Abstract
AbstractEver since the first deep ice cores were drilled, it has been a challenge to determine their original, in-situ orientation. In general, the orientation of an ice core is lost as the drill is free to rotate during transport to the surface. For shallow ice cores, it is usually possible to match the adjacent core breaks, which preserves the orientation of the ice column. However, this method fails for deep ice cores, such as the EastGRIP ice core in Northeast Greenland. We provide a method to reconstruct ice core orientation using visual stratigraphy and borehole geometry. As the EastGRIP ice core is drilled through the Northeast Greenland Ice Stream, we use information about the directional structures to perform a full geographical re-orientation. We compared the core orientation with logging data from core break matching and the pattern of the stereographic projections of the crystals’ c-axis orientations. Both comparisons agree very well with the proposed orientation method. The method works well for 441 out of 451 samples from a depth of 1375–2120 m in the EastGRIP ice core. It can also be applied to other ice cores, providing a better foundation for interpreting physical properties and understanding the flow of ice.
Highlights
1.1 Why we drill ice coresDeep ice core drilling projects are usually undertaken for the purpose of climate reconstruction, which was first demonstrated by Dansgaard and others (1969)
Unlike many ice streams where ice is entering the trunk of fast flow by a tributary system, the ice entering the Northeast Greenland Ice Stream (NEGIS) passes through the well-developed shear margins (Fahnestock and others, 2001; Joughin and others, 2001)
We measured the tilt of cloudy bands (α′) for every visual stratigraphy sample, and plot one point per 165 cm-long sample (Fig. 6a)
Summary
Deep ice core drilling projects are usually undertaken for the purpose of climate reconstruction, which was first demonstrated by Dansgaard and others (1969). The reconstruction of the past climate from ice core samples assumes that the snow in the ice sheet interior does not melt, and that annual layers accumulate horizontally, which leads to a continuous record (Cuffey and Paterson, 2010). The stratigraphic layers may be disturbed, which can lead to climate reconstruction errors, in particular for the deep parts of the ice column (Dahl-Jensen and others, 2013). 1.2 Northeast Greenland Ice Stream and EastGRIP drill site. This is a central location in the onset region of the Northeast Greenland Ice Stream (NEGIS). Observations by Franke and others (2020) support these findings and indicate the existence of subglacial landforms, shaped by the activity of the ice stream
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