Fjord insertion into continental margins driven by topographic steering of ice

Abstract

Fjords commonly punctuate continental edges formerly occupied by Quaternary ice sheets, reaching kilometre depths and extending many tens of kilometres inland1,2. These features must have been created by late Cenozoic ice sheets, because rivers cannot erode bedrock much below sea level. Ice sheets drain primarily through fjords3,4; therefore, widespread fjord insertion may have altered ice-sheet size, shape and dynamics. Here, we use a two-dimensional ice-sheet model to simulate the incision of fjords through a coastal mountain range. We show that topographic steering of ice and erosion proportional to ice discharge are sufficient to form fjords. Within one million years, kilometre-deep fjords punched through the mountain range owing to a robust positive feedback initiated by ice being steered towards mountain passes. Enhanced erosion beneath thicker, faster ice deepens these passes, amplifying the topographic steering. Simulated fjords are deepest through the highest topography and drain a large fraction of the interior ice. Ice sheets simulated on landscapes with existing fjords are generally smaller and exhibit longer response times and larger responses to climate changes, suggesting that modern ice sheets are more sensitive to climate fluctuations than Early Quaternary ice sheets.

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Figure 1: Baffin Island fjorded topography.
Figure 2: Evolution of model topography.
Figure 3: Modelled bed and ice flow after 1.2 Myr and ice discharge evolution.
Figure 4: Modelled topographic and ice surface evolution with a randomly crenulated coastal mountain range.

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Acknowledgements

This research was partially financially supported by NSF (EAR-0644966; EAR 0549566). Support for M.A.K. came from CU postdoctoral research funds and N. Kessler. We appreciate careful reading of an early manuscript by Y. Axford, M. M. Berlin, D. Hess, M. Duhnforth, D. Ward, B. Hallet and T. Stern.

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Correspondence to Mark A. Kessler.

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Kessler, M., Anderson, R. & Briner, J. Fjord insertion into continental margins driven by topographic steering of ice. Nature Geosci 1, 365–369 (2008). https://doi.org/10.1038/ngeo201

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