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

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 the last glacial. 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 almost 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 found minerals are related to terrestrial dust, such as quartz, feldspar, mica, and hematite. We further found carbonaceous particles, dolomite, and gypsum in high abundance. Rare minerals are e.g., rutile, anatase, epidote, titanite, and grossular. 2D impurity imaging with 20 &mu;m resolution revealed that Na, Mg and Sr are mainly at grain boundaries. Dust-related analytes, such as Al, Fe, and Ti, are also located in the grain interior forming clusters of insoluble impurities. Cloudy bands are thus clearly distinguishable in the chemical data. We present novel vast micron-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.</p

Folded ice stratigraphy in North East Greenland: A three dimensional structural analysis

Advances in radio-echo sounding technology over the last two decades made it possible to map complex englacial structures in the lower part of the Greenland and Antarctic Ice Sheet. Deformation structures are made visible by distorted isochrones acting as radar reflectors. Decoding the formation history of these structures offers an excellent possibility to reconstruct past ice movements, and thus provides an additional archive about processes on the earth's surface in the past. In this study, we use ultra-wideband ice-penetrating radar data to map the deformation of the radar stratigraphy in Northern Greenland. We construct 3-dimensional horizons from folded radar layers of features which show no apparent link to the current velocity field or the regional bed topography. Furthermore, we are able to constrain the geometry and spatial extend of folds, which suggests that they were formed in several stages and in a different ice-dynamic setting than the present one in Northern Greenland

Issues with fracturing ice during an ice drilling project in Greenland (EastGRIP)

Drilling an ice core through an ice sheet (typically 2000 to 3000 m thick) is a technical challenge that nonetheless generates valuable and unique information on palaeo-climate and ice dynamics. As technically the drilling cannot be done in one run, the core has to be fractured approximately every 3 m to retrieve core sections from the bore hole. This fracture process is initiated by breaking the core with core-catchers which also clamp the engaged core in the drill head while the whole drill is then pulled up with the winch motor. This standard procedure is known to become difficult and requires extremely high pulling forces (Wilhelms et al. 2007), in the very deep part of the drill procedure, close to the bedrock of the ice sheet, especially when the ice material becomes warm (approximately -2°C) due to the geothermal heat released from the bedrock. Recently, during the EastGRIP (East Greenland Ice coring Project) drilling we observed a similar issue with breaking off cored sections only with extremely high pulling forces, but started from approximately 1800 m of depth, where the temperature is still very cold (approximately -20°C). This has not been observed at other ice drilling sites. As dependencies of fracture behaviour on crystal orientation and grain size are known (Schulson & Duval 2009) for ice, we thus examined the microstructure in the ice samples close to and at the core breaks. First preliminary results suggest that these so far unexperienced difficulties are due to the profoundly different c-axes orientation distribution (CPO) in the EastGRIP ice core. In contrast to other deep ice cores which have been drilled on ice domes or ice divides, EastGRIP is located in an ice stream. This location means that the deformation geometry (kinematics) is completely different, resulting in a different CPO (girdle pattern instead of single maximum pattern). Evidence regarding additional grain-size dependence will hopefully help to refine the fracturing procedure, which is possible due to a rather strong grain size layering observed in natural ice formed by snow precipitation. --------------------- Wilhelms, F.; Sheldon, S. G.; Hamann, I. & Kipfstuhl, S. Implications for and findings from deep ice core drillings - An example: The ultimate tensile strength of ice at high strain rates. Physics and Chemistry of Ice (The proceedings of the International Conference on the Physics and Chemistry of Ice held at Bremerhaven, Germany on 23-28 July 2006), 2007, 635-639 Schulson, E. M. & Duval, P. Creep and Fracture of Ice. Cambridge University Press, 2009, 40

Comment on “Exceptionally high heat flux needed to sustain the Northeast Greenland Ice Stream” by Smith-Johnsen et al. (2020)

Smith-Johnsen et al. (The Cryosphere, 14, 841–854, https://doi.org/10.5194/tc-14-841-2020, 2020) model the effect of a potential hotspot on the Northeast Greenland Ice Stream (NEGIS). They argue that a heat flux of at least 970 mW m−2 is required to have initiated or to control NEGIS. Such an exceptionally high heat flux would be unique in the world and is incompatible with known geological processes that can raise the heat flux. Fast flow at NEGIS must thus be possible without the extraordinary melt rates invoked in Smith-Johnsen et al. (2020)

Melt in the Greenland EastGRIP ice core reveals Holocene warm events

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

Deformation Features and Disturbances in the Stratigraphy of the EastGRIP Ice Core

