Recent ice sheet snow accumulation and firn storage of meltwater inferred by ground and airborne radars
Recent surface mass balance changes in space and time over the polar ice sheets need to be better constrained in order to estimate the ice-sheet contribution to sea-level rise. The mass balance of any ice body is obtained by subtracting mass losses from mass gains. In response to climate changes of...
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| Format: | Text |
| Language: | English |
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University of Utah
2016
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| Online Access: | https://dx.doi.org/10.26053/0h-1kbw-bx00 https://collections.lib.utah.edu/ark:/87278/s6cr92n8 |
| Summary: | Recent surface mass balance changes in space and time over the polar ice sheets need to be better constrained in order to estimate the ice-sheet contribution to sea-level rise. The mass balance of any ice body is obtained by subtracting mass losses from mass gains. In response to climate changes of the recent decades, ice-sheet mass losses have increased, making ice-sheet mass balance negative and raising sea level. In this work, I better quantify the mass gained by snowfall across the polar ice sheets; I target specific regions over both Greenland and West Antarctica where snow accumulation changes are occurring due to rising air temperature. Southeast Greenland receives 30% of the total snow accumulation of the Greenland ice sheet. In this work, I combine internal layers observed in ice-penetrating radar data with firn cores to derive the last 30 years of accumulation and to measure the spatial pattern of accumulation toward the southeast coastline. Below 1800 m elevation, in the percolation zone, significant surface melt is observed in the summer, which challenges both firn-core dating and internal-layer tracing. While firn-core drilling at 1500 m elevation, liquid water was found at ~20-m depth in a firn aquifer that persisted over the winter. The presence of this water filling deeper pore space in the firn was unexpected, and has a significant impact on the ice sheet thermal state and the estimate of mass balance made using satellite altimeters. Using a 400-MHz ice-penetrating radar, the extent of this widespread aquifer was mapped on the ground, and also more extensively from the air with a 750-MHz airborne radar as part of the NASA Operation IceBridge mission. Over three IceBridge flight campaigns (2011-2013), based on radar data, the firn aquifer is estimated to cover ~30,000 km2 area within the wet-snow zone of the ice sheet. I use repeated flightlines to understand the temporal variability of the water trapped in the firn aquifer and to simulate its lateral flow, following the gentle surface slope (< 1) and undulated topography of the ice sheet surface toward the ablation zone of the ice sheet. The fate of this water is currently unknown; water drainage into crevasses and at least partial runoff is inferred based on the analysis of radar profiles from different years. I also present results from a field expedition in West Antarctica, where data collection combined high-frequency (2-18 GHz) radar data and shallow (< 20 m) firn cores from Central West Antarctica, crossing the ice divide toward the Amundsen Sea. The radar-derived accumulation rates show a 75% increase (+0.20 m w.eq. y-1) of net snow accumulation from the ice divide, toward the Amundsen Sea for a 70-km transect, assuming annual isochrones being detected in the radar profile. On the Ross Sea side of the divide, with accumulation rates less than 0.25 m w.eq. y-1 and significant wind redistribution, only a multi-annual stratigraphy is detected in the radar profile. Using radar, I investigated the small-scale variability within a radius of ~1.5 km of one firn-core site, and I find that the averaged variation in accumulation-rate in this area is 0.1 m w.eq. y-1 in the upper 25-m of the firn column, which is 20% of the average accumulation rate. |
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