EastGRIP Research Articles

Combining traditional and novel techniques to increase our understanding of the lock-in depth of atmospheric gases in polar ice cores – results from the EastGRIP region

Abstract. We investigate the lock-in zone (LIZ) of the East Greenland Ice Core Project (EastGRIP) region, northeastern Greenland, in detail. We present results from the firn air-pumping campaign of the S6 borehole, forward modeling, and a novel technique for finding the lock-in depth (LID, the top of the LIZ) based on the visual stratigraphy of the EastGRIP ice core. The findings in this work help to deepen our knowledge of how atmospheric gases are trapped in ice cores. CO2, δ15N, and CH4 data suggest that the LID lies around 58 to 61 m depth. With the pixel value intensity and bright-spot analysis based on visual stratigraphy, we can pinpoint a change in ice properties to exactly 58.3 m depth, which we define as the optical lock-in depth (OLID). This visual change in ice properties is caused by the formation of rounded and enclosed air bubbles that alter the measured refraction of the light pathways. The results for the LID and OLID agree accurately on the depth. We furthermore use the visual stratigraphy images to obtain information on the sharpness of the open- to closed-porosity transition. Combining traditional methods with the independent optical method presented here, we can now better constrain the bubble closure processes in polar firn.

Post-depositional modification on seasonal-to-interannual timescales alters the deuterium-excess signals in summer snow layers in Greenland

Abstract. We document the isotopic evolution of near-surface snow at the East Greenland Ice Core Project (EastGRIP) ice core site in northeast Greenland using a time-resolved array of 1 m deep isotope (δ18O, δD) profiles. The snow profiles were taken from May–August during the 2017–2019 summer seasons. An age–depth model was developed and applied to each profile, mitigating the impacts of stratigraphic noise on isotope signals. Significant changes in deuterium excess (d) are observed in surface snow and near-surface snow as the snow ages. Decreases in d of up to 5 ‰ occur during summer seasons after deposition during two of the three summer seasons observed. The d always experiences a 3 ‰–5 ‰ increase after aging 1 year in the snow due to a broadening of the autumn d maximum. Models of idealized scenarios coupled with prior work indicate that the summertime post-depositional changes in d (Δd) can be explained by a combination of surface sublimation, forced ventilation of the near-surface snow down to 20–30 cm, and isotope-gradient-driven diffusion throughout the column. The interannual Δd is also partly explained with isotope-gradient-driven diffusion, but other mechanisms are at work that leave a bias in the d record. Thus, d does not just carry information about source-region conditions and transport history as is commonly assumed, but also integrates local conditions into summer snow layers as the snow ages through metamorphic processes. Finally, we observe a dramatic increase in the seasonal isotope-to-temperature sensitivity, which can be explained solely by isotope-gradient-driven diffusion. Our results are dependent on the site characteristics (e.g., wind, temperature, accumulation rate, snow properties) but indicate that more process-based research is necessary to understand water isotopes as climate proxies. Recommendations for monitoring and physical modeling are given, with special attention to the d parameter.

Spatial and temporal stable water isotope data from the upper snowpack at the EastGRIP camp site, NE Greenland, sampled in summer 2018

Abstract. Stable water isotopes stored in snow, firn and ice are used to reconstruct climatic parameters. The imprint of these parameters at the snow surface and their preservation in the upper snowpack are determined by a number of processes influencing the recording of the environmental signal. Here, we present a dataset of approximately 3800 snow samples analysed for their stable water isotope composition, which were obtained during the summer season next to the deep drilling site of the East Greenland Ice Core Project in northeast Greenland (75.635411° N, 36.000250° W). Sampling was carried out every third day between 14 May and 3 August 2018 along a 39 m long transect. Three depth intervals in the top 10 cm were sampled at 30 positions with a higher resolution closer to the surface (0–1 and 1–4 cm depth vs. 4–10 cm). The sample analysis was carried out at two renowned stable water isotope laboratories that produced isotope data with the overall highest uncertainty of 0.09 ‰ for δ18O and 0.8 ‰ for δD. This unique dataset shows the strongest δ18O variability closest to the surface, damped and delayed variations in the lowest layer, and a trend towards increasing homogeneity towards the end of the season, especially in the deepest layer. Additional information on the snow height and its temporal changes suggests a non-uniform spatial imprint of the seasonal climatic information in this area, potentially following the stratigraphic noise of the surface. The data can be used to study the relation between snow height (changes) and the imprint and preservation of the isotopic composition at a site with 10–14 cm w.e. yr−1 accumulation. The high-temporal-resolution sampling allows additional analyses on (post-)depositional processes, such as vapour–snow exchange. The data can be accessed at https://doi.org/10.1594/PANGAEA.956626 (Zuhr et al., 2023a).

