Displays

There is a general consensus in the scientific community that Greenlandic ice cores do not allow for reconstruction of past atmospheric carbon dioxide (CO₂) concentrations due to artifacts likely caused by in-situ production of excess CO₂ from both organic and inorganic carbon compounds within the ice archive. In the case of Antarctic ice cores such processes are thought to be insignificant, making Antarctic ice cores the only direct archive of past atmospheric CO₂ concentrations beyond modern observations. However, with increasing numbers of high-precision CO₂ reconstructions from multiple Antarctic ice cores – mostly covering specific time intervals during the last 130 ka – it has become evident that offsets in CO₂ are not unique to Greenland ice cores. Over the last decade evidence is mounting that small systematic offsets of typically 2-10 ppm exist among different Antarctic CO₂ records covering the same time period. Because CO₂ is well-mixed within the atmosphere different ice cores should agree with each other within their measurement uncertainty, independent of the ice core drilling site. The unambiguous detection of such offsets between different ice cores is only possible in the absence of strong atmospheric trends, such as during interglacial periods. Here, we take a closer look at CO₂ offsets among records available for the Holocene and the Last Interglacial and investigate their long-term evolution. We present unpublished CO₂ data from multiple ice cores, including Talos Dome and EPICA Dome C, and discuss possible offset producing mechanisms. We speculate that Antarctic ice cores are also subject to slowly progressing in-situ production of CO₂ over many millennia, similar to Greenlandic ice cores, however to a much smaller extent and limited to about 10 ppm. We further note a tendency for higher offsets in the case of high accumulation sites. Despite all possible mechanisms that have the potential to alter CO₂ concentrations within the ice archive, we highlight that the overall integrity of the ice core-based CO₂ reconstruction is not in question, as all records generally share the same common signal. However, the absolute CO₂ levels should be interpreted with care and in light of such potential offsets.

How to cite: Nehrbass-Ahles, C., Schmitt, J., Bereiter, B., Eggleston, S., Mächler, L., Silva, L., Stocker, T., and Fischer, H.: Offsets among ice core derived CO2 reconstructions covering the Holocene and Last Interglacial, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21734, https://doi.org/10.5194/egusphere-egu2020-21734, 2020.

Methane (CH₄) is the third most powerful greenhouse gas. However, its warming potential is two orders of magnitude higher than of carbon dioxide and its residence time in the atmosphere is only 9.1 ± 0.9 years. It makes CH₄ a good indicator of rapid climate variations both under natural conditions and due to the anthropogenic influence.

The Elbrus ice core was drilled in 2009 on the Western Plato (43°20’53.9’’N, 42°25’36.0’’E) at elevation 5115 m a.s.l. It is 182 m long and is dated back to 280 ± 400 CE (Common Era). The CH₄ mixing ratios were analyzed using a continuous flow analysis (CFA) system paired with optical-feedback cavity-enhanced absorption spectroscopy. The measurements campaign was organized at Institut des Géosciences de l'Environnement (IGE), Grenoble, France. This is a first high-resolution mid-latitude CH₄ record. The record aims to better constrain the past evolution of mid-latitude methane sources.

Here we present preliminary results of the methane concentration measurements of the Elbrus ice core in high-resolution (CFA CH₄ record). We observe in situ production (max level 2900 ppb) and a baseline. We inspect a potential origin of the multiple spikes in the high-resolution record. Supposedly, either an in-situ production in the dust-rich layers occurred or a gas dissolution in the melt layers took place. However, the possibility of in-situ production during continuous gas extraction has to be further studied. The identified melt layers can serve as an indicator of interrupted stable water isotopic signal and may be supportive in the regional temperature reconstructions based on the Elbrus ice core record. A cleaned off the spikes record is inspected for the natural variability of the CH₄ baseline concentration related to the short-term climate and methane emissions variability.

How to cite: Vladimirova, D., Faïn, X., Ginot, P., Kutuzov, S., and Mikhalenko, V.: Continuous (CFA) CH4 record of the Elbrus ice core, Caucasus (preliminary results), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20586, https://doi.org/10.5194/egusphere-egu2020-20586, 2020.

Ice sheets are reliable archives of atmospheric impurities such as aerosols and gasses of both natural and anthropogenic origin. Impurity records from Greenland ice cores reveal much information about previous atmospheric conditions and long-range transport in the Northern hemisphere going back more than a hundred thousand years.

Here we present the data from the upper 1,411 m from the EGRIP ice core, measuring conductivity, dust, sodium, calcium, ammonium, and nitrate. These records contain information about ocean sources, transport of terrestrial dust, soil and vegetation emissions as well as biomass burning, volcanic eruptions, etc., covering approximately the past 15,000 years. This newly obtained data set is unique as it provides the first high-resolution information about several thousands of years of the mid-Holocene period in Greenland that none of the previous impurity records from the other deep Greenland ice cores had managed to cover before due to brittle ice. This will contribute to further understanding of the atmospheric conditions for the pre-industrial period.

The ammonium record contains peaks significantly higher than the background level. These peaks are caused by biomass burning or forest fires emitting plumes of ammonia large enough so that they can extend to the free troposphere and be efficiently transported all the way to the Greenland ice sheet. Here we present preliminary results of the wild fire frequency covering the entire Holocene, where the wild fires are defined as outliers in the ammonium record of annual means.

How to cite: Jensen, C. M., Erhardt, T., Sinnl, G., and Fischer, H.: First continuous high-resolution aerosol record from the East Greenland Ice Core Project (EGRIP), covering the last 15,000 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15202, https://doi.org/10.5194/egusphere-egu2020-15202, 2020.

Records of past aerosol deposition to the polar ice sheets have enabled us to study variability in different parts of the earth system in great temporal detail over past glacial cycles. Furthermore, the high temporal resolution of ice-core aerosol records has been the basis for precise dating of climate records using annual layer counting. Nonetheless, the intermittent character of show deposition and especially the redistribution of snow on the surface of the ice sheet intrinsically affects the preservation of climate signals in the ice. This strongly limits how representative a climate record from a single ice core can be. It has been well established that even though seasonal variability might be preserved in an ice-core aerosol record, the inter annual variability of that record is different from a different core from the same site.

