Regional and temporal variation of accumulation around NGRIP derived from ground based ice-penetrating radar

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Steinhage, D. , Eisen, O. and Johnsen, S. F. (2004): Regional and temporal variation of accumulation around NGRIP derived from ground based ice-penetrating radar , International Symposium on Arctic Glaciology, 23-27 August 2004, Geilo, Norway. .
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During the last two decades several deep ice cores have been obtained from Greenland as well as Antarctica in order to study the palaeo-climate preserved in the ice. These ice cores have been drilled in places which are believed to have had stable conditions in the past.The representativeness of a deep ice-core can be evaluated by mapping theinternal structure of the ice sheet by ice-penetrating radar (IPR).This has the advantage over common point-sampling with firn cores thatthe conditions surrounding the drill site are quasi-continuously mapped.During the summer 2003, a ground-based IPR survey around the North-GRIPdeep ice core drilling site (75 06'N, 42 20'W, 2960 m a.s.l) has beencarried out using using a shielded 250 MHz IPR system. Thedrill site is located on an ice divide, roughly 300 km NWN of the summit ofthe Greenland ice sheet. More than 430 km of profiles have been measured,covering an area of 10 km by 10 km, centered on thedrilling location, and eight profiles extending beyond this grid.As a by-product the detailed topography along the IPR profiles has beenmeasured by kinematic GPS. Seven internal horizons within the upper 120 mof the ice sheet have been continuously tracked. Based on the age-depth anddensity-depth distribution of a firn core we dated the internal layers andderived the regional and temporal distribution of accumulation in thevicinity of NGRIP. The distribution of accumulation shows a relativelysmoothly increasing trend from east to west from 120 to 200 kg m-2 a-1 over adistance of 50 km across the ice divide. The general trend is overlain bysmall-scale variations on the order of 2.5 kg m-2 a-1 per km, i.e. around 1%of the accumulation mean. In addition we present the temporal variation ofthe accumulation pattern and discuss implications for ice core interpretations.

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