On the importance of the humidity flux for the surface mass balance in the accumulation zone of the Greenland Ice Sheet

Abstract. It is highly uncertain how the humidity flux between the snow surface and the atmosphere contributes to the surface mass balance (SMB) of the interior Greenland Ice Sheet (GrIS). Due to sparse observations, evaluations of the simulated humidity flux are limited. Model-based estimates of the humidity flux contribution to the SMB are, therefore, unconstrained and even disagree in magnitude and sign. In this study, we evaluate the regional climate model MAR at the EGRIP (East Greenland Ice-Core Project) site in the accumulation zone of the GrIS. We use a combined dataset of continuous one-level bulk estimates of the humidity flux covering the period of May 2016–August 2019 and eddy-resolving eddy-covariance humidity flux measurements from all four summer seasons. In summer, we document a bias of too little sublimation (−1.3 W m−2, −1.65 mm w.e.) caused by a cold bias in both air and surface temperature, leading to a reduced humidity gradient. In winter, MAR overestimates vapor deposition by about 1 order of magnitude. This is a consequence of an overestimated temperature gradient in too stable atmospheric conditions compared to observations. Both systematic errors cause a large discrepancy in the annual net humidity flux between the model and observations of −9 mm w.e. yr−1. Remarkably, the simulated net annual humidity flux contributes positively to the SMB, contrary to observations documenting a net sublimation flux. We correct the systematic errors by applying a simple but effective correction function to the simulated latent heat flux. Using this correction, we find that 5.1 % of the annual mass gain at the EGRIP site sublimates again, and 4.3 % of the total mass gain is deposited vapor from the near-surface air. The estimated net humidity flux contribution to the annual SMB is about −1 % (net sublimation) compared to +5.6 % for the uncorrected simulation. In summer, the corrected MAR simulation shows that vapor deposition accounts for 9.6 % of the total mass gain and that 31 % of the total mass gain at the EGRIP site sublimates again. The net fluxes contribute to −32 % of the summer SMB. These results demonstrate that the humidity flux is a major driver of the summer SMB in the accumulation zone of the GrIS and highlight that even small changes could increase its importance for the annual SMB in a warming climate.

Evaluating the 11-year solar cycle and short-term 10Be deposition events with novel excess water samples from the East Greenland Ice-core Project (EGRIP)

Abstract. 10Be is produced by the interaction between galactic cosmic rays (GCRs) and solar energetic particles (SEPs) with the Earth's atmospheric constituents. The flux of GCRs is modulated by the varying strength of the magnetic fields of the Earth and the Sun. Measurement of 10Be concentrations from polar ice cores is thus a valuable tool to reconstruct the variations in the geomagnetic field and solar activity levels. The interpretation of 10Be records is, however, complicated by non-production-related effects on the 10Be deposition rate caused by climate- or weather-induced variability. Furthermore, volcanic eruptions have been proposed to lead to short-term 10Be deposition enhancements. In this study, we test the use of excess meltwater from continuous flow analysis (CFA) to measure 10Be, allowing less time-consuming and more cost-effective sample preparation. We compare two records obtained from CFA and discrete samples from the East Greenland Ice core Project (EGRIP) S6 firn core, reaching back to 1900 CE. We find that the two records agree well and that the 10Be record from CFA samples agrees as well as the discrete samples with other records from Greenland. Furthermore, by subtracting the theoretically expected GCR-induced signal, we investigate the high-frequency variability in the 10Be records from Greenland and Antarctica after 1951 CE, focusing on SEP events and volcanic eruptions. Finally, we use the 10Be records from Greenland and Antarctica to study the 11-year solar cycles, allowing us to assess the suitability of the CFA samples for the reconstruction of solar activity. This result opens new opportunities for the collection of continuous 10Be records with less time-consuming sample preparation, while saving an important portion of the ice cores for other measurements.