Until now most of the investigations have focused on inter annual representatives. This is mostly due to limited sample availability as multiple long records are needed for investigations on longer time scales. However, with the prospect of new high-resolution records over the Holocene from the EastGRIP ice core, understanding the representativeness of this record on decadal time scales is an important question. To tackle this problem, we use high-resolution aerosol records from multiple closely spaced ice cores from the EastGRIP deep ice core drill site. The records approximately cover the last millennium and are sub-seasonally resolved enabling the study of interannual to decadal variability over multiple aerosol species. All records are dated using annual layer counting and cross dating to the EastGRIP deep ice core using volcanic match points. In the presented pilot study, we focus on records of sea-salt and dust related aerosol species as well as on episodic aerosol signals from volcanos and wildfires.

How to cite: Erhardt, T., Jensen, C., Hörhold, M., and Fischer, H.: Representativeness of decadal-scale climate signals in ice-core aerosol records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14381, https://doi.org/10.5194/egusphere-egu2020-14381, 2020.

Deposition of dust on the Antarctic continent is controlled by many factors, such as the primary supply of dust particles from the continents [1], the long range transport, the hydrological cycle and the snow accumulation rate [2, 3]. Thus, the study of mineral dust in ice cores gives the possibility to reconstruct past climatic and environmental conditions.

Generally, when an ice core sample is melted, soluble elements dissolve in water, while insoluble elements remain in the solid phase. Other elements, such as iron, calcium, potassium and sulfur, typically partition between the soluble and the insoluble fractions. However recent studies have shown how the dust record may be chemically and physically altered in deep ice cores [4, 5], posing a challenge in the interpretation of the climatic signal that may lie within such samples. In particular, relative abundance of specific elements was shown to be different when comparing shallow and deep dust samples, suggesting that post depositional processes are taking place.

In this study we present a comparison between samples belonging to the Talos Dome ice core analyzed through two different techniques: instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectrometry (ICP-MS). While the former is used to investigate only the insoluble fraction of dust, as it can only be applied to solid samples, the latter is used to assess the elemental composition of both the total and the soluble fraction of dust. We determined 45 elements through ICP-MS and 39 through INAA, with a good overlapping of the elements between the two techniques. Besides the determination of major elements, the high sensibility of both techniques also permitted the determination of trace elements. Among these, rare earth elements (REE) are of particular importance as they have been widely used as a geochemical tracer of aeolian dust sources [6]. We here present depth profiles for each analysed element, covering discrete portions of the entire ice core.

Bibliography

[1] Petit, Jean-Robert, et al. "Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica." Nature 399.6735 (1999): 429-436.

[2] Lambert, Fabrice, et al. "Dust-climate couplings over the past 800,000 years from the EPICA Dome C ice core." Nature 452.7187 (2008): 616.

[3] Wegner, Anna, et al. "The role of seasonality of mineral dust concentration and size on glacial/interglacial dust changes in the EPICA Dronning Maud Land ice core." Journal of Geophysical Research: Atmospheres 120.19 (2015): 9916-9931.

[4] Baccolo, Giovanni, et al. “The contribution of synchrotron light for the characterization of atmospheric mineral dust in deep ice cores: Preliminary results from the Talos Dome ice core (East Antarctica).” Condensed Matter 3, no. 3 (2018): 25.

[5] De Angelis, Martine, et al. “Micro-investigation of EPICA Dome C bottom ice: Evidence of long term in situ processes involving acid-salt interactions, mineral dust, and organic matter.” Quaternary Science Reviews 78 (2013): 248-265.

[6] Gabrielli, Paolo, et al. “A major glacial-interglacial change in aeolian dust composition inferred from Rare Earth Elements in Antarctic ice.” Quaternary Science Reviews 29, no. 1-2 (2010): 265-273.

How to cite: Di Stefano, E., Baccolo, G., Gabrielli, P., Ellis, A., Delmonte, B., and Maggi, V.: Soluble/insoluble fractionation of elements in mineral dust from Antarctic samples, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15986, https://doi.org/10.5194/egusphere-egu2020-15986, 2020.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) offers minimally destructive ice core impurity analysis at micron-scale resolution. This technique is especially suited for exploring closely spaced layers of ice within samples collected at low accumulation sites or in regions of highly compressed and thinned ice. Accordingly, LA-ICP-MS promises invaluable insights in the analysis of a future “Oldest ice core” from Antarctica. However, in contrast to ice core melting techniques, taking into account the location of impurities is crucial to avoid misinterpretation of ultra-fine resolution signals obtained from newly emerging laser ablation technologies. Here we present first results from a new LA-ICP-MS setup developed at the University of Venice, based on a customized two-volume cryogenic ablation chamber optimized for fast wash-out times. We apply our method for high-resolution chemical imagining analysis of impurities in samples from intermediate and deep sections of the Talos Dome and EPICA Dome C ice cores. We discuss the localization of both soluble and insoluble impurities within the ice matrix and evaluate the spatial significance of a single profile along the main core axis. With this, we aim at establishing a firm basis for a future deployment of the LA-ICP-MS in an “Oldest Ice Core”. Moreover, our work illustrates how LA-ICP-MS may offer new means to study the impurity-microstructure interplay in deep polar ice, thereby promising to advance our understanding of these fundamental processes.

How to cite: Bohleber, P., Roman, M., Barbante, C., Stenni, B., and Delmonte, B.: Towards an improved understanding of high-resolution impurity signals in deep Antarctic ice cores, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8537, https://doi.org/10.5194/egusphere-egu2020-8537, 2020.

Ice cores provide a wealth of insights into past changes in climate and atmospheric composition.

Obtaining information on past polar temperature changes is important to document climate variations beyond instrumental records, and to test our understanding of past climate variations, including the Earth system response to astronomical forcing.

Since the 1960s, major breakthrough in ice core science have delivered a matrix of quantitative Greenland and Antarctic ice core records.

Temperature reconstructions from polar ice cores document past polar amplification, and provide quantitative constraints to test climate models.

Climate information from the air and ice preserved in deep ice cores has been crucial to unveil the tight coupling between the carbon cycle and climate and the role of past changes in atmospheric greenhouse gas composition in the Earth system response to astronomical forcing.

Ice core constraints on past changes in ice sheet topography are also key to characterize the contribution of the Greenland and Antarctic ice sheets to past sea level changes.

The construction of a common chronological framework for Greenland and Antarctic ice core records has unveiled the bipolar sequence of events during the glacial-interglacial cycle, and the interplay between abrupt change and the response of the climate system to astronomical forcing.

