Radio-Echo Sounding Over Polar Ice Masses

Peer reviewedPublisher PD

Satellite remote sensing for ice sheet research

Potential research applications of satellite data over the terrestrial ice sheets of Greenland and Antarctica are assessed and actions required to ensure acquisition of relevant data and appropriate processing to a form suitable for research purposes are recommended. Relevant data include high-resolution visible and SAR imagery, infrared, passive-microwave and scatterometer measurements, and surface topography information from laser and radar altimeters

Glacier dynamics near the calving front of Bowdoin Glacier, northwestern Greenland

To better understand recent rapid recession of marine-terminating glaciers in Greenland, we performed satellite and field observations near the calving front of Bowdoin Glacier, a 3 km wide outlet glacier in northwestern Greenland. Satellite data revealed a clear transition to a rapidly retreating phase in 2008 from a relatively stable glacier condition that lasted for >20 years. Ice radar measurements showed that the glacier front is grounded, but very close to the floating condition. These results, in combination with the results of ocean depth soundings, suggest bed geometry in front of the glacier is the primary control on the rate and pattern of recent rapid retreat. Presumably, glacier thinning due to atmospheric and/or ocean warming triggered the initial retreat. In situ measurements showed complex short-term ice speed variations, which were correlated with air temperature, precipitation and ocean tides. Ice speed quickly responded to temperature rise and a heavy rain event, indicating rapid drainage of surface water to the bed. Semi-diurnal speed peaks coincided with low tides, suggesting the major role of the hydrostatic pressure acting on the calving face in the force balance. These observations demonstrate that the dynamics of Bowdoin Glacier are sensitive to small perturbations occurring near the calving front

Southern Ocean warming: Increase in basal melting and grounded ice loss

We apply a global finite element sea ice/ice shelf/ocean model (FESOM) to the Antarctic marginal seas to analyze projections of ice shelf basal melting in a warmer climate. The model is forced with the atmospheric output from two climate models: (1) the Hadley Centre Climate Model (HadCM3) and (2) Max Planck Institute’s ECHAM5/MPI-OM. Results from their 20th-century simulations are used to evaluate the modeled present-day ocean state. Sea-ice coverage is largely realistic in both simulations. Modeled ice shelf basal melt rates compare well with observations in both cases, but are consistently smaller for ECHAM5/MPI-OM. Projections for future ice shelf basal melting are computed using atmospheric output for IPCC scenarios E1 and A1B. While trends in sea ice coverage, ocean heat content, and ice shelf basal melting are small in simulations forced with ECHAM5 data, a substantial shift towards a warmer regime is found in experiments forced with HadCM3 output. A strong sensitivity of basal melting to increased ocean temperatures is found for the ice shelves in the Amundsen Sea. For the cold-water ice shelves in the Ross and Weddell Seas,decreasing convection on the continental shelf in the HadCM3 scenarios leads to an erosion of the continental slope front and to warm water of open ocean origin entering the continental shelf. As this water reaches deep into the Filchner-Ronne Ice Shelf (FRIS) cavity, basal melting increases by a factor of three to six compared to the present value of about 100 Gt/yr. Highest melt rates at the deep FRIS grounding line causes a retreat of > 200km, equivalent to an land ice loss of 110 Gt/yr

A Second Large Subglacial Impact Crater in Northwest Greenland?

