Catastrophic floods and denudation

Working out the rate at which landscapes evolve depends on some means of dating surfaces formed at different stages in the cutting down of topography.   Modern studies rely to a large extent on the build up of isotopes, such as 10Be, that form in minerals no more than a metre or so beneath the Earth’s surface when they are exposed to cosmic ray bombardment.  If such transmuted nuclides stay in place, for instance on a relic surface or a series of alluvial terraces, cosmogenic isotope analysis dates the formation of that surface.  No matter how precise such surface dating can be, and currently there is a slop of around 20% either side of an age, there is a limit to the number of suitable surfaces.  So, the continual degradation of landscape can only be sampled at a few isolated times.  At best, an average denudation rate over long periods is all that geomorphologists can hope for.  The same goes for analysis of the range of times during which grains in a sediment were exposed to cosmic rays, before they were eroded, transported and finally protected from bombardment when they were buried in alluvium, that is another approach to timing erosion.  Average rates of erosion are useful in assessing some aspects of landscape development, but they are not much good for judging how it took place.  Standing in some awesome scenery easily gives the impression that it must have evolved by some continuous, steady process, and there is a long tradition dating back to James Hutton that views surface processes in that way.  Changes in rates have been seen as responses to “rejuvenation”, either by falls in the base-level of erosion or tectonic uplift to add gravitational potential to a region that makes flowing water more energetic.

Another approach is to look at the actual transfer of mass, either carried by rivers during different seasons or in the volumes of sediments that were deposited by recognisable individual events, such as a flood.  In large river basins that have a low average gradient it is well-accepted that occasional floods don’t have much effect on sediment movement in the long term, but most of the sediment moved in such basins is alluvium already supplied by earlier processes.  In mountainous areas rivers carry material directly from bedrock and the regolith that lies on it.  Anyone who has witnessed flash floods in a normally crystal clear mountain river knows their awesome power.  They become mud torrents studded with boulders that even fly through the air; they are debris flows rather than streams in the normally accepted sense.  Such flows are episodic, but frequently annual, and exert a major influence over denuding the landscape.  Yet over millennia, they too should maintain a consistent down wearing.  In the Appalachian mountains of the eastern USA denudation rates seem to average out between 2.5 to 5 centimetres per thousand years.  However, four catastrophic Appalachian storms in the late 20th century, related to hurricanes, had an astonishing effect on erosion there (Eaton, L.S. et al. 2003.  Role of debris flows in long-term landscape denudation in the central Appalachians of Virginia.  Geology, v. 31, p. 339-342).  Carbon-14 dating of ancient mass-flow deposits formed in Virginia by comparable storms indicates recurrences in particular drainage basins around every 2500 to 3500 years.  During that time the average rate of erosion would have denuded the surface by a measurable amount (5 to 10 cm), yet the recent storms removed between 47 to 63% of that expected during periods measured in millennia.  The Appalachians are well vegetated, and therefore well protected from the effects of extreme floods compared with the surfaces of really big mountains such as the Himalaya and rugged areas in arid regions.  The obvious question is, “Are average denudation rates, no matter how precise, very relevant to the way landscapes actually develop?”  It is an important one, because weathering of debris from mountains is regarded by many geochemists as a means of taking carbon dioxide from the atmosphere – silicate weathering that involves CO2 dissolved in rainwater locks atmospheric carbon in bicarbonate ions that carbonate-secreting creatures in the sea can sequester to deep storage when they die.  If about half the erosion of mountains is in widely separated catastrophes, which shift and then dump debris in a matter of days, then it is possible that the sums based on equating rates of weathering with those of erosion are not entirely valid.  Weathering of continental silicates is one means of forcing global cooling by reducing the greenhouse effect, and understandably mountain rivers teem with researchers sampling the water and sediment load, especially in the Himalaya.  If the bulk of debris shed to plains, such as those of the Indus, Ganges and Brahmaputra, never had time to be weathered at high altitude because it moved in catastrophic pulses, then maybe the sampling should be done somewhere else.  Processes in the vast alluvial tracts far below high mountains are slower and more constant wit time, so maybe looking at groundwater that moves through them might add to the current research..


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