International efforts have started to obtain the oldest ice cores (hopefully back to 1,5 million years) from Antarctica, in order to understand the reasons for the major shifts in the response of the climate system to astronomical forcing at that time, leading to more intense and longer glacial periods. 

How to cite: Masson-Delmotte, V.: Astronomical forcing and climate : insights from ice core records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22651, https://doi.org/10.5194/egusphere-egu2020-22651, 2020.

Gases preserved in ice cores provide a potential direct archive for atmospheric oxygen. Yet, oxygen-to-nitrogen ratios in ice cores (expressed as δO₂/N₂) are modified by a number of processes related to gas trapping and gas losses in the ice. Such complications have long hindered the use of ice core δO₂/N₂ to derive true atmospheric oxygen concentrations. Recently, a persistent decline in δO₂/N₂, observed in four different ice cores (GISP2, Vostok, Dome F, and EDC), is interpreted to reflect decreasing atmospheric O₂ concentrations over the late Pleistocene (Stolper et al., 2016). The rate of δO₂/N₂ change is -8.4±0.2 ‰/Myr (1σ). Using new measurements made on EDC samples stored at -50 °C and therefore free from gas loss, Extier et al (2018) confirms the decrease in δO₂/N₂ with a slope of -7.0±0.6‰/Myr (1σ).

Here, we present new δO₂/N₂ measurements made on 1.5-million-year-old blue ice cores from Allan Hills Blue Ice Areas, East Antarctica. We use argon-to-nitrogen ratios (δAr/N₂) in the ice to correct for the fractionations during bubble close-off and gas losses. In those processes, δAr/N₂ is fractionated in a fashion similar to δO₂/N₂ (Huber et al., 2006; Severinghaus and Battle, 2006). Paired δO₂/N₂-δAr/N₂ values measured from the same sample were classified into three different time slices: 1.5 Ma (million years old), 950 ka, and 490 ka. Between 950 ka and 490 ka, we observe a decline in δO₂/N₂ similar to that observed in the aforementioned deep ice cores. This observation gives us confidence in the validity of the Allan Hills blue ice δO₂/N₂ records. Between 1.5 Ma and 950 ka, however, there is no statistically significant trend in ice core δO₂/N₂. Our results show a surprising lack of variability from 1.5 to 0.95 Ma; even during the past ~0.9 Ma, the rate of decline was very slow.

How to cite: Yan, Y., Bender, M., Brook, E., Clifford, H., Kemeny, P., Kurbatov, A., Mackay, S., Mayewski, P., Ng, J., Severinghaus, J., and Higgins, J.: Oxygen-to-nitrogen ratios in 1.5-million-year-old ice cores from Allan Hills Blue Ice Areas: implications for the long-term atmospheric oxygen concentrations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12753, https://doi.org/10.5194/egusphere-egu2020-12753, 2020.

For several decades, ice-core water-isotope research was focused on retrieving and interpreting single cores, measured on increasingly finer resolution and higher analytic precision. However, not only the sampling resolution or analytical precision limits the ability to recover the climate signal, but also the way the climatic signal is imprinted in the isotopic composition profile obtained from ice cores. Therefore, despite three decades of Antarctic ice-coring and dozens of firn cores, especially the temperature evolution in the low accumulation region of East Antarctica during the last millennium is still barely known.  In the recent years, strong progress has been made in the understanding of the isotopic signal formation based on process studies, snow pits, snow trenches and replicate cores. Using this knowledge, we will review the limits of temperature reconstructions based on theoretical considerations, empirical signal-to-noise ratio estimates and forward models of the signal formation. We will further discuss new avenues for sharpening the ability to recover high-resolution temperature signals from firn and ice cores by optimally combining multiple cores and by combining isotope with impurity records.

How to cite: Laepple, T., Münch, T., Casado, M., Hörhold, M., Freitag, J., Werner, M., and Dallmayr, R.: Theoretical limits and new approaches to reconstruct temperature from the isotopic composition of ice cores in low-accumulation regions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20166, https://doi.org/10.5194/egusphere-egu2020-20166, 2020.

Stable water isotope ratios are routinely used to infer past climatic conditions in palaeoclimate archives. In particular, oxygen isotope ratios in precipitation co-vary with temperature in high latitudes, and have been established as indicators for past temperature changes in ice-cores. The timescales for which this holds, and the validity of spatial/temporal regression slopes are difficult to constrain based on the observational record.

Here, surface climate and oxygen isotope ratio variability are compared across an ensemble of millennial-long simulations with the isotope-enabled version of the Hadley Centre Coupled Model version 3 (iHadCM3). The ensemble consists, amongst others, of paired experiments. One half were performed as conventional palaeoclimate equilibrium simulations for the Last Glacial Maximum (LGM, orbital and trace gas concentrations of 21kyrs BP), the mid Holocene (conditions 6kyrs BP) and the pre-industrial period (PI, 1850CE) analogously to the simulations in the Palaeoclimate Modeling Intercomparison Project. The second half of the ensemble is additionally perturbed by radiative forcing variations from solar variability and volcanic forcing as for the last millennium. Each simulation is continued for at least 1050 years.

We find that global mean surface temperature and precipitation decrease significantly in all considered climate states (LGM, 6k, PI). Post-volcanic temperature reduction is fairly consistent across the globe, but weak in Antarctica. In the PI state, we find a significant increase in the AMOC strength after eruptions. This does not occur for the LGM state. No significant responses to solar forcing were detectable in the isotopic record. Correlating precipitation-weighted δ¹⁸O (δ¹⁸O_pr) at these locations with surface temperature across the globe shows strong linear relationships and teleconnections. In Greenland, δ¹⁸O_pr, at the decadal scale, shows high correlations across the Northern hemisphere for the PI simulations, but this spatial representativeness is smaller in the LGM.

We finally examine the detectability of strong interannual volcanic impacts in the climate and isotope record at ice core drill sites in West and East Antarctica, Greenland, the European Alps and the Tibet Plateau. At all locations, modeled isotope and climate variance is higher in the naturally forced simulations. On annual time scales, we find only weak imprints of sub-supervolcanic eruptions in annual δ¹⁸O_pr at most locations compared to interannual variability, with the exception of the Tibet plateau. We extend this epoch analysis to high-resolution ice core records to assess the consistency between modeled and measured isotope variations for prominent volcanic eruptions over the last millennium.

