Does decreasing paraglacial sediment supply slow knickpoint retreat?

Jansen, John D.; Fabel, Derek; Bishop, Paul; Xu, Sheng; Schnabel, Christoph; Codilean, Alexandru T.

doi:10.1130/g32018.1

Cited by 91 publications

(104 citation statements)

References 24 publications

(34 reference statements)

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“…Concurrently, high fluxes of channelized meltwater with elevated hydraulic potential gradient are forced along low-lying terrain corridors, flushing debris and promoting maximum bedrock erosion at t 2 . After further ice retreat (t 3 ), high fluxes of meltwater continue until final deglaciation (t 4 ), but in contrast to t 2 , erosion rates along the gorges decline because the subaerial rivers are unpressurized and sediment 'tools' necessary for bedrock erosion are largely trapped in proglacial lakes 28 .…”

Section: Resultsmentioning

confidence: 99%

“…Typical field evidence for subglacial meltwater erosion includes anastomosing channels, irregular valley long profiles and topography that amplifies hydraulic potential 2,9,18,20 . Secondly, other workers attribute inner gorges to subaerial pre or postglacial river incision in response to base-level fall or shifts in sediment supply [25][26][27][28] . A third explanation sees inner gorges as palimpsest fluvial forms whose relief can deepen over multiple glacial cycles.…”

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confidence: 99%

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Inner gorges cut by subglacial meltwater during Fennoscandian ice sheet decay

et al. 2014

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The century-long debate over the origins of inner gorges that were repeatedly covered by Quaternary glaciers hinges upon whether the gorges are fluvial forms eroded by subaerial rivers, or subglacial forms cut beneath ice. Here we apply cosmogenic nuclide exposure dating to seven inner gorges along B500 km of the former Fennoscandian ice sheet margin in combination with a new deglaciation map. We show that the timing of exposure matches the advent of ice-free conditions, strongly suggesting that gorges were cut by channelized subglacial meltwater while simultaneously being shielded from cosmic rays by overlying ice. Given the exceptional hydraulic efficiency required for meltwater channels to erode bedrock and evacuate debris, we deduce that inner gorges are the product of ice sheets undergoing intense surface melting. The lack of postglacial river erosion in our seven gorges implicates subglacial meltwater as a key driver of valley deepening on the Baltic Shield over multiple glacial cycles.

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Section: Resultsmentioning

confidence: 99%

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confidence: 99%

Inner gorges cut by subglacial meltwater during Fennoscandian ice sheet decay

et al. 2014

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“…If a knickpoint is retreating steadily through time, as would be expected if associated with a normal flow regime, the exposure age of samples from fluvial surfaces would become progressively younger with decreasing distance from the modern knickpoint location (28)(29)(30). If, on the other hand, a knickpoint retreated a large distance in a short period, such as during an extreme flood event, the exposure ages would be expected to cluster around the time of that significant erosion event (19).…”

Section: Significancementioning

confidence: 99%

Erosion during extreme flood events dominates Holocene canyon evolution in northeast Iceland

