Mass balance and ice flow along the north-northwest ridge of the Greenland ice sheet at NorthGRIP

The North Greenland Icecore Project (NorthGRIP) deep drilling site (75degrees05'47" N, 42degrees19'42" W) is located at the north-northwest ridge of the Greenland ice sheet, 320 km from Summit. A strain net has been established around the NorthGRIP site and surveyed with global positioning system. Our results show that ice flows with a horizontal surface velocity of 1.329 +/- 0.015 m a(-1) along the ridge. Estimated principal surface strain rates at NorthGRIP are epsilon(1) = (-0.4 +/- 0.6) x 10(-5) a(-1) and epsilon(2) = (7.1 +/- 0.6) x 10(-5) a(-1), in the directions along and transverse to the north-northwest ridge, respectively, i.e. ice is compressed along the ridge but stretched transverse to the ridge. Possible implications of the observed flow pattern for the stratigraphy are discussed. The average thickening rate in the strain-net area is found to be partial derivativeH/partial derivativet = 0.00 +/- 0.04 m a(-1), in agreement with previous estimates of mass balance in high-elevation areas of Greenland.