collapse of the Larsen A and B ice shelves, in 1995 and 2002 AD respectively, has resulted in accelerated mass loss and an increased Antarctic Peninsula contribution to global mean sea-level rise. Understanding the drivers of Antarctic ice shelf collapse is critical in quantifying future predictions of Antarctic mass balance and sea-level rise. We demonstrate that proxies from Palmer Antarctic Peninsula ice core capture Larsen Ice Shelf surface melt. Utilizing proxies, we identify past periods that are likely to have been associated with more extreme and increased frequency of Larsen melt events. Our results suggest that ice shelf melt has occurred in the past, however, considering that that δ18O has been higher since the 1970s than any time throughout the core, our findings suggest that the warming and melt events have become greater in the recent past compared to the past 391 years.

Improved understanding of decadal surface mass balance, recent climate variability, and future ice core sites along coastal West Antarctica using NASA Operation IceBridge airborne snow radar (New ice archives)

• Julia Andreasen (University of Minnesota)

The coastal margin of the West Antarctic Ice Sheet (WAIS) is a dynamic and critical region where ice, ocean, and atmosphere converge. Persistent regional ice loss is of global concern for sea level rise. Despite its importance, direct climate observations along coastal WAIS are extremely limited. Ice core records are largely unavailable along the coast from the Ross Sea to the Amundsen Sea, restricting observational surface mass balance (SMB) constraints to the continent’s inland regions. To address this gap, we utilize NASA Operation IceBridge (OIB) airborne snow radar data to place new observational constraints on coastal WAIS SMB (e.g. snow accumulation). At eleven coastal WAIS ice rises, we investigate observed layering in OIB radargrams to automatically detect bright subsurface reflectors likely representing annual snow layers. We then apply depth-density corrections to calculate layer thicknesses and thus annual accumulation rates. This radar-derived snow accumulation timeseries is then compared to regional reanalysis precipitation and other variables from 1979 to the time of the OIB overflight (2016 for many locations). Analysis at three ice rises located on the western, central, and eastern regions of the WAIS coast demonstrates the range of climate forcings affecting interannual snowfall variability along this dynamic coastline. Radar-derived multi-decadal records of interannual snow accumulation variability at these locations are integral to improving the accuracy of ice sheet surface mass balance calculations in the vicinity of ice shelves, validating climate reanalysis products, and exploring fundamental climate variability, trends, and extremes. Spatiotemporal correlations between multi-decadal radar snow accumulation time series and reanalysis variables (e.g., geopotential height, temperature, etc.) will provide new constraints on the climate processes driving surface mass balance patterns and trends along the poorly-sampled WAIS coast. Additionally, this data may provide a better understanding of regional ocean-atmosphere influences on snowfall, particularly at future ice core locations on coastal ice rises. Extracting information about this region through OIB and reanalysis data is the first step in better understanding the WAIS coast and its recent climate history. Future ice cores at several of these ice rises would expand spatial and temporal climate perspective beyond what can be gained from snow accumulation alone–adding insight into past fluctuations in temperature, sea ice conditions, winds, atmospheric river frequency and intensity, and more.

Sub-Antarctic ice core records - potential sites for deep ice core drilling (New ice archives)

• Liz Thomas (British Antarctic Survey)

The sub-Antarctic is one of the most data-sparse regions on earth. Several glaciated Antarctic and sub-Antarctic islands appear suitable for deep ice core drilling; however, little is known about their glaciology or vulnerability to atmospheric warming. Here we present stable water isotope and geochemical records from four shallow ice (firn) cores (14 to 24 m), drilled as part of the Antarctic Circumnavigation Expedition (ACE), together with complementary ground-penetrating radar (GPR). The cores were drilled at locations on Bouvet Island (54∘25′19′′ S, 03∘23′27′′ E) in the South Atlantic, Peter I Island (68∘51′05′′ S, 90∘30′35′′ W) in the Bellingshausen Sea, Mount Siple (73∘43′ S, 126∘66′ W) on the Amundsen Sea coast, West Antarctica, and Cape Hurley, on the eastern side of the Mertz Glacier (67∘33′ S, 145∘18′ E), East Antarctica. All cores have been annual layer counted and compared with reanalysis data and satellite observations. Despite evidence of surface melting, the records reliably capture local and regional changes in surface temperature, sea ice and atmospheric circulation over the past 20-30 years. The GPR profiles indicate uniform internal structure and, except for Bouvet Island, indicate ice thicknesses more than ~60 m. Thus, demonstrating the potential for future deep ice core drilling to retrieve centennial-scale climate reconstructions from these unique sites.

