Fjord insertion into continental margins driven by topographic steering of ice

Fjords commonly punctuate continental edges formerly occupied by Quaternary ice sheets, reaching kilometre depths and extending many tens of kilometres inland. 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 fjords; 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. The introduction of fjords probably enabled strong co-evolution of landscape and glacial dynamics through the late Cenozoic era. Recent thermochronologic studies in British Columbia suggest rapid fjord incision soon after the onset of Northern Hemisphere glaciation. Cosmogenic radionuclide studies reveal sharp spatial gradients in erosion rates, with rates in fjords four orders of magnitude higher than on interfjord uplands. This illustrates the strong control exerted by fjorded topography on spatial variations in erosion, probably reflecting in part the thermal state of the bed. This correlation between topography and bed thermal state would have been much weaker early in fjord development when topographic differences were small. Although conceptual models of fjord formation exist, the feedback mechanisms responsible for fjords have not been modelled. Whereas numerical models of glacier erosion have shown small-scale topographic steering and suggest that ice convergence can cause valley overdeepening, fjord formation has not been investigated. In these models, erosion depends on an intermediate-timescale average basal sliding and subglacial hydrology. Acknowledging our poor knowledge of how to model Barnes Ice Cap Metres asl