5 results on '"Alley, R. B"'
Search Results
2. Enhanced Firn Densification in High‐Accumulation Shear Margins of the NE Greenland Ice Stream.
- Author
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Riverman, K. L., Alley, R. B., Anandakrishnan, S., Christianson, K., Holschuh, N. D., Medley, B., Muto, A., and Peters, L. E.
- Subjects
SOIL densification ,ICE streams ,GLACIERS ,DENSITY - Abstract
Firn thickness across the NE Greenland Ice Stream is a function of accumulated strain, with thinner firn in the high‐strain margins of the ice stream. We present a novel technique for extracting firn density from previously collected seismic reflection profiles and apply this technique across both shear margins of NE Greenland Ice Stream. Firn is up to 30 m thinner in the vicinity of the ice stream shear margins. Snow accumulation rates across the ice stream were calculated from airborne ice‐penetrating radar data, calibrated with ground‐based firn density measurements from a shallow core. We find that accumulation is ~20% higher in the shear margins compared to the surroundings. The higher density firn adjacent to shear margins is due to high along‐flow stresses that accelerate firn densification and develops despite the higher accumulation rate favoring lower density. These firn density variations influence subglacial hydropotential by changing the ice surface slope and overburden pressure and may influence subglacial water flow. These results demonstrate the importance of high‐resolution firn surveys in studies of shear‐margin dynamics. Plain Language Summary: Snow that falls on ice sheets is squeezed to ice under its own weight. Old snow during its transformation to ice is called firn. Ice sheets in Greenland and Antarctica are covered in 10–100 m of firn, and many studies on ice sheets require knowledge of firn properties. Observations of the firn, however, are logistically difficult and expensive to make. We present a new technique for estimating firn properties from previously collected seismic surveys. We then use this technique to show that firn is unexpectedly thin along the edges of the fast‐moving Northeast Greenland Ice Stream. Ice and firn stretch as they move into the ice stream and speed up. This causes the firn to change to ice more rapidly, contributing to formation of subtle troughs in the surface along the edge of the ice stream, which trap some extra drifting snow. Properly accounting for this extra snow and the thinner firn allows us to more accurately calculate the weight of the ice. Water under the ice moves to where there is the least weight above it. Ice moves more rapidly where more water lubricates the bed, so this new knowledge contributes to better understanding of the motion of the ice stream. Key Points: We present a novel survey of firn densification across an ice stream shear margin, with new methods for extracting firn velocity profiles from previously collected active seismic reflection profilingFirn thickness across the NE Greenland Ice Stream is a function of accumulated strain and thins by up to 30 m within high‐strain shear margins despite high accumulation thereFirn thickness variations are sufficient to impact subglacial hydropotential, affecting routing of subglacial water flow, with implications for shear margin stability [ABSTRACT FROM AUTHOR] more...
- Published
- 2019
- Full Text
- View/download PDF
Catalog
3. Initial results from geophysical surveys and shallow coring of the Northeast Greenland Ice Stream (NEGIS).
- Author
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Vallelonga, P., Christianson, K., Alley, R. B., Anandakrishnan, S., Christian, J. E. M., Dahl-Jensen, D., Gkinis, V., Holme, C., Jacobel, R. W., Karlsson, N. B., Keisling, B. A., Kipfstuhl, S., Kjær, H. A., Kristensen, M. E. L., Muto, A., Peters, L. E., Popp, T., Riverman, K. L., Svensson, A. M., and Tibuleac, C. more...
