Unravelling Antarctica’s past through the stratigraphy of a deep ice core: an image-analysis study of the EPICA-DML line-scan images

Polar ice research has undergone great progress in the last six decades. One of its recent technological achievements has been the development of new techniques for digital image recording and analysis of ice-core stratigraphy and microstructure. In this work we investigate one such image records, namely the line-scan image records of the EPICA-DML (European Project for Ice Coring in Antarctica, Dronning Maud Land) deep ice core. These images provide a multiscale depiction of the stratigraphy and structure of the Antarctic Ice Sheet. While previous studies have focused on the ice-core optical stratigraphy on the micro- and mesoscale (<1 mm and 10−3–1 m, respectively), in this work we present several methods to obtain fast and reliable information on the ice-core stratigraphy on the macroscale (1–103 m), including the full ice-sheet thickness. The paleoclimatic relevance of the ice-core optical stratigraphy on the macroscale is demonstrated through the comparison of the line-scan grey-value record of the EPICA-DML deep ice core with its mineral dust record, which is used as a proxy for microinclusions and for several other types of climate proxies. Additionally, we introduce a novel method to estimate the macroscopic air-bubble concentration (including number and size of bubbles) in ice cores, which is simpler, faster, and almost as reliable as painstaking microscopic studies. After a brief excursion on the relation between macroscopic and mesoscopic measures of optical stratigraphy, we close this work by making the case for a multi-measure analysis of ice-core line-scan images, which enables us to obtain a broad perspective of the optical stratigraphy of the whole ice core, with relevance for paleoclimate and ice-sheet-flow studies.This research is supported by the Spanish Government through the María de Maeztu excellence accreditation 2018–2022 (Ref. MDM-2017-0714), and by the Basque Government through the BERC 2018–2021 programme. SHF acknowledges support from the Spanish Ministry of Science, Innovation, and Universities (MCIU) through the project iMechPro (RTI2018–100696–B–I00), and from the Ramón y Cajal grant RYC–2012–12167 of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO

Direct evidence for continuous radar reflector originating from changes in crystal-orientation fabric

International audienceThe origin of a strong continuous radar reflector observed with airborne radio-echo sounding (RES) at the EPICA deep-drilling site in Dronning Maud Land, Antarctica, is identified as a transition in crystal fabric orientation from a vertical girdle to an increased single-pole orientation seen along the ice core. The reflector is observed with a 60 ns and 600 ns long pulse at a frequency of 150 MHz, spans one pulse length, is continuous over 5 km, and occurs at a depth of about 2025?2045 m at the drill site. Changes in conductivity as reflector origin are excluded by investigating the ice-core profile, synthetic RES data, and a RES profile with different electromagnetic polarisation azimuths. The reflector's magnitude shows maximum values for polarisation parallel to the nearby ice divide and disappears for polarisation perpendicular to it, identifying the orientation of the girdle to lie in the vertical plane parallel to the ice divide. Observations allow us to extrapolate the crystal orientation feature along the reflector in space, with implications for ice-sheet dynamics and modeling

Micro-deformation of the NEEM ice core: implications for stratigraphic interpretation

第2回極域科学シンポジウム　氷床コアセッション 11月16日（水） 国立極地研究所 2階大会議

Direct evidence for radar reflector originating from changes in crystal-orientation fabric

International audienceThe origin of a strong continuous radar reflector observed with airborne radio-echo sounding (RES) at the EPICA deep-drilling site in Dronning Maud Land, Antarctica, is identified as a transition in crystal fabric orientation from a vertical girdle- to increased single-pole orientation seen along the ice core. The reflector is observed with a 60 ns and 600 ns long pulse at a frequency of 150 MHz, spans one pulse length, is continuous over 5 km, and occurs at a depth of about 2020?2030 m at the drill site. Changes in conductivity as reflector origin are excluded by investigating the ice-core profile and synthetic RES data. Our observations allow to extrapolate the crystal orientation feature along the reflector in space, with implications for ice-sheet dynamics. As the conductivity profile of the EPICA shows no distinctive peak at this depths, we exclude changes in conductivity as the reflector origin. This is supported by application of numerical forward modelling of electromagnetic wave propagation, based on the conductivity profile, which is able to reproduce nearby reflections, but fails to reproduce this one. Because of background noise, the permittivity profile based on dielectric does not show prominent signals at these depths. We therefore interpret the observed reflector to originate from this change in crystal fabric

