Rivers clay mineral transport

Some of the clays that enter the ocean are transported by river input, but the vast majority of the riverine particles are too large to travel fer and, hence, settle to the seafloor close to their point of entry on the continental margins. The most abundant clay minerals are illite, kaolinite, montmorillonite, and chlorite. Their formation, geographic source distribution and fete in the oceans is the subject of Chapter 14. In general, these minerals tend to undergo little alteration until they are deeply buried in the sediments and subject to metagenesis. 

Weathered fragments of continental crust comprise the bulk of marine sediments. These particles are primarily detrital silicates, with clay minerals being the most abmidant mineral type. Clay minerals are transported into the ocean by river runoff, winds, and ice rafting. Some are authigenic, being produced on and in the seafloor as a consequence of volcanic activity, diagenesis and metagenesis. 

Direct evidence supporting the occurrence of reverse weathering has proven difficult to obtain for two reasons. First, the same kinds of clay minerals produced by this process are also transported to the ocean as part of the suspended load in river runoff. Second, the rate of reverse weathering is so slow that laboratory studies of this process are difficult to conduct. 

Rivers transport clay minerals primarily as part of their suspended load (silts and clays). The silt-size fraction is composed of quartz, feldspars, carbonates, and polycrystalline rocks. The clay-sized fraction is dominated by the clay minerals illite, kaolinite, chlorite, and montmorillonite. In addition to suspended particles, rivers carry as a bed load larger size fractions. The bed load constitutes only 10% of the total river load of particles and is predominantly quartz and feldspar sands. 

River transport of clay minerals into the ocean is spatially and temporally variable. The global annual suspended load of river sediment into coastal waters currently averages 12.6 X 10 ton. This flux is approximately 10% less than was delivered before humans began damming rivers. (One notable exception is the Mississippi River, whose sediment load has increased due to very high rates of soil erosion. The riverine sediments deposited in the mouth of the Mississippi River form one of the world s largest deltas.)... 

The dominant clay mineral at high latitudes is chlorite. In addition to ice rafting, lithogenous materials are transported in the polar oceans by rivers and winds. Polar seas are also characterized by diatomaceous oozes due to the occurrence of upwelling supported by divergence at 60°N and 60°S. 

The first use of natural resources was to obtain water, salt, and other natural materials to make tools and weapons. Larger cities were located where there was a source of water such as a river. Salt was used as a flavoring and to preserve food. A major discovery around 9000 b.c.e. was that clay minerals could be heated to make pottery so that food and water could be stored and transported much more easily. 

The particles transported by large rivers are a complex mixing of primary minerals, carbonates, clays, oxides and biogenic remains. The assessment of adsorption processes in controlling the levels of trace elements in large rivers has been documented by a couple of studies that will be described below. 

Sediment-laden rivers flowing over flat terrain commonly develop extensive floodplains. At times, floodplains coalesce into broad depositional alluvial plains such as the Llanos of South America. The sediments in those deposits weather chemically. Less stable minerals in the sediment are broken down and alluvial soils develop. Eventually, only the most stable minerals such as quartz remain, and the clays are transformed into cation-deficient varieties. Sediment in such rivers, especially the sand, may go through many cycles of deposition, weathering, and erosion before it is transported out of the system. Compositionally, this sediment resembles that derived from transport-limited erosion. Elemental fractionation between the original bedrock and erosion products still occurs because of the permanent burial of some cation-rich material and the uninterrupted transport of much of the fine-grained suspended sediment out of the system (Johnsson etal., 1988 StaUard, 1985,1988). 

These rock fragments are generally referred to as sediments when they have been dropped by the transporting agents. The components of sediments include small fragments (gravel, sand, or silt size), new minerals (mainly clays), and dissolved portions of the source rock (e.g., dissolved salts in river and ocean water). 

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