The inclusion of natural forcing in the simulations alleviates the discrepancy between modeled and observed isotope variability. However, the gap cannot be closed completely. This suggests that improving our understanding of the signal formation process, the dynamical origins of isotope signatures, and model biases at all latitudes is important to constrain the regional to global representativeness of stable water isotopes in ice cores.

How to cite: Rehfeld, K., Kirschner, M., Holloway, M., and Sime, L.: Quantifying the influence of natural forcing on oxygen isotope variability in alpine and polar ice core sites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19341, https://doi.org/10.5194/egusphere-egu2020-19341, 2020.

It is important to understand the magnitude and rate of past sea ice changes, as well as their timing relative to abrupt shifts in other components of Earth’s climate system. Furthermore, records of past sea ice over the last few centuries are urgently needed to assess the scale of natural (internal) variability over decadal timescales. By continuously recording past atmospheric composition, polar ice cores have the potential to document changing sea ice conditions if atmospheric chemistry is altered.  Sea salt aerosol, specifically sodium (Na), and bromine enrichment (Br_enr, Br/Na enriched relative to seawater ratio) are two ice core sea ice proxies suggested following this premise.

Here we aim to move beyond a conceptual understanding of the controls on Na and Br_enr in ice cores by using process-based modelling to test hypotheses. We present results of experiments using a 3D global chemical transport model (p-TOMCAT) that represents marine aerosol emission, transport and deposition. Critically, the complex atmospheric chemistry of bromine is also included. Three fundamental issues will be examined: 1) the partitioning of Br between gas and aerosol phases, 2) sea salt aerosol production from first-year versus multi-year sea ice, and 3) the impact of increased acidity in the atmosphere due to human activity in the Arctic.

How to cite: Rhodes, R., Yang, X., and Wolff, E.: Exploring ice core sea ice proxies through process-based modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9013, https://doi.org/10.5194/egusphere-egu2020-9013, 2020.

Ice cores constitute an important record of the past surface mass balance (SMB) of the ice sheets, with SMB ultimately modulating the ice sheets’ sea level impact. For the Antarctic Ice Sheet (AIS), SMB is dominated by snow accumulation and strongly controlled by atmospheric circulation. Large-scale atmospheric depressions collect warmth and moisture from further north that they then release over the AIS in the form of widespread accumulation or focused atmospheric rivers. This implies that snow deposited at the surface of the AIS should show strongly coupled SMB and surface air temperatures (SAT) variations. Ice cores do not record SAT directly but their d¹⁸O record is often used as a temperature proxy.

Here, using the PAGES 2k Network ice core compilations of SMB and d¹⁸O of Thomas et al. (2017) and Stenni et al. (2017), we obtain a weak correlation between SMB and d¹⁸O over historical timescales, and an equivalently weak correlation between SMB and SAT based on the Nicolas & Bromwich (2014) SAT reconstructions. However, we calculate a strong and positive SMB-SAT correlation in the majority of regions of the AIS using Global Climate Models (GCM) and the regional model RACMO2.3p2.

To resolve the discrepancy between measured and modeled signals, we show that averaging the ice core records in close spatial proximity increases their SMB-SAT correlation. This increase in measured SMB-SAT correlation likely results from noise present in the ice core records, but is not enough to match the strong correlation calculated in the models. On the model side, the high spatial resolution of the RACMO2.3p2 model allows us to highlight a number of areas of the AIS where SMB and SAT are not strongly correlated. We describe how wind-driven processes acting on the SMB and SAT locally, through Foehn and katabatic effects, can overwhelm the large-scale atmospheric input that induces the positive SMB-SAT correlations. In particular, we focus on Dronning Maud Land, East Antarctica, where each ice promontory clearly shows this wind-driven snow redistribution. Nevertheless, those regions displaying a low SMB-SAT correlation cover only a small fraction of the AIS and are not sufficient to explain the model-data discrepancy, suggesting a critical role of processes at a scale smaller than the one resolved by the regional model.

References:

Thomas, E. R., 2017, Regional Antarctic snow accumulation over the past 1000 years, Climate of the Past, 13, 1491–1513.

Stenni, B. et al., 2017, Antarctic climate variability on regional and continental scales over the last 2000 years, Climate of the Past, 13, 1609–1634.

Nicolas, J. P. & Bromwich, D. H., 2014, New reconstruction of Antarctic near-surface temperatures: Multidecadal trends and reliability of global reanalyses, Journal of Climate, 27, 8070–8093.

How to cite: Cavitte, M. G. P., Dalaiden, Q., Goosse, H., Lenaerts, J. T. M., and Thomas, E. R.: Examining the strength of the link between surface temperature and surface mass balance in ice cores and models over the last centuries in Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-801, https://doi.org/10.5194/egusphere-egu2020-801, 2020.

Air inclusions trapped in polar ice provide unique records of the past atmospheric composition ranging from key greenhouse gases to short-lived trace gases like ethane and propane. Provided the analyzed species concentrations and their isotopic fingerprints accurately reflect past atmospheric composition, valuable constraints can be put onto biogeochemical cycles. However, it is already known that not all drill sites or specific time intervals are equally suitable to derive artefact-free gas records; e.g., CO₂ data from Greenland ice is overprinted by CO₂ ‘in situ’ production due to impurities in the ice, and only the cleaner Antarctic ice allows to reconstruct past atmospheric CO₂.

Until recently, CH₄ artefacts in polar ice were only detected on melt affected samples or for short spikes related to exceptional impurity deposition events (Rhodes et al 2013). However, careful comparison of CH₄ records obtained using different extraction methods revealed disagreements among Greenland CH₄ records and initiated targeted experiments.

Here, we report experimental findings of CH₄ artefacts occurring in dust-rich sections of Greenland ice cores. The artefact production happens during the melt extraction step (‘in extractu’) of the classic wet extraction technique and typically reaches 20 ppb in dusty stadial ice which causes erroneous reconstructions of the interhemispheric CH₄ difference and strongly affects the hydrogen isotopic signature of CH₄ (Lee et al. 2020). The measured CH4 excess is proportional to the amount of mineral dust in the ice. Knowing the empirical relation between produced CH4 and the dust concentration of a sample allows a first-order correction of existing CH4 data sets and to revise previous interpretations.