Seasonal temperatures in West Antarctica during the Holocene (Holocene and last 2000 year climate forcings and variability)

• Tyler Jones (Institute of Arctic and Alpine Research)

Disentangling the drivers of mean annual temperature change in Antarctica requires an understanding of seasonal temperature change. A high-resolution climate record capable of resolving summer and winter seasons could address long-standing questions about the role of orbitally-driven insolation in driving Antarctic mean-annual temperature change. What drives summer and winter climate in West Antarctica? How does this relate to insolation intensity, seasonally integrated insolation, or season duration? Here, we present a continuous record of water isotope ratios from the West Antarctic Ice Sheet (WAIS) Divide ice core that reveals both summer and winter temperature change though the last 11,000 years. We use complex (HadCM3) and intermediate (energy balance) modeling to interpret the records. Observed summer temperatures increased through the early-to-mid Holocene, reached a plateau at 4 to 2 ka, and then decreased to the present. The observed changes are explained primarily by changes in maximum summer insolation. In the early to mid-Holocene, additional summer warming results from the retreat and thinning of the WAIS. The magnitude of observed summer temperature change constrains the lowering of the WAIS surface to less than 100 m since the early Holocene, consistent with geologic records. Importantly, annual mean temperatures cannot be fully explained by orbital forcing and ice sheet elevation change alone; in the early Holocene, large wintertime temperature excursions overwhelm the summer signal. These winter excursions indicate that regional heat transport anomalies, rather than local thermodynamics, can dominate the annual mean.

The Mount Brown South ice core: initial results and current research from a new, high resolution ice core in the Indian Ocean sector of East Antarctica. (Holocene and last 2000 year climate forcings and variability)

• Tessa Vance

High resolution ice core records are crucial to understanding past climate variability in Antarctica and the Southern Ocean. While existing long records tell us about how the climate has changed over centuries to millennia, only ice cores that resolve past climate at annual and ideally seasonal resolution can provide insight into features such as the Southern Annular Mode (SAM), westerly wind variability, sea ice extent and changes in snowfall accumulation rate. Records such as this that span more than a few tens or hundreds of years are rare across Antarctica, and in East Antarctica in particular. For the observation-sparse Indian Ocean sector spanning Enderby Land to East Wilkes Land (~45-160 East), only two annually resolved records >2000 years exist. This includes the Law Dome record, which has been crucial for developing proxy climate records relevant to Australia and the southwest Pacific. In the 2017-18 austral summer, we drilled a new high-resolution ice core record in Wilhelm II Land to complement the emerging and existing proxy records from the Law Dome ice core and to enhance our understanding of past climate variability in the observations sparse Indian Ocean sector. The past climate records now being developed from the new Mount Brown South ice core include trace chemistry analysed by both discrete and continuous flow analysis (CFA), water stable isotopes (discrete and CFA), physical stratigraphy, volcanic tracers (cryptotephra), persistent organic pollutants and snowfall accumulation rates. Interim dating of the record suggests a high annual accumulation rate of 0.3 metres ice equivalent and that the 295 metre record will span around 1200 years to present. Initial atmospheric analysis shows the ice core preserves signals of atmospheric variability from the mid-latitudes of the southern Indian Ocean, as well as signals of large-scale variability such as the El Niño-Southern Oscillation. Work in progress suggests the ice core is subject to episodic moisture intrusions, and likely has a less uniform accumulation regime than the Law Dome ice core to its east. The episodic nature of the accumulation regime increases the complexity of dating the record. However, this complexity is currently being exploited to develop proxies of regional moisture and impurities transport at the seasonal and annual scale, providing insight into synoptic variability in the data sparse southern Indian Ocean and its impacts on meridional moisture transport and ultimately surface mass balance.

Variation of the solar activity over the last 1300 years based on two new 10Be high-resolution records from Dome C and Talos Dome (Antarctica) (Holocene and last 2000 year climate forcings and variability)

• Mélanie Baroni (CEREGE, Aix-Marseille Université, CNRS, IRD, INRAE, Collège de France)

