scholarly journals Pervasive diffusion of climate signals recorded in ice-vein ionic impurities

2021 ◽  
Vol 15 (4) ◽  
pp. 1787-1810
Author(s):  
Felix S. L. Ng
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Impurity Concentration ◽  
Ice Core ◽  
Ice Cores ◽  
Original Theory ◽  
Soluble Ions ◽  
Spatial Fluctuations ◽  
Climate Signals ◽  
Physical Interactions

Abstract. A theory of vein impurity transport conceived two decades ago predicts that signals in the bulk concentration of soluble ions in ice migrate under a temperature gradient. If valid, it would mean that some palaeoclimatic signals deep in ice cores (signals from vein impurities as opposed to matrix or grain-boundary impurities) suffer displacements that upset their dating and alignment with other proxies. We revisit the vein physical interactions to find that a strong diffusion acts on such signals. It arises because the Gibbs–Thomson effect, which the original theory neglected, perturbs the impurity concentration of the vein water wherever the bulk impurity concentration carries a signal. Thus, any migrating vein signals will not survive into deep ice where their displacement matters, and the palaeoclimatic concern posed by the original theory no longer stands. Simulations with signal peaks introduced in shallow ice at the GRIP and EPICA Dome C ice-core sites, ignoring spatial fluctuations of the ice grain size, confirm that rapid damping and broadening eradicates the peaks by two-thirds way down the ice column. Artificially reducing the solute diffusivity in water (to mimic partially connected veins) by 103 times or more is necessary for signals to penetrate into the lowest several hundred metres with minimal amplitude loss. Simulations incorporating grain-size fluctuations on the decimetre scale show that these can cause the formation of new, non-migrating solute peaks. The deep solute peaks observed in ice cores can only be explained by widespread vein disconnection or a dominance of matrix or grain-boundary impurities at depth (including their recent transfer to veins) or signal formation induced by grain-size fluctuations; in all cases, the deep peaks would not have displaced far. Disentangling the different signal contributions – from veins, the ice matrix, grain boundaries, and grain-size fluctuations – will aid robust reconstruction from ion records.

scholarly journals Pervasive diffusion of climate signals recorded in ice-vein ionic impurities

2020 ◽  
Author(s):  
Felix S. L. Ng
Keyword(s):  
Grain Boundary ◽  
Impurity Concentration ◽  
Ice Core ◽  
Ice Cores ◽  
Original Theory ◽  
Soluble Ions ◽  
Climate Signals ◽  
Physical Interactions ◽  
Ionic Impurities ◽  
Strong Diffusion

Abstract. A theory of vein impurity transport conceived two decades ago predicts that signals in the bulk concentration of soluble ions in ice migrate under a temperature gradient. If valid, it would mean that some palaeoclimatic signals deep in ice cores (signals from vein impurities as opposed to matrix/grain-boundary impurities) suffer displacements that upset their dating and alignment with other proxies. We revisit the vein physical interactions to show that a strong diffusion prevents such signals from surviving into deep ice. It arises because the Gibbs–Thomson effect, which the original theory had neglected, perturbs the impurity concentration of the vein water wherever the bulk impurity concentration carries a signal. Thus no distinct vein signals will reach a depth where their displacement matters; accordingly, the palaeoclimatic concern posed by the original theory no longer stands. Simulations with signal peaks introduced in shallow ice at the GRIP and EPICA Dome C ice-core sites confirm that rapid damping and broadening eradicates their form by two-thirds way down the ice column; artificially reducing the solute diffusivity in water (to mimic partially-connected veins) by 103 times or more is necessary for signals to penetrate into the lowest several hundred metres with minimal loss of amplitude. The deep solute peaks observed in ice cores can only be explained by widespread vein disconnection or a dominance of matrix/grain-boundary impurities at depth (including their recent transfer to veins); in either case, the deep peaks would not have displaced far. Decomposing the vein and matrix impurity contributions will aid robust reconstruction from ion records.