To shed light on the underlying mechanism, we analyzed samples for other short-chain alkanes ethane (C₂H₆) and propane (C₃H₈). The production of CH₄ was always tightly accompanied with C₂H₆ and C₃H₈ production at amounts exceeding the past atmospheric background levels derived from low-dust samples. Independent of the produced amounts, CH₄, C₂H₆, and C₃H₈ were produced in molar ratios of roughly 16:2:1, respectively. The simultaneous production at these ratios does not point to an anaerobic methanogenic origin which typically exhibits methane-to-ethane ratios of >>100. Such alkane patterns are indicative of abiotic degradation of organic matter as found in sediments.

We found this specific alkane pattern not only for dust-rich samples but also for samples that were affected by surface melting from the last interglacial (NEEM ice core) with low dust concentrations. This implies that the necessary precursor is an impurity also present in low-dust ice and the step leading to the production of the alkanes could then be activated when a sufficient boundary condition is met for the production, e.g. by melt/refreeze of surface snow.

How to cite: Schmitt, J., Lee, J., Edwards, J., Brook, E., Blunier, T., Mühl, M., Seth, B., Beck, J., and Fischer, H.: Coupled artefact production of methane, ethane, and propane in polar ice cores, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3583, https://doi.org/10.5194/egusphere-egu2020-3583, 2020.

Central Antarctic Plateau sites display a strong contrast in deep firn gas ages with relatively high accumulation sites (South Pole, EPICA DML) showing very old (about a century) gas ages in the open porosity of deep firn on one side, and very young (few decades) gas ages and an absence of deep firn δ¹⁵N plateau (indicative of remaining gas transport) at low accumulation rate sites (Dome C, Dome F, Vostok) on the other side. Multi-tracer results from an intermediate accumulation site named "Lock-in" will be presented. At this fairly low accumulation rate site (~3.6 cm water equivalent / year), very old air ages were obtained in deep firn but the lock-in zone looks narrower than at South Pole. Analytical results, as well as gas transport and densification modelling results will be discussed in terms of variability of gas-trapping characteristics on the central Antarctic Plateau and degree of understanding of the underlying mechanisms.

How to cite: Martinerie, P., Fourteau, K., Chappellaz, J., Orsi, A., Faïn, X., Lee, G., Landais, A., and Sturges, W.: Variability of gas-trapping characteristics on the central Antarctic Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9860, https://doi.org/10.5194/egusphere-egu2020-9860, 2020.

Paleoclimate reconstructions from ice core records can be hampered due to the lack of a reliable chronology, especially when the stratigraphy is disturbed and conventional dating methods are not readily applied. The noble gas radioisotopes ⁸¹Kr and ³⁹Ar can in these cases provide robust constraints as they yield absolute, radiometric ages. ⁸¹Kr (half-life 229 ka) covers the time span from 50-1300 ka, which is particularly relevant for polar ice cores, whereas ³⁹Ar (half-life 269 a) with a dating range of 50-1400 a is suitable for high mountain glaciers. For a long time the use of ⁸¹Kr and ³⁹Ar for dating of ice samples was hampered by the lack of a detection technique that can meet its extremely small abundance at a reasonable sample size. Here, we report on ⁸¹Kr and ³⁹Ar dating of Antarctic and Tibetan ice cores with the detection method Atom Trap Trace Analysis (ATTA), using 5-10 kg of ice for ⁸¹Kr and 2-5 kg for ³⁹Ar. Among others, we measured ⁸¹Kr in the lower section of Taldice ice core, which is difficult to date by conventional methods, and in the meteoric bottom of the Vostok ice core in comparison with an age scale derived from hydrate growth. Moreover, we have obtained an ³⁹Ar profile for an ice core from central Tibet in combination with a timescale constructed by layer counting. The presented studies demonstrate how the obtained ⁸¹Kr and ³⁹Ar ages can complement other methods in developing an ice core chronology, especially for the bottom part.

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

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

[3] L. Tian, ⁸¹Kr Dating at the Guliya Ice Cap, Tibetan Plateau, Geophysical Research Letters, (2019)

[4] http://atta.ustc.edu.cn

How to cite: Ritterbusch, F., Chu, Y.-Q., Crotti, I., Dong, X.-Z., Gu, J.-Q., Hu, S.-M., Jiang, W., Landais, A., Lipenkov, V., Lu, Z.-T., Shao, L., Stenni, B., Team, T., Tian, L., Tong, A.-M., Wang, W.-H., and Zhao, L.: Constraining ice core chronologies with 39Ar and 81Kr, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21651, https://doi.org/10.5194/egusphere-egu2020-21651, 2020.

The Dye3 core was drilled at Dye3 (65°11’N, 43°50’W) in 1979 – 1981. The core has been analyzed for numerous components over the last decades. We measured remaining sections, the Younger Dryas and a larger portion of the last glacial, in a continuous flow setup in fall 2019. Here we focus on gas measurements. We measured methane, δ¹⁵N, δ⁴⁰Ar, and the elemental ratio of Ar and N₂. We present the continuous flow setup for measuring those components in parallel and first results with a focus on the exact timing of changes in methane and δ¹⁵N and δ⁴⁰Ar at the Younger Dryas and Dansgaard-Oeschger transitions.

How to cite: Blunier, T., Venkatesh, J., Soestmeyer, D. A., Liisberg, J. B., Rhodes, R., Menking, J. A., Severinghaus, J. P., Harlan, M., Kjær, H. A., and Vallelonga, P.: New data from the 40 year old Dye3 core, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13557, https://doi.org/10.5194/egusphere-egu2020-13557, 2020.

Mineral dust in ice cores provides insight into past atmospheric circulation patterns provided that the source(s) of these aerosols can be identified. Isotopes of strontium, neodymium and lead are frequently used for source discrimination in ice cores, while those of hafnium much less so. This is because of the extremely low (1-5 ng) amounts of Hf present in 5-10 mg dust samples usually available for isotopic analyses from the dustiest periods of past glaciations, e.g. the Last Glacial Maximum. The use of ¹⁷⁶Hf/¹⁷⁷Hf isotopic ratios in dust fingerprinting is crucial in situations when Sr-Nd isotopes are inconclusive in source identification.

The overall Hf budget is dominated by the heavy mineral zircon in silt-sized, wind-blown material, while it is significantly depleted in the finer (<5 µm) fractions and the effects of other minerals (apatite, sphene, monazite, xenotime and clay minerals) become increasingly important. Since the major hosts of Hf are refractory heavy minerals, the complete digestion of dust material is crucial in determining reliable Hf isotope ratios.