Room: Centre de Congrès Le Régent Documenting the solar activity before the instrumental period is important to improve our knowledge of the solar-dynamo process which generates the cyclically-varying solar magnetic field. Cosmogenic nuclides, such as beryllium-10 (10Be) and carbon-14 (14C), measured in ice cores and tree rings, respectively, provide information on the variation of past solar activity. We present two new 10Be records, covering the last 1300 years, from the Dome C and Talos Dome ice cores drilled in the frame of the VOLSOL and TALDICE projects, respectively. We based the Dome C chronology on the WAIS Divide chronology (WD2014, Sigl et al., Nature, 2015), the Talos Dome chronology is based on a combination of WD2014 and AICC2012 (Veres et al., Clim. Past, 2013, Bazin et al., Clim. Past, 2013). Sub-annual resolution was achieved at the Dome C site through the 1700 10Be measurements that were made on the first 58 m of the ice core, while the 450 samples measured on the first 140 m of the Talos Dome ice core allowed an average resolution of 2.5 - 3 years. The five minima of solar activity (Oort, Wolf, Spörer, Maunder and Dalton) are detected and characterized by a 10Be concentration increase of ca. 20% above average in agreement with previous studies of ice cores drilled at South Pole and Dome Fuji in Antarctica (Bard et al., EPSL, 1997; Horiuchi et al., Quat. Geochrono., 2008) and at NGRIP and Dye3 in Greenland (Berggren et al., GRL, 2009). We propose a reconstruction of the 10Be production signal based on our two new 10Be records which is in a very good agreement with the 14C production obtained from new tree rings records covering the last millennium (Brehm et al., Nat. Geo., 2021). This shows, firstly, that the chronologies of Dome C and Talos Dome are well established, and secondly, that the production signal obtained, independently, from these two cosmogenic nuclides records, is robust. At Dome C, the high resolution allows the detection of the 11-year solar cycle. We will show the importance of applying a correction to 10Be ice core records in order to remove the volcanic disturbance on the 10Be deposition (Baroni et al., JGR Atm., 2019). This correction is based on the relationship between the 10Be and the sulphate concentrations obtained from the exact same samples. We will discuss the persistence of solar cycles throughout the last millennium and the comparison with the 11-yrs solar cycles detected in annual tree-ring 14C data over the same period (Brehm et al., Nat. Geo., 2021).

Abrupt early Holocene changes in ice dynamics inferred from the Skytrain ice core sea salt record (Holocene and last 2000 year climate forcings and variability)

• Mackenzie Grieman (Reed College)

Ice cores drilled at sites closer to the Antarctic coast typically have relatively higher concentrations of salts because they are closer to sea salt sources (i.e., the open ocean, sea ice, and frost flowers). Holocene Antarctic ice core sodium variability has been attributed to changes in sea ice extent. In this study, sea salt was analyzed in the top 436 m (last 12,000 years before 1950) of an ice core from Skytrain Ice Rise, Antarctica (79$^\circ$S, 78$^\circ$W). The Skytrain ice core was drilled as part of the WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial (WACSWAIN) project. Unlike other Antarctic ice core records, Holocene Skytrain ice core sodium increased abruptly by $\sim$40 ppb (a factor of $\sim$2 increase) and became much more variable in the early Holocene from $\sim$8000 to 7500 years BP (before 1950). This atypical sea salt increase could be indicative of a change in regional ice sheet dynamics in the Weddell Sea Sector. This possible change in sea salt sources is further investigated using the pTOMCAT atmospheric model and HYSPLIT air mass back trajectory analysis. The Skytrain ice core Holocene stable water isotope signal ($\delta$$^{18}$O) similarly increased abruptly by $\sim$3‰ from $\sim$8600 to 8300 years before present, about 500 years before the change in sea salt. We interpret these increases in terms of changes in the ice sheet elevation and ice shelf extent. Model reconstructions of the West Antarctic Ice Sheet (WAIS) assume that it retreated monotonically during the Holocene. Our findings support an abrupt regional change in ice dynamics in the early Holocene.

High resolution sulfur isotopes from ice cores: improved estimates of the volcanic forcing of climate (Holocene and last 2000 year climate forcings and variability)

• Andrea Burke (University of St Andrews)

The record of the volcanic forcing of climate over the past 2500 years is reconstructed primarily from sulfate concentrations in ice cores. Of particular interest are stratospheric eruptions, as these afford sulfate aerosols the longest residence time and largest dispersion in the atmosphere, and thus the greatest impact on radiative forcing. Sulfur isotopes can be used to distinguish between stratospheric and tropospheric volcanic sulfate in ice cores since stratospheric sulfur aerosols are exposed to UV radiation which imparts a mass independent fractionation (Savarino et al., 2003). Thus, sulfur isotopes in ice cores provide a means to identify stratospheric eruptions and calculate the proportion of sulfate deposited from a volcanic event that came the stratosphere, allowing us to refine the historic record of explosive volcanism and its forcing of climate. Here we present high-resolution (sub-annual) sulfur isotope data from both Greenland and Antarctica across a suite of unidentified eruptions from the anomalously cold decades of the 530s, 1450s and 1600s to investigate the climate forcing potential of these eruptions. Savarino, J., Romero, A., Cole Dai, J., Bekki, S., & Thiemens, M. H. (2003). UV induced mass‐independent sulfur isotope fractionation in stratospheric volcanic sulfate. Geophysical Research Letters, 30(21). http://doi.org/10.1029/2003GL018134