Bridging the Gap Between Experimental and Natural Fabrics: Modeling Ice Stream Fabric Evolution and its Comparison With Ice‐Core Data

AbstractFabrics, also known as textures or crystallographic preferred orientations, reveal information about the deformation history of the flow of polycrystalline materials, including glacial ice, olivine in the mantle, and feldspar and quartz in the crust. Ice fabrics can have an order‐of‐magnitude effect on the ease of flow in ice sheets. However, due to the choice of ice core drill site locations, the outputs of fabric models have mostly been compared to observations from the least dynamic regions of the ice sheet (ice divides). Recently, fabric data from an active ice stream have become available from ice cores drilled at the East Greenland Ice‐core Project (EGRIP). In this work, we present a novel approach that combines satellite‐derived velocity data with the fabric evolution model SpecCAF, a continuum model based on dominant dislocation creep. We directly compare model output to ice‐core observations from EGRIP, allowing us to examine the quantitative predictions of calibrated fabric models alongside those of active ice streams for the first time. As the model is calibrated against laboratory experiments, it provides a surrogate to compare the evolution of fabrics under laboratory and natural conditions. The predictions show good qualitative agreement with the observed fabric patterns and orientations, suggesting that a fabric between a girdle and horizontal maximum, orientated perpendicular to the flow direction, is the characteristic ice stream fabric. A discrepancy in fabric strength is attributed to the lower strain rates of ice sheets compared to laboratory experiments, revealing a significant strain‐rate dependence in the processes controlling fabric evolution.

Recent decline in atmospheric Pb deposition and isotopic constraints on changes in source contributions in snow from northwestern Greenland

We present lead (Pb) concentrations and isotope ratios in a continuous series of 38 snow samples from a 1.9-m snow pit, covering the period from winter 2012 to summer 2017, at the East Greenland Ice-core Project (EGRIP) ice core drill site in northwestern Greenland. Pb concentrations were highly variable, ranging from 1.53 to 94.9 pg g−1 (mean value of 10.6 pg g−1), with higher concentrations during winter and spring and lower concentrations during summer and fall. Our results show a substantial reduction in the Pb concentration of ∼50% between the 2000s and 2010s, reaching a level close to that observed in the mid-18th century, that is, the time of the Industrial Revolution. Remarkably low radiogenic Pb isotope compositions were observed in our samples compared to previously reported values during the 2000s. The Pb isotope mixing model results indicated a decreasing Chinese contribution from the 2000s onwards, while Europe/Russia emerged as a relatively more important contributor to the anthropogenic Pb input to central Greenland during the corresponding period. Thus, we hypothesized that the reduction in Pb pollution in central Greenland is largely due to a decreasing contribution from Chinese sources in response to the effectiveness of stringent emission control measures in China.

Chemical and visual characterisation of EGRIP glacial ice and cloudy bands within

Abstract. Impurities in polar ice play a critical role in ice flow, deformation, and the integrity of the ice core record. Especially cloudy bands, visible layers with high impurity concentrations, are prominent features in ice from glacial periods. Their physical and chemical properties are poorly understood, highlighting the need to analyse them in more detail. We bridge the gap between decimetre and micrometre scales by combining the visual stratigraphy line scanner, fabric analyser, microstructure mapping, Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry 2D impurity imaging. We classified approximately 1300 cloudy bands from glacial ice from the East Greenland Ice-core Project (EGRIP) ice core into seven different types. We determine the localisation and mineralogy of more than 1000 micro-inclusions at 13 depths. The majority of the minerals found are related to terrestrial dust, such as quartz, feldspar, mica, and hematite. We further found carbonaceous particles, dolomite, and gypsum in high abundance. Rutile, anatase, epidote, titanite, and grossular are infrequently observed. The 2D impurity imaging at 20 µm resolution revealed that cloudy bands are clearly distinguishable in the chemical data. Na, Mg, and Sr are mainly present at grain boundaries, whereas dust-related analytes, such as Al, Fe, and Ti, are located in the grain interior, forming clusters of insoluble impurities. We present novel vast micrometre-resolution insights into cloudy bands and describe the differences within and outside these bands. Combining the visual and chemical data results in new insights into the formation of different cloudy band types and could be the starting point for future in-depth studies on impurity signal integrity and internal deformation in deep polar ice cores.

