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2022 ◽  
Vol 277 ◽  
pp. 107366
Author(s):  
Alexander R. Simms ◽  
Louise Best ◽  
Ian Shennan ◽  
Sarah L. Bradley ◽  
David Small ◽  
...  

2022 ◽  
Author(s):  
Thomas Hudson ◽  
Sofia-Katerina Kufner ◽  
Alex Brisbourne ◽  
Michael Kendall ◽  
Andrew Smith ◽  
...  

Abstract The slip of glaciers over the underlying bed is the dominant mechanism governing the migration of ice from land into the oceans, contributing to sea-level rise. Yet glacier slip remains poorly understood or constrained by observations. Here we observe both frictional shear-stress and slip at the bed of an ice stream, using 100,000 repetitive stick-slip icequakes from Rutford Ice Stream, Antarctica. Basal shear-stresses and slip-rates vary from 10^4 to 10^7 Pa and 0.2 to 1.5 m day^(-1), respectively. Friction and slip vary temporally over the order of hours and spatially over 10s of meters, caused by corresponding variations in ice-bed interface material and effective-normal-stress. Our findings also suggest that the bed is substantially more complex than currently assumed in ice stream models and that basal effective-normal-stresses may be significantly higher than previously thought. The observations also provide previously unresolved constraint of the basal boundary conditions of ice dynamics models. This is critical for constraining the primary contribution of ice mass loss in Antarctica, and hence the endeavour to reduce uncertainty in sea-level rise projections.


Author(s):  
Rebecca Schlegel ◽  
Tavi Murray ◽  
Andrew M. Smith ◽  
Alex M. Brisbourne ◽  
Adam D. Booth ◽  
...  
Keyword(s):  

2021 ◽  
Author(s):  
Ian Lee ◽  
Sridhar Anandakrishnan ◽  
Richard Alley ◽  
Sofia-Katerina Kufner ◽  
Andrew Smith ◽  
...  

2021 ◽  
Author(s):  
◽  
Laurine van Haastrecht

<p>The Siple Coast ice streams, which drain the West Antarctic Ice Sheet into the Ross Ice Shelf, are susceptible to temporal changes in flow dynamics. The Kamb Ice Stream on the Siple Coast, stagnated approximately 160 years ago, thought to partially be the result of basal water diversion. The character of its subglacial environment can exert an important control on long- and short-term ice sheet and ice stream fluctuations. Were the Kamb Ice Stream to reactivate in response to subglacial or future climate change, it would have the potential to contribute more substantially to ice discharge into the Ross Ice Shelf. Therefore, it is important to characterise the present-day subglacial environment and climatic conditions that may reactivate this flow. This study investigates the present-day subglacial conditions of the Kamb Ice Stream and how these conditions may be affected by environmental perturbations. Due to the difficult nature of making direct observations of ice sheet basal conditions, other methods are employed to investigate the response of the Kamb Ice Stream to environmental change. Active source seismic surveying data obtained during the 2015/16 and 2018/19 austral summer seasons provides an instantaneous snapshot of the present-day basal conditions. Flowline and whole-continent numerical ice sheet modelling is used to investigate the longer-term response of the Kamb Ice Stream and the West Antarctic Ice Sheet. Amplitude analysis of seismic lines indicate saturated till beneath the Ross Ice Shelf in the vicinity of the grounding zone, which is supported by retreat rates of the Kamb Ice Stream grounding zone post-stagnation. Seismic reflection imaging suggests potential dewatered till conditions beneath the grounded Kamb Ice Stream. Flowline modelling of the Kamb Ice Stream indicates that changes to the water content of the subglacial sediments appear to be self regulating, with high reversibility over centennial timescales. Oceanic temperature forcings are the key driver of change of the Kamb Ice Stream, and the ice stream is susceptible to topographic pinning points in 2D and lateral drag. Future glaciological change is more likely to occur in response to oceanic than to atmospheric temperature perturbations. Results from 3D continent-wide modelling experiments also find that precipitation increases offset the effect of air temperature perturbations and influence subglacial conditions, indicating more dynamic ice stream behaviour on the Siple Coast. This study has worked to re-enforce and strengthen our existing understanding of the Kamb Ice Stream and its sensitivity to environmental change. Future work using higher-resolution simulations and a higher density of observational data may help refine these results.</p>


