DISSOLVED MATTER DISCHARGE
AND MECHANICAL AND CHEMICAL EROSION
Prof. O . A . ALEKIN and Dr. L . V . BRAZHNJKOVA
Leningrad, USSR,
Hydrometeorological Institute
ABSTRACT
The present report is based on the authors' long-term studies in which comprehensive data of the net of the Hydrometeorological Service of the USSR and other data
from pertinent literature have been used.
The report gives detailed quantitative characteristics of the outflow by rivers of
matter dissolved in water throughout the territory of the USSR.
The total quantity of salts discharged from a given basin as well as the quantity of
salts discharged from its unit of area are reported here.
These data refer to all the continents of the earth as well. The conditions of the
formation of the discharge of dissolved matter in connection with the general complex
of physical and geographical conditions are also considered briefly.
On the basis of the dissolved matter discharge, the rate of chemical erosion for the
territory of the USSR and that approximately for the entire land area of the earth have
been estimated. The rate of chemical erosion for the territory of the URSS is compared
to that of mechanical erosion and the observed regularities are accounted for.
RÉSUMÉ
Le présent rapport est basé sur les études à long terme des auteurs dans lesquelles
les données du réseau du Service hydrométéorologique et d'autres données de la
littérature se référant au sujet en question ont été utilisées.
Le rapport donne les caractéristiques quantitatives détaillées de l'écoulement par
l'eau des rivières des matières qui y sont dissoutes dans tout le territoire de l'URSS.
La quantité totale de sels emportés d'un bassin donné ainsi que les quantités de
sels emportés par unité de surface sont présentées dans cette note.
Ces données se réfèrent aussi à tous les continents du globe. On considère aussi
brièvement les conditions de formation du débit solide dissous en rapport avec l'ensemble complexe des conditions physiques et géographiques.
On estime aussi le taux d'érosion chimique sur le territoire de l'URSS et, en approximation, sur toute la surface de la terre, en se basant sur le débit solide dissous d'après
les chiffres du rapport. Le taux d'érosion chimique pour l'URSS est comparé à celui
de l'érosion mécanique et des lois sont tirées de cette comparaison.
In the process of the water cycle an abundance of dissolved matters are transferred
by water; these matters are quantitatively characterized by the value of the so-called
"dissolved matters discharge". Taking into account this value it is possible to judge
the intensity of various erosive and accumulative processes, viz., earth surface denudation, chemical weathering processes, karst formation, soils lixiviation, chemical
elements migration, and also accumulation of salts in the ocean, the formation of
chemical sediments and soil salinity of arid zones, and in general the formation of
chemical composition of the weathering crust.
At present we possess some reliable information only on the principal component
of dissolved matters discharge—on ion discharge—but for a restricted number of
countries: USSR, USA, Sweden, Bulgaria and some others. Dissolved matters discharge
for the USSR territory was estimated by the authors C 1,2,3 ), who used an abundance of
data, collected by the network of hydrological stations of the Chief Administration of
the Hydrometeorological Service of the USSR and some published information as well.
The total value of ion discharge for the whole USSR territory is 384 million t/year
(table 1). The average value of ion discharge index for the whole territory of the USSR
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equals 17.8 t/km 2 per year. Minimum salts quantity comes from the Pacific basin
(31 million t). It is explained by a considerable humidity, by properly washed up podzol
soils and by permafrost. Minimum of ion discharge—9.8 t/km 2 —is observed on this
basin as well.
Maximum quantity of salts (201 million t) comes from the large basin of the Arctic
Ocean, constituting the principal part of the USSR territory. Maximum values of ion
discharge index, however, belong to the Aral Sea basin (64.5 t/km 2 ), where chemical
denudation processes are very intense in the mountain regions.
The study of ion discharge from the territory of the USSR gives an opportunity to
determine the following principles, which are of a general character for all the rivers
of the world:
(1) Absolute value of ion discharge mainly depends on the volume of river discharge
or basin water yield.
(2) Seasonal variations of ion discharge are closely connected with the regime of
river discharge and in general repeat the hydrograph shape.
(3) Ions of hydrocarbon and calcium are transported by rivers in the maximum
quantity, ions of chlorine and magnesium—in the minimum quantity.
(4) Ion discharge index which characterizes denudation, depends on the local
humidity rate and basin petrography. Its maximum values are observed in mountain
regions (up to 200 t/km 2 per year). Small values of ion discharge index are typical of
plain northern territories and southern arid zones (less than 10 t/km 2 ). This is caused by
an insignificant runoff in aried zones and in the northern regions by excessive humidity
and the deposited soils, which are poor in salts.