Diatoms in ice cores for reconstructing past wind variability in the Southern Hemisphere Westerly Wind belt (Holocene and last 2000 year climate forcings and variability)

• Dieter Tetzner (British Antarctic Survey)

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. Here, we present multiple records of diatoms preserved in a set of ice cores retrieved from the southern Antarctic Peninsula and the Ellsworth Land region. The diatom abundances and species assemblages from these ice cores represent the regional variability in wind strength and atmospheric circulation patterns. We use this novel proxy to produce an annual-to-quinquennial reconstruction of winds in the Pacific sector of the Southern Hemisphere Westerly Wind belt over the last 400 years. This wind reconstruction allows exploring the link between the recent increase in wind strength, greenhouse gases and ozone depletion in the atmosphere.

Timing of Holocene volcanic eruptions and their radiative aerosol forcing (Holocene and last 2000 year climate forcings and variability)

• Michael Sigl (OCCR/University of Bern)

Room: Centre de Congrès Le Régent Volcanic eruptions play a dominant role in driving climate, in ways beyond the established short-term influence on surface air temperatures. In order to mitigate and adapt to the climate effects of future large volcanic eruptions we need to better quantify the risk of these eruptions including (a) the probability of their occurrence and (b) their expected climatic impact. The observational record of the timing of volcanic eruptions, their locations, magnitudes of sulphate aerosol injection is incomplete which limits our understanding of the sensitivity of the Earth system to volcanism and the vulnerability of social and economic systems to the climate impact of past and future eruptions. Here we use an array of synchronized, accurately dated, high-resolution ice-core aerosol records from Greenland and Antarctica to reconstruct the timing, sulphur injections and source locations of 850 volcanic eruptions occurring during the Holocene (i.e., the past 11,500 years) to answer the questions: 1) What is the likelihood of a stratospheric sulphur injection as large as that from the colossal eruption of Tambora in 1815 to occur somewhere on the globe within the next 50 years? 2) How has subaerial volcanic activity changed in space and time throughout the Holocene? 3) How did major eruptions affect global climate and humans through time? We demonstrate in case studies (e.g., McConnell et al., 2020; Pearson et al., 2022) how novel geochemical tools (e.g., sulphur isotopes, cryptotephra) allow to constrain source parameters of the eruptions (location, plume injection height, stratospheric vs. tropospheric aerosol formation) that control their effects on climate, and propose exact dates for the largest eruptions of the Holocene with the help of ultra-long tree-ring chronologies. Finally, we generate global-scale, space-and-time resolved stratospheric aerosol properties for climate models (HolVol1.0; Sigl et al., 2021; Abbott et al., 2021) to simulate the volcanic influence on Holocene climate evolution and examine to which extent simulated climate responses agree with those inferred from proxy records. References: Abbott, P. M. et al. Volcanic climate forcing preceding the inception of the Younger Dryas: Implications for tracing the Laacher See eruption. *Quaternary Sci Rev* 274, (2021). McConnell, J. R. et al. Extreme climate after massive eruption of Alaska's Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom. *P Natl Acad Sci USA* 117, 15443-15449, doi:10.1073/pnas.2002722117 (2020). Pearson, C. et al. Geochemical ice-core constraints on the timing and climatic impact of Aniakchak II (1628 BCE) and Thera (Minoan) volcanic eruptions. *P Natl Acad Sci USA Nexus* (2022). Sigl, M., et al. Volcanic stratospheric sulfur injections and aerosol optical depth during the Holocene (past 11,500 years) from a bipolar ice core array. *Earth Syst. Sci. Data* 2022, 1-45, doi:10.5194/essd-2021-422 (2022).

