A quantitative record of seasonally-varying densification rates at Summit, Greenland, from 2004-2008, using Borehole Optical Stratigraphy

Estimates of ice sheet mass balance from altimetry data require knowledge or assumptions about the firn density profile. Recently, it has been shown that firn densification rates can vary seasonally and therefore cannot be taken as a constant. Here we present a monthly record of firn densification rates from Summit, Greenland, from 2004-2008. In 2004, we drilled four 30 meter-long boreholes spaced 50 meters apart at Summit, Greenland. We determine densification rates from these boreholes using Borehole Optical Stratigraphy (BOS). This technique allows us to measure how firn densification rates change over the entire 30 m profile, rather than at discrete depth increments. Additionally, the sampling of 4 independent boreholes allows us to explore the spatial variability in firn densification on short length scales. To determine the thermal forcing of seasonality in densification rates, we measured firn temperatures from the surface to 10 meters. We find a seasonal trend in firn densification rates, corresponding to the thermal forcing experienced by the system, with higher densification rates in firn experiencing warmer temperatures.