SCOPE 42 - Biogeochemistry of Major World Rivers

12.1 INTRODUCTION

Rivers and creeks are motors of the petrogenetic cycle of the Earth. Apart from glaciers, running water is the most effective agent to erode the surface of the Earth. The most important geological principles of riverine erosion, transport and sedimentation were recognized in the nineteenth century (see Neumayr 1895 for a summary); but mineralogical and petrological analyses of river sediments did not start earlier than 150 years ago, and were mainly focused on ancient sediments ( e.g. Sorby 1856; Cayeux 1906). Sedimentologi cal, mineralogical and chemical data on major world rivers have been published since the first decades of this century (Katzer 1903; Clarke and Steiger 1914; Russell 1937; Holmes and Hearn 1942; Potter et al. 1975; Gibbs 1967; Naidu 1966; Depetris and Griffin 1968; Quakernaat 1968).

In recent years mineralogical investigations of river sediments have been intensified mainly by obtaining river sediments from areas where formerly no analyses existed. Additionally, new aspects as, for example, plate tectonics were introduced to the discussion of the provenance of river sediments.

This study aims at establishing a synopsis of the source, composition, and behavior of minerals in world rivers.

12.2 TRANSPORT OF RIVER SEDIMENTS

Running waters erode particles only up to a certain size corresponding to the water velocity. Hard rocks must be disintegrated prior to erosion by physical, chemical and/or biological processes. The eroded material is composed of single minerals, polycrystalline grains and rock fragments. Most of the coarse-grained particles are transported as bedload forming ripples or sand-waves of different sizes. Due to the high water velocities of most rivers, part of the coarse material is transported in suspension. Meade et al. (1979) observed in the lower Amazon that 30% of the suspended sediments have grain sizes larger than 31µm and even 8% larger than 125µm An increase in sphericity occurs during the transport generally only in grain sizes larger than 1 mm, as shown by Cameron and Blatt (1971) for the Elk Creek in North Dakota.

Striking changes in sediment composition occur in high relief headwaters. Gravels, several centimeters in diameter, are formed after short transport distances. Cameron and Blatt (1971) showed that schist fragments disappear after a transport distance of a few kilometers because of their , weak consistency. In contrast, hard volcanic rock fragments do not show any decrease in their quantitative distribution. Mack (1981) stated that sedimentary and low grade metamorphic rock fragments break down within the first 25 km in a river with high relief. The amount of rock fragments in the sand fraction of sediments of the Mississippi River lowland course, however, does not decrease over a distance of about 1200 km between Cairo and the delta (Russell 1937).

Both mechanisms of transport¾bedload and suspension¾do not significantly affect the original shape of particles smaller than 1mm. This explains the high angularity of sediments in large rivers even after being carried several thousands of kilometers through lowlands (Irion 1976a; see also Figure 12.1).

The majority of particles transported by rivers are carried in suspension. The major part of the suspended load is of silt and clay size with minor contributions of sands. Clay minerals are most important and generally dominant in the fraction smaller than 6 µm but all the other known minerals¾with only a few exceptions¾may also occur in river suspended solids (Figure 12.2). Since the contact of the suspended solids and bedload with the water during transport is relatively short (not exceeding a few months) the chemical alteration is restricted to surface exchanges (Irion 1976b), to dissolation of salts¾and to a certain extent of carbonate minerals.

Figure 12.1 Scanning electron micrograph of a sediment sample from lower Amazon River, taken near the mouth of Rio Xingú. The angularity of the sand is very high and does not show its long transport way from the Andes to the lower Amazon (magnification = 100 times)

Figure 12.2 Fraction analysis of Mississippi alluvial mud from levee on the east side of New Madrid Bend, Kentucky (after Potter et al. 1975)

12.3 SEDIMENT LOAD OF THE WORLD RIVERS

In general the erosion rates and hence the average suspended sediment load of rivers depend¾when human activities are not taken into account¾on the following factors:

1. Age and morphology of the surfaces.
2. Composition of the bedrock material.
3. Tectonic uplifting.
4. Climate and vegetation cover .

