Description: The firn core array of the North Greenland Traverse (NGT) provides unique climatic information for North Greenland until the mid 1990ies. In order to extend this climate record into more recent time, some of the drill sites were revisited and extension cores were drilled. The record is a composite of snow liners (from the surface to 1.38 m depth) and a consecutive firn core from 1.38 m depth to 30.64 m depth with a diameter of 75 cm. Density was measured by micro-focus Computer Tomography with a resolution of 0.1mm and subsequently smoothed with a Gaussian filter with sigma 2cm. Stable oxygen isotopes (δ18O) were measured on discrete samples using Cavity Ring-Down spectroscopy with an L2120-i and L2130-i by Picarro Instruments. The δ18O concentration with depth is provided with a precision of better than 0.1 per mil. Using raw, un-calibrated dielectrical profiling (DEP) data from the field (NEEM Set-up), volcanic tie points were derived. Together with the smoothed density data and the isotopic composition the record was dated by layer counting. The annual mean values of δ18O were obtained based on this dating. Based on the density measurements and the dating, the annual accumulation rate in water equivalent depth was derived.

Description: The firn core array of the North Greenland Traverse (NGT) provides unique climatic information for North Greenland until the mid 1990ies. In order to extend this climate record into more recent time, some of the drill sites were revisited and extension cores were drilled. The record is a composite of snow liners (from the surface to 1.6 m depth) and a consecutive firn core from 1.6 m depth to 30.98 m depth with a diameter of 75 cm. Density was measured by micro-focus Computer Tomography with a resolution of 0.1mm and subsequently smoothed with a Gaussian filter with sigma 2cm. Stable oxygen isotopes (δ18O) were measured on discrete samples using Cavity Ring-Down spectroscopy with an L2120-i and L2130-i by Picarro Instruments. The δ18O concentration with depth is provided with a precision of better than 0.1 per mil. Using raw, un-calibrated dielectrical profiling (DEP) data from the field (NEEM Set-up), volcanic tie points were derived. Together with the smoothed density data and the isotopic composition the record was dated by layer counting. The annual mean values of δ18O were obtained based on this dating. Based on the density measurements and the dating, the annual accumulation rate in water equivalent depth was derived.

Description: The firn core array of the North Greenland Traverse (NGT) provides unique climatic information for North Greenland until the mid 1990ies. In order to extend this climate record into more recent time, some of the drill sites were revisited and extension cores were drilled. The record consists of a firn core from 0 to 17m m depth with a diameter of 75 cm. Density was measured by micro-focus Computer Tomography with a resolution of 0.1mm and subsequently smoothed with a Gaussian filter with sigma 2cm. Stable oxygen isotopes (δ18O) were measured on discrete samples using Cavity Ring-Down spectroscopy with an L2120-i and L2130-i by Picarro Instruments. The δ18O concentration with depth is provided with a precision of better than 0.1 per mil. Using raw, un-calibrated dielectrical profiling (DEP) data from the field (NEEM Set-up), volcanic tie points were derived. Together with the smoothed density data and the isotopic composition the record was dated by layer counting. The annual mean values of δ18O were obtained based on this dating. Based on the density measurements and the dating, the annual accumulation rate in water equivalent depth was derived.

Description: The firn core array of the North Greenland Traverse (NGT) provides unique climatic information for North Greenland until the mid 1990ies. In order to extend this climate record into more recent time, some of the drill sites were revisited and extension cores were drilled. The record is a composite of snow liners (from the surface to 1.51 m depth) and a consecutive firn core from 1.31 m depth to 30.98 m depth with a diameter of 75 cm. Density was measured by micro-focus Computer Tomography with a resolution of 0.1mm and subsequently smoothed with a Gaussian filter with sigma 2cm. Stable oxygen isotopes (δ18O) were measured on discrete samples using Cavity Ring-Down spectroscopy with an L2120-i and L2130-i by Picarro Instruments. The δ18O concentration with depth is provided with a precision of better than 0.1 per mil. Using raw, un-calibrated dielectrical profiling (DEP) data from the field (NEEM Set-up), volcanic tie points were derived. Together with the smoothed density data and the isotopic composition the record was dated by layer counting. The annual mean values of δ18O were obtained based on this dating. Based on the density measurements and the dating, the annual accumulation rate in water equivalent depth was derived.

Description: Here, we present internal reflection horizons (IRHs) picked in radargrams in the Dome Fuji region, Antarctica based on 22 radar profiles collected with the airborne radio-echo sounding (RES) system of the AWI mounted on its Basler BT-67 aircraft during the 2016/17 Antarctic season. 6 or 7 IRHs are traced in each radargram. The IRHs are then conneced to the Dome Fuji ice core and used to transfer the age-depth scale from the ice core to the large Dome Fuji region. The age-depth information of the IRHs are then input to a 1D ice flow model to recostruct the age field in the lower part of ice, and to evaluate basal thermal state.