Variations in tropospheric oxidant levels during the Holocene and late Glacial (Holocene and last 2000 year climate forcings and variability)

• Sergey Gromov (Max Planck Institute for Chemistry)

Room: Centre de Congrès Le Régent The reconstruction of past changes in the temperature, composition and dynamics of the atmosphere is of great interest, also with regard to predictions of future climate change. There are discrepancies between models and climate proxies on temperature variations over the Holocene, and also the temporal evolution of key greenhouse gases like CH4 is not understood. Variations of atmospheric oxidants in the past atmosphere are particularly poorly constrained because they are not stable in any paleo-climate archive. We present measurements of the clumped isotope composition of atmospheric O2 (abundance of 18O18O denoted by ∆36) extracted from a Greenland ice core covering the Holocene and late glacial periods, and in the present atmosphere. The data provide new constraints on upper tropospheric temperatures and oxidant levels in the past. In the glacial period ∆36 was higher than in the Late Holocene, because of the lower temperatures and oxidant levels during this period. ∆36 shows pronounced millennial-scale variability over the Holocene, with Mid Holocene ∆36 values being lower than in the Late Holocene, and even lower compared to present-day air. Simulations of ∆36 in the 3D atmospheric chemistry model EMAC are used to investigate the contribution of changing temperatures, oxidant precursors and atmospheric transport on oxidant levels in the different climate states. Our data suggest a maximum in oxidant levels during the Mid Holocene, consistent with the well-established Mid Holocene CH4 minimum. However, an increase in oxidants alone cannot account for the unexpectedly low ∆36 values in the Mid Holocene. Middle and upper tropospheric temperatures must also have been warmer than today. The ∆36 data imply that a number of key atmospheric processes must have varied considerably over the Holocene, which is generally considered a climatically stable period.

Deciphering the chemical stratigraphy with LA-ICP-MS for the “Oldest Ice Challenge” – Where do we go from here? (The Oldest Ice challenge, and the preservation of climatic signals in the deepest ice)

• Pascal Bohleber (Ca'Foscari University of Venice)

The success of the “Oldest Ice Challenge” crucially depends on extracting paleoclimatic information at sufficiently high resolution while avoiding misinterpretation by post-depositional signal alteration. The record of chemical impurities provides an important set of paleoclimate proxies, but its preservation is challenged especially in deep ice e.g., by crystal growth, diffusion and chemical reactions. Ice core impurity analysis with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has a two-fold value in this context. Combining micro-destructive sampling with micron-scale resolution, LA-ICP-MS can be used for stratigraphic analysis with line profiles along the main core axis and for mapping the spatial impurity distribution via 2D imaging. A combined approach promises to extract stratigraphic signals while considering constraints from post-depositional processes. However, current limitations arise primarily from the need for an improved signal interpretation in concert with a call for further technical innovation. Here we present first results that already highlight a way to fully capitalizing on the high-resolution impurity analysis once the “Oldest Ice” has been retrieved. The 2D chemical images afforded by LA-ICP-MS can provide the “ground truth” for assessing the spatial significance of single line profiles and have already revealed a close association with grain boundaries for impurity species such as Na and Mg. In order to trace the development of this impurity-grain boundary association, we show how the imaging can in principle also be extended to firn samples, potentially opening a door for investigating signal formation with the imaging approach. In some new images recorded from Greenland and Antarctic ice, widely dispersed aggregates of particles become visible for primarily dust-related species such as Al and Ti. Combining image segmentation and elemental ratios allows us to investigate both the localization and composition of terrestrial dust, showing consistency in a preliminary validation against cryo-Raman spectroscopy. Aiming at a more detailed glacio-chemical characterization in the future, we show how the combination of laser ablation with “time of flight” mass spectrometry greatly expands the range of elements retrieved during a single image acquisition, even when analyzing highly pure Antarctic ice. A further technical innovation which is yet to be realized would be a large cryocell with imaging capabilities to avoid the destructive preparation of cm-sized ice strips and to make decimeter-sized images feasible. Proceeding along this roadmap may ultimately provide a new tool for deciphering the deep ice chemical stratigraphy – in the “Oldest Ice” and elsewhere.

New estimates of chemical diffusion rates in the EPICA Dome C ice core (The Oldest Ice challenge, and the preservation of climatic signals in the deepest ice)

• Rachael Rhodes (University of Cambridge)

In order to extract climatically relevant chemical signals from the deepest, oldest Antarctic ice, we must first understand the degree to which chemical ions diffuse within solid ice. Volcanic sulfate peaks are the ideal target for such an investigation because they are high amplitude, short duration (~3 years) events with a quasi-uniform structure. Here we present analysis of the EPICA Dome C sulfate record over the last 600 kyr, extending previous work which focused only on the Holocene. We first identify volcanic peaks and isolate them from the non-sea salt sulfate background to reveal the effects of diffusion: amplitude damping and broadening of peaks in the time domain with increasing depth/age. Sulfate peak shape is also altered by the thinning of ice layers with depth. Both processes must be simulated in order to derive effective diffusion rates. This is achieved by running a forward model to diffuse idealised sulfate peaks at different rates while also accounting for ice thinning. Our simulations suggest effective diffusion rates of sulfate ions on the order of 10-8 m2 yr-1 in the Holocene ice, in agreement with previous work. The effective diffusion rates of the Holocene are higher than for any other time interval. Furthermore, there doesn’t appear to be a significant difference in time-averaged diffusion rates between glacial and interglacial periods despite variations in ice chemistry. Implications for the preservation of volcanic sulfate peaks, and also climatic signals, in the upcoming Beyond EPICA ice core will be discussed.

