Using a composite flow law to model deformation in the NEEM deep ice core, Greenland – Part 2: The role of grain size and premelting on ice deformation at high homologous temperature

Ernst-Jan N. Kuiper,Johannes H. P. de Bresser,Gill M. Pennock,Ilka Weikusat,Martyn R. Drury,Jan Eichler

doi:10.5194/tc-14-2449-2020

Abstract

Abstract. The ice microstructure in the lower part of the North Greenland Eemian Ice Drilling (NEEM) ice core consists of relatively fine-grained ice with a single maximum crystallographic preferred orientation (CPO) alternated by much coarser-grained ice with a partial (great circle) girdle or multi-maxima CPO. In this study, the grain-size-sensitive (GSS) composite flow law of Goldsby and Kohlstedt (2001) was used to study the effects of grain size and premelting (liquid-like layer along the grain boundaries) on strain rate in the lower part of the NEEM ice core. The results show that the strain rates predicted in the fine-grained layers are about an order of magnitude higher than in the much coarser-grained layers. The dominant deformation mechanisms, based on the flow relation of Goldsby and Kohlstedt (2001), between the layers is also different, with basal slip rate limited by grain boundary sliding (GBS-limited creep) being the dominant deformation mechanism in the finer-grained layers, while GBS-limited creep and dislocation creep (basal slip rate limited by non-basal slip) contribute both roughly equally to bulk strain in the coarse-grained layers. Due to the large difference in microstructure between finer-grained ice and the coarse-grained ice at premelting temperatures (T>262 K), it is expected that the fine-grained layers deform at high strain rates, while the coarse-grained layers are relatively stagnant. The difference in microstructure, and consequently in viscosity, between impurity-rich and low-impurity ice can have important consequences for ice dynamics close to the bedrock.

Highlights

As a consequence of anthropogenic global warming, it is expected that global mean sea level (GMSL) rise will accelerate in the few decades and centuries (e.g., IPCC, 2014; Kopp et al, 2017)
The layers of alternating grain size in deeper North Greenland Eemian Ice Drilling (NEEM) samples allow for the study of the effect of grain size on strain rate and crystallographic preferred orientation (CPO) development in the high-temperature regime where enhanced creep and recrystallization can be explained by the occurrence of premelting along grain boundaries
We explore the effect of grain size on the dominant deformation mechanism and the total strain rate, it is beyond the scope of this study to derive a stress-depth model for NEEM because this requires knowledge of the rheology, which is the property that is investigated here

Summary

Introduction

As a consequence of anthropogenic global warming, it is expected that global mean sea level (GMSL) rise will accelerate in the few decades and centuries (e.g., IPCC, 2014; Kopp et al, 2017). A full understanding and description of ice deformation mechanisms in polar ice sheets is required. Ice in the lower part of polar ice sheets is of particular interest. This ice is expected to deform much faster than the ice closer to the surface, due to the relatively high temperatures and shear stress increase towards the bedrock. The high variation in deformation rates in the ice near the bedrock are often shown by borehole surveys (e.g., Gow and Williamson, 1976; Paterson, 1983; Morgan et al, 1998; Thorsteinsson et al, 1999; Weikusat et al, 2017). Other sources of heat can be important, like strain heating (e.g., Krabbendam, 2016) or latent heat released by refreezing of meltwater

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Conclusion