scholarly journals Mechanical Properties of Dye 3 Greenland Deep Ice Cores

1984 ◽  
Vol 5 ◽  
pp. 1-8 ◽  
Author(s):  
Nobuhiko Azuma ◽  
Akira Higashi
Keyword(s):  
Mechanical Properties ◽  
Grain Size ◽  
Power Law ◽  
Ice Core ◽  
Ice Cores ◽  
Uniaxial Stress ◽  
Strain Rates ◽  
Compression Tests ◽  
Polycrystalline Ice ◽  
Relationship Of

Uniaxial compression tests were carried out with specimens cut from several deep ice cores obtained at Dye 3, Greenland, in 1980 and 1981. The power law relationship of = Αση was obtained between the uniaxial strain-rate and the uniaxial stress σ. In a range of strain-rates between 10−8 and 10−7 s−1, the value of the power n for samples with strong single maximum fabric was approximately 4, significantly larger than the value of 3 which has been generally accepted from experiments using artificial polycrystalline ice. A work-hardening effect was found in the ice-core samples taken from a depth of 1900 m, which had a smaller grain size than the others. Recrystallization occurred when the temperature of the specimen was raised during the test and this ultimately caused the formation of the so-called diamond pattern ice fabric.

scholarly journals A model of crystal-size evolution in polar ice masses

2014 ◽  
Vol 60 (221) ◽  
pp. 463-477 ◽  
Author(s):  
Felix NG ◽  
T.H. Jacka
Keyword(s):  
Crystal Growth ◽  
Dislocation Density ◽  
Grain Boundary ◽  
Crystal Size ◽  
Boundary Migration ◽  
Ice Core ◽  
Ice Cores ◽  
Grain Boundary Migration ◽  
Shallow Subsurface ◽  
The Mean

AbstractIn the deep ice cores drilled at the GRIP, NGRIP and GISP2 sites in Greenland and at Byrd Station and the summit of Law Dome in Antarctica, the mean crystal size increases with depth in the shallow subsurface and reaches steady values at intermediate depth. This behaviour has been attributed to the competition between grain-boundary migration driven crystal growth and crystal polygonization, but the effects of changing crystal dislocation density and non-equiaxed crystal shape in this competition are uncertain. We study these effects with a simple model. It describes how the mean height and width of crystals evolve as they flatten under vertical compression, and as crystal growth and polygonization compete. The polygonization rate is assumed to be proportional to the mean dislocation density across crystals. Migration recrystallization, which can affect crystal growth via strain-induced grain boundary migration but whose impact on the mean crystal size is difficult to quantify for ice at present, is not accounted for. When applied to the five ice-core sites, the model simulates the observed crystal-size profiles well down to the bottom of their steady regions, although the match for Law Dome is less satisfactory. Polygonization rate factors retrieved for the sites range from 10–5 to 10–2 a–1. We conclude that since crystal size and dislocation density evolve in a strongly coupled manner, consistent modelling requires multiple differential equations to track both of these variables. Future ice-core analysis should also determine crystal size in all three principal directions.

Diffusion of climatic signals in ice cores by vein migration

2020 ◽  
Author(s):  
Felix S. L. Ng
Keyword(s):  
Grain Growth ◽  
Impurity Concentration ◽  
Diffusion Mechanism ◽  
Ice Core ◽  
Ice Cores ◽  
Spatial Gradient ◽  
Stochastic Motion ◽  
Advection Diffusion Equation ◽  
Peak Broadening ◽  
Climatic Signals

