River clay minerals

To construct an alternative model of Amazon River water, we assume that equilibrium with kaolinite (a clay mineral, Al2Si205 (OH)4) and hematite (ferric oxide, Fe203) controls the aluminum and iron concentrations ... 

Reference materials that represent the primary deep-sea and coastal depositional environments and biological materials would solve many of the problems that radiochemists face in analysis of sediments from these settings. Radiochemists require reference materials comprising the primary end member sediment and biological types (calcium carbonate, opal, and red clay from the deep-sea and carbonate-rich, silicate-rich, and clay mineral-rich sediments from coastal environments and representative biological materials). Additional sediment reference material from a river delta would be valuable to test the release of radionuclides that occurs as riverine particles contact seawater. 

Earth s crust is a source of particles produced as a consequence of weathering and volcanic activity. Weathering of continental rocks generates terrigenous particles that are carried into ocean via rivers, glaciers, and winds. As shown in Table 13.2, the most abundant mineral types are quartz, plagioclase, and clay minerals. The most abimdant... 

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. 

Clay minerals are important to the crustal-ocean-atmosphere fectory, not just for their abundance, but because they participate in several biogeochemical processes. For example, the chemical weathering reactions responsible for their formation are accompanied by the uptake and release of cations and, thus, have a large impact on the chemical composition of river and seawater. This includes acid/base buffering reactions, making clay minerals responsible for the long-term control of the pH of seawater and, hence, of importance in regulating atmospheric CO2 levels. 

Most cation exchange occurs in estuaries and the coastal ocean due to the large difference in cation concentrations between river and seawater. As riverborne clay minerals enter seawater, exchangeable potassium and calcium are displaced by sodium and magnesium because the Na /K and Mg /Ca ratios are higher in seawater than in river water. Trace metals are similarly displaced. 

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 chemical weathering of crustal rock was discussed in Chapter 14 from the perspective of clay mineral formation. It was shown that acid attack of igneous silicates produces dissolved ions and a weathered solid residue, called a clay mineral. Examples of these weathering reactions were shown in Table 14.1 using CO2 + H2O as the acid (carbonic acid). Other minerals that undergo terrestrial weathering include the evaporites, biogenic carbonates, and sulfides. Their contributions to the major ion content of river water are shown in Table 21.1. 

The overall effect of the terrestrial weathering reactions has been the addition of the major ions, DSi, and alkalinity to river water and the removal of O2, and CO2 from the atmosphere. Because the major ions are present in high concentrations in crustal rocks and are relatively soluble, they have become the most abimdant solutes in seawater. Mass-wise, the annual flux of solids from river runoff (1.55 x 10 g/y) in the pre-Anthropocene was about three times greater than that of the solutes (0.42 x 10 g/y). The aeolian dust flux (0.045 X 10 g/y) to the ocean is about 30 times less than the river solids input. Although most of the riverine solids are deposited on the continental margin, their input has a significant impact on seawater chemistry because most of these particles are clay minerals that have cations adsorbed to their surfaces. Some of these cations are desorbed... 

The clay minerals carried by rivers into the ocean represent a net annual addition of 5.2 X 10 mEq of cation exchange capacity. Most of these exchange sites are occupied by calcivun. Within a few weeks to months following introduction into seawater, sodium, potassium, and magnesium displace most of the calcium. As shown in Table 21.7, this uptake removes a significant fraction of the river input of sodium, magnesium, and potassium. 

At the prevailing pH in the Namibian groundwaters, the predicted solubility of carnotite is low and close to saturation. From one hole in the Tubas deposit, carnotite saturation is close to 0 and predicted to be over saturated around the water-table zone and in the near-surface upper 2m of the gypcrete. Where Eh is positive carnotite is predicted to be nearsaturation. This indicates that carnotite accumulation at or above the regional water-table can occur by upward diffusion of uranyl carbonate species with possible precipitation due to nucleation on clay minerals or gypsum, as evidenced in the Tubas River. 

Many investigations have reported the presence of zeolites at the deep ocean bottom (Biscaye, 1965 Heath, 1969 Bonatti, 1963 Sheppard and Gude, 1971 Jacobs, 1970 Morgenstein, 1967 among others). Most of the alkali zeolites are represented except the silica-poor species natrolite and analcite. Rex and Martin (1966) indicate that detrital potassium feldspar is not stable under ocean floor conditions. Zeolites are found in most ocean basins where wind-carried volcanic ash predominates over detrital river-born clay mineral sediments. In these sediments phillipsite is particularly evident and it is known to continue to grow in the sediment column to depths of more than a meter (Bernat, t al.,... 

There is also evidence that new clay mineral phases form. Comparisons of the clay phases carried down with a river with the clay phases deposited in the ocean outside the river mouth are suggestive but not altogether conclusive since it is hard to exclude completely the influence of rates of sedimentation, currents, etc. 

The mineralogy of the suspended matter carried by rivers is not well documented. There are numerous analyses either of the clay fraction or of sands carried by rivers, but only a few total quantitative analyses are reported in the literature. As examples, the average mineralogical composition of two large river systems, the Amazon and the Mississippi, are presented in Table 9.9. This table also includes the mean mineralogical composition of shales for comparison with river suspended sediments. The overall average of 300 samples of shales analyzed by Shaw and Weaver (1965) is 30.8% quartz, 4.5% feldspar, 60.9% clay minerals, and... 

Amseth R.W. (1982) Carbonate and clay mineral reactions in a modern mixing zone environment, Salt River Estuary, St. Croix, U.S. Virgin Islands. Ph.D. dissertation, Northwestern Univ. 

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