scholarly journals Modeling enhanced firn densification due to strain softening

2021 ◽  
Author(s):  
Falk M. Oraschewski ◽  
Aslak Grinsted
Keyword(s):  
Strain Softening ◽  
High Strain ◽  
Accumulation Rate ◽  
Ice Core ◽  
Strain Rates ◽  
Overburden Pressure ◽  
North East ◽  
The North ◽  
Horizontal Strain ◽  
Power Law Creep

Abstract. In the accumulation zone of glaciers and ice sheets snow is transformed into glacial ice by firn densification. Classically, this processes is assumed to solely depend on temperature and overburden pressure which is controlled by the accumulation rate. However, exceptionally thin firn layers have been observed in the high-strain shear margins of ice streams. Previously, it has been proposed that this firn thinning can be explained by an enhancement of firn densification due to the effect of strain softening inherent to power-law creep. This hypothesis has not been validated, and the greater firn densities in the presence of horizontal strain rates have not yet been reproduced by models. Here, we develop a model that corrects the firn densification rate predicted by classical, climate-forced models for the effect of strain softening. With the model it is confirmed that strain softening dominates the firn densification process when high strain rates are present. Firn densities along a cross section of the North-East Greenland ice stream (NEGIS) are reproduced with good agreement, validating the accuracy of the developed model. Finally, it is shown that strain softening has significant implications for ice core dating and that it considerably affects the firn properties over wide areas of the polar ice sheet, even at low strain rates. Therefore, we suggest that, besides temperature and accumulation rate, horizontal strain rates should generally be considered as a forcing parameter in firn densification modelling.

How to model enhanced firn densification due to strain softening

2021 ◽  
Author(s):  
Falk Oraschewski ◽  
Aslak Grinsted
Keyword(s):  
Strain Softening ◽  
Accumulation Rate ◽  
Strain Rates ◽  
Ice Streams ◽  
North East ◽  
The North ◽  
General Importance ◽  
Model Extension ◽  
Horizontal Strain ◽  
Power Law Creep

<p>Most classical firn densification models merely consider temperature and accumulation rate as variable input parameters. However, in locations with high horizontal strain rates, such as the shear margins of ice streams, a reduced firn thickness can be observed. This is explained by an enhancement of power-law creep due to the effect of strain softening, which is not yet captured by existing firn models. We present a model extension that corrects the densification rate, predicted by any classical, climate-forced firn model, for the effect of strain softening caused by horizontal strain rates. With the presented model firn densities measured along a cross-section of the North-East Greenland ice stream (NEGIS) are reproduced with good agreement, validating the accuracy of the developed model. The results further indicate the general importance of considering strain rates in firn densification modeling and pave the way for the development of a firn model that inherently uses temperature, accumulation rate and horizontal strain rates as forcing parameters.</p>

scholarly journals Modelling of firn densification in the presence of horizontal strain rates

2021 ◽  
Author(s):  
Falk M. Oraschewski
Keyword(s):  
Strain Softening ◽  
Accumulation Rate ◽  
Pure Shear ◽  
Greenland Ice Sheet ◽  
Strain Rates ◽  
Ice Streams ◽  
North East ◽  
The North ◽  
Horizontal Strain ◽  
Power Law Creep

The densification of polar firn that is subjected to horizontal strain rates is studied. A model for the enhanced densification of the firn by strain softening is developed. Strain softening describes an acceleration of power-law creep in the presence of high horizontal strain rates, which was suggested to explain the occurrence of exceptionally thin firn in the shear margins of ice streams. With the model the effect of strain softening is compared to other strain-driven densification mechanisms, like pure shear and strain heating, and to potential variations of temperature and accumulation rate. Thereby, strain softening is identified to dominate firn densification at high strain rates. A recorded density profile along a cross-section of the North-East Greenland ice stream (NEGIS) is reproduced with the presented model with good agreement in the shear margins. There, the thinning of the firn correlates with the location and magnitude of the shear margin troughs, which indicates that their formation is caused by strain softening. In regions with low strain rates the model overestimates the densification rate. Because of a particularly strong sensitivity of the model to low strain rates and the presence of non-zero strain rates on large parts of the Greenland Ice Sheet (GrIS), it is suggested that empirically tuned densification models already implicitly consider moderate horizontal strain rates. Besides the temperature and the accumulation rate, the effective horizontal strain rate is therefore proposed as a third forcing parameter, that needs to be considered in the development of a physics-based firn densification model.

scholarly journals Is there 1.5 million-year old ice near Dome C, Antarctica?

