In-situ measurements of temporal and spatial variability in firn compaction rates at Summit, Greenland, using Borehole Optical Stratigraphy

Quantitative understanding of firn compaction is important for remote-sensing mass-balance studies, which seek to measure and interpret the changing height of the ice sheet surface; the surface can rise due to snow accumulation, and fall due to ice flow or increased densification rates. Quantitative knowledge of all 3 processes is essential. Evidence suggests that the rate of densification, which is thermally activated, undergoes a seasonal cycle, related to the seasonal cycle of temperature. In this ongoing project we have made detailed measurements of the temporal and spatial variations of firn compaction using Borehole Optical Stratigraphy (BOS), which uses a borehole camera to record details of a borehole wall. We tracked these details over time to determine vertical motion and strain, which in the shallow depth of our study is dominated by firn compaction. We profiled an array of 4 boreholes near Summit Camp, central Greenland, on a monthly basis over 2 winters and 3 summers. Here, we give an overview of the borehole measurement and data reduction system, present data and analysis from this winter-over experiment, and discuss the implications of temporal changes in firn densification for remote sensing mass-balance studies.