Here we introduce a closed vessel ammonium bifluoride (NH₄HF₂) digestion method (220 °C), which is a fast and low blank (0.5 ng for Sr, 0.2 ng for Nd, and <25 pg for Hf) technique for dust dissolution, prior to column chemistry for combined Hf-Sr-Nd isotope analyses. Repeated measurements of the Hf isotope ratios of USGS geological reference materials (AGV-2, BCR-2 and GSP-2) demonstrate that raw, non fractionation corrected ¹⁷⁶Hf/¹⁷⁷Hf ratios are accurate within 5-50 ppm, while the JMC-475 fractionation corrected values are accurate to 5-10 ppm, compared to reference values using our ion exchange chemistry setup. This methodology also allows separating Sr and Nd from the same samples, and analysing the ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopic compositions. Here we discuss mass spectrometry issues (including sensitivity) of TIMS and two different MC-ICP-MS instruments, and major limitations on dust sample size for Hf-Sr-Nd isotope analyses. Furthermore, the mineralogical background of Hf isotopic compositions, including zircon depletion effects and clay mineralogy (illite) control will be demonstrated. Hf isotope data obtained from four NorthGRIP ice core samples will be presented.

This study was financially supported by the FWF Austria through a Lise Meitner grant (project nr. M 2503-N29) and the European Regional Development Fund in the project of GINOP-2.3.2.-15-2016-00009 ‘ICER’.

How to cite: Ujvari, G., Klötzli, U., Horschinegg, M., Wegner, W., Hippler, D., Tepe, N., Kiss, G., Horváth, A., and Svensson, A.: Hafnium (and Sr-Nd) isotope analysis of mineral dust: from sample digestions to mass spectrometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15726, https://doi.org/10.5194/egusphere-egu2020-15726, 2020.

The ice in the deepest and therefore oldest parts of polar ice cores is highly compressed and therefore annual layers, although potentially preserved, can be thinned to a millimeter level or even below. However, for many palaeoclimate studies these are the most interesting sections. Within the WACSWAIN project we aim to investigate the basal part of an ice core recently drilled to bedrock at the Skytrain ice rise in West Antarctica to obtain unique information on the state of the Filchner-Ronne ice shelf during the last interglacial. To achieve this we have set up a system to perform high resolution laser-ablation ICP-MS measurements using a cryocell stage on selected segments of the deepest parts of the ice cores.

Here we present first results of system performance including assessment of measurement sensitivity and precision with respect to analyses of the most relevant components, namely sodium, calcium and aluminium. We also report on the development and the performance of a matrix matched calibration method using flash-freezed water samples of known composition to convert relative signal intensities into concentrations. This especially focuses on homogeneity and reproducibility of the in-house produced standard. Finally, the results of laser ablation ICP-MS results are compared to parallel low resolution data from continuous flow analysis of the Skytrain core to evaluate the capabilities of the method in terms of improving depth resolution.

How to cite: Hoffmann, H., Wolff, E., Day, J., Grieman, M., Humby, J., and Gibson, S.: Setup and first testing of Laser Ablation - ICP-MS measurements for high resolution chemical ice core analyses at University of Cambridge , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19625, https://doi.org/10.5194/egusphere-egu2020-19625, 2020.

The Beyond EPICA Oldest Ice (BEOI) project will drill an ice core dating back to 1.5 million-years (1.5 Myr) ago. This ice core is of particular interest to the scientific community as it will be the only one covering the climate history of the Mid Pleistocene Transition, when glacial-interglacial cycles changed from a 40 Kyr to 100 Kyr cyclicity, and for which causes are not well understood currently. Obtaining useful climatic information beyond 800 Kyr represents an analytical challenge due to the fact that the deepest section of the ice core is very compact and the amount of sample available is very low.

Current analytical methods for the determination of organics in ice are characterized by a large number of steps that requires large amounts of sample for a single analysis. This results in the loss of the high time resolution desired from ice cores which is particularly problematic for deeper (i.e. older) records where the ice is more compact.

This work aims at combining the growing field of microfluidics with improvements to conventional mass spectrometry to allow for continuous analysis of organics in ice cores, melted in continuous on a melting-head. In fact, microfluidic is a powerful technology in which, only a small amount of liquid (10^-9-10^-18 liters) is manipulated and controlled with an extremely high precision. The method invokes a three-step process: (1) the melted ice core sample is sent to a nebulizer to produce aerosol, then (2) the aerosol is dried to remove water content and concentrate the sample, and (3) the aerosol is sent to a mass spectrometer for continuous analysis through a modified electrospray ionization (ESI) probe.

This novel system, once operational, can be applied to a range of ice cores but is especially useful for older ice cores given the stratification of deeper segments. It will allow the research community to measure organic compounds with a high time resolution, even in the oldest of ice, to retrieve paleoclimatic information that would otherwise be lost using traditional methods.

How to cite: FIlippi, D. and Giorio, C.: Microfluidic device for continuous-flow analysis of organics in oldest ice, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19976, https://doi.org/10.5194/egusphere-egu2020-19976, 2020.

The study of the deep portions of ice cores still represents a poorly explored field due to the presence of processes acting in the lowermost layers and possibly affecting the preservation of the original climatic signal. For the 1620 m TALDICE ice core, drilled at Talos Dome (East Antarctica), the high-resolution climate reconstruction and chronology definition are available only until the depth of ~1450 m (150 kyr BP) (Stenni et al., 2011, Bazin et al., 2013). Our aim is to investigate the portion below 1460 m depth to the bottom of the core, where radargrams show the presence of an unconformity in the ice sheet, to define a preliminary chronology and identify a discernible climatic signal.