Baynes

Attal

Niedermann

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

108

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Extreme flood events have the potential to cause catastrophic landscape change in short periods of time (10 0 to 10 3 h). However, their impacts are rarely considered in studies of long-term landscape evolution (>10 3 y), because the mechanisms of erosion during such floods are poorly constrained. Here we use topographic analysis and cosmogenic 3 He surface exposure dating of fluvially sculpted surfaces to determine the impact of extreme flood events within the Jökulsárgljúfur canyon (northeast Iceland) and to constrain the mechanisms of bedrock erosion during these events. Surface exposure ages allow identification of three periods of intense canyon cutting about 9 ka ago, 5 ka ago, and 2 ka ago during which multiple large knickpoints retreated large distances (>2 km). During these events, a threshold flow depth was exceeded, leading to the toppling and transportation of basalt lava columns. Despite continuing and comparatively largescale (500 m 3 /s) discharge of sediment-rich glacial meltwater, there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive event because the vertical knickpoints have not diffused over time. We provide a model for the evolution of the Jökulsárgljúfur canyon through the reconstruction of the river profile and canyon morphology at different stages over the last 9 ka and highlight the dominant role played by extreme flood events in the shaping of this landscape during the Holocene.bedrock erosion | extreme floods | knickpoints | Iceland | cosmogenic 3 He E xtreme floods in both terrestrial and extraterrestrial environments can cause abrupt landscape change that can have longterm consequences (1-5), especially when a geomorphic threshold is exceeded (6). The timescale over which this change is visible is controlled by the ability and efficiency of background processes to reshape the landscape. As a result, progress in understanding both short-term and long-term landscape evolution requires better knowledge of bedrock channel erosion processes and thresholds over the different scales at which geomorphological processes operate (7-10).The majority of research into extreme flood events has focused on the interpretation of deposited sediments (e.g., refs. 11 and 12) and the reconstruction of the hydraulic conditions prevailing during such events (e.g., refs. 13-15). Further work has defined the geomorphic impact of extreme flood events in proglacial areas close to the source of the flood water (e.g., refs. 16 and 17). Studies that examine the processes of bedrock erosion, especially large canyon formation, during extreme flood events can help establish a diagnostic link between formation processes and morphology in canyons in both terrestrial and extraterrestrial settings, but they remain scarce (e.g., refs. 18-20). Here, evidence for bedrock landscape change during extreme floods along the course of the Jökulsá á Fjöllum River (northeast Iceland) is used to test whether the contemporary landscape morphology reflects erosion during rare extreme eve...

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“…This is known as the cover effect, which can be divided into a static cover effect, where shielding of the bed is due to stationary sediment, and a dynamic cover effect, where mobile particles interact and reduce the number of bed impacts (Turowski et al, 2007). There is now evidence for the importance of both the tools and the cover effect in a variety of field settings and over a range of spatial and temporal scales Beer, Turowski, Fritschi, & Rieke-Zapp, 2015;Cook, Turowski, & Hovius, 2013;Cowie et al, 2008;Hobley, Sinclair, Mudd, & Cowie, 2011;Jansen et al, 2011;Johnson, Whipple, Sklar, & Hanks, 2009;Turowski, Hovius, Hsieh, Lague & Chen, 2008;Turowski, Hovius, Wilson, & Horng, 2008;Wilson et al, 2013;Yanites et al, 2011).…”

Section: Introductionmentioning

confidence: 99%

The influence of sediment thickness on energy delivery to the bed by bedload impacts

Turowski

Bloem

2015

Geodinamica Acta

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Fluvial bedrock erosion rates due to impacting sediment particles are thought to be proportional to the energy delivered to the bedrock. When sediment particles cover the bed, they reduce the energy transmitted to the bed by an impacting particle. We measured the decline of energy transferred through sediment cover of increasing thickness in laboratory experiments. The energy arriving at the bed is a function both of the cover thickness and the grain size of the covering sediment. Using a simple stochastic model of cover distribution, the experimental results were upscaled to the reach scale. Although cover thickness influences energy delivery heavily at a given point, when averaging over the whole bed, cover-free areas dominate total energy delivery, making partial energy transfer through the cover negligible when a small or intermediate fraction of the bed is covered by sediment. Partial energy delivery through the bed cover is not negligible when a large fraction or the complete bed is already covered, but in this situation, an erosion threshold may become important. On grounds of the presented data, we expect that the areal distribution of sediment in a bedrock channel dominates total energy delivery and that partial energy delivery to the bed through a sediment layer can be neglected for most modelling purposes.

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Does decreasing paraglacial sediment supply slow knickpoint retreat?

Cited by 91 publications

References 24 publications

Inner gorges cut by subglacial meltwater during Fennoscandian ice sheet decay

Inner gorges cut by subglacial meltwater during Fennoscandian ice sheet decay

Erosion during extreme flood events dominates Holocene canyon evolution in northeast Iceland

The influence of sediment thickness on energy delivery to the bed by bedload impacts

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