Milliman and Meade (1983) estimated the annual suspended load of the world rivers to be about 13.5 x 10⁹ t. The distribution of the sediment loads of different areas is very heterogeneous, and high sediment transport rates are concentrated in regions near the Equator and in Arctic regions. Some of the most important examples of river sediment load are described below, largely based on the synopsis of Milliman and Meade (1983).

Among the major world rivers, the Huanghe delivers the highest average concentration of sediment load (22000 mg/l) into the sea, followed by the Nile which (before the construction of the Aswan Dam) carried 3700 mg/l and the Ganges/Brahmaputra with 1700 mg/l. The concentrations of other major rivers are for: the Purari River (1040 mg/l), the Fly River (390 mg/l), the Mississippi (360 mg/l), the Mekong (340 mg/l), the Po and Danube (both with 325 mg/l), the Yukon (310 mg/l), the Amazon (200 mg/l), the Orinoco (190 mg/l), the Niger (78 mg/l), the São Francisco (62 mg/l), the Rhine (47 mg/I), the Elbe (36mg/l), the Weser (35mg/l) and the Zaïre (34mg/l). Amazonian rivers, as far as they drain old land surfaces in the eastern Amazon basin, carry less than 5 mg/l mineral suspension, but in other parts of the lowland, where soft Tertiary sediments crop out, the suspension load regionally exceeds 1000 mg/l (particularly in the upper reaches of Rio Juruá and Rio Purus). The suspended load of one single river may temporarily change in a wide range. For example Pickup et al. (1979) published data on the Alice River (Papua New Guinea) within a minimum of 1 mg/l and a maximum of 1100 mg/l.

Average erosion rates may be calculated from the sediment load of the rivers and from the size of the drainage areas. High erosion rates are calculated for the Andes, where values of 917 t/km²/year were published by NEDECO (1973). In contrast to this, erosion rates in eastern Amazon lowlands are generally below 10 t/km²/year. In tropical Africa, for the relatively low relief of the Zaïre River drainage area, erosion rates of 23 t/km²/year can be calculated. The highest erosion rate in the tropics exceeds 1000 t/km²/year. In Taiwan, values even higher than 1000 t have been determined. Pickup (1977) found an average sediment load of 5000 mg/l, corresponding to an erosion rate of 16500 t/km²/year in the Aure River (a tributary of the Purari River).

In the temperate climate zone the erosion rates are by far lower. Milliman and Meade (1983) calculated an average erosion rate of 50 t/km²/year for Europe and 8 t/km²/year for the Euroasiatic Arctic. In the Arctic/glacial regions, however, the erosion rates again exceed 1000 t/km²/year.

12.4 MINERALOGY AND PETROLOGY OF RIVER SEDIMENT LOAD

12.4.1 NON-CLAY MINERALS AND ROCK FRAGMENTS

The mineralogy and petrology of suspended matter and bedload in rivers mainly depends on grain size. Particles smaller than about 6µm are dominantly clays. Coarser sediment particles are mostly quartz, feldspars, carbonates or polycrystalline rock fragments. In river sediments all known minerals, with only a few exceptions, and fragments of all outcropping rocks occur. An extreme example is a creek in northern Spain where the highly soluble mineral halite forms gravel (Wagner, pers. comm). Other easily soluble minerals (e.g. gypsum) are more common in river sediments (e.g. Rio Huallaga, Peru). Carbonates, the next least soluble group of minerals, are widespread in many rivers. Hahn (1969) found 25% calcite and 12% dolomite in Rhine sediments just upstream of its inflow into Lake Constance. In contrast to this the calcium carbonate mineral content in many other rivers is low and in the Amazon River calcium carbonate minerals occur only in the Andean headwaters and in the subandean lowlands (see below).

Feldspars, the most widespread group of rock-forming minerals, are very common in river sediments. Potter (1978), who analysed sands from the major world rivers (see below), found maximum feldspar contents of 28% in the Mekong River and very low contents in the Orinoco River (1%). Krook (1979) determined 36% feldspars in the lower reach of the Amazon River , whereas in the same area, Franzinelli and Potter (1983) found in river sands contents of less than 10% , but rock fragments included in the same sample may have contributed additional feldspar. An average river sand, when delivered to the ocean, contains 11% feldspars (Potter 1978).