Here we present the new TALDICE δ¹⁸O_atm record in the air bubbles, in association with the new high-resolution δ¹⁸O_ice and δD_ice profiles and an ⁸¹Kr radiometric date. New 46 measurements of δ¹⁸O_atm  allowed to increase the resolution of the available profile from 1357 to 1553.95 m depth and to extend the record till the bottom of the core at 1617 m depth. The comparison between the δ¹⁸O_atm profile of TALDICE and the one of EPICA Dome C (EDC) ice core (Extier et al., 2018) allows to solidly define a preliminary age-depth relationship for the TALDICE core until 1500 m depth, where the gas age is estimated to be ~200 kyr BP. Below 1500 m, supplementary δ¹⁸O_atm measurements will be needed to identify older precession cycles and to extend the age-depth relationship further back in time. On the other hand, the high-resolution isotopic profiles in the ice (¹⁸O/¹⁶O and D/H ratios) obtained below the depth of 1528 m and compared with the EDC ones suggest that the climatic signal in the ice is preserved until to the lower level of 1547.8 m, which is dated back to 343 kyr BP. However, the lack of similarities with the EDC water isotopes record below this depth, in spite of the ⁸¹ Kr radiometric age 459 ± 50 kyr BP at the depth of 1574-1578 m, indicates the missing of the MIS 11 in the isotopic profiles. Moreover, the increase of high-frequency variability in the δ¹⁸O_ice and δD_ice below 1547.8 m depth implies that this part of the core lays in an area of the ice sheet characterized by different properties in comparison to the ice above.

Additional δ¹⁸O_atm, ⁴⁰Ar, δ¹⁸O_ice, and δD_ice measurements will be performed in the lowermost portion of the core and the results will be compared with the new ⁸¹Kr radiometric dating at the depth of 1560-1564 m and 1614-1619 m to better constrain the chronology and to investigate the ice properties in the deeper portion of the core.

How to cite: Crotti, I., Barbante, C., Frezzotti, M., Jiang, W., Landais, A., Lu, Z.-T., Ritterbusch, F., Stenni, B., and Yang, G.-M.: New δ18Oatm, δ18Oice and δDice profiles from deep ice of the TALDICE core, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4179, https://doi.org/10.5194/egusphere-egu2020-4179, 2020.

The atmospheric processes determining the isotopic composition of precipitation on the Antarctic plateau are yet to be fully understood, as well as the post-depositional processes altering the snow pristine isotopic signal. Improving the comprehension of these physical mechanisms is of crucial importance for interpreting the isotopic records from ice cores drilled in the low accumulation area of Antarctica, e.g., the upcoming Beyond EPICA drilling at Little Dome C.

Up to now, few records of the isotopic composition of precipitation in Antarctica are available, most of them limited in time or sampling frequency. Here we present a 9-year long δ¹⁸O and δD record (2008-2016) of precipitation at Concordia base, East Antarctica. The snow is collected daily on a raised platform (1 m), positioned in the clean area of the station; the precipitation collection is still being carried out each year by the winter over personnel.

A significant positive correlation between isotopes in precipitation and 2-m air temperature is observed at both seasonal and interannual scale; the lowest temperature and isotopic values are usually recorded during winters characterized by a strongly positive Southern Annular Mode index.

To improve the understanding of the mechanisms governing the isotopic composition of precipitation, we compare the isotopic data of Concordia samples with on-site observations, meteorological data from the Dome C AWS of the University of Wisconsin-Madison, as well as with high-resolution simulation results from the isotope-enabled atmospheric general circulation models ECHAM5-wiso and ECHAM6-wiso, nudged with the ERA-Interim and ERA5 reanalyses respectively.

How to cite: Stenni, B., Dreossi, G., Casado, M., Scarchilli, C., Landais, A., Del Guasta, M., Grigioni, P., Casasanta, G., Werner, M., Masiol, M., Cauquoin, A., and Ciardini, V.: A Nine-year series of daily oxygen and hydrogen isotopic composition of precipitation at Concordia station, East Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8610, https://doi.org/10.5194/egusphere-egu2020-8610, 2020.

The goal of the SNOWISO project is to quantify the role of the post-depositional processes, which are influencing the isotopic composition of the surface snow and hence the ice core water isotope climate signal. Here we are reporting on findings from field campaigns carried out at EastGRIP over the four summers 2016-2019. We have collected a suite of observations containing the isotopic composition of the surface snow and the snowpack, together with direct observations of atmospheric water vapor isotopes and fluxes between the snow surface and the atmosphere. To support the analysis of the isotopic data we also collected meteorological observations comprising of atmospheric temperature and humidity gradients alongside with sub-surface and snow surface temperature along with atmospheric temperature and humidity gradients. With this dataset we are able to document significant changes in the snow isotopic composition, which are driven by post-depositional processes. The changes in the snow surface isotopic composition is observed to occur on time scales ranging from diurnal to several days. The changes in the snow surface isotopic composition is observed to occur on time scales ranging from diurnal to several days. We can show that the changes in the snow surface is consistent with the flux of the isotopologues between the snow surface and the atmosphere. This gives us confidence that we will be able to develop parameterizations of post-depositional effects, and model their influence on the ice core isotopic climate signal.

How to cite: Steen-Larsen, H. C., Hörhold, M., Wahl, S., Hughes, A., Faber, A.-K., Zuhr, A., Sveinbjørnsdottir, A., Behrens, M., and Kipfstuhl, S.: Quantifying the role of post-depositional processes on the isotopic composition of surface snow – new findings from the SNOWISO project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11774, https://doi.org/10.5194/egusphere-egu2020-11774, 2020.

The reconstruction of past temperatures based on ice core records relies on the quantitative but empirical relationship of stable water isotopes and annual mean temperature. However, its relation varies through space and time. On the East Antarctic Plateau, temperature reconstructions from ice cores are poorly constrained or even fail on decadal and smaller time scales. The observed discrepancy between annual mean temperature and isotopic composition partly relies on surface processes altering the signal after deposition but also, to a great deal, on spatially coherent processes prior to or during deposition. However, spatial coverage over larger areas on the East Antarctic Plateau is challenging. We here present in-situ measurements of the isotopic composition of surface snow with unprecedented statistical quality and coverage. 1m surface snow profiles were collected during an overland traverse between Kohnen station and Plateau Station, covering a 1200km long transect. We explore regional differences of the temperature-isotope relationship and discuss possible mechanisms affecting the isotopic composition in areas with accumulation rates lower than 60mmWEa^-1.

How to cite: Hörhold, M., Weinhart, A., Kipfstuhl, S., Freitag, J., Micha, G., Werner, M., and Lohmann, G.: Spatial variability of surface snow isotopic composition on the East Antarctic Plateau and implications for climate reconstructions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13653, https://doi.org/10.5194/egusphere-egu2020-13653, 2020.

Ice cores represent one of the most important palaeoclimate archives, which record, among many other parameters, changes in stable oxygen and hydrogen isotopic composition and soluble ionic impurities. While impurities serve, for example, as proxies for sea ice, marine biological activity and volcanism, records of isotopic composition are the major proxy for the reconstruction of natural polar temperature variability. The latter is based on the temperature-dependent distillation and fractionation of the isotopic composition of water vapour along its atmospheric pathway and empirically determined relationships thereof.