Simulating ice core 10Be on the glacial–interglacial timescale

10Be ice core measurements are an important tool for paleoclimate research, e.g., allowing for the reconstruction of past solar activity or changes in the geomagnetic dipole field. However, especially on multi-millennial timescales, the share of production and climate-induced variations of respective 10Be ice core records is still up for debate. Here we present the first quantitative climatological model of the 10Be ice concentration up to the glacial–interglacial timescale. The model approach is composed of (i) a coarse resolution global atmospheric transport model and (ii) a local 10Be air–firn transfer model. Extensive global-scale observational data of short-lived radionuclides as well as new polar 10Be snow-pit measurements are used for model calibration and validation. Being specifically configured for 10Be in polar ice, this tool thus allows for a straightforward investigation of production- and non-production-related modulation of this nuclide. We find that the polar 10Be ice concentration does not immediately record the globally mixed cosmogenic production signal. Using geomagnetic modulation and revised Greenland snow accumulation rate changes as model input, we simulate the observed Greenland Summit (GRIP and GISP2) 10Be ice core records over the last 75 kyr (on the GICC05modelext timescale). We show that our basic model is capable of reproducing the largest portion of the observed 10Be changes. However, model–measurement differences exhibit multi-millennial trends (differences up to 87% in case of normalized to the Holocene records) which call for closer investigation. Focusing on the (12–37) b2k (before the year AD 2000) period, mean model–measurement differences of 30% cannot be attributed to production changes. However, unconsidered climate-induced changes could likely explain the model–measurement mismatch. In fact, the 10Be ice concentration is very sensitive to snow accumulation changes. Here the reconstructed Greenland Summit (GRIP) snow accumulation rate record would require revision of +28% to solely account for the (12–37) b2k model–measurement differences

Constraints on post-depositional isotope modifications in East Antarctic firn from analysing temporal changes of isotope profiles

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 interannual 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 data 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 ‰ 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

A search in north Greenland for a new ice-core drill site

This is the published version. Copyright International Glaciological SocietyA new deep ice-core drilling site has been identified in north Greenland at 75.12 ° N, 42 .30 ° W, 316 km north-northwest (NNW) of the GRIP drill site on the summit of the ice sheet. The ice thickness here is 3085 m; the surface elevation is 2919 m. The North GRIP (NG RIP) site is identified so that ice of Eemian age (115- 130 ka BP, calendar years before present ) is located as far above bedrock as possible and so the thickness of the Eemian layer is as great as possible. An ice-flow model, similar to the one used to date the GRIP ice core, is used to simulate the flow along the NNW-trending ice ridge. Surface and bedrock elevations, surface accumulation-rate distribution and radio-echo sounding along the ridge have been used as model input. The surface accumulation rate drops from 0.23 mice equivalent year 1 at GRIP to 0.19 mice equivalent year- 1 50 km from GRIP. Over the following 300 km the accumulation is relatively constant, before it starts decreasing again further north. Ice thicknesses up to 3250 m bring the temperature of the basal ice up to the pressure-melting point 100- 250 km from GRIP. The NGRIP site is located 316 km from GRIP in a region where the bedrock is smooth and the accumulation rate is 0.19 m ice equivalent year 1 • The modeled basal ice here has always been a few degrees below the pressure-melting point. Internal radio-echo sounding horizons can be traceq between the GRIP and NGRIP sites, allowing us to date the ice down to 2300 m depth (52 ka BP ). An ice-flow model predicts that the Eemian-age ice will be located in the depth range 2710 - 2800 m, which is 285 m above the bedrock. This is 120 m further above the bedrock, and the thickness of the Eemian layer of ice is 20 m thicker, than at the GRIP ice-core ite