2021 ◽  
Author(s):  
◽  
Laurine van Haastrecht

<p>The Siple Coast ice streams, which drain the West Antarctic Ice Sheet into the Ross Ice Shelf, are susceptible to temporal changes in flow dynamics. The Kamb Ice Stream on the Siple Coast, stagnated approximately 160 years ago, thought to partially be the result of basal water diversion. The character of its subglacial environment can exert an important control on long- and short-term ice sheet and ice stream fluctuations. Were the Kamb Ice Stream to reactivate in response to subglacial or future climate change, it would have the potential to contribute more substantially to ice discharge into the Ross Ice Shelf. Therefore, it is important to characterise the present-day subglacial environment and climatic conditions that may reactivate this flow. This study investigates the present-day subglacial conditions of the Kamb Ice Stream and how these conditions may be affected by environmental perturbations. Due to the difficult nature of making direct observations of ice sheet basal conditions, other methods are employed to investigate the response of the Kamb Ice Stream to environmental change. Active source seismic surveying data obtained during the 2015/16 and 2018/19 austral summer seasons provides an instantaneous snapshot of the present-day basal conditions. Flowline and whole-continent numerical ice sheet modelling is used to investigate the longer-term response of the Kamb Ice Stream and the West Antarctic Ice Sheet. Amplitude analysis of seismic lines indicate saturated till beneath the Ross Ice Shelf in the vicinity of the grounding zone, which is supported by retreat rates of the Kamb Ice Stream grounding zone post-stagnation. Seismic reflection imaging suggests potential dewatered till conditions beneath the grounded Kamb Ice Stream. Flowline modelling of the Kamb Ice Stream indicates that changes to the water content of the subglacial sediments appear to be self regulating, with high reversibility over centennial timescales. Oceanic temperature forcings are the key driver of change of the Kamb Ice Stream, and the ice stream is susceptible to topographic pinning points in 2D and lateral drag. Future glaciological change is more likely to occur in response to oceanic than to atmospheric temperature perturbations. Results from 3D continent-wide modelling experiments also find that precipitation increases offset the effect of air temperature perturbations and influence subglacial conditions, indicating more dynamic ice stream behaviour on the Siple Coast. This study has worked to re-enforce and strengthen our existing understanding of the Kamb Ice Stream and its sensitivity to environmental change. Future work using higher-resolution simulations and a higher density of observational data may help refine these results.</p>


2021 ◽  
Author(s):  
Hannah Petrie ◽  
Christian Eide ◽  
Haflidi Haflidason ◽  
Timothy Watton

Conceptual geological models of the shallow subsurface which integrate geological and geotechnical information are important for more strategic data acquisition and engineering at offshore wind sites. Utsira Nord is an offshore wind site in the Norwegian North Sea. It covers an area of 23 km x 43 km within the Norwegian Channel palaeo ice stream, with an average water depth of 267 m making the site a candidate for floating offshore wind. The goal of this study is to present a preliminary conceptual geological model for the site, which combines an overview of previous knowledge about the complex ice streaming history of the Norwegian Channel with key observations from high resolution bathymetric data, 2D acoustic data, and shallow cores. Despite limited data, four geotechnical provinces can be defined: 1) exposed glacimarine to marine sediments, 2) buried to exposed subglacial traction till, 3) buried lodgment till and 4) shallowly buried to exposed crystalline bedrock. The model serves as a basis for planning site surveys at Utsira Nord and as a reference for offshore wind sites on other formerly glaciated coasts where palaeo ice stream systems are common, such as the northern coastlines of the United States and the United Kingdom.


Boreas ◽  
2021 ◽  
Author(s):  
Michelle S. Gauthier ◽  
Andy Breckenridge ◽  
Tyler J. Hodder