Exploring the role of snow metamorphism on the isotopic composition of the surface snow at EastGRIP

Abstract. Stable water isotopes from polar ice cores are invaluable high-resolution climate proxy records. Recent studies have aimed to improve our understanding of how the climate signal is stored in the stable water isotope record by addressing the influence of post-depositional processes on the isotopic composition of surface snow. In this study, the relationship between surface snow metamorphism and water isotopes during precipitation-free periods is explored using measurements of snow-specific surface area (SSA). Continuous daily SSA measurements from the East Greenland Ice Core Project site (EastGRIP) during the summer seasons of 2017, 2018 and 2019 are used to develop an empirical decay model to describe events of rapid decrease in SSA linked to snow metamorphism. We find that SSA decay during precipitation-free periods at the EastGRIP site is best described by the exponential equation SSA(t)=(SSA0-22)⋅e-αt+22, and has a dependency on wind speed. The relationship between surface snow SSA and snow isotopic composition is primarily explored using empirical orthogonal function analysis. A coherence between SSA and deuterium excess is apparent during 2017 and 2019, suggesting that processes driving change in SSA also influence snow deuterium excess. By contrast, 2018 was characterised by a covariance between SSA and δ18O highlighting the inter-annual variability in surface regimes. Moreover, we observed changes in isotopic composition consistent with fractionation effects associated with sublimation and vapour diffusion during periods of rapid decrease in SSA. Our findings support recent studies which provide evidence of isotopic fractionation during sublimation, and show that snow deuterium excess is modified during snow metamorphism.

Improved estimation of the bulk ice crystal fabric asymmetry from polarimetric phase co-registration

Abstract. The bulk crystal orientation in ice influences the flow of glaciers and ice streams. The ice c-axes fabric is most reliably derived from ice cores. Because these are sparse, the spatial and vertical distribution of the fabric in the Greenland and Antarctic ice sheets is largely unknown. In recent years, methods have been developed to determine fabric characteristics from polarimetric radar measurements. The aim of this paper is to present an improved method to infer the horizontal fabric asymmetry by precisely determining the travel-time difference using co-polarised phase-sensitive radar data. We applied this method to six radar measurements from the East Greenland Ice-core Project (EastGRIP) drill site on Greenland's largest ice stream to give a proof of concept by comparing the results with the horizontal asymmetry of the bulk crystal anisotropy derived from the ice core. This comparison shows an excellent agreement, which is a large improvement compared to previously used methods. Our approach is particularly useful for determining the vertical profile of the fabric asymmetry in higher resolution and over larger depths than was achievable with previous methods, especially in regions with strong asymmetry.

Accelerating ice flow at the onset of the Northeast Greenland Ice Stream

Mass loss near the ice-sheet margin is evident from remote sensing as frontal retreat and increases in ice velocities. Velocities in the ice sheet interior are orders of magnitude smaller, making it challenging to detect velocity change. Here, we analyze a 35-year record of remotely sensed velocities, and a 6-year record of repeated GPS observations, at the East Greenland Ice-core Project (EastGRIP), located in the middle of the Northeast-Greenland Ice Stream (NEGIS). We find that the shear margins of NEGIS are accelerating, indicating a widening of the ice stream. We demonstrate that the widening of the ice stream is unlikely to be a response to recent changes at the outlets of NEGIS. Modelling indicates that the observed spatial fingerprint of acceleration is more consistent with a softening of the shear margin, e.g. due to evolving fabric or temperature, than a response to external forcing at the surface or bed.

Melt in the Greenland EastGRIP ice core reveals Holocene warm events

Abstract. We present a record of melt events obtained from the East Greenland Ice Core Project (EastGRIP) ice core in central northeastern Greenland, covering the largest part of the Holocene. The data were acquired visually using an optical dark-field line scanner. We detect and describe melt layers and lenses, seen as bubble-free layers and lenses, throughout the ice above the bubble–clathrate transition. This transition is located at 1150 m depth in the EastGRIP ice core, corresponding to an age of 9720 years b2k. We define the brittle zone in the EastGRIP ice core as that from 650 to 950 m depth, where we count on average more than three core breaks per meter. We analyze melt layer thicknesses, correct for ice thinning, and account for missing layers due to core breaks. Our record of melt events shows a large, distinct peak around 1014 years b2k (986 CE) and a broad peak around 7000 years b2k, corresponding to the Holocene Climatic Optimum. In total, we can identify approximately 831 mm of melt (corrected for thinning) over the past 10 000 years. We find that the melt event from 986 CE is most likely a large rain event similar to that from 2012 CE, and that these two events are unprecedented throughout the Holocene. We also compare the most recent 2500 years to a tree ring composite and find an overlap between melt events and tree ring anomalies indicating warm summers. Considering the ice dynamics of the EastGRIP site resulting from the flow of the Northeast Greenland Ice Stream (NEGIS), we find that summer temperatures must have been at least 3 ± 0.6 ∘C warmer during the Early Holocene compared to today.