Description: The isotopic composition of water in ice sheets is extensively used to infer past climate changes. In low-accumulation regions their interpretation is however challenged by poorly constrained effects that may influence the initial isotope signal during and after deposition of the snow. This is reflected in snow-pit isotope data from Kohnen Station, Antarctica, which exhibit a seasonal cycle but also strong inter-annual variations that contradict local temperature observations. These inconsistencies persist even after averaging many profiles and are thus not explained by local stratigraphic noise. Previous studies have suggested that post-depositional processes may significantly influence the isotopic composition of East Antarctic firn. Here, we investigate the importance of post-depositional processes within the open-porous firn (〉 10 cm depth) at Kohnen Station by separating spatial from temporal variability. To this end, we analyse 22 isotope profiles obtained from two snow trenches and examine the temporal isotope modifications by comparing the new with published trench data extracted 2 years earlier. The initial isotope profiles undergo changes over time due to downward-advection, firn diffusion and densification in magnitudes consistent with independent estimates. Beyond that, we find further modifications of the original isotope record to be unlikely, or small in magnitude (〈〈 1 per mil RMSD). These results show that the discrepancy between local temperatures and isotopes most likely originates from spatially coherent processes prior to or during deposition, such as precipitation intermittency or systematic isotope modifications acting on drifting or loose surface snow.

Description: In order to interpret the paleoclimatic record stored in the air enclosed in polar ice cores, it is crucial to understand the fundamental lock-in process. Within the porous firn, bubbles are sealed continuously until the respective horizontal layer reaches a critical porosity. Present-day firn models use a postulated temperature dependence of this value as the only parameter to adjust to the surrounding conditions of individual sites. However, no direct measurements of the firn microstructure could confirm these assumptions. Here we show that the critical porosity is a universal constant by providing a statistically solid data set of µm-resolution 3D X-ray computer tomographic measurements for ice cores representing different extremes of the temperature and accumulation ranges. We demonstrate why indirect measurements yield misleading data and substantiate our observations by applying percolation theory as a theoretical framework for bubble trapping. Incorporation of our results does significantly influence the dating of trace gas records, changing gas age-ice age differences by up to more than 1000 years. This will help resolve inconsistencies, such as differences between East Antarctic d15N records (as a proxy for firn height) and model results. We expect our findings to be the basis for improved firn air and densification models, leading to lower dating uncertainties. The reduced coupling of proxies and surrounding conditions may allow for more sophisticated reinterpretations of trace gas records in terms of paleoclimatic changes and will foster the development of new proxies, such as the air content as a marker of local insolation.

Description: Density is a fundamental property of polar snowpacks and of particular importance for various applications such as remote sensing, surface-mass-balance estimates and paleoclimatic ice-core studies. As a result, there is growing interest in quantifying and understanding the stratigraphy of the snowpack and its spatial variability. Melting and refreezing significantly impact the density layering. Melt frequencies observed in polar ice cores are commonly used as markers for extreme summer temperatures. Moreover, the evolution of a seasonal cycle in density with depth is not well-understood as all of these aspects require extensive field data. To provide a larger sample size, we analyzed snow cores up to 5 m depth from two sampling sites on the Greenlandic and East Antarctic plateaus and determined the spatial distribution of melt features along two orthogonal trench walls of 60 m length and 4 m depth for the Greenland site. We quantify how the observed variability of melt features is strongly influencing the ability to interpret single-core melt records. In particular, prominent vertical melt pipes indicate deep penetration of water in cold Greenlandic plateau firn. For the spatial variability in density, we find a much larger homogeneity of the snowpack along the main wind direction and an imprint of surface features orthogonal to it. Thus, within a certain area (such as the footprint of an altimeter) the stratigraphic variability is directionally dependent and estimation of a representative profile (e.g. from snow cores or modeling) is not straightforward.

Description: We analysed crystal-preferred orientation of c-axis and microstructure data from the Alpine ice core KCC at an unprecedented resolution and coverage of any Alpine ice core. We find that an anisotropic single-maximum fabric develops as early as 25 m depth in firn under vertical compression and strengthens under simple shear conditions towards the bedrock at 72 m depth. The analysis of continuously measured intervals with subsequent thin section samples from several depths of the ice core reveals a high spatial variability in the crystal orientation and crystal size on the 10 cm-scale as well as within a few centimeters. We quantify the variability and investigate the possible causes and links to other microstructural properties. Our findings support the hypothesis that the observed variability is a consequence of strain localisation on small spatial scales with influence on fabric and microstructure. From a methodological perspective, the results of this study lead us to challenge whether single thin sections from ice cores provide representative parameters for their depth to be used to infer the fabric development in a glacier on the large scale. Previously proposed uncertainty estimates for fabric and grain size parameters do not capture the observed variability. This might therefore demand a new scale-sensitive statistical approach.

Description: Stable water isotope composition (δ¹⁸O, δD, d excess) and accumulation rates of six firn cores from Union Glacier, Ellsworth Mountains, West Antarctica