Using a composite flow law to model deformation in the NEEM deep ice core, Greenland – Part 1: The role of grain size and grain size distribution on deformation of the upper 2207 m

Abstract

Abstract. The effect of grain size on strain rate of ice in the upper 2207 m in the North Greenland Eemian Ice Drilling (NEEM) deep ice core was investigated using a rheological model based on the composite flow law of Goldsby and Kohlstedt (1997, 2001). The grain size was described by both a mean grain size and a grain size distribution, which allowed the strain rate to be calculated using two different model end-members: (i) the microscale constant stress model where each grain deforms by the same stress and (ii) the microscale constant strain rate model where each grain deforms by the same strain rate. The model results predict that grain-size-sensitive flow produces almost all of the deformation in the upper 2207 m of the NEEM ice core, while dislocation creep hardly contributes to deformation. The difference in calculated strain rate between the two model end-members is relatively small. The predicted strain rate in the fine-grained Glacial ice (that is, ice deposited during the last Glacial maximum at depths of 1419 to 2207 m) varies strongly within this depth range and, furthermore, is about 4–5 times higher than in the coarser-grained Holocene ice (0–1419 m). Two peaks in strain rate are predicted at about 1980 and 2100 m depth. The prediction that grain-size-sensitive creep is the fastest process is inconsistent with the microstructures in the Holocene age ice, indicating that the rate of dislocation creep is underestimated in the model. The occurrence of recrystallization processes in the polar ice that did not occur in the experiments may account for this discrepancy. The prediction of the composite flow law model is consistent with microstructures in the Glacial ice, suggesting that fine-grained layers in the Glacial ice may act as internal preferential sliding zones in the Greenland ice sheet.

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

“…The increase in strain rate coincides with the increase in temperature along the NEEM ice core (Fig. 1 in Kuiper et al, 2020). The relative contribution of GBS-limited creep and dislocation creep to the bulk strain rate of the modified composite flow law is roughly equal for the ice core sections that were assigned a mean grain diameter of 30 mm just below the stratigraphic disruptions at 2209.6 and 2262.2 m depth.…”

“…The difference in microstructure of impurity-rich glacial and low-impurity interglacial ice in the premelting zone can have important consequences for ice dynamics close to the bedrock. Since the impurity content in the Greenland ice sheet is approximately 10 times higher than the impurity content in the Antarctic ice sheet (e.g., Legrand and Mayewski, 1997), this effect of strain partitioning in the premelting zone is likely to be stronger in the Greenland ice sheet than in the Antarctic ice sheet.…”

“…The composite flow law describes the deformation of polycrystalline ice as a combination of grain-size-sensitive (GSS) and grain-size-insensitive (GSI) deformation mechanisms, where the dominant deformation mechanism depends on the temperature, stress and grain size. Compared to our companion paper (Kuiper et al, 2020), which describes ice deformation at relatively low temperatures in the upper 2207 m of the NEEM ice core, the emphasis in this paper is on possible differences in the controlling deformation mechanism close to the melting point, that is, in the premelting regime below 2207 m depth. The results from flow law calculations were combined with CPO data to study deformation mechanisms in the lower part of the NEEM ice core.…”

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

“…This observation suggests intragranular distortion is more significant at lower temperatures. Subgrain rotation is a process that involves an increase in the misorientation across a subgrain boundary forming via the progressive addition of dislocations (White, 1976;Lallemant, 1985). New grains will form as the misorientation across the subgrain boundary becomes large enough, with the subgrain boundary eventually dividing its parent grain (Poirier and Nicolas, 1975;Guillope and Poirier, 1979;Urai et al, 1986;Halfpenny et al, 2006;Gomez-Rivas et al, 2017).…”

The role of grain-size evolution on the rheology of ice: Implications for reconciling laboratory creep data and the Glen flow law

“…This composite law has been used to model the rheology of ice satellites (Barr & McKinnon, 2007) and the relative contribution of GBS and dislocation creep in ice sheets (Kuiper et al, 2020). Here the creep mechanisms are assumed to be independent and each term on the right-hand-side of Eq.…”