| 1. |
- Doyle, Sam H, et al.
(författare)
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Ice tectonic deformation during the rapid in situ drainage of a supraglacial lake on the Greenland Ice Sheet
- 2013
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Ingår i: The Cryosphere. - : Copernicus GmbH. - 1994-0416 .- 1994-0424. ; 7:1, s. 129-140
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Tidskriftsartikel (refereegranskat)abstract
- We present detailed records of lake discharge, ice motion and passive seismicity capturing the behaviour and processes preceding, during and following the rapid drainage of a 4 km2 supraglacial lake through 1.1-km-thick ice on the western margin of the Greenland Ice Sheet. Peak discharge of 3300 m3 s−1 coincident with maximal rates of vertical uplift indicates that surface water accessed the ice–bed interface causing widespread hydraulic separation and enhanced basal motion. The differential motion of four global positioning system (GPS) receivers located around the lake record the opening and closure of the fractures through which the lake drained. We hypothesise that the majority of discharge occurred through a 3-km-long fracture with a peak width averaged across its wetted length of 0.4 m. We argue that the fracture's kilometre-scale length allowed rapid discharge to be achieved by combining reasonable water velocities with sub-metre fracture widths. These observations add to the currently limited knowledge of in situ supraglacial lake drainage events, which rapidly deliver large volumes of water to the ice–bed interface.
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| 2. |
- Gusmeroli, Alessio, et al.
(författare)
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Vertical seismic profiling of glaciers : appraising multi-phase mixing models
- 2013
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Ingår i: Annals of Glaciology. - 0260-3055 .- 1727-5644. ; 54:64, s. 115-123
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the speed of compressional waves in a polythermal glacier by, first, predicting them from a simple three-phase (ice, air, water) model derived from a published ground-penetrating radar study, and then comparing them with field data from four orthogonally orientated walkaway vertical seismic profiles (VSPs) acquired in an 80 m deep borehole drilled in the ablation area of Storglaciaren, northern Sweden. The model predicts that the P-wave speed increases gradually with depth from 3700 m s(-1) at the surface to 3760 m s(-1) at 80 m depth, and this change is almost wholly caused by a reduction in air content from 3% at the surface to <0.5% at depth. Changes in P-wave speed due to water content variations are small (<10 m s(-1)); the model's seismic cold-temperate transition surface (CTS) is characterized by a 0.3% decrease downwards in P-wave speed (about ten times smaller than the radar CTS). This lack of sensitivity, and the small contrast at the CTS, makes seismically derived water content estimation very challenging. Nevertheless, for down-going direct-wave first arrivals for zero- and near-offset VSP shots, we find that the model-predicted travel times and field observations agree to within 0.2 ms, i.e. less than the observational uncertainties.
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| 3. |
- Kulessa, Bernd, et al.
(författare)
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Seismic evidence for complex sedimentary control of Greenland Ice Sheet flow
- 2017
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 3:8
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Tidskriftsartikel (refereegranskat)abstract
- The land-terminating margin of the Greenland Ice Sheet has slowed down in recent decades, although the causes and implications for future ice flow are unclear. Explained originally by a self-regulating mechanism where basal slip reduces as drainage evolves from low to high efficiency, recent numerical modeling invokes a sedimentary control of ice sheet flow as an alternative hypothesis. Although both hypotheses can explain the recent slowdown, their respective forecasts of a long-term deceleration versus an acceleration of ice flow are contradictory. We present amplitude-versus-angle seismic data as the first observational test of the alternative hypothesis. We document transient modifications of basal sediment strengths by rapid subglacial drainages of supraglacial lakes, the primary current control on summer ice sheet flow according to our numerical model. Our observations agree with simulations of initial postdrainage sediment weakening and ice flow accelerations, and subsequent sediment restrengthening and ice flow decelerations, and thus confirm the alternative hypothesis. Although simulated melt season acceleration of ice flow due to weakening of subglacial sediments does not currently outweigh winter slowdown forced by self regulation, they could dominate over the longer term. Subglacial sediments beneath the Greenland Ice Sheet must therefore be mapped and characterized, and a sedimentary control of ice flow must be evaluated against competing self-regulation mechanisms.
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| 4. |
- Schannwell, Clemens, et al.
(författare)
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An automatic approach to delineate the cold-temperate transition surface with ground-penetrating radar on polythermal glaciers
- 2014
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Ingår i: Annals of Glaciology. - 0260-3055 .- 1727-5644. ; 55:67, s. 89-96
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Tidskriftsartikel (refereegranskat)abstract
- Ground-penetrating radar has been widely used to map the thermal structure of polythermal glaciers. Hitherto, the cold temperate transition surface (CTS) in radargrams has been identified by a labour-intensive and subjective manual picking method. We introduce a new automatic approach for picking the CTS that uses the difference in signal power exhibited by the cold and temperate ice layers. We compare our automatically computed CTS depths with manual picks. Our results show very good agreement between the two methods in most areas (r(2) > 0.7). RMSEs computed at each trace in two-way travel-time from three test sites range from 14 to 19 ns (2.4-3.2 m). The proposed automated method mostly fails in areas showing a rather gradual transition in signal power at the CTS. In some areas, high power originating from non-water sources is misinterpreted by the automatic picking method as 'temperate ice'.
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| 5. |
- Shahbaz, Muhammad, et al.
(författare)
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Spatial heterogeneity of soil carbon exchanges and their drivers in a boreal forest
- 2022
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Ingår i: Science of the Total Environment. - : Elsevier BV. - 1879-1026 .- 0048-9697. ; 831
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Tidskriftsartikel (refereegranskat)abstract
- Boreal forests have a large impact on the global greenhouse gas balance and their soils constitute an important carbon (C) reservoir. Mature boreal forests are typically a net CO2 sink, but there are also examples of boreal forests that are persistent CO2 sources. The reasons remain often unknown, presumably due to a lack of understanding of how biotic and abiotic drivers interact to determine the microbial respiration of soil organic matter (SOM). This study aimed at identifying the main drivers of microbial SOM respiration and CO2 and CH4 soil chamber-fluxes within dry and wet sampling areas at the mature boreal forest of Norunda, Sweden, a persistent net CO2 source. The spatial heterogeneity of the drivers was assessed with a geostatistical approach combined with stepwise multiple regression. We found that heterotrophic soil respiration increased with SOM content and nitrogen (N) availability, while the SOM reactivity, i.e., SOM specific respiration, was determined by soil moisture and N availability. The latter suggests that microbial activity was N rather than C limited and that microbial N mining might be driving old-SOM decomposition, which was observed through a positive correlation between soil respiration and its δ13C values. SOM specific heterotrophic respiration was lower in wet than in dry areas, while no such dependencies were found for chamber-based soil CO2 fluxes, implying that oxygen depletion resulted in lower SOM reactivity. The chamber-based soil CH4 flux differed significantly between the wet and dry areas. In the wet area, we observed net CH4 emission that was positively related to soil moisture and NH4+-N content. Taken together, our findings suggest that N availability has a strong regulatory effect on soil CO2 and CH4 emissions at Norunda, and that microbial decomposition of old-SOM to release bioavailable N might be partly responsible for the net CO2 emission at the site.
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