Microstructure, micro-inclusions, and mineralogy along the EGRIP (East Greenland Ice Core Project) ice core – Part 2: Implications for palaeo-mineralogy

Abstract. Impurities in polar ice do not only allow the reconstruction of past atmospheric aerosol concentrations but also influence the physical properties of the ice. However, the localisation of impurities inside the microstructure is still under debate and little is known about the mineralogy of solid inclusions. In particular, the general mineralogical diversity throughout an ice core and the specific distribution inside the microstructure is poorly investigated; the impact of the mineralogy on the localisation of inclusions and other processes is thus hardly known. We use dust particle concentration, optical microscopy, and cryo-Raman spectroscopy to systematically locate and analyse the mineralogy of micro-inclusions in situ inside 11 solid ice samples from the upper 1340 m of the East Greenland Ice Core Project ice core. Micro-inclusions are more variable in mineralogy than previously observed and are mainly composed of mineral dust (quartz, mica, and feldspar) and sulfates (mainly gypsum). Inclusions of the same composition tend to cluster, but clustering frequency and mineralogy changes with depth. A variety of sulfates dominate the upper 900 m, while gypsum is the only sulfate in deeper samples, which however contain more mineral dust, nitrates, and dolomite. The analysed part of the core can thus be divided into two depth regimes of different mineralogy, and to a lesser degree of spatial distribution, which could originate from different chemical reactions in the ice or large-scale changes in ice cover in northeast Greenland during the mid-Holocene. The complexity of impurity mineralogy on the metre scale and centimetre scale in polar ice is still underestimated, and new methodological approaches are necessary to establish a comprehensive understanding of the role of impurities. Our results show that applying new methods to the mineralogy in ice cores and recognising its complexity, as well as the importance for localisation studies, open new avenues for understanding the role of impurities in ice cores.

Airborne ultra-wideband radar sounding over the shear margins and along flow lines at the onset region of the Northeast Greenland Ice Stream

Abstract. We present a high-resolution airborne radar data set (EGRIP-NOR-2018) for the onset region of the Northeast Greenland Ice Stream (NEGIS). The radar data were acquired in May 2018 with the Alfred Wegener Institute's multichannel ultra-wideband (UWB) radar mounted on the Polar 6 aircraft. Radar profiles cover an area of ∼24 000 km2 and extend over the well-defined shear margins of the NEGIS. The survey area is centered at the location of the drill site of the East Greenland Ice-Core Project (EastGRIP), and several radar lines intersect at this location. The survey layout was designed to (i) map the stratigraphic signature of the shear margins with radar profiles aligned perpendicular to ice flow, (ii) trace the radar stratigraphy along several flow lines, and (iii) provide spatial coverage of ice thickness and basal properties. While we are able to resolve radar reflections in the deep stratigraphy, we cannot fully resolve the steeply inclined reflections at the tightly folded shear margins in the lower part of the ice column. The NEGIS is causing the most significant discrepancies between numerically modeled and observed ice surface velocities. Given the high likelihood of future climate and ocean warming, this extensive data set of new high-resolution radar data in combination with the EastGRIP ice core will be a key contribution to understand the past and future dynamics of the NEGIS. The EGRIP-NOR-2018 radar data products can be obtained from the PANGAEA data publisher (https://doi.pangaea.de/10.1594/PANGAEA.928569; Franke et al., 2021a).

Microstructure, micro-inclusions, and mineralogy along the EGRIP ice core – Part 1: Localisation of inclusions and deformation patterns