- Subjects
GEOPHYSICAL surveys ,ICE streams ,GLACIAL crevasses ,ELECTRIC conductivity - Abstract
The Northeast Greenland Ice Stream (NEGIS) is the sole interior Greenlandic ice stream. Fast flow initiates near the summit dome, and the ice stream terminates approximately 1000 km downstream in three large outlet glaciers that calve into the Greenland Sea. To better understand this important system, in the summer of 2012 we drilled a 67m firn core and conducted ground-based radio-echo sounding (RES) and active-source seismic surveys at a site approximately 150 km downstream from the onset of streaming flow (NEGIS firn core, 75°37.61' N, 35°56.49' W). The site is representative of the upper part of the ice stream, while also being in a crevasse-free area for safe surface operations. Annual cycles were observed for insoluble dust, sodium and ammonium concentrations and for electrolytic conductivity, allowing a seasonally resolved chronology covering the past 400 yr. Annual layer thicknesses averaged 0.11m ice equivalent (i.e.) for the period 1607-2011, although accumulation varied between 0.08 and 0.14mi.e., likely due to flowrelated changes in surface topography. Tracing of RES layers from the NGRIP (North Greenland Ice Core Project) ice core site shows that the ice at NEGIS preserves a climatic record of at least the past 51 kyr. We demonstrate that deep ice core drilling in this location can provide a reliable Holocene and late-glacial climate record, as well as helping to constrain the past dynamics and ice-lithosphere interactions of the Greenland Ice Sheet. [ABSTRACT FROM AUTHOR] more...
- Published
- 2014
- Full Text
- View/download PDF
4. Review Ice sheet mass balance and sea level.
- Author
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Allison, I., Alley, R. B., Fricker, H. A., Thomas, R. H., and Warner, R. C.
- Subjects
ICE sheets ,ICE streams ,ABSOLUTE sea level change ,CLIMATE change ,GEOGRAPHIC information systems ,MASS budget (Geophysics) - Abstract
Determining the mass balance of the Greenland and Antarctic ice sheets (GIS and AIS) has long been a major challenge for polar science. But until recent advances in measurement technology, the uncertainty in ice sheet mass balance estimates was greater than any net contribution to sea level change. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (AR4) was able, for the first time, to conclude that, taken together, the GIS and AIS have probably been contributing to sea level rise over the period 1993-2003 at an average rate estimated at 0.4mmyr
-1 . Since the cut-off date for work included in AR4, a number of further studies of the mass balance of GIS and AIS have been made using satellite altimetry, satellite gravity measurements and estimates of mass influx and discharge using a variety of techniques. Overall, these studies reinforce the conclusion that the ice sheets are contributing to present sea level rise, and suggest that the rate of loss from GIS has recently increased. The largest unknown in the projections of sea level rise over the next century is the potential for rapid dynamic collapse of ice sheets. [ABSTRACT FROM AUTHOR] more...- Published
- 2009
- Full Text
- View/download PDF
5. Decoding the dipstick: Thickness of Siple Dome, West Antarctica, at the Last Glacial Maximum.
- Author
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Waddington, E. D., Conway, H., Steig, E. J., Alley, R. B., Brook, E. J., Taylor, K. C., and White, J. W. C.
- Subjects
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PHYSICAL geography , *ICE , *GEOLOGY , *EARTH sciences - Abstract
Ice thickness in West Antarctica at the Last Glacial Maximum (LGM) is poorly known, yet is key information for understanding ice streams and interpreting ice cores. Although trim lines, moraine limits, and exposure-age dating provide geologic constraints on ice thickness near the Transantarctic Mountains and in Marie Byrd Land, lack of exposed bedrock hampers traditional geologic methods in a central, ∼2 × 106 km² region. Here we infer ice-sheet thickness changes in the central Ross Sea Embayment by using a transient ice-flow model to find combinations of accumulation-rate and ice-sheet thickness histories that match the depth-age relationship and the measured layer-thickness pattern in the Siple Dome ice core. After we reject unreasonable accumulation-rate histories, the remaining history pales indicate thinning of 200–400 m since the LGM. Our estimate is lower than previous reconstructions that were constrained by geologic evidence from the Transantarctic Mountains and by marine data from the Ross Sea floor, which indicate that a grounded ice sheet extended to the continental shelf margin during the last glacial period. Low surface elevations in the central Ross Sea Embayment during the LGM do not preclude thicker ice along its boundaries. However, if this grounded ice sheet came over 1000 km from interior West Antarctica, as is usually assumed, then it had very low surface slope, requiring a very slippery bed. Alternatively, the grounded Ross Sea ice might have flowed from the Transantarctic Mountains and western Marie Byrd Land. [ABSTRACT FROM AUTHOR] more...
- Published
- 2005
- Full Text
- View/download PDF
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