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May 15th, 2008

Formation of deep fjords simpler than previously thought

Mark Kessler (INSTAAR), Bob Anderson (INSTAAR), and Jason Briner (former INSTAAR, now Univ at Buffalo) used a numerical model of ice-sheet behavior to discover that a single feedback loop explains a long-standing geomorphic enigma: why do fjords often extend to depths well below sea level and cut deeply into continental edges? They found that the well known feedback between preferential ice flow and erosion was sufficiently strong by itself.

The team used a carefully constructed but relatively simple two-dimensional model to sidestep the complexity of glacial erosional processes that had stymied previous modeling attempts. They simulated a coastal mountain range having four shallow gaps or mountain passes. They found that ice preferentially flowed through the lowest pass. Because thicker ice increases ice discharge and erosion non-linearly, subtle initial differences in landscape relief became greatly exaggerated as time passed in the model. Over a few million years, small valleys and mountain passes were eroded into the dramatic features seen today, with fjords plunging a kilometer below sea level, and tens of kilometers inland.

The team's new simulations also help resolve a second long-standing enigma: the origin of nunataks - apparent ice-free areas on formerly glaciated coastal mountain ranges. Their simulations show that glacial erosion of such high areas will be minimal, implying that an absence of glacial landforms does not conclusively demonstrate a lack of ice cover. In other words, the summit areas only provide evidence of ice cover and ice thickness when they contain glacial erratics that can be cosmogenically dated.

Finally, the researchers pose the question of how greatly the presence of fjords will modify the size, extent, and dynamics of ice sheets. Ice sheets developed prior to the full insertion of fjords into the landscape may well have looked and acted quite differently than their more recent counterparts. The team's results were published in Nature Geoscience.

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