Abstract. Impurities deposited in polar ice enable the reconstruction of the atmospheric aerosol concentration of the past. At the same time they impact the physical properties of the ice itself such as its deformation behaviour. Impurities are thought to enhance ice deformation, but observations are ambiguous due to a shortage of comprehensive microstructural analyses. For the first time, we systematically analyse micro-inclusions in polar fast flowing ice, i.e. from the East Greenland Ice Core Project ice core drilled through the Northeast Greenland Ice Stream. In direct relation to the inclusions we derive the crystal preferred orientation, fabric, grain size, and microstructural features at 10 depths, covering the Holocene and Late Glacial. We use optical microscopy to create microstructure maps to analyse the in situ locations of inclusions in the polycrystalline, solid ice samples. Micro-inclusions are more variable in spatial distribution than previously observed and show various distributional patterns ranging from centimetre-thick layers to clusters and solitary particles, independent of depth. In half of all samples, micro-inclusions are more often located at or close to the grain boundaries by a slight margin (in the areas occupied by grain boundaries). Throughout all samples we find strong indications of dynamic recrystallisation, such as grain islands, bulging grains, and different types of sub-grain boundaries. We discuss the spatial variability in micro-inclusions, the link between spatial variability and mineralogy, and possible effects on the microstructure and deformation behaviour of the ice. Our results emphasise the need for holistic approaches in future studies, combining microstructure and impurity analysis.

An unmanned aerial vehicle sampling platform for atmospheric water vapor isotopes in polar environments

Abstract. Above polar ice sheets, atmospheric water vapor exchange occurs across the planetary boundary layer (PBL) and is an important mechanism in a number of processes that affect the surface mass balance of the ice sheets. Yet, this exchange is not well understood and has substantial implications for modeling and remote sensing of the polar hydrologic cycle. Efforts to characterize the exchange face substantial logistical challenges including the remoteness of ice sheet field camps, extreme weather conditions, low humidity and temperature that limit the effectiveness of instruments, and dangers associated with flying manned aircraft at low altitudes. Here, we present an unmanned aerial vehicle (UAV) sampling platform for operation in extreme polar environments that is capable of sampling atmospheric water vapor for subsequent measurement of water isotopes. This system was deployed to the East Greenland Ice-core Project (EastGRIP) camp in northeast Greenland during summer 2019. Four sampling flight missions were completed. With a suite of atmospheric measurements aboard the UAV (temperature, humidity, pressure, GPS) we determine the height of the PBL using online algorithms, allowing for strategic decision-making by the pilot to sample water isotopes above and below the PBL. Water isotope data were measured by a Picarro L2130-i instrument using flasks of atmospheric air collected within the nose cone of the UAV. The internal repeatability for δD and δ18O was 2.8 ‰ and 0.45 ‰, respectively, which we also compared to independent EastGRIP tower-isotope data. Based on these results, we demonstrate the efficacy of this new UAV-isotope platform and present improvements to be utilized in future polar field campaigns. The system is also designed to be readily adaptable to other fields of study, such as measurement of carbon cycle gases or remote sensing of ground conditions.

The role of sublimation as a driver of climate signals in the water isotope content of surface snow: laboratory and field experimental results

Abstract. Ice core water isotope records from Greenland and Antarctica are a valuable proxy for paleoclimate reconstruction, yet the processes influencing the climate signal stored in the isotopic composition of the snow are being challenged and revisited. Apart from precipitation input, post-depositional processes such as wind-driven redistribution and vapor–snow exchange processes at and below the surface are hypothesized to contribute to the isotope climate signal subsequently stored in the ice. Recent field studies have shown that surface snow isotopes vary between precipitation events and co-vary with vapor isotopes, which demonstrates that vapor–snow exchange is an important driving mechanism. Here we investigate how vapor–snow exchange processes influence the isotopic composition of the snowpack. Controlled laboratory experiments under forced sublimation show an increase in snow isotopic composition of up to 8 ‰ δ18O in the uppermost layer due to sublimation, with an attenuated signal down to 3 cm snow depth over the course of 4–6 d. This enrichment is accompanied by a decrease in the second-order parameter d-excess, indicating kinetic fractionation processes. Our observations confirm that sublimation alone can lead to a strong enrichment of stable water isotopes in surface snow and subsequent enrichment in the layers below. To compare laboratory experiments with realistic polar conditions, we completed four 2–3 d field experiments at the East Greenland Ice Core Project site (northeast Greenland) in summer 2019. High-resolution temporal sampling of both natural and isolated snow was conducted under clear-sky conditions and demonstrated that the snow isotopic composition changes on hourly timescales. A change of snow isotope content associated with sublimation is currently not implemented in isotope-enabled climate models and is not taken into account when interpreting ice core isotopic records. However, our results demonstrate that post-depositional processes such as sublimation contribute to the climate signal recorded in the water isotopes in surface snow, in both laboratory and field settings. This suggests that the ice core water isotope signal may effectively integrate across multiple parameters, and the ice core climate record should be interpreted as such, particularly in regions of low accumulation.