2017 ◽  
Author(s):  
Frédéric Parrenin ◽  
Marie G. P. Cavitte ◽  
Donald D. Blankenship ◽  
Jérome Chappellaz ◽  
Hubertus Fischer ◽  
...  
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Ice Sheets ◽  
Atmospheric Composition ◽  
Regional Environment ◽  
Ice Flow ◽  
True Nature ◽  
Flow Modelling ◽  
North East ◽  
Geothermal Flux

Abstract. Ice sheets provide exceptional archives of past changes in polar climate, regional environment and global atmospheric composition. The oldest dated deep ice core drilled in Antarctica has been retrieved at EPICA Dome C (EDC), reaching ~ 800,000 years. Obtaining an older paleoclimatic record from Antarctica is one of the greatest challenges of the ice core community. Here, we use internal isochrones, identified from airborne radar coupled to ice-flow modelling to estimate the age of basal ice along transects in the Dome C area. Three glaciological properties are inverted from isochrones: surface accumulation rate; geothermal flux; and the exponent of the Lliboutry velocity profile. We find that old ice (> 1 Myr, 1 million years) likely exists in two regions: one ~ 40 km south-west of Dome C along the ice divide to Vostok, close to a secondary dome that we name "Little Dome C" (LDC); and a second region named "North Patch" (NP) located 10–30 km north-east of Dome C, in a region where the geothermal flux is apparently relatively low. Our work demonstrates the value of combining radar observations with ice flow modelling to accurately represent the true nature of ice flow, and the formation of ice-sheet architecture, in the centre of large ice sheets.

Rate Dependent Deformation of Semi-Crystalline Polypropylene Near Room Temperature

1997 ◽  
Vol 119 (3) ◽  
pp. 216-222 ◽  
Author(s):  
E. M. Arruda ◽  
S. Ahzi ◽  
Y. Li ◽  
A. Ganesan
Keyword(s):  
Plastic Deformation ◽  
Strain Rate ◽  
Room Temperature ◽  
Strain Softening ◽  
High Strain ◽  
Constant Strain Rate ◽  
Strain Rates ◽  
High Strain Rates ◽  
Rate Dependent ◽  
Static Conditions

We examine the strain rate dependent, large plastic deformation in isotropic semi-crystalline polypropylene at room temperature. Constant strain rate uniaxial compression tests on cylindrical polypropylene specimens show very little true strain softening under quasi-static conditions. At high strain rates very large amounts (38 percent) of apparent strain softening accompanied by temperature rises are recorded. We examine the capability of a recently proposed constitutive model of plastic deformation in semi-crystalline polymers to predict this behavior. We neglect the contribution of the amorphous phase to the plastic deformation response and include the effects of adiabatic heating at high strain rates. Attention is focused on the ability to predict rate dependent yielding, strain softening, strain hardening, and adiabatic temperature rises with this approach. Comparison of simulations and experimental results show good agreement and provide insight into the merits of using a polycrystalline modeling assumption versus incorporating the amorphous contribution. Discrepancies between experiments and model predictions are explained in terms of expectations associated with neglecting the amorphous deformation.

scholarly journals A first chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core

2013 ◽  
Vol 9 (6) ◽  
pp. 2713-2730 ◽  
Author(s):  
S. O. Rasmussen ◽  
P. M. Abbott ◽  
T. Blunier ◽  
A. J. Bourne ◽  
E. Brook ◽  
...  
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Conductivity Measurement ◽  
Ice Cores ◽  
Heat Diffusion ◽  
Ice Age ◽  
Age Difference ◽  
Volcanic Origin ◽  
The North ◽  
North Greenland

Abstract. A stratigraphy-based chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core has been derived by transferring the annual layer counted Greenland Ice Core Chronology 2005 (GICC05) and its model extension (GICC05modelext) from the NGRIP core to the NEEM core using 787 match points of mainly volcanic origin identified in the electrical conductivity measurement (ECM) and dielectrical profiling (DEP) records. Tephra horizons found in both the NEEM and NGRIP ice cores are used to test the matching based on ECM and DEP and provide five additional horizons used for the timescale transfer. A thinning function reflecting the accumulated strain along the core has been determined using a Dansgaard–Johnsen flow model and an isotope-dependent accumulation rate parameterization. Flow parameters are determined from Monte Carlo analysis constrained by the observed depth-age horizons. In order to construct a chronology for the gas phase, the ice age–gas age difference (Δage) has been reconstructed using a coupled firn densification-heat diffusion model. Temperature and accumulation inputs to the Δage model, initially derived from the water isotope proxies, have been adjusted to optimize the fit to timing constraints from δ15N of nitrogen and high-resolution methane data during the abrupt onset of Greenland interstadials. The ice and gas chronologies and the corresponding thinning function represent the first chronology for the NEEM core, named GICC05modelext-NEEM-1. Based on both the flow and firn modelling results, the accumulation history for the NEEM site has been reconstructed. Together, the timescale and accumulation reconstruction provide the necessary basis for further analysis of the records from NEEM.