The mineral showing the highest contribution to river sands is quartz. Young (1976) analysed quartz from different sources, ranging from low metamorphic to crystalline rocks, and could distinguish between mono- and polycrystalline quartz grains. Highest contents of quartz were found in sands of tropical lowland rivers. The sands of the Negro River (Brazil) consist almost exclusively of quartz. In contrast, quartz values are very low in rivers draining areas where volcanic basic pyroclastics dominate. According to Potter (1978) quartz contributes 60% to the sands of world rivers.

Besides the rock-forming light minerals, heavy minerals are considered to be important in obtaining information on the origin of the sediments (Füchtbauer 1964). The percentage of heavy minerals varies from more than 90% in river sand placers to less than 1% in highly mature sediments. The average contents are around 4%.

Individual grains in river sands can be divided into monomineralic grains and grains with polycrystalline structures. The latter are generally rock fragments. Most of them are formed from sedimentary, metamorphic, volcanic or plutonic rocks. Rock fragments may contribute more than 60% to river sediments (Moose River, see Potter 1978) or less than 1% (Rio Negro, see Franzinelli and Potter 1983).

12.4.2 CLAY MINERALS

The dominant clay minerals in river suspended loads are illite, smectite, chlorite and kaolinite. Illite, the most common clay mineral, is found in almost every river suspension. Potter et al. (1975) estimated the illite content of Mississippi River sediments (in percentage of the total clay minerals in the < 2 µm fraction) as high as 24%; Müller and Stoffers (1974) measured 74% illite in the Danube River just upstream of its entrance to the Black Sea. The Po River, receiving its material mainly from the Alps, has an illite content of 58% when draining into the Mediterranean Sea (Quakernaat 1968). Naidu and Mowatt (1983) analysed sediments of the Mackenzie River (66% illite) and of the Yukon River (41% illite). The Amazon River and the Orinoco River have 20-25% illite, whereas rivers of tropical Africa (the Nile and the Niger) contain only small proportions of illite. In the Huanghe and Mekong Rivers the illite content is estimated to be about 20% , and in Oceanian rivers (the Mahakam, Sepik and Strickland Rivers) the illite content is between 40% and 60% .No illite has been detected in rivers draining the volcano-rich island of Java (Ci Tanduy, Serayu, and Proga Rivers) and in some tropical lowland rivers in the Amazon basin. The main representative of these rivers in the Amazon basin is the Negro River, where most samples (Irion 1987) lack illite. It is estimated that the average illite content of world rivers ranges from 45% to 60% .

Chlorite, mainly derived from metamorphic rocks and from slates, is much less abundant than illite. Chlorite is found in the Danube River (2%) and in the Po River (20%). In the Mississippi, the Yukon and the Mackenzie Rivers, it contributes 6%, 26% and 15% , respectively. In the Amazon River there is about 20% chlorite, whereas in the Orinoco River only traces of chlorite are found. Chlorite is absent in rivers draining the Brazilian east coast, but the Paraná River carries about 16% in its suspended load (Depetris 1968). In tropical Africa chlorite could be found neither in Nile River nor in Niger River sediments. Chlorite content in the Ganges is very high (25%) but in the Godavari River its content is as low as 5% (Naidu et al. 1985). In the Huanghe and Mekong Rivers the chlorite content is estimated to be 20% to 25% , and similar amounts are present in the Mahakam and Sepik Rivers. In the Strickland River the chlorite content is higher than 40% .Highest amounts of chlorite are found in the Ok Binai and Leonhard Schulze Rivers in Papua New Guinea, where values of more than 50% have been recorded. In the Amazon basin the highest value is 50% (in the Napo River), while the rivers draining exclusively lowland areas do not have any chlorite. Thus the variability of chlorite in world rivers ranges from 0% to more than 50% (average 15% to 25%).

Smectite is formed mainly on the Earth's surface, with its subgroup member montmorillonite being the most abundant. Many authors think that it is more convenient to give smectite data together with all other expandable 14Å-minerals. When smectite is formed from triformic highly charged minerals, in particular from mica or illite, the highly charged nature remains. Therefore it is possible to get some information on the origin of this mineral by determining the inner layer electric charge.