The Center for Oldest Ice Exploration (COLDEX) and Opportunities for International Collaboration (The Oldest Ice challenge, and the preservation of climatic signals in the deepest ice)

• Edward Brook (Oregon State University)

Room: Centre de Congrès Le Régent The Center for Oldest Ice Exploration (COLDEX) is a US NSF Science and Technology Center (STC) funded in 2021 and a new US contribution to IPICS Oldest Ice Goals. COLDEX is a multi-institution collaboration to find and analyze the oldest ice preserved in the Antarctic ice sheet. COLDEX is headquartered in the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University. COLDEX research goals are underpinned by several decades of international research on cores drilled through the polar ice sheets. This work has revealed how the composition of Earth’s atmosphere and climate are linked on many time scales, from ice-age cycles to abrupt climate changes, and provides the groundwork for our understanding of human impacts on climate and the environment. However, the existing ice core data do not extend far enough back in time to reveal how the Earth system behaves under warmer than present conditions. Reaching these time periods is critical for understanding our future, and is also a significant challenge, requiring a coordinated approach and sustained collaboration of numerous research groups. COLDEX also addresses challenges in making polar science more equitable for people from diverse backgrounds and perspectives, and in making scientific knowledge from our work relevant, useful, and accessible to educators, policymakers, students, and a broad range of communities. COLDEX activities include searching for a site for a 1.5 Ma ice core in the East Antarctic interior with new airborne data and modelling, development of existing and new ice margin sections where ice as old as 2.6 Ma has already been recovered, new rapid access tools and radar technologies, a broad program of ice core analysis, research on the social network of the center and its links to stakeholders and the public, education and professional development focused on ice core science and scientists and a focus on improving equity, diversity and inclusivity in our field. COLDEX welcomes new partners, community involvement and international collaboration, including sharing information about site selection, developing new blue ice archives, deciphering the stratigraphy of complex old ice sections, new analytical developments, rapid access technologies and professional development programs for early career researchers.

Assessing changes in the carbon cycle during Dansgaard-Oeschger events from cosmogenic radionuclides (Biogeochemical Cycles in the Earth system – data and models)

• Florian Adolphi (Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany)

Room: Centre de Congrès Le Régent The last glacial cycle was characterized by strong millennial-scale climate variability. Rapid transitions between cold stadial and warm interstadial conditions (known as Dansgaard-Oeschger Events) had widespread effects on the Earth system. Changes in the carbon cycle accompanied these climate swings, but the underlying processes remain unresolved. Radiocarbon (14C) provides a tool to assess some of the proposed mechanisms. Combining measured atmospheric 14C with independent 14C-production rate estimates (e.g., from 10Be or geomagnetic field records) produces a residual signal that informs about past carbon cycle changes. We will present new data from New Zealand subfossil kauri-trees that provide a high-precision, high-resolution and truly atmospheric 14C-record for large parts of MIS-3. In combination with ice core 10Be and geomagnetic field data, we investigate the imprint of Dansgaard-Oeschger events on the carbon cycle and discuss potential mechanisms using carbon-cycle modelling.

Sulfur isotope compositions as a tracer of marine biogenic and terrestrial sulfate in the Skytrain Ice Core from 0 - 125 ka BP (Biogeochemical Cycles in the Earth system – data and models)

• Helena Pryer (University of Cambridge, UK)

Room: Centre de Congrès Le Régent Analysis of the sulfur (S) isotope composition of ice cores represents a novel method to trace the variable sources of sulfate transported to Antarctica. Non-sea-salt (nss) sulfate is thought to be dominated by marine biogenic inputs, and relatively stable fluxes in the EPICA Dome C core have been interpreted to show that marine productivity around Antarctica remained constant between glacial and interglacial periods. However, recent work from Dome Fuji highlighted the possibility of a substantial terrestrial sulfate component during glacial periods. Building on this research, we present the first S isotope dataset covering an entire glacial cycle (0-125 ka BP) from the Skytrain Ice Rise in West Antarctica. We find that S isotope compositions are significantly depleted in the heavier isotope and show more variability during the Last Glacial compared to the Holocene or Last Interglacial period. The S isotope values also display linear relationships with water isotope ratios and nss-magnesium concentrations in the ice, suggesting a climate-driven increase in the flux of isotopically-light terrestrial sulfate during the Last Glacial. Given the relatively stable nss-sulfate concentrations in the Skytrain Ice Core, this increase in terrestrial sulfate potentially implies a significant reduction in the flux of marine biogenic sulfate, especially during the Last Glacial Maximum and Marine Isotope Stage 4. These findings provide new insights into the key controls on the sources of sulfate delivered to the West Antarctic Ice Sheet and a better understanding of how the S cycle interacts with the climate system over glacial-interglacial timescales.