<p>Ice-core analysis shows that climatic signals carried by dissolved impurities (e.g., sulphate) in the water veins exhibit peak broadening and damping with depth into the ice. Such diffusion distorts the signals progressively, limiting the retrievability of the past climatic variations, notably their time resolution. A mechanism put forward for the diffusion invokes continuous differential grain growth in creating gradients in impurity concentration in the vein water (Barnes et al., 2003). Separately, a mechanism known as “anomalous diffusion” has been proposed (Rempel et al., 2001) where vertical temperature gradients in the ice drive the migration of chemical peaks without diffusion — this migration causes age offset between the signals and the ice. Here, we show that climatic signals diffuse because of constant dynamical evolution of the vein network in polycrystalline ice that accompanies grain-boundary migration. In this new mechanism, the stochastic motion of vein segments carrying solute leads to a net diffusive transport of impurities when there is spatial gradient in the bulk impurity concentration or porosity. By modelling this phenomenon with a statistical-mechanical formulation in three dimensions, we find that the diffusivity <em>κ</em> for the bulk impurity concentration is given by <em>κ</em> = <em>K</em>(<em>T</em>)/3c, where <em>K</em> is the temperature-dependent grain growth rate and c (≈ 2 to 3) is a geometry constant, and that <em>κ</em> is independent of the mean grain size. The description of porosity follows an advection-diffusion equation that includes the other processes of Rempel et al. (2001) and Barnes et al. (2003). Our calculations for the Greenland summit ice cores and the EPICA ice core predict diffusivities of <em>κ</em> ∼ 10<sup>–8</sup> – 10<sup>–7 </sup>m<sup>2</sup> yr<sup>–1</sup>, which can explain the observed amount of peak broadening. Further including into this theory the regelative transport of the solute by water flow along the veins reveals a correction of ≈ 10% for the signal migration speed predicted by Rempel et al. (2001). Besides contributing a new diffusion mechanism, our study highlights the importance of grain-scale recrystallisation processes for understanding bulk ice properties.</p>

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

2020 ◽  
Author(s):  
Ilka Weikusat ◽  
David Wallis ◽  
Steven Franke ◽  
Nicolas Stoll ◽  
Julien Westhoff ◽  
...  
Keyword(s):  
Grain Size ◽  
Standard Procedure ◽  
Ice Core ◽  
Ice Sheet ◽  
Ice Cores ◽  
Orientation Distribution ◽  
Geothermal Heat ◽  
Ice Dynamics ◽  
The Core ◽  
Pulling Forces

<p>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.</p><p> </p><p>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.</p><p> </p><p>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.</p><p> </p><p>---------------------</p><p>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), <strong>2007</strong>, 635-639</p><p>Schulson, E. M. & Duval, P. Creep and Fracture of Ice. Cambridge University Press, <strong>2009</strong>, 401</p>

Microstructure through an Ice Sheet

2013 ◽  
Vol 753 ◽  
pp. 481-484 ◽  
Author(s):  
Tobias Binder ◽  
Ilka Weikusat ◽  
Johannes Freitag ◽  
Christoph S. Garbe ◽  
Dietmar Wagenbach ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Driving Forces ◽  
Boundary Migration ◽  
Ice Core ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Cores ◽  
Grain Size Analysis ◽  
Size Analysis

Ice cores through an ice sheet can be regarded as a sample of a unique natural deformation experiment lasting up to a million years. Compared to other geological materials forming the earth‘s crust, the microstructure is directly accessible over the full depth. Controlled sublimation etching of polished ice sections reveals pores, air bubbles, grain boundaries and sub-grain boundaries at the surface. The microstructural features emanating at the surface are scanned. A dedicated method of digital image processing has been developed to extract and characterize the grain boundary networks. First preliminary results obtained from an ice core drilled through the Greenland ice sheet are presented. We discuss the role of small grains in grain size analysis and derive from the shape of grain boundaries the acting driving forces for grain boundary migration.

Representativeness of decadal-scale climate signals in ice-core aerosol records

2020 ◽  
Author(s):  
Tobias Erhardt ◽  
Camilla Jensen ◽  
Maria Hörhold ◽  
Hubertus Fischer
Keyword(s):  
High Resolution ◽  
Time Scales ◽  
Decadal Variability ◽  
Ice Core ◽  
Ice Cores ◽  
Aerosol Deposition ◽  
High Temporal Resolution ◽  
Annual Layer ◽  
Decadal Scale ◽  
Climate Signals

<p>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.</p><p>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.</p>

scholarly journals On the role of grain growth, recrystallization and polygonization in a continuum theory for anisotropic ice sheets

2004 ◽  
Vol 39 ◽  
pp. 49-52 ◽  
Author(s):  
Luca Placidi ◽  
Sérgio H. Faria ◽  
Kolumban Hutter
Keyword(s):  
Grain Boundary ◽  
Grain Growth ◽  
Boundary Migration ◽  
Ice Core ◽  
Continuum Theory ◽  
Ice Cores ◽  
Grain Boundary Migration ◽  
Bending Stresses ◽  
Continuum Description ◽  
Continuous Diversity

