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

During the summer 2003, a ground-based ice penetrating radar (IPR) survey around the North-GRIP deep ice core drilling site (75deg06'N, 42deg20'W, 2957 m) has been carried out using a shielded 250 MHz radar system. The drill site is located on an ice divide, roughly 300 km NNW of the summit of the Greenland ice sheet. More than 430 km of profiles have been measured, covering an area of 10 km by 10 km, with a grid centered on the drilling location, and eight profiles extending beyond this grid. Seven internal horizons within the upper 120 m of the ice sheet have been continuously tracked, containing the last 400-500 a of accumulation history. Based on the age-depth and density-depth distribution of the deep core, the internal layers have been dated and the regional and temporal distribution of accumulation in the vicinity of North-GRIP has been derived. The distribution of accumulation shows a relatively smoothly increasing trend from east to west from 145 to 200 kg m^-2 a^-1 over a distance of 50 km across the ice divide. The generaltrend is overlaid by small-scale variations on the order of 2.5 kg m^-2 a^-1 per km, i.e. around 1.5% of the accumulation mean. The temporal variations of the seven periods defined by the seven tracked isochrones are on the order of 4% of the mean of the last 400 a, i.e. at North-GRIP 7 kg m^-2 a^-1. If the regional accumulation pattern has been stable for the last several thousand years during the Holocene and ice flow has been comparable to today, then advective effects along the particle trajectory upstream of North-GRIP do not have any significant effect on the interpretation of climatically induced changes in accumulation rates derived from the deep ice core over the last 10 ka.