Smectite (montmorillonite) contents in the Danube River amount to 14%, and in the Po River to 22%¾both are Alpine streams. The smectite content of the Mississippi is even higher (53% ). The Mackenzie River and the Yukon River have different contents: 6% and 21% , respectively ('expandable clay minerals'; Naidu and Mowatt 1983).

In the Amazon River smectite contents of 40-50% are found. In contrast, the values for the Orinoco are low (< 10%) . Rivers of tropical Africa are characterized by very different smectite contents. In the Nile River the smectite content is about 90%; in the Niger River it is about 30% .In the Ganges River 'expandable minerals' are missing whereas in the Godavari River they contribute between 66% and 81% .The Huanghe and Mekong Rivers have smectite contents of about 40% and 10% , respectively. In Mahakam and Sepik River sediments about 30% is present, and in the Strickland River only 10% smectite has been found. A group of large rivers draining the western Amazonian lowlands display smectite contents of > 80% (e.g. the Juruá and the Purus Rivers). The average smectite content of world rivers is estimated to range between 30% and 40%.

Mixed layer minerals are widespread in soils and metamorphic rocks. But in river sediments, when other clay minerals are dominant, they are difficult to determine. Most mixed layer minerals may be determined as smectite-like clays. However, in some rivers, when higher percentages of mixed-Iayer minerals are present, the determination is possible. For example Quakernaat (1968) described mixed layer minerals in rivers of northern Italy and Irion (1987) described them in some rivers of the mountainous drainage areas in Papua New Guinea. Rectorite belongs to the group of mixed layer minerals and is found in some small rivers.

Kaolinite is also formed mainly on the Earth's surface. It is an 'end member' of the weathered clay minerals. Main structural differences in this group exist between well-ordered and disordered kaolinites. The Danube River contains 10% kaolinite, and in Po River sediments no kaolinite could be found. In the Mississippi River the kaolinite content is 16% , and in the Mackenzie and Yukon River sediments kaolinite amounts to 13% and 12% respectively. The values in the Amazon River sediments do not exceed 14%, whereas in the lowland drainage system of the Amazon many rivers exhibit kaolinite contents of > 90% (Negro and Xingú Rivers). In the Orinoco the kaolinite content is about 40% .In tropical Africa the Niger River sediments are characterized by more than 50% kaolinite, whereas the Nile River has only traces of this mineral. The Ganges River sediments lack kaolinite whereas in the Godavari River, Naidu et al. (1985) determined kaolinite contents of between 7% and 16%.

The Huanghe and Mekong Rivers are poor in kaolinite with values of <10% .The Mahakam and Sepik Rivers have similar amounts, while in the Strickland River sediments only traces of kaolinite are present. In summary, the content of kaolinite in major world rivers varies from traces to nearly 100% in the clay fraction of the suspended load. The mean kaolinite content may be in the order of 10% .

Halloysite is a clay mineral formed mainly on volcanic material and does not normally contribute significant amounts to river sediments. It is found in rivers draining the surface of Mt Bosawi, a volcano in eastern Papua New Guinea. In tropical regions gibbsite, an Al-hydroxide mineral of clay-like characteristics, is sometimes present in river suspended solids.

12.5 PROVENANCE OF RIVER SEDIMENTS 

12.5.1 NON-CLAY MINERALS AND ROCK FRAGMENTS

I As already recognized by Leonardo da Vinci, coarse-grained river sands and gravels may often easily be related to their source rock by their color (e.g. red sandstone, white limestone) and by their shape when derived from slates. The crystallinity and mono/polycrystalline character of quartz grains may indicate if they were eroded from a crystalline rock or from a metamorphic schist (summarized in Pettijohn et al. 1987). Feldspars and particularly rock fragments may give important details on the composition of the source rocks.

Weathering plays an important role in disintegrating the bedrock and preparing it for erosion. However, in the resulting sand and coarse fractions its geochemical/mineralogical alteration is restricted. James et al. (1981) analysed the relative alteration of microcline and sodic plagioclase in semi-arid and humid climates. They found some dependence of limonite plus kaolinite on feldspars of humid climates and of limonite plus smectite on feldspars of arid climates. Their conclusion was, however, that the potential of using occurrences of detrital feldspars for paleoclimate interpretations is limited. In addition, the chemical alteration of feldspars, which occurs during weathering, may also take place during diagenesis and feldspar composition is therefore not necessarily valuable for paleoclimatological assessment studies. This may also be the case for alteration of other minerals. As will be shown later, it is very important to bear in mind that minerals in rivers are not mainly eroded from soils, which are in equilibrium with the climate with regard to their mineral composition (Gibbs 1977), but are derived mostly from old sediments or sedimentary deposits and from poorly altered bedrocks.