The computation of other components of dissolved matters discharge besides ion
discharge can be made quite approximately, because its study, from a quantitative
point of view, is not sufficient. It can be evaluated by the following values for the
whole territory of the USSR: organic matters—80 million t/year, mineral colloids
(Fe2C>3, AI2O3, SiÛ2 etc).—20 million t, microelements—0.76 million t and nonorganic biogenetic matters—2 million t.
The calculation of the total amount of dissolved matters for the whole world is
very difficult because of the absence of necessary information on river water quality of
different continents. That is why some attempts have been made to calculate this
value using an indirect way. Thus, some calculations have been performed by
Murray( 7 ), Reade( 10 ), Penk( 8 ), Clarke( 4 ) and some others, and quite recently by
D.A. Livingstone( 5 ). (Table 2).
TABLE 2
Dissolved Matters Discharge
Name of Author,
Date of Publication
F.W. Clarke, 1924
G.A. Maksimovich, 1949
O.A. Alekin and L.V. Brazhnikova,
1960
D.A. Livingstone, 1963
from the whole
] a n dof the giobe
million t/year
t/km2
per
year
2735
3750
26.4
30.0
2543
3905
19.1
—
In the process of the comparison of these figures, however, it is necessary to take
into account the difference in the number of components, included in the analysis of
water. Thus, F. Clarke, besides principal ions, includes silicon, ferrum, aluminium and
37
nitrates, total about 15%. The same components are included in the analyses of
D. Livingstone, but hydrocarbonates are expressed in the form of HCOg, and O. Alekin
expresses them as COg.
For the calculation of ion discharge from the land of the globe the authors used the
method of analogies by ion discharge indices. The land of the globe was divided into
regions and zones by physiographic features, the value of ion discharge index was
accepted by analogy with the territory of the USSR, studied in details during recent
years.
The ion discharge value, calculated for the whole land-area of the earth, reaches
2543 million t/year.from which 2316 million t are transported into the oceans (table 3).
According to our calculations the ion discharge index for the land-area of the earth
equals 19.1 t/km 2 per year and 22.9 t/km 2 is the ion discharge index for the part of
land with stream flow into the oceans. This index is a very important geochemical
constant, which characterizes erosive and accumulative functions of the continental
runoff. It may serve as standard characteristics for the rate of chemical erosion of
individual regions, The autors composed maps of chemical erosion of the USSR
territory and also maps of the global land surface in respect of the average value of
erosion. At the comparison of ion discharge index values and denudation values one
should take into account the income of CO2 from the atmosphere, resulting in dissolving
of lime-stones and industrial activities of man, causing water pollution.
Distribution of ion discharge values over continents, according to our calculations,
corresponds to the areas of continents though no exact proportionality is observed of
course. Maximum values of ion discharge index are observed on the continents with
excessive humidity, 25.7 t/km 2 for Europe and 24.7 t/km 2 for South America. Maximum value of this index is observed in Australia, the territory of which being occupied
by deserts for almost 50%.
The other components of dissolved matters discharge may be estimated quite
approximately.
By analogy of the USSR territory they may have the following approximate values:
organic matters—720 million t, mineral colloids—175 million t, microelements—
36 million t, non-organic biogenetic matters—18 million t. Consequently, the total
discharge of dissolved matters for the land-area of the earth with stream flow
into oceans may be calculated as 3300 million t, or 2600 million t organic matters excluded.
The data on river discharge from continents provide us to estimate the average
value of chemical composition of river water for the land-area of the earth, this value
being an important geochemical characteristic F. Clarke calculated the probable
composition for the land-area of the earth on the basis of the known chemical
composition of river discharge from the territory of North America. Taking into
account the average composition of the waters of North America after D. Livingstone,
whose data are close to the data of F. Clarke, and noting the average composition of
river waters on the USSR territory, we estimated the average composition of river
waters, which can be accepted as an average composition of the total river discharge
from continents into the oceans, because of the absence of more reliable data (table 4).
Average mineralization of river waters of the earth (taking into account seven ions) equals
63.5 mg/1, and if hydrocarbonates are expressed as HCOg, it increases up to 89 mg/1.
Taking into account the average composition of river water of the areas with river
discharge into the ocean it is possible to determine the amount of matters depositing
on the bottom of the ocean. It is possible to assume that concerning the carbonate
system components in the ocean at present a certain stable equilibrium is obviously
observed; in this case the amount of calcium carbonate deposited from the saturated
and supersaturated surface layer of water is compensated by these ions transported by
river waters.
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If we accept the discharge of 6 principal ions from continents into the ocean equal
to 2316 million t, then the amount of transported ions will be as follows: CO3826 million t, Ca"-494 million t, Mg"-120 million t. But if Ca" and CO'g are deposited
completely, then Mg" deposits partially in an amount which is determined by the
difference of equivalents CO'3-Ca", that is the amount of M g " equals 35.5 million t.