Local-scale deposition of surface snow on the Greenland ice sheet

Abstract. Ice cores from polar ice sheets and glaciers are an important climate archive. Snow layers, consecutively deposited and buried, contain climatic information from the time of their formation. However, particularly low-accumulation areas are characterised by temporally intermittent precipitation, which can be further redistributed after initial deposition, depending on the local surface features at different spatial scales. Therefore, the accumulation conditions at an ice core site influence the quantity and quality of the recorded climate signal in proxy records. This study aims to characterise the local accumulation patterns and the evolution of the snow height to describe the contribution of the snow (re-)deposition to the overall noise level in climate records from ice cores. To this end, we applied a structure-from-motion photogrammetry approach to generate near-daily elevation models of the surface snow for a 195 m2 area in the vicinity of the deep drilling site of the East Greenland Ice-core Project in northeast Greenland. Based on the snow height information we derive snow height changes on a day-to-day basis throughout our observation period from May to August 2018 and find an average snow height increase of ∼ 11 cm. The spatial and temporal data set also allows an investigation of snow deposition versus depositional modifications. We observe irregular snow deposition and erosion causing uneven snow accumulation patterns, a removal of more than 60 % of the deposited snow, and a negative relationship between the initial snow height and the amount of accumulated snow. Furthermore, the surface roughness decreased by approximately a factor of 2 throughout the spring and summer season at our study site. Finally, our study shows that structure from motion is a relatively simple method to demonstrate the potential influences of depositional processes on proxy signals in snow and ice.

A portable lightweight in situ analysis (LISA) box for ice and snow analysis

Abstract. There are enormous costs involved in transporting snow and ice samples to home laboratories for “simple” analyses in order to constrain annual layer thicknesses and identify accumulation rates of specific sites. It is well known that depositional noise, incurred from factors such as wind drifts, seasonally biased deposition and melt layers can influence individual snow and firn records and that multiple cores are required to produce statistically robust time series. Thus, at many sites, core samples are measured in the field for densification, but the annual accumulation and the content of chemical impurities are often represented by just one core to reduce transport costs. We have developed a portable “lightweight in situ analysis” (LISA) box for ice, firn and snow analysis that is capable of constraining annual layers through the continuous flow analysis of meltwater conductivity and hydrogen peroxide under field conditions. The box can run using a small gasoline generator and weighs less than 50 kg. The LISA box was tested under field conditions at the East Greenland Ice-core Project (EastGRIP) deep ice core drilling site in northern Greenland. Analysis of the top 2 m of snow from seven sites in northern Greenland allowed the reconstruction of regional snow accumulation patterns for the 2015–2018 period (summer to summer).

Upstream flow effects revealed in the EastGRIP ice core using Monte Carlo inversion of a two-dimensional ice-flow model

Abstract. The Northeast Greenland Ice Stream (NEGIS) is the largest active ice stream on the Greenland Ice Sheet (GrIS) and a crucial contributor to the ice-sheet mass balance. To investigate the ice-stream dynamics and to gain information about the past climate, a deep ice core is drilled in the upstream part of the NEGIS, termed the East Greenland Ice-core Project (EastGRIP). Upstream flow can introduce climatic bias into ice cores through the advection of ice deposited under different conditions further upstream. This is particularly true for EastGRIP due to its location inside an ice stream on the eastern flank of the GrIS. Understanding and ultimately correcting for such effects requires information on the atmospheric conditions at the time and location of snow deposition. We use a two-dimensional Dansgaard–Johnsen model to simulate ice flow along three approximated flow lines between the summit of the ice sheet (GRIP) and EastGRIP. Isochrones are traced in radio-echo-sounding images along these flow lines and dated with the GRIP and EastGRIP ice-core chronologies. The observed depth–age relationship constrains the Monte Carlo method which is used to determine unknown model parameters. We calculate backward-in-time particle trajectories to determine the source location of ice found in the EastGRIP ice core and present estimates of surface elevation and past accumulation rates at the deposition site. Our results indicate that increased snow accumulation with increasing upstream distance is predominantly responsible for the constant annual layer thicknesses observed in the upper part of the ice column at EastGRIP, and the inverted model parameters suggest that basal melting and sliding are important factors determining ice flow in the NEGIS. The results of this study form a basis for applying upstream corrections to a variety of ice-core measurements, and the inverted model parameters are useful constraints for more sophisticated modelling approaches in the future.