Slatt and Eyles (1981) said about the breakdown of the parent rock under glacial conditions: ' As a result of abrasion, glacial clasts and lithic fragments are subjected to shear stress so that fractures propagate along intracrystal and intercrystal planes of weakness resulting in a continuous grain-size spectrum of lithic fragments and monomineralic grains irrespective of the type of source rock'. But with subsequent aquatic transport sorting occurs and the differences between typical glacial sands and non-glacial sands disappear.

Some of the heavy minerals are strongly related to their source rock types. For example, cassiterite derives only from granite; augite and magnetite from basic igneous rocks; and andalusite, garnet and sillimanite from metamorphic rocks. Highly mature sands, which have been repeatedly reworked in geological history , may contain only rutile, tourmaline and zircon. In Tertiary sediments deposited north of the Alps Füchtbauer (1964) showed the possibility to determine source areas of sediment bodies by comparing heavy mineral distributions.

12.5.2 CLAY MINERALS

The < 6 µm fraction of river sediments consists predominantly of clay minerals, some quartz, organic matter and accessory compounds. The minerals in the coarser fractions are mostly rock-forming minerals crystallized from magmas or formed under deep burial, whereas the clay minerals largely originate from processes which take place on the Earth's surface itself. Kaolinite, smectite, vermiculite and mixed layer minerals are chiefly formed during weathering processes by neoformation from bedrocks or by precipitation from solution. Illite is formed by a minor change of its structure from mica during weathering processes as well. Together with chlorite, mica originates predominantly from bedrocks. Micas occur in most rock types from granite to metamorphic rocks, and chlorite is widespread in metamorphic rocks, slates and similar rock formations. Clay minerals are major components of recent and ancient sedimentary deposits.

In principle, climate can be divided into two categories: tropical and temperate/Arctic, with the subtropics forming the transitional zone.

As an example of tropical weathering, the clay mineral associations of rivers from the Amazon basin are discussed first. In the lowland areas a humid tropical climate may have persisted since Tertiary times. During this period a thick kaolinite-rich regolith has formed on coarse-grained Cretaceous sediments and on the coarse crystalline basement of the Guyana and Brazilian shields. In other areas, where fine-grained sediments of Tertiary age are predominant, thick smectite-rich horizons are widespread (Irion 1976b). The kaolinite-rich horizons are generally subjected to low erosion rate, whereas the material of the smectite-rich horizons is carried away with relative ease. Hence, rivers draining areas with kaolinite-rich weathering horizons have low suspended sediment loads (e.g. the Negro), whereas those rivers draining areas with a smectite-rich regolith carry, generally high amounts of suspended solids (e.g. the Purus).

The same regularity is found in Africa where high suspended loads are linked with high smectite concentrations, and low amounts of suspended solids are linked with proportionally high kaolinite loads. As mentioned above, the suspended load of the Nile, before the construction of the Aswan dam, amounted to 3700 mg/l of which > 80% was contributed by smectite. The Niger has a suspended load rich in kaolinite with a moderate content of smectite (Martins 1982). It carries an average sediment load of 78 mg/l. But the Zaïre, a river with high kaolinite percentages, has an average suspended sediment load of about 30 mg/l only (Eisma and van Bennekom 1978).

Similar geological settings are found on the Indian subcontinent. The Godavari River drains large parts of these areas 70% of its suspended load consists of expandable 14Å-minerals (Naidu et al. 1985). These are predominantly smectites. The suspended solids reach a concentration of 1140 mg/l (Bikshamaiah and Subramanian 1980) and are therefore similar to the smectite-rich rivers of Africa and of the Amazon basin.

