Temperate ice in the shear margins of the Antarctic Ice Sheet: Controlling processes and preliminary locations

Meyer, Colin R.; Minchew, Brent

doi:10.1016/j.epsl.2018.06.028

Cited by 49 publications

(78 citation statements)

References 40 publications

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“…In the ice stream shear margin, where fast flow transitions to the stagnant ice ridge, a region of temperate ice forms (Figures a 1 to a 3 ). This is consistent with previous work (Haseloff et al, , ; Jacobson & Raymond, ; Meyer & Minchew, ; Perol & Rice, ; Schoof, ; Suckale et al, ), even though the mechanism by which the ice stream shear margin is localized in these earlier studies differs from the mechanism considered here.…”

Section: Resultssupporting

confidence: 93%

Englacial Pore Water Localizes Shear in Temperate Ice Stream Margins

2019

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The margins of fast‐moving ice streams are characterized by steep velocity gradients. Some of these gradients cannot be explained by a temperature‐dependent viscosity alone. Laboratory data suggest that water in the ice‐grain matrix decreases the ice viscosity; we propose that this causes the strong localization of shear in temperate ice stream margins. However, the magnitude of weakening and its consequences for ice stream dynamics are poorly understood. Here we investigate how the coupling between temperate ice properties, ice mechanics, and drainage of melt water from the ice stream margin alters the dynamics of ice streams. We consider the steady‐state ice flow, temperature, water content, and subglacial water drainage in an ice stream cross section. Temperate ice dynamics are modeled as a two‐phase flow, with gravity‐driven water transport in the pores of a viscously compacting and deforming ice matrix. We find that the dependence of ice viscosity on meltwater content focuses the temperate ice region and steepens the velocity gradients in the ice stream margin. It provides a possible explanation for the steep velocity gradients observed in some ice stream shear margins. This localizes heat dissipation there, which in turn increases the amount of meltwater delivered to the ice stream bed. This process is controlled by the permeability of the temperate ice and the sensitivity of ice viscosity to meltwater content, both of which are poorly constrained properties.

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Section: Resultssupporting

confidence: 93%

Englacial Pore Water Localizes Shear in Temperate Ice Stream Margins

2019

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“…We propose WIS stagnation is being driven by the growth of a frozen fringe in the ice stream margins. This hypothesis is consistent with declining water pressure, as originally proposed by Alley and others (1994) and plausibly related to the subglacial hydrology in the vicinity of active shear margins (Meyer and others, 2018b; Meyer and Minchew, 2018). The growth of a frozen fringe is expected to ultimately decelerate ice flow by creating a large anchor of frozen sediments with extreme resistance to sliding (Meyer and others, 2018a).…”

Section: Discussionsupporting

confidence: 89%

Glacier sliding, seismicity and sediment entrainment

Lipovsky¹,

Meyer

Zoet

et al. 2019

Ann. Glaciol.

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The evolution of glaciers and ice sheets depends on processes in the subglacial environment. Shear seismicity along the ice–bed interface provides a window into these processes. Such seismicity requires a rapid loss of strength that is typically ascribed to rate-weakening friction, i.e., decreasing friction with sliding or sliding rate. Many friction experiments have investigated glacial materials at the temperate conditions typical of fast flowing glacier beds. To our knowledge, however, these studies have all found rate-strengthening friction. Here, we investigate the possibility that rate-weakening rock-on-rock friction between sediments frozen to the bottom of the glacier and the underlying water-saturated sediments or bedrock may be responsible for subglacial shear seismicity along temperate glacier beds. We test this ‘entrainment-seismicity hypothesis’ using targeted laboratory experiments and simple models of glacier sliding, seismicity and sediment entrainment. These models suggest that sediment entrainment may be a necessary but not sufficient condition for the occurrence of basal shear seismicity. We propose that stagnation at the Whillans Ice Stream, West Antarctica may be caused by the growth of a frozen fringe of entrained sediment in the ice stream margins. Our results suggest that basal shear seismicity may indicate geomorphic activity.

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“…The inversion is run with spatially constant priors. Based on the tabulated values of the temperature-dependent flow-rate parameter (MacAyeal and others, 1995; Cuffey and Paterson, 2010) and an understanding of ice temperature in rapidly-deforming regions of ice sheets (Meyer and Minchew, 2018), we estimate the ice temperature to be −10°C, leading to a spatially constant value of the flow-rate parameter A 0 = 1.15 × 10 −8 a −1 kPa −3 (Cuffey and Paterson, 2010). We prescribe the prior of the slipperiness coefficient to be spatially constant at c 0 = 0.01 m a −1 kPa −3 .…”

Section: Bindschadler and Macayeal Ice Streamsmentioning

confidence: 99%

A new approach to inferring basal drag and ice rheology in ice streams, with applications to West Antarctic Ice Streams

et al. 2020

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Drag at the bed and along the lateral margins are the primary forces resisting flow in outlet glaciers. Simultaneously inferring these parameters is challenging since basal drag and ice viscosity are coupled in the momentum balance, which governs ice flow. We test the ability of adjoint-based inverse methods to infer the slipperiness coefficient in a power-law sliding law and the flow-rate parameter in the constitutive relation for ice using a regularization scheme that includes coefficients weighted by surface strain rates. Using synthetic data with spatial variations in basal drag and ice rheology comparable to those in West Antarctic Ice Streams, we show that this approach allows for more accurate inferences. We apply this method to Bindschadler and MacAyeal Ice Streams in West Antarctica. Our results show relatively soft ice in the shear margins and spatially varying basal drag, with an increase in drag with distance upstream of the grounding line punctuated by localized areas of relatively high drag. We interpret soft ice to reflect a combination of heating through viscous dissipation and changes in the crystalline structure. These results suggest that adjoint-based inverse methods can provide inferences of basal drag and ice rheology when regularization is informed by strain rates.

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Temperate ice in the shear margins of the Antarctic Ice Sheet: Controlling processes and preliminary locations

Cited by 49 publications

References 40 publications

Englacial Pore Water Localizes Shear in Temperate Ice Stream Margins

Englacial Pore Water Localizes Shear in Temperate Ice Stream Margins

Glacier sliding, seismicity and sediment entrainment

A new approach to inferring basal drag and ice rheology in ice streams, with applications to West Antarctic Ice Streams

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