scholarly journals Glacial Stage Ice-Core Records from the Subtropical Dunde Ice Cap, China

1990 ◽  
Vol 14 ◽  
pp. 288-297 ◽  
Author(s):  
L.G. Thompson ◽  
E. Mosley-Thompson ◽  
M.E. Davis ◽  
J.F. Bolzan ◽  
J. Dai ◽  
...  
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Major Change ◽  
Late Glacial ◽  
Ice Cap ◽  
The North ◽  
Glacial Stage ◽  
Isotope Record ◽  
Temperature Proxy

The first ice-core record of both the Holocene and Wisconsin/Würm Late Glacial Stage (LGS) from the subtropics has been extracted from three ice cores to bedrock from the Dunde ice cap on the north-central Qinghai-Tibetan Plateau. Ice thicknesses at the ice-cap summit average 138 m, the bedrock surface is relatively flat, surface and basal temperatures are −7.3 and −4.7°C, respectively and the ice cap exhibits radial flow away from the summit dome. These records reveal a major change in the climate of the plateau ∼10 000 years ago and suggest that LGS conditions were colder, wetter and dustier than Holocene conditions. This is inferred from the more negative δ18O ratios, increased dust content, decreased soluble aerosol concentrations, and reduced ice-crystal sizes, which characterize the LGS part of the cores. Total β radioactivity from shallow ice cores indicates that over the last 24 years the average accumulation rate has been ∼400 mm a−1 at the summit. The ice cores have been dated using a combination of annual layers in the insoluble dust and δ18O in the upper sections of core, visible dust layers which are annual, and ice-flow modeling. The oxygen-isotope record which serves as a temperature proxy indicates that the last 60 years have been the warmest in the entire record.

scholarly journals Is there 1.5-million-year-old ice near Dome C, Antarctica?

2017 ◽  
Vol 11 (6) ◽  
pp. 2427-2437 ◽  
Author(s):  
Frédéric Parrenin ◽  
Marie G. P. Cavitte ◽  
Donald D. Blankenship ◽  
Jérôme Chappellaz ◽  
Hubertus Fischer ◽  
...  
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Ice Sheets ◽  
Atmospheric Composition ◽  
Regional Environment ◽  
Ice Flow ◽  
True Nature ◽  
Flow Modelling ◽  
North East ◽  
Geothermal Flux

Abstract. Ice sheets provide exceptional archives of past changes in polar climate, regional environment and global atmospheric composition. The oldest dated deep ice core drilled in Antarctica has been retrieved at EPICA Dome C (EDC), reaching ∼ 800 000 years. Obtaining an older paleoclimatic record from Antarctica is one of the greatest challenges of the ice core community. Here, we use internal isochrones, identified from airborne radar coupled to ice-flow modelling to estimate the age of basal ice along transects in the Dome C area. Three glaciological properties are inferred from isochrones: surface accumulation rate, geothermal flux and the exponent of the Lliboutry velocity profile. We find that old ice (> 1.5 Myr, 1.5 million years) likely exists in two regions: one ∼ 40 km south-west of Dome C along the ice divide to Vostok, close to a secondary dome that we name Little Dome C (LDC), and a second region named North Patch (NP) located 10–30 km north-east of Dome C, in a region where the geothermal flux is apparently relatively low. Our work demonstrates the value of combining radar observations with ice flow modelling to accurately represent the true nature of ice flow, and understand the formation of ice-sheet architecture, in the centre of large ice sheets.

scholarly journals A review of the brittle ice zone in polar ice cores

2014 ◽  
Vol 55 (68) ◽  
pp. 72-82 ◽  
Author(s):  
Peter D. Neff
Keyword(s):  
Atmospheric Pressure ◽  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Snow Accumulation ◽  
Air Bubbles ◽  
Polar Ice ◽  
Overburden Pressure ◽  
Trapped Air ◽  
Borehole Temperatures