AbstractWe outline how to incorporate microscale effects of polycrystalline ice into a continuum description. Actually, analyses of ice cores in Antarctica show that different microstructures generally produce different responses, i.e. a non-uniform distribution of c axes gives rise to anisotropic behaviour. It has been recognized that, to describe certain microstructural processes, like recrystallization or polygonization, we need a parameter able to switch them on (e.g. dislocation density or its associated lattice distortion energy). With this in mind, balance equations for a continuum theory of an anisotropic ice sheet undergoing recrystallization have been recently proposed. In this work, we examine relations for some constitutive quantities, in order to take into account the effects of grain-boundary migration, nucleation and polygonization. We check our assumptions by explicit comparison with the first 1200 m of the Byrd (Antarctica) ice core. Current literature usually gives a relation between normal grain growth and grain boundary migration rate. Here, an equation for normal grain growth which also incorporates the influence of polygonization is suggested. It is based on experimental data from the same core in Antarctica. Polygonization is a microscopic process, but here we present a continuum description of the bending stresses which promote the fragmentation of crystallites in terms of the theory of mixtures with continuous diversity.

scholarly journals Location and distribution of micro-inclusions in the EDML and NEEM ice cores using optical microscopy and in situ Raman spectroscopy

2017 ◽  
Vol 11 (3) ◽  
pp. 1075-1090 ◽  
Author(s):  
Jan Eichler ◽  
Ina Kleitz ◽  
Maddalena Bayer-Giraldi ◽  
Daniela Jansen ◽  
Sepp Kipfstuhl ◽  
...  
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Large Scale ◽  
Flow Behavior ◽  
Boundary Migration ◽  
Ice Cores ◽  
Impurity Effect ◽  
Polar Ice

Abstract. Impurities control a variety of physical properties of polar ice. Their impact can be observed at all scales – from the microstructure (e.g., grain size and orientation) to the ice sheet flow behavior (e.g., borehole tilting and closure). Most impurities in ice form micrometer-sized inclusions. It has been suggested that these µ inclusions control the grain size of polycrystalline ice by pinning of grain boundaries (Zener pinning), which should be reflected in their distribution with respect to the grain boundary network. We used an optical microscope to generate high-resolution large-scale maps (3 µm pix−1, 8 × 2 cm2) of the distribution of micro-inclusions in four polar ice samples: two from Antarctica (EDML, MIS 5.5) and two from Greenland (NEEM, Holocene). The in situ positions of more than 5000 µ inclusions have been determined. A Raman microscope was used to confirm the extrinsic nature of a sample proportion of the mapped inclusions. A superposition of the 2-D grain boundary network and µ-inclusion distributions shows no significant correlations between grain boundaries and µ inclusions. In particular, no signs of grain boundaries harvesting µ inclusions could be found and no evidence of µ inclusions inhibiting grain boundary migration by slow-mode pinning could be detected. Consequences for our understanding of the impurity effect on ice microstructure and rheology are discussed.

Mechanical Properties of Dye 3 Greenland Deep Ice Cores

1984 ◽  
Vol 5 ◽  
pp. 1-8 ◽  
Author(s):  
Nobuhiko Azuma ◽  
Akira Higashi
Keyword(s):  
Mechanical Properties ◽  
Grain Size ◽  
Power Law ◽  
Ice Core ◽  
Ice Cores ◽  
Uniaxial Stress ◽  
Strain Rates ◽  
Compression Tests ◽  
Polycrystalline Ice ◽  
Relationship Of

Uniaxial compression tests were carried out with specimens cut from several deep ice cores obtained at Dye 3, Greenland, in 1980 and 1981. The power law relationship of= Ασηwas obtained between the uniaxial strain-rateand the uniaxial stress σ. In a range of strain-rates between 10−8and 10−7s−1, the value of the power n for samples with strong single maximum fabric was approximately 4, significantly larger than the value of 3 which has been generally accepted from experiments using artificial polycrystalline ice. A work-hardening effect was found in the ice-core samples taken from a depth of 1900 m, which had a smaller grain size than the others. Recrystallization occurred when the temperature of the specimen was raised during the test and this ultimately caused the formation of the so-called diamond pattern ice fabric.

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