Connection between climatic state and ice softness derived from deformation measurements of the Greenlandic NEEM borehole

Deformation measurements from the northern Greenlandic NEEM borehole reveal a strong correlation between ice softness and δ18O, such that isotopically warmer ice (high δ18O) is stiffer and isotopically colder ice (low δ18O) is softer. This holds down to millennial time scales (Dansgaard-Oeschger events) for both the borehole closure rate and the shear rate. By assuming that the flow in the vicinity of the drill site can be approximated by the shallow ice approximation, we establish a relation between the scalar flow enhancement factor E and the δ18O. The enhancement factor computed directly from the observed shear rate shows a significant scatter; however, it can be fit reasonably well by a piece-wise linear function with the following assumptions: (1) for isotopically warm ice with δ18O ≥ -38‰, E = 1 holds; (2) for isotopically cold ice with δ18O < -38‰, E increases linearly with decreasing δ18O, and the slope is different for ice from the last (Weichselian/Wisconsin) and penultimate (Saalian/Illinoian) glacial period, respectively. While the fitted enhancement factor for the penultimate glacial period does not reach values larger than 3, it reaches values up to 40 for the last glacial period. This difference is crucial to allow reproducing the observed horizontal velocity profile by the fitted enhancement factor with reasonable accuracy.