AbstractMaintaining ice-core quality through the brittle ice zone (BIZ) remains challenging for polar ice-core studies. At depth, increasing ice overburden pressurizes trapped air bubbles, causing fracture of cores upon exposure to atmospheric pressure. Fractured ice cores degrade analyses, reducing resolution and causing contamination. BIZ encounters at 18 sites across the Greenland, West and East Antarctic ice sheets are documented. The BIZ begins at a mean depth of 545 ± 162 m (1 standard deviation), extending to depths where ductile clathrate ice is reached: an average of 1132 ± 178 m depth. Ice ages in this zone vary with snow accumulation rate and ice thickness, beginning as young as 2 ka BP at Dye-3, Greenland, affecting ice >160 ka BP in age at Taylor Dome, Antarctica, and compromising up to 90% of retrieved samples at intermediate-depth sites. Effects of pressure and temperature on the BIZ are explored using modeled firn-column overburden pressure and borehole temperatures, revealing complex associations between firn densification and BIZ depth, and qualitatively supporting expected thinning of the BIZ at low ice temperatures due to shallower clathrate stability. Mitigating techniques for drilling, transport, sampling and analysis of brittle ice cores are also discussed.

scholarly journals Glacial Stage Ice-Core Records from the Subtropical Dunde Ice Cap, China

1990 ◽  
Vol 14 ◽  
pp. 288-297 ◽  
Author(s):  
L.G. Thompson ◽  
E. Mosley-Thompson ◽  
M.E. Davis ◽  
J.F. Bolzan ◽  
J. Dai ◽  
...  
Keyword(s):  
Accumulation Rate ◽  
Ice Core ◽  
Ice Cores ◽  
Major Change ◽  
Late Glacial ◽  
Ice Cap ◽  
The North ◽  
Glacial Stage ◽  
Isotope Record ◽  
Temperature Proxy

The first ice-core record of both the Holocene and Wisconsin/Würm Late Glacial Stage (LGS) from the subtropics has been extracted from three ice cores to bedrock from the Dunde ice cap on the north-central Qinghai-Tibetan Plateau. Ice thicknesses at the ice-cap summit average 138 m, the bedrock surface is relatively flat, surface and basal temperatures are −7.3 and −4.7°C, respectively and the ice cap exhibits radial flow away from the summit dome. These records reveal a major change in the climate of the plateau ∼10 000 years ago and suggest that LGS conditions were colder, wetter and dustier than Holocene conditions. This is inferred from the more negative δ18O ratios, increased dust content, decreased soluble aerosol concentrations, and reduced ice-crystal sizes, which characterize the LGS part of the cores. Total β radioactivity from shallow ice cores indicates that over the last 24 years the average accumulation rate has been ∼400 mm a−1 at the summit. The ice cores have been dated using a combination of annual layers in the insoluble dust and δ18O in the upper sections of core, visible dust layers which are annual, and ice-flow modeling. The oxygen-isotope record which serves as a temperature proxy indicates that the last 60 years have been the warmest in the entire record.

Nonlinear Analysis Techniques for Shear Band Formation at High Strain-Rates

1992 ◽  
Vol 45 (3S) ◽  
pp. S82-S94 ◽  
Author(s):  
Athanasios E. Tzavaras
Keyword(s):  
Strain Rate ◽  
Shear Strain ◽  
Nonlinear Analysis ◽  
Strain Rate Sensitivity ◽  
Strain Softening ◽  
High Strain ◽  
Rate Sensitivity ◽  
Strain Rates ◽  
High Strain Rates ◽  
Analysis Techniques

One of the most striking manifestations of instability in solid mechanics is the localization of shear strain into narrow bands during high speed, plastic deformations of metals. According to one theory, the formation of shear bands is attributed to effective strain-softening response, which results at high strain rates as the net outcome of the influence of thermal softening on the, normally, strain-hardening response of metals. Our objective is to review some of the insight obtained by applying nonlinear analysis techniques on simple models of nonlinear partial differential equations simulating this scenario for instability. First, we take up a simple system, intended as a paradigm, that describes isothermal shear deformations of a material exhibiting strain softening and strain-rate sensitivity. As it turns out, for moderate amounts of strain softening strain-rate sensitivity exerts a dissipative effect and stabilizes the motion. However, once a threshold is exceeded, the response becomes unstable and shear strain localization occurs. Next, we present extensions of these results to situations where explicit thermal effects are taken into account.

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