Based on the mentioned examples and additional data from other rivers it can be concluded that tropical rivers deliver far more smectite than kaolinite to the world's oceans. Because of the low stability of the clay mineral chlorite it generally does not occur in tropical rivers exclusively draining lowlands. Out of the three large African rivers only Zaïre's suspended solids contain some chlorite. Likewise, only small amounts of chlorite are found in the Godavari River while in the suspended solids of the Amazonian lowland rivers this mineral has not been detected at all.

Rivers draining young high mountain ranges are generally rich in clay minerals originating from bedrocks, slates and less consolidated sediment deposits (e.g. the Huallaga, Madeira and Napo, Huanghe, Mekong, Mahakam, Sepik, Strickland and Fly rivers¾see Figure 12.3). Climatic influences are hardly of any importance. The suspended solids of rivers draining young mountains are generally rich in chlorite and illite regardless of whether they are located in the tropics (e.g. Amazon, Mahakam) or in higher latitudes (e.g. Mackenzie and Yukon rivers). A specific group of tropical rivers are those which drain volcanic areas of high relief. These are predominantly rivers in Oceania. Apart from smectite, halloysite and minerals similar to halloysite characterize their solids in suspension.

The clay mineral association of rivers from the temperate climatic belt and from the cold regions depends to a larger extent on the bedrock composition of their drainage areas, as in case of the tropical lowland rivers. The weathering horizons in these areas are generally thin and most of the clay minerals of the rivers originate from slates and from old sedimentary deposits. This does not exclude the possibility that, in limited areas, the clay minerals formed in soils under recent climatic conditions are dominant in rivers (Konta 1984). So far, not many studies have concentrated on clay mineral distributions in river suspended solid of temperate and cold regions. Quakernaat (1968) studied clay mineral distribution in north Italian rivers and creeks. In a relatively small area the illite content varied between 20% and 70% of the total clay minerals, whereas smectite reached amounts of 40% .Mississippi sediments are rich in smectite (Potter et al. 1975) but this mineral is not in equilibrium with the present climate of its drainage basin. Potter et al. (1975) concluded that the majority of smectite has its origin in Cretaceous to Pleistocene sediments which are subjected to erosion. We found, for example, a small river in Central Europe carrying exclusively kaolinite which may have been formed from Mesozoic sediments during the Tertiary (Irion 1984).

Figure 12.3 X-ray diffractograms of the clay fractions of some South-East Asian and Oceanian rivers

The dominant clay minerals in Alaskan rivers are illite and chlorite (Naidu and Mowatt 1983). But minerals originating generally from weathering horizons or soils occur as well. Expandable 14Å-minerals reach values of up to 26% and the highest kaolinite content of rivers studied by Naidu and Mowatt (1983) is 15% .Both minerals may be related to the erosion of old sediments formed under warmer climatic conditions.

12.6 ALTERATION OF MINERALS AND ROCK FRAGMENTS DURING TRANSPORT, AND THEIR DEPENDENCE ON PLATE TECTONICS

The low alteration rate of minerals during transport may be demonstrated by the Amazon River. In the Huallaga, an Andean tributary of the Amazon, gypsum occurs in relatively high amounts together with calcite and dolomite. Both carbonate minerals are found in many other Andean rivers. In the subandean lowlands gypsum has disappeared, but calcite and dolomite are still present in minor amounts. At Iquitos (already some distance from the Andes) the carbonate minerals are completely dissolved due to the low ionic strength of the Amazon water. With decreasing electrolyte content in the waters of the Amazon main stem, between the subandean lowland and the delta, the cation concentrations on the surfaces of the clay minerals decrease as well. This is particularly the case for calcium.

Clay mineral associations in the Amazon River below Iquitos change only due to sediment mixing after its confluence with the Madeira. Apart from this no change has been observed between the foothills of the Andes and the delivery into the Atlantic Ocean. Similar observations have been published by Potter et al. (1975) for the Mississippi River and for Sepik River/Papua New Guinea (Irion 1987).

