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ScienceWeek
EARTH SCIENCE: ON RIVER FLOODING
The following points are made by Chris Paola (Nature 2003 425:459):
1) One of the quirks of the Earth's stratigraphic recording system is that it operates in fits and starts, archiving surface conditions for a while and then suddenly switching off. The sedimentary "record" button is evidently pretty finicky. It is sensitive to external controls, but also to the internal dynamics of the sediment-deposition system. An intriguing example of how the recording process works is given by Aalto et al.(1), who demonstrate how changes in the way that rainwater is delivered to a river system, driven by the El Nino/Southern Oscillation (ENSO) climate cycle in the Pacific Ocean, can switch one of the major processes of sedimentation on and off.
2) River flooding seems a fairly straightforward process -- so much so that one could easily overlook the fact that no obvious natural law decrees how often and by what means a river should flood, or, for that matter, that it should flood at all. An alluvial river channel (one formed from its own sediment) and its adjoining flood plain are self-formed from the interplay of water, sediment, and vegetation. The dynamics of flooding results from the way in which the channel–flood-plain system organizes itself.
3) In many alluvial rivers, sedimentation associated with flooding creates "natural levees", low ridges that run along the edges of the channel. These levees are naturally formed counterparts of the familiar artificial levees built to control flooding in populated areas. They raise the "overtopping" threshold of the channel and in so doing create a pressure-head that helps drive water onto the flood plain once the levee is breached. So, rather than gradually inundating the flood plain, water can enter as a relatively focused, narrow flow that often carries with it a good deal of suspended sediment. Once the flow leaves the breach (the "crevasse"), it expands and dumps its load in the form of a fan, colorfully referred to as a crevasse splay. The deposits of these fans fill much of the flood plain around the main channel with distinctive sheets, usually of sand or silt(2-5).
4) An interesting twist concerns the effects of clear water (also referred to as "black water") on the flood plain. Clear water can come directly from rainfall, or it can be channel-derived water that has deposited its charge of sediment elsewhere. In either case, if the flood plain is inundated with clear water, and if the channel flood wave rises slowly enough, the water pooled on the flood plain produces back-pressure that works with the levee system to contain the channel flow. This back-pressure becomes more effective as the flood plain becomes higher. In addition, inundating the flood plain with clear, particle-free water inhibits sedimentation there, instead sending the sediment on downriver.
References (abridged):
1. Aalto, R. et al. Nature 425, 493-497 (2003)
2. Aslan, A. & Autin, W. J. J. Sedim. Res. 69, 800-815 (1999)
3. Blum, M. D. & Törnqvist, T. Sedimentology 47 (suppl. 1), 2-48 (2000)
4. Slingerland, R. & Smith, N. D. Geology 26, 435-438 (1998)
5. Smith, N. D., Cross, T. A., Dufficy, J. P. & Clough, S. R. Sedimentology 36, 1-23 (1989)
Nature http://www.nature.com/nature
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ON RIVER CARBON AND THE CARBON CYCLE
In its most general outline, the term "carbon cycle", in geochemistry and Earth science, refers to the movement of carbon from an atmospheric inorganic state to a biospheric organic state and then back to an atmospheric inorganic state. In detail, there are several pathways from biospheric organic carbon to atmospheric inorganic carbon, one of which, of great importance, is the movement of organic carbon into the hydrosphere, principally via rivers that empty into oceans, with oceanic dissolved organic carbon a reservoir for movement of organic carbon to an atmospheric inorganic state. Concerning the transfer of dissolved organic carbon from rivers to oceans, there are puzzles that have not yet been solved.
The following points are made by Wolfgang Ludwig (Nature 2001 409:466):
1) The author points out that dissolved organic carbon in the oceans is one of the largest reservoirs in the global carbon cycle, this reservoir comparable in size to all of the carbon in terrestrial plants, or to all of the carbon in form of carbon dioxide in the atmosphere. The input of terrestrial organic carbon from rivers, the main source of most constituents of sea water, could fill the oceanic reservoir in only a few thousand years, which (according to radiocarbon dating) is apparently the average age of oceanic organic carbon. But although there ought to be a great amount of terrestrial-derived organic carbon in the oceans, geochemical studies indicate there is apparently very little, and the fate of river-transported carbon ("riverine carbon") once it enters the oceans is unclear.
2) Almost all of the organic carbon on Earth is created via photosynthesis, whether on land or in water, but on land the process produces characteristic markers, so that terrestrial carbon should be traceable after it has entered the oceans. For example, many land plants synthesize certain compounds, such as *lignin or *tannin, which are absent in marine *phytoplankton. In principle, therefore, detecting these biomarkers in the oceans can reveal if carbon had a terrestrial origin. The other widely used method involves measuring the ratio between the two stable carbon isotopes, C-13 and C-12, in the bulk organic matter. Most land plants produce carbon that is more depleted in C-13 than carbon produced by marine phytoplankton, which results in higher isotopic ratios in marine than in terrestrial carbon.
3) Raymond and Bauer (2001) now present an analysis of organic materials in four rivers (Amazon [BR], Hudson [New York, US], (York [Virginia, US], Parker [Massachusetts, US] by radiocarbon dating (carbon-14, carbon-13 measurements), and they report the organic carbon in these rivers is up to several thousand years old [*Note #1]. This is in sharp contrast with the general belief that most of the organic carbon in rivers should be relatively "fresh". The particulate organic carbon (i.e., the fraction retained on a filter) was especially old (C-14-depleted). From these results, and laboratory evidence that suggests selective degradation of young (C-14-rich) dissolved organic carbon over the residence times of river and coastal waters, Raymond and Bauer conclude that pre-aging and degradation may alter significantly the structure, distribution, and quantities of terrestrial organic matter before its delivery to the oceans. The implication is that the absence of riverine carbon in the oceans is only apparent and due to the fact that we have not been able to distinguish riverine carbon from marine-generated carbon.
Nature http://www.nature.com/nature
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Notes:
Note #1: Carbon-14 dating depends on the decay of carbon-14 to nitrogen. Carbon-14 is continually formed in nature by the interaction of neutrons with nitrogen-14 in the Earth's atmosphere, the required neutrons produced by cosmic rays interacting with the atmosphere. The carbon-14 from this reaction is converted to carbon dioxide by reaction with atmospheric oxygen and mixed and uniformly distributed with the atmospheric carbon dioxide containing stable carbon-12. Since living organisms use atmospheric carbon dioxide either directly or indirectly, their systems contain the constant ratio of carbon-12 to carbon-14 that exists in the atmosphere. Death of an organism terminates the equilibrium process: no fresh carbon dioxide is added to the dead substance, and the carbon-14 present in the dead substance decays with a half-life of 5730 years, while carbon-12 in the dead substance remains what it was at death. Measurement of the carbon-14 activity at a given time thus allows calculation of the time elapsed after the death of the organism.
lignin: A complex organic polymer and major component of wood.
tannin: A complex astringent substance occurring widely in plants, particularly in leaves, unripe fruits, and tree bark.
phytoplankton: Small, usually microscopic, aquatic plants capable of photosynthesis; e.g., unicellular algae. Phytoplankton and plankton are not equivalent. The term "plankton" is a general designation for various drifting microscopic aquatic organisms in the upper regions of the oceans, both photosynthetic and non-photosynthetic.
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