Following the discovery of the Hiawatha impact crater beneath the northwest margin of the Greenland Ice Sheet, we explored satellite and aerogeophysical data in search of additional such craters. Here we report the discovery of a possible second subglacial impact crater that is 36.5 km wide and 183 km southeast of the Hiawatha impact crater. Although buried by 2 km of ice, the structure's rim induces a conspicuously circular surface expression, it possesses a central uplift and it causes a negative gravity anomaly. The existence of two closely-spaced and similarlysized complex craters raises the possibility that they formed during related impact events. However, the second structure's morphology is shallower, its overlying ice is conformal and older, and such an event can be explained by chance. We conclude that the identified structure is very likely an impact crater, but it is unlikely to be a twin of the Hiawatha impact crater

Satellite-derived submarine melt rates and mass balance (2011–2015) for Greenland's largest remaining ice tongues

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in The Cryosphere 11 (2017): 2773-2782, doi:10.5194/tc-11-2773-2017.Ice-shelf-like floating extensions at the termini of Greenland glaciers are undergoing rapid changes with potential implications for the stability of upstream glaciers and the ice sheet as a whole. While submarine melting is recognized as a major contributor to mass loss, the spatial distribution of submarine melting and its contribution to the total mass balance of these floating extensions is incompletely known and understood. Here, we use high-resolution WorldView satellite imagery collected between 2011 and 2015 to infer the magnitude and spatial variability of melt rates under Greenland's largest remaining ice tongues – Nioghalvfjerdsbræ (79 North Glacier, 79N), Ryder Glacier (RG), and Petermann Glacier (PG). Submarine melt rates under the ice tongues vary considerably, exceeding 50 m a−1 near the grounding zone and decaying rapidly downstream. Channels, likely originating from upstream subglacial channels, give rise to large melt variations across the ice tongues. We compare the total melt rates to the influx of ice to the ice tongue to assess their contribution to the current mass balance. At Petermann Glacier and Ryder Glacier, we find that the combined submarine and aerial melt approximately balances the ice flux from the grounded ice sheet. At Nioghalvfjerdsbræ the total melt flux (14.2 ± 0.96 km3 a−1 w.e., water equivalent) exceeds the inflow of ice (10.2 ± 0.59 km3 a−1 w.e.), indicating present thinning of the ice tongue.Nat Wilson, Fiammetta Straneo, and Patrick Heimbach were supported by NASA NNX13AK88G and NSF OCE 1434041

Refining Greenland geothermal heat flux through stable isotope analysis

Geothermal heat flux is an important control on the dynamics of glaciers and ice sheets. In Greenland however, only few direct observations of geothermal heat flux exist. The exact spatial distribution and magnitude of heat flux in Greenland is therefore largely unknown. Many studies have attempted to constrain heat flux in Greenland indirectly by modelling it based on other observable variables, such as the seismic and magnetic structure of the Greenland lithosphere, or through techniques that extrapolate the existing measurements onto models of the Greenland lithology. Various estimates of Greenand heat flux have been produced this way, however many do not agree well with each other and show large inter- estimate variability both in terms of magnitude and spatial distribution of estimated heat flux values. Stable isotope composition of basal meltwater has previously not been considered in efforts to constrain Greenland geothermal heat flux. The ice layers in the Greenland ice sheet show large differences in δ18O values resulting from changes in climate throughout their deposi- tional history. If different ice layers are in contact with the bed, then spatial differences in geothermal heat flux will affect the local meltrates these layers experience at the ice sheet base and hence modulate the amount of meltwater each layer contributes into the subglacial drainaige system. If the δ18O values of the melting ice layers are sufficiently different, the isotopic composition of the mixed meltwater that flows through the subglacial hydrological system will be different for different spatial distributions of geothermal heat flux. By simulating the basal meltwater production in Greenland based on different published estimates of Greenland geothermal heat flux, I show in this thesis that different heat fluxes result in differences in the age distribution of the basal ice. In particular, the presence and extent of Eemian ice in central northern Greenland shows substantial differences for different heat flux estimates. As Eemian ice, being interglacial ice, shows higher δ18O val- ues than ice from the last glacial period, the modelled differences in Eemian extent result in detectable differences in the isotopic composition of the basal meltwater in North-east Greenland on the order of few permille. Stable isotope composition of basal meltwater might thus have the potential to contribute to the discussion about a heat flux hotspot in central northern Greenland.Master's Thesis in Earth ScienceGEOV399MAMN-GEO