The EastGRIP (East GReenland Ice core Project) Ice Core is drilled in a highly dynamic area, the North East Greenland Ice Stream (NEGIS), with a surface velocity of 55 m/yr, representing high flow rates in this area. All previous deep ice cores in Greenland were mainly drilled to find undisturbed ice for climate reconstruction. In contrast, the main purpose of this ice core is to increase our understanding of ice flow and linking it to deformation features shown in the physical properties. Some of these deformation features can be made visible using the line scanner device. It scans a polished ice core slab illuminated by an indirect light source (similar to dark field microscopy) and thus makes internal features (e.g. impurities, bubbles, hydrates and partly grain boundaries) visible, creating a 10x165 cm image of the core. Light traveling though the core is reflected and scattered at these features thus causing the camera to detect a bright section where the impurity content is high (“cloudy bands”), whereas ice with a low impurity concentration will not reflect light and contribute a dark layer in the image (“clear bands”). This is used to make layering, i.e. the stratigraphy, visible. Ice from the last glacial period has a well layered stratigraphy resulting from fairly regular annual dust storms in spring to summer. As deformation increases and deformation modes change towards the bottom of the core, these layers will show disturbances and folding. A strong relationship between the δ18O of the water isotopes in the ice core and the impurity concentration, derived from the visual intensity of different layers, can be observed. The correlation of these two makes way for a very precise correlation of the visual stratigraphy and their δ18O age, to analyze differences in deformation modes of ice from different climatic periods. Main deformation in the upper part of the ice sheet is pure shear (stretching along the horizontal and thinning in the vertical) and simple shear in the bottom parts. The gradual change from pure to simple shear is seen in the development of small scale disturbances in the layers, such as wavy patterns. The evolution of these features into z- and s-folds is expected in greater depth. Hereby the layer will deform into a z- or s-shape, overturning a section of the layer. Wavy features, such as the ones seen here, have not been observed in other cores and could be associated to the highly dynamic drill site in NEGIS

NEGIS - A unique feature in Greenland?

Reliable knowledge of ice discharge dynamics for the Greenland Ice Sheet via its ice streams is essential if we are to understand its stability under future climate scenarios. Little however is known about the paleo ice-sheet configuration in areas still covered by ice. Here we use radio-echo sounding data to decipher the regional deformation history of the north-eastern Greenland Ice Sheet from its internal stratigraphy. We map folds deep below the surface that we attribute to the activity of a now-extinct ice stream, which shows strong similarities to the Northeast Greenland Ice Stream. We propose that locally this ancient ice flow regime reached much further inland than today’s and was ceased in the Holocene. The new insight that major ice streams may abruptly disappear will affect future approaches to understanding and modelling the response of Earth’s ice sheets to global warming

Holocene ice-stream shutdown and drainage basin reconfiguration in northeast Greenland

Reliable knowledge of ice discharge dynamics for the Greenland ice sheet via its ice streams is essential if we are to understand its stability under future climate scenarios. Currently active ice streams in Greenland have been well mapped using remote-sensing data while past ice-stream paths in what are now deglaciated regions can be reconstructed from the landforms they left behind. However, little is known about possible former and now defunct ice streams in areas still covered by ice. Here we use radio-echo sounding data to decipher the regional ice-flow history of the northeastern Greenland ice sheet on the basis of its internal stratigraphy. By creating a three-dimensional reconstruction of time-equivalent horizons, we map folds deep below the surface that we then attribute to the deformation caused by now-extinct ice streams. We propose that locally this ancient ice-flow regime was much more focused and reached much farther inland than today’s and was deactivated when the main drainage system was reconfigured and relocated southwards. The insight that major ice streams in Greenland might start, shift or abruptly disappear will affect future approaches to understanding and modelling the response of Earth’s ice sheets to global warming

Impurities throughout the EGRIP ice core – a microstructural perspective

Impurities in polar ice cores are analyzed for various reasons, ranging from the reconstruction of the climate of the past to the absolute positioning of age markers. In particular, microstructural impurity research provides insights into the internal deformation of ice and post-depositional stratigraphy changes. However, most stud- ies offer limited snapshots of impurity characteristics at a few specific depth regimes, highlighting the need to determine the localization and chemistry of impurities throughout one ice core with complementary methods. We report a detailed investigation of solid and dissolved impurities throughout the 2120 m long East Green- land Ice Core Project (EGRIP) ice core. Using microstructure mapping and confocal Cryo-Raman spectroscopy, we analyzed solid micro-inclusions inside 25 solid ice samples covering the last 50 ka. Micro-inclusions are heterogeneously distributed inside the ice matrix and in Holocene ice, as an upper limit assumption, between 22.3 and 42.4% are located in the vicinity of grain boundaries. We identified the mineralogy of more than 1600 solid inclusions. Most are terrestrial dust minerals, such as quartz, feldspar, mica, carbonaceous particles, and sulfate minerals, such as gypsum. Less common minerals are e.g., dolomite, hematite, nitrates, rutile, and anatase. However, the upper 900 m are characterized by various sulfate minerals, while gypsum is the domi- nant sulfate species below. In the deepest 400 m of the core, we expose the mineralogy inside and surrounding distinct cloudy bands. Aiming at a holistic picture of soluble and insoluble impurities, we combined two meth- ods for the first time: We further analyzed most samples with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Due to recent adaptions, LA-ICP-MS now enables us to image the 2D distribution of elements, such as Na, Mg, Al, and Fe, with a resolution of up to 10 microns showing element-depended dif- ferences in localization. For example, Na is primarily located at grain boundaries, and Al indicates dispersed particle clusters. Mg, and to some extent also Fe, are found in both regimes. Our results illustrate the merit of combining cryo-Raman spectroscopy and LA-ICP-MS to obtain new insights into small-scale deformation, chemical stratigraphy, and processes in deep ice and the future potential to enhance our understanding of impurities by exploiting such a multi-method approach