A preliminary record of changes in Southern Hemisphere atmospheric OH abundance from 14CO in glacial ice (Law Dome, Antarctica, 1870 AD to present) (Biogeochemical Cycles in the Earth system – data and models)

• Peter Neff (University of Minnesota)

Room: Centre de Congrès Le Régent Hydroxyl, OH, is the main tropospheric oxidant and determines the lifetime of methane and most other trace gases in the atmosphere, thereby controlling the amount of greenhouse warming produced by these gases. Changes in OH concentration ([OH]) in response to large changes in reactive trace gas emissions (which may occur in the future) are uncertain. Measurements of 14C-containing carbon monoxide (14CO) and other tracers such as methyl chloroform over the last ≈25 years have been successfully used to monitor changes in average [OH], but there are no observational constraints on [OH] further back in time. Reconstructions of 14CO from ice cores at sites with very high snow accumulation rates can provide such constraints, as rapid snow burial limits in-situ production of 14CO by cosmic rays directly in the ice. A joint US and Australian team sampled and measured firn air and ice at Law Dome, Antarctica (2018-19 season, site DE08-OH, 1.2 m a-1 ice-equivalent snow accumulation), to a maximum depth of 240 m. Trapped air was extracted from the ice using an on-site large-volume ice melting system. Preliminary comparisons of methane measured in the samples to existing ice core records and atmospheric measurements suggest ice core air sample ages spanning from the 1870s to the early 2000s. Firn-air samples from the snow surface to 81 m depth capture air from the early 2000s to present. Analyses of [CO] and halocarbons in the samples show a relatively low and stable procedural CO blank and demonstrate that the samples are unaffected by ambient air inclusion. 14CO analyses in these firn and ice core air samples have been successfully completed. Corrections for in-situ 14CO production, validated against direct atmospheric measurements for the more recent samples, have allowed us to develop a preliminary 14CO history. This history will be interpreted with the aid of the GEOS-Chem chemistry-transport model to place the first observational constraints on the variability of Southern Hemisphere [OH] since ≈1870 AD.

Story-telling the last glacial inception: CO2, AMOC weakening and orbital triggers (Glacial / interglacial dynamics, interglacials, and sea level)

• Lucas Silva (Climate and Environmental Physics, University of Bern)

The transition from glacial to interglacial periods has been deeply studied, largely relying on the last deglaciation and its associated availability of a vast array of precisely dated high-resolution climate indicators. Its counterpart transition, glacial inception, has received much less attention, mostly due to the lack of a sufficient number of well-resolved proxies covering the last glacial inception at the end of the last interglacial (129–116 ka). As a consequence, there are significant knowledge gaps concerning overall driving mechanisms, lead/lag relationships and the role of each orbital parameter. Specifically, a long-standing issue has been why does CO2 lag Antarctic Temperature by five thousand years during glacial inception, in sharp contrast to an otherwise strong correlation throughout the 800 ka record. Here, using a dry-extraction technique, we present a reconstruction of past atmospheric CO2 concentrations from the EDC ice core for the period 135–105 ka at centennial resolution. Focusing on the glacial inception, we suggest that the CO2 drawdown at the end of the last interglacial was triggered by a weakening of the Atlantic Meridional Overturning Circulation (AMOC). After cascading events in the Northern Hemisphere that led to an unusually mild climate amidst growing ice sheets, a sudden reduction in North Atlantic Deep Water formation possibly triggered the establishment of a glacial AMOC. The northward expansion of Antarctic Bottom Water that likely followed may have helped create the necessary conditions for enhanced deep ocean storage of CO2. A coeval quick expansion of the Northern ice sheets and associated positive feedback mechanisms possibly helped to sustain the CO2 decrease. We suggest that the establishment of a deep ocean reservoir was a necessary condition for CO2 to drop from the long and stable plateau that we observe during the last interglacial, ultimately explaining the lag between CO2 and Antarctic temperature.

