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

Abstract

Abstract. The ice microstructure in the lower part of the North Greenland Eemian Ice Drilling (NEEM) ice core consists of relatively fine grained glacial ice with a single maximum crystallographic preferred orientation (CPO) alternated by much coarser grained Eemian ice with a partial girdle type of 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 on strain rate in the lower part of the NEEM ice core. The results show that the strain rates predicted in the fine grained glacial layers are about an order of magnitude higher than in the much coarser grained Eemian layers. The dominant deformation mechanisms between the layers is also different with basal slip accommodated by grain boundary sliding (GBS-limited creep) being the dominant deformation mechanism in the glacial layers, while GBS-limited creep and dislocation creep (basal slip accommodated by non-basal slip) contribute both roughly equally to bulk strain in the coarse grained layers. Due to the large difference in microstructure between the impurity-rich glacial ice and the impurity-depleted Eemian ice at premelting temperatures (T>262 K), it is expected that the fine grained layers deform mainly by simple shear at high strain rates, while the coarse grained layers are relatively stagnant. The difference in microstructure, and consequently in viscosity, between glacial and interglacial ice at temperatures just below the melting point can have important consequences for ice dynamics close to the bedrock.

Temperature and strain controls on ice deformation mechanisms: insights from the microstructures of samples deformed to progressively higher strains at −10, −20 and −30 °C

“…Goldsby and Kohlstedt (1997) suggest a general importance of GBS on the basis of the constitutive law parameters required to fit the mechanical data from experimentally deformed fine-grained ice. Recent studies suggest GBS in fine-grained ice layers has a key role in controlling the Greenland ice flow (Kuiper et al, 2019a(Kuiper et al, , 2019b by applying the Goldsby-Kohlstedt flow law Kohlstedt, 1997, 2001) to modelling the deformation in the NEEM (North Greenland Eemian Ice Drilling) deep ice core. The grain size reduction resulting from dynamic recrystallization is thought to cause mechanical weakening by increasing the strain rate contribution of grain size sensitive deformation mechanisms (De Bresser et al, 2001).…”

Seismic Anisotropy of Temperate Ice in Polar Ice Sheets

“…Ice near the bedrock was found frozen to the bed at the GISP2 and GRIP ice cores. But for the Byrd, EDC, EDML, NEEM, and Siple Dome cores, the ice was found at, or very close to, the pressure melting point (Kuiper et al, 2020). In three of these ice cores—Byrd (Gow & Williamson, 1976), EDC (EPICA community members, 2004), and EDML (Weikusat et al, 2017; Wilhelms et al, 2014)—subglacial water was encountered close to the bedrock.…”

“…Shear localization at the base of the ice sheet is enhanced by the softening of ice here due to the geothermal heat flow (Dahl-Jensen et al, 1998). The effect of heating increases at high temperature because the activation energy for ice deformation increases significantly toward its melting point (Barnes et al, 1971;Goldsby & Kohlstedt, 2001;Kuiper et al, 2020;Souchez et al, 1993) and because of the potential presence of water (Gudlaugsson et al, 2016). In temperate glaciers, where liquid water coexists with glacier ice, a melt content of 1% implies an increase in the effective strain rate by a factor of 3 (Duval, 1977;Lliboutry & Duval, 1985).…”