However, temperature is by far not the only driver of isotopic composition changes. A single isotopic ice-core record will comprise variations caused by a multitude of processes, from variable atmospheric circulation and moisture pathways to the intermittency of precipitation and finally to the mixing and re-location of surface snow by wind drift (stratigraphic noise). Taken together, these additional processes constitute a large amount of noise in the single isotope record, which masks the true temperature-related variability. Averaging a sufficient number of records to reduce overall noise is one means to allow for quantitative reconstructions, but its effectiveness depends on the spatial scales of the involved processes. Here, we discuss an alternative approach. Assuming that major impurity species exhibit a seasonal cycle and are mainly also, along with the isotopic composition, deposited by precipitation and redistributed by wind, a large portion of their interannual variability should be linked, which would offer the possibility of using the impurities to correct the variability of the isotopic records.

In this contribution, we present the "ideal" dataset for testing this idea. We sampled and analysed isotopic composition and major impurity species on a four metre deep and 50 metre long trench at Kohnen Station, East Antarctica. This enables us to study the two-dimensional structure and relationship of both proxies to learn about their deposition mechanisms, their seasonality, and to test the ability of a combined isotope–impurity approach to reconstruct local temperatures by comparing so obtained temperature reconstructions with the local weather station data.

How to cite: Münch, T., Hörhold, M., Freitag, J., Behrens, M., and Laepple, T.: Testing the ideal ice-core record for past temperature reconstructions using combined isotope and impurity analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15866, https://doi.org/10.5194/egusphere-egu2020-15866, 2020.

The iconic curve of D in water showing the 8 glacial/interglacial cycles from the EPICA Dome C ice
core is now a reference in paleoclimate. It shows past temperature variability back to 800 ka over the
3200 m deep ice core with a 55 cm resolution. However, the millennial and centennial scale
variability gets more challenging to observe in the deepest part of the core. Indeed, the time
resolution worsens when going deeper in the ice because of the ice thinning: it is larger than 200
years at 2500 m depth. Furthermore, isotopic diffusion affects the signal at the bottom of the ice
core. Pol et al., (2010) have thus shown that the sub-millennial MIS (Marine Isotopic Stage) 19 signal
(3157-3181 m deep) is erased because of diffusion and high resolution doesn’t add any further
information at this depth. In this study we want to better characterize the increase of the isotopic
diffusion with depth by providing new high resolution water isotopes at several intervals over the
EPICA ice core (EDC).
We present here published high resolution (11 cm) d18O measurements over the EDC ice core as
well as new records of high resolution (11 cm) D over MIS 7;13 and 14). We use spectral analyses to
determine at which depth the isotopic diffusion erases the sub-millennial variability. We also show
that cold periods exhibit a larger variability of water isotopes than interglacial periods.
The information obtained here is crucial for the new project Beyond EPICA oldest ice core, which has
the goal of analyzing a 1.5 Ma old ice core. In the deepest part, 1 m of ice core could represent
10 000 years of climate archive.

How to cite: Grisart, A., Vinther, B., Gkinis, V., Popp, T., Stenni, B., Pol, K., Masson Delmotte, V., Jouzel, J., Casado, M., Laepple, T., Horhold, M., Prie, F., Minster, B., Fourre, E., and Landais, A.: High frequency water isotopes records during glacial/interglacial cycles on EPICA Dome C ice core., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20679, https://doi.org/10.5194/egusphere-egu2020-20679, 2020.

The El Nino-Southern Oscillation (ENSO) drives interannual variability of rainfall, ecosystems and floods in many parts of the world. Climates in the Tibetan Plateau (TP) called as the “water tower” may be impacted by ENSO, but the character of ENSO impact and its mechanism are still not well understood. Here we present the isotopic profiles (δ¹⁸O) from a new Zangsegangri (ZSGR) ice core drilled in 2013 in the central TP covering 200 years to understand the ENSO impact on the TP climate. The imprint of ENSO is evidenced at annual scale as recorded in ice core. This ice core δ¹⁸O record also reveal contributions of south/north moisture sources change with the transition of El nino/La nina events which are triggered by the tropical sea surface temperature, associated with the change of convections along the moisture transport paths. These rapid changes lead to the variation of ZSGR ice core δ¹⁸O, namely El Nino events result in lower δ¹⁸O in the ZSGR ice core record. The mechanism of ENSO impact on the ZSGR ice core δ¹⁸O are quantified with LMDZiso model. The significant impact of ENSO activity on the Tibetan ice core record during the past centuries implies the importance of ENSO in land surface processes in the TP.

How to cite: Gao, J., Yao, T., Wu, G., and Risi, C.: ENSO modulates the variability of ice core δ18O in the central Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22237, https://doi.org/10.5194/egusphere-egu2020-22237, 2020.

Ice in polar ice sheets once experience a state of firn at near-surface depths. Therefore, it is important to understand physical processes of firn formation, metamorphism and deformation for ice core studies. We investigated firn through measurement of tensorial values of the dielectric permittivity at microwave and millimeter-wave frequencies. This method can detect presence and strength of anisotropic structure in the geometry of pore spaces and ice matrix. We applied the method to many firn cores drilled at both ice sheets. We find that firn that have shorter residence time at the near-surface depths does not form strong vertical anisotropy that is caused by vertical movement of moistures. In contrast, firn that have longer residence time at the near-surface depths tend to form vertical anisotropy. When density exceeds  ~600 kg/m³, a common feature of firn at many polar sites is that there are evolution of vertically elongated features of pore spaces in firn despite growth of vertical compression. We hypothesize an explanation as follows. As firn becomes denser, air within firn needs escape paths to upward directions as compared to sinking firn. In firn, porous structure tend to have vertically elongated structure because of this vertical escape movement of air. The observed phenomena of the grow th of the vertical dielectric anisotropy
can be understood by this vertical movement of the air w ithin firn.

How to cite: Fujita, S., Fukui, K., Hirabayashi, M., Iizuka, Y., Matoba, S., Miyamoto, A., Motoyama, H., Saito, T., and Suzuki, T.: Evolution in geometry of firn in ice sheets detected by dielectric anisotropy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9202, https://doi.org/10.5194/egusphere-egu2020-9202, 2020.