It has been expected that the composition of river sediments shows imprints of the plate tectonical framework of their drainage area. In this case the ratio of quartz, feldspars and rock fragments should be related to the tectonical history of the drainage areas (Potter 1978). On the trailing edge of a rising mobile belt, quartz should be enriched due to the long distance to the source area. In contrast to this assumption, in the vicinity of collision rims of plates, rock fragments should be present in higher percentages in the sediments. Potter (1978) showed in a study of near margin sands of 36 major world rivers that at the trailing edge the percentage of quartz averages 71% , feldspar 9% and rock fragments 20% , whereas for collision areas values of 36% , 17% and 47% , respectively, have been found. Yet most rivers from the trailing edges have a quartz content of more than 90% and drain cratons which are located in lowlands of the tropics. Therefore high quartz contents may also be explained by intensive weathering in the tropics. For the collision rim Potter (1978) analysed sands mostly from temperate to cold areas where low weathering gradients prevail. In summary it may be concluded that the quartz/feldspar/rock fragment relation in modern river sands is governed by the combination of several factors such as composition of the source rocks, climate, altitude of the drainage area and tectonical stability rather than by a single factor.

12.7 CONCLUSION AND SUMMARY

The sediment load of the world's rivers to the oceans is about 15 x 10⁹ t/year (Milliman and Meade 1983), more than 85% of this material consists of minerals. About 13.5 x 10⁹ t are carried in suspension with the remainder being transported as bottom load. A high (but unknown) volume of sediments may be deposited on inland floodplains (Meade 1982; Irion 1976c).
River sediments originate from the near surface, exposed igneous, volcanic and sedimentary rocks. Some of these are easily eroded, whereas others, especially the crystalline and highly metamorphic rocks, are affected by streams only when altered in the surface layers. Additional sources of river sediments are soils which inherited their mineral content (with some alteration) from bedrocks or which in the tropics may consist completely of newly formed minerals.

In river sediments the whole range of known minerals can be found: salt minerals in very restricted areas, carbonates, heavy minerals; feldspars, quartz and clay minerals. In addition, amorphous particles of volcanic origin and of skeletons and shells from organisms contribute to river sediments. Organic matter is generally present in a range between 1% and 15%.

It is obvious, that the grain-size variability in river sediments is very high, ranging from gravels with some decimeters in diameter or even from rock debris to particles of less than l µm. But a closer view reveals that large rivers generally transport only particles of sand- (63-2000 µm), silt- (2- 63 µm) and clay-size (< 2 µm), where sand is carried mainly as bottom load and silt plus clay as suspended matter.

The alteration of the transported minerals and rock fragments is mainly restricted to the upper reaches. The grain size of the sediments is reduced by abrasion due to the collision of individual particles. Dissolution of easily soluble minerals (e.g. rock salt and gypsum) but also of carbonates takes place. Rock fragments behave according to their resistance to abrasion. Fragments of soft rocks disappear after short transport, whereas fragments from hard rocks generally do not undergo further alteration when a grain size of < 1 mm is reached. Whether or not alteration of rock fragments proceeds in very large tropical rivers remains uncertain. Since abrasion of < 1 mm particles is small even when transported over several thousand kilometers, the angular character is commonly preserved. In the lowland reaches of rivers, variations in the mineral composition of the river sediments are, with few exceptions, the consequence of mixing with sediments of incoming waters. This is implied in, for example, the clay mineral variation in the main stem of the Amazon River in Brazil, where changes are related to the confluence with the Madeira, which delivers a large amount of sediments to the main river.

Due to the intensive and deep weathering processes in tropical lowlands, the mineral composition of land surfaces is altered. The newly formed minerals are mainly kaolinite and smectite. Carbonates, feldspars and most of the other minerals are dissolved. Quartz may be dissolved, or have its shape changed to more rounded particles. Consequently, rivers draining exclusively tropical lowlands have suspended loads where the fine fraction is predominantly made up of smectite and/or kaolinite, and the bedload is formed by quartz with some heavy minerals. In subtropical and temperate zones, the climate may also have a significant influence on mineral composition of river sediments, especially when the bedrock itself is difficult to erode, and when there has been sufficient time for soil formation. In those areas the main constituents of the river sediments originate from the soils and therefore reflect the climatic conditions under which the soils have been formed. But it has to be stressed that most of the world's river sediments are derived from mountainous areas, floodplains and areas where soil cover is not important for their genesis. For a few areas only, mineral composition is a function of the present climatic conditions of their drainage areas.

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