Did the Ronne Ice Shelf and West Antarctic Ice Sheet retreat in the last interglacial? (Glacial / interglacial dynamics, interglacials, and sea level)

• Eric Wolff (University of Cambridge, UK)

The largest uncertainty in long-term sea level projections (as shown by the recent IPCC reports) is the fate and stability of the West Antarctic Ice Sheet (WAIS). Models include different physics, and range widely in their predictions. The timescales for ice sheets are long, so observational data from the recent past provide little constraint, and we need to look at the past behaviour of WAIS. The last interglacial (LIG) is particularly relevant because Antarctic temperature was higher than present and some models predict the complete loss of WAIS and of the large adjacent ice shelves adjacent. Within the WACSWAIN (WArm Climate Stability of the West Antarctic ice sheet in the last INterglacial) project, we drilled (in 2019) 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. Our analyses show that the core is continuous through the last glacial period, and most of the last interglacial. There are flow disturbances at the base of the core, with the very warmest part of the LIG missing, although older ice lies below it. There is also a small flow disturbance at the top of the LIG. However our analyses, including discrete measurements of CH4 and δ18Oatm, show that the ice is continuous between 106 and 126 ka, allowing us to interpret what happened during the bulk of the LIG. In the LIG, the record of marine ions in the ice suggest that the Ronne Ice Shelf was present at least from 126 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. The ice shelf may have partly retreated later in the LIG but was never absent. We see somewhat higher water isotope ratios in the LIG than the Holocene, possibly consistent with drawdown of WAIS in sectors other than the Weddell region.

Sensitivity of climate to the West Antarctic Ice Sheet extent in past interglacials (Glacial / interglacial dynamics, interglacials, and sea level)

• Elizabeth Keller (GNS Science)

Earth system models often have difficulty reproducing the warmth of past interglacials, particularly at the polar regions. CO2 concentrations and orbital configurations do not by themselves reproduce regional climate for interglacial periods such as e.g. MIS 5e, MIS 11, and MIS 31. The West Antarctic ice sheet (WAIS) extent is a key source of uncertainty. Here we use an intermediate complexity model, UVic ESCM, to simulate three different past interglacials (MIS 5e / 127 ka, MIS 31 / 1.07 Ma, mid-Pliocene warm period / 3.2 Ma) with four different ice sheet configurations (present-day, WAIS absent, and two intermediate states) and compare to paleoclimate ice core and sediment proxy data to evaluate their consistency. These experiments offer insight into the sensitivity of climate to the interactions between CO2 concentrations, orbital forcing, and Antarctic ice sheet extent. Model outputs will be used as boundary conditions for Southern Ocean high-resolution regional ocean model simulations to explore the impact of differences in circulation around the Antarctic continent.

Characterizing the source of mineral dust in Ice from the Allan Hills, East Antarctica during the Last Interglacial Period (Glacial / interglacial dynamics, interglacials, and sea level)

• Austin Carter (Scripps Institution of Oceanography)

Room: Centre de Congrès Le Régent Variations in the source of mineral dust entrained in ice provide insight into past Earth surface conditions and atmospheric transport pathways. The concentration, composition, and transport of mineral dust is dependent on the climate-regime, with markedly higher dust fluxes during glacial periods compared to interglacial periods. Dust deposited on the East Antarctic Ice Sheet during glacial periods uniformly points to sources in southern South America; however, characterizing dust provenance during the last interglacial period (~129-115 ka) is analytically challenging due to low quantities of material in the ice. Previous work using high-volume, peripheral East Antarctic ice from Taylor Glacier has indicated that dust deposition during the last interglacial period was distinct with a young volcanic composition characteristic of the West Antarctic Rift System. To further constrain source region, this study probes the mineral dust record contained within high-volume and high-temporally sampled ice from the Allan Hills Blue Ice Area, another peripheral East Antarctic site. Samples span the end of the penultimate glacial period (~145-136 ka), subsequent deglaciation (~136-129 ka), and the last interglacial period with a temporal resolution of ~one thousand years per sample. This peripheral ice from the Ross Sea sector may reflect a unique climate history due to its proximity to both the Southern Ocean and the West Antarctic Ice Sheet. Here we present initial (1) trace and major element concentrations, (2) grain size distribution and concentrations, and (3) particle morphology and elemental mapping using an inductively coupled plasma mass spectrometer, a Coulter Counter, and a scanning electron microscope, respectively. Future work will include measuring the strontium, neodymium, and lead isotopic compositions of the dust and conducting Earth System model simulations to probe the sensitivity of dust transport to ice sheet extent. This study helps provide insight into the source and transport mechanism of dust to the peripheral portion of the East Antarctic Ice Sheet across the transition from the penultimate glacial maximum to the last interglacial period.