Ice flow between the Greenland Ice Core Project and Greenland Ice Sheet Project 2 boreholes in central Greenland

The ice flow between the Greenland Ice Core Project (GRIP) and Greenland Ice Sheet Project 2 (GISP2) boreholes in the Summit region of central Greenland is modeled with a steady state finite element model. The model calculates the free ice sheet surface and the coupled ice flow and temperature fields along the flow line between the boreholes. Three-dimensional effects are expressed in terms of the divergence of the flow and included in the coupled stress and temperature fields. In addition to ice-core data, several geophysical surface programs have provided data that are used to constrain the model. The modeled isochrones resemble the bedrock structure along the flow line but rise at the divide. The rise at the divide is not seen in internal layers found by surface radio echo sounding. An ice particle, originally deposited at the surface, moves through different stress and temperature regimes. A set of trajectory lines are followed from the surface to the boreholes, in order to follow the variation of stress deviators, temperature, and deformation along with the ice movement. These variations along a trajectory line constitute a history of the corresponding ice layer. The difference between the history of ice layers found in the GRIP and GISP2 boreholes is mainly due to the shear stress, which is 3-4 times higher for GISP2 ice than for GRIP ice. Furthermore, the GRIP ice experiences a nonzero longitudinal stress deviator much longer than the GISP2 ice.