2021 ◽  
Author(s):  
◽  
Theo Calkin

<div>The grounding line of the Siple Coast incorporates six major ice streams, which together drain around a third of the West Antarctic Ice Sheet. Previously, the ~2000 km-long feature had only been sampled and directly observed at Whillans Ice Stream. This thesis examines glaciomarine sediment and processes operating at the presently stagnant Kamb Ice Stream (KIS) grounding zone ~3.3 km seaward of the modern grounding line (Lat. -82.78, Long. -155.16), where the ice is 590 m thick and overlies a 30 m thick water column. KIS-GZ is the planned site for a deep drilling project in 2023. The sea floor was accessed using a hot water drill in the 2019/20 Antarctic field season. A remotely operated submersible (‘Icefin’) was deployed under the ice shelf, which provided 800 m of sea floor video toward the grounding line. A small number of short (~0.6 m) gravity cores were collected from the seafloor, one of which was examined in this study. <br></div><div><br></div><div>The Icefin video imagery was processed using Structure-from-Motion (SfM) software, enabling the identification of two previously unrecognised sea floor sedimentary facies. One is defined by ubiquitous cm-scale ripples in fine-medium sand, where the ripples are aligned with the prevailing tidal currents flowing parallel to the grounding line. Observed current speeds are too low for the ripples to be generated under the modern oceanographic regime. The second facies is defined by abundant dropstones in mediumcoarse sand. A transition zone separates the two facies. Previously unidentified decimetre-scale bedforms are present in the transition zone and near the borehole.</div><div><br></div><div>The lithological, geochemical, and microfossil properties of the gravity core were analysed. The core sediment is sandy diamicton with weak stratification defined by decimetre-scale changes in clast abundance. Mineral counts, zircon ages, Nd/Sr isotopes, and an immature composition indicate this sediment is sourced within the Kamb catchment. The core also contains reworked late Oligocene-late Miocene diatoms. Quaternary diatoms are absent. Ramped pyrolysis radiocarbon dating was attempted on 19 carbon fractions obtained from samples at four depths. Two pyrolysis fractions yielded ages of 31.5-33.2 ka, while the rest did not contain measurable radiocarbon. This likely reflects the reworking of radiocarbon-dead material into the sediment and can only be considered a maximum age for deposition.</div><div><br></div><div>Together, the sediment and video data suggest deposition of the sea floor sediment at the core site occurred subsequent to the stagnation of KIS ~160 years ago. I assume that sediment concentrations are relatively uniform along the length of the ice stream and calculate that up to ~2.7 m of diamicton was rapidly deposited at the core site as the grounding line retreated and englacial sediment melted out and settled through the water column. Accumulation in recent decades has been comparatively low. During the period of reduced sedimentation, the sea floor diamicton has been reworked to varying degrees to form ripples and winnowed lag deposits, resulting in a textural patchwork at km scale.</div>


2021 ◽  
Author(s):  
◽  
Theo Calkin

<div>The grounding line of the Siple Coast incorporates six major ice streams, which together drain around a third of the West Antarctic Ice Sheet. Previously, the ~2000 km-long feature had only been sampled and directly observed at Whillans Ice Stream. This thesis examines glaciomarine sediment and processes operating at the presently stagnant Kamb Ice Stream (KIS) grounding zone ~3.3 km seaward of the modern grounding line (Lat. -82.78, Long. -155.16), where the ice is 590 m thick and overlies a 30 m thick water column. KIS-GZ is the planned site for a deep drilling project in 2023. The sea floor was accessed using a hot water drill in the 2019/20 Antarctic field season. A remotely operated submersible (‘Icefin’) was deployed under the ice shelf, which provided 800 m of sea floor video toward the grounding line. A small number of short (~0.6 m) gravity cores were collected from the seafloor, one of which was examined in this study. <br></div><div><br></div><div>The Icefin video imagery was processed using Structure-from-Motion (SfM) software, enabling the identification of two previously unrecognised sea floor sedimentary facies. One is defined by ubiquitous cm-scale ripples in fine-medium sand, where the ripples are aligned with the prevailing tidal currents flowing parallel to the grounding line. Observed current speeds are too low for the ripples to be generated under the modern oceanographic regime. The second facies is defined by abundant dropstones in mediumcoarse sand. A transition zone separates the two facies. Previously unidentified decimetre-scale bedforms are present in the transition zone and near the borehole.</div><div><br></div><div>The lithological, geochemical, and microfossil properties of the gravity core were analysed. The core sediment is sandy diamicton with weak stratification defined by decimetre-scale changes in clast abundance. Mineral counts, zircon ages, Nd/Sr isotopes, and an immature composition indicate this sediment is sourced within the Kamb catchment. The core also contains reworked late Oligocene-late Miocene diatoms. Quaternary diatoms are absent. Ramped pyrolysis radiocarbon dating was attempted on 19 carbon fractions obtained from samples at four depths. Two pyrolysis fractions yielded ages of 31.5-33.2 ka, while the rest did not contain measurable radiocarbon. This likely reflects the reworking of radiocarbon-dead material into the sediment and can only be considered a maximum age for deposition.</div><div><br></div><div>Together, the sediment and video data suggest deposition of the sea floor sediment at the core site occurred subsequent to the stagnation of KIS ~160 years ago. I assume that sediment concentrations are relatively uniform along the length of the ice stream and calculate that up to ~2.7 m of diamicton was rapidly deposited at the core site as the grounding line retreated and englacial sediment melted out and settled through the water column. Accumulation in recent decades has been comparatively low. During the period of reduced sedimentation, the sea floor diamicton has been reworked to varying degrees to form ripples and winnowed lag deposits, resulting in a textural patchwork at km scale.</div>


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