A composite flow law to model deformation in the NEEM deep ice core, Greenland - The role of grain size, grain size distribution and premelting on ice deformation

Results from a rheological model based on the composite flow law of Goldsby and Kohlstedt (1997, 2001) applied to mean grain size and grain size distributions in the North Greenland Eemian Ice Drilling (NEEM) deep ice core predict that grain-size-sensitive flow produces almost all of the deformation in the upper 2207 m, while dislocation creep hardly contributes to deformation. The difference in calculated strain rate between two model end-members is relatively small: (i) the micro-scale constant stress model where each grain deforms by the same stress and (ii) the micro-scale constant strain rate model where each grain deforms by the same strain rate. The predicted strain rate in the fine-grained Glacial ice (that is, ice deposited during the Last Glacial Maximum now at depths of 1419 to 2207 m) varies strongly within this depth range. Predicted strain rate 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 of depth. See also interpretations in publication (Kuiper et al. 2020 TC)