Thus, the amount of deposited salts, containing principal ions, equals 1362 million t.
It is worthy noting, that the calculated amount of deposited M g " equals 6.6
weighed % of the total deposit of C a " and Mg",that is about the same ratio, observed
in carbonate deposits of the seas.
Besides principal ions, Fe, Al, and Si, combined in the group of mineral colloids,
and transported by rivers, will evidently deposit too. The amount of these elements in
oxidate form will equal 360 million t.
Two other groups, composing dissolved matters discharge—microelements and
non-organic biogenetic matters—will also deposit, but it is difficult to judge the form
of this deposition. Thus, taking into account the total river discharge from the continents into the oceans, the amount of deposition will be as follows: 1362 + 360 + 36
+ 18 = 1776 million t.
The estimation of values of specific ions, especially for Asia and Africa, is an important problem, concerning further research of dissolved matters discharge, together
with the precision of ion discharge value.
TABLE 4
Average Chemical Composition of River Discharge From Continents
Area
Rivers of the USSR
Units
Ca"
Mg"Na'+K'
HCO3
mg/1
%
16.5
15.4
21
15.9
4.06
3.8
5
3.8
7.67
7.1
10.4
7.8
58.2
54.2
68
51.4
12.2
11.3
20
15.1
8.76
8.2
8
6
107.4
100
132.4
100
mg%
15.6
3.7
6.8
53.4
13.3
7.2
100
%
Rivers of
North America
Rivers of the USSR
and North America
mg/1
SO4
CI'
Total
Chemical and mechanical erosion has much in common by their origin because
these kinds of erosion represent various forms of the same phenomenon-water erosion
of the earth's surface. That is why it is essential to establish genetic relations between
ion discharge and suspended sediment discharge.
The study of relations between mean long-term value of specific discharge of suspended sediments and ion discharge index of the USSR rivers shows, that if we
consider 178 rivers, the ratio for 67% of them is within the limits from 10:1 to 0.5:1,
and for 70% of mountain rivers it is within the limits from 10:1 to 1:1. The analysis of
factors, influencing the inter-relations of specific discharge of suspended sediments
and ion discharge index in rivers permits us to conclude that the given limits of this
ratio are not occasional, but they are connected with the chemical composition of
products, being formed during the whole history of the earth's crust in the process of
chemical weathering of aluminosilicates. The ratio between insoluble and dissoluble
products of weathering for the aluminosilicates, viz. feldspar, is mainly within the
limits from 3:1 up to 5:1.
40
Besides weathering processes, forming general conditions, the ratio between products of mechanical and chemical erosion is greatly influenced also by the transported
destructive materials and soil particles which increase this ratio and also by salt solution
contained in sedimentary rocks which on the contrary decrease this ratio. No matter
how schematic these calculations would be, they permit, to a certain degree, to explain
the observed inter-relations of specific discharge of suspended sediments and ion
discharge index.
If after the consideration of individual river basins, which are effected by local
conditions, we shall examine larger areas, we shall see, that the pointed principle is
confirmed. Thus, for the global earth surface the suspended sediment discharge after
G. V. Lopatin (6) is evaluated as 19700 million t per year. Assuming an ion discharge
which equals 2500 million t, the ratio between mechanical and chemical erosion is 4.9,
i.e. it is within the limits, observed for the products of chemical weathering of aluminosilicates.
The fact that the ratio of the accumulated masses of insoluble and dissoluble
products of weathering of the earth's crust is the same as the previous one, is of a great
interest. Taking into account Poldervaart's data ( 9 ), we calculated, that the total
amount of insoluble sedimentary rocks equals 13 • 10 17 t, and the total amount of
redeposited and dissolved salts equals 4 • 10 17 t, i.e. their ratio is 3:1. The obtained
ratio of the amount of insoluble and dissoluble sedimentary rocks of the earth's
crust, though the calculation is rather schematic, has much in common with the ratio
of insoluble and dissoluble weathering products of aluminosilicates which ranges from
5:1 to 2:1.
The calculated amount of carbonates, contained in sedimentary rocks, in general
corresponds to the amount of carbonates, transported by rivers into the ocean at
present (about 1400 million t), if we accept, that the transportation of carbonates by
river increases gradually. The accumulation of carbonates in sedimentary rocks in this
case will take 1 billion of years.
The given characteristics of chemical erosion values show the close connection of
natural phenomena and processes, which may be expressed even in the quantitative
relations.
REFERENCES
C1) ALEKIN, O.A. and BRAZHNIKOVA, L.V., Novye dannye po stoku rastvorennykh
veschestv s territorii SSSR. New data on dissolved matters discharged from the
USSR territory. Report of the Academy of Sciences of the USSR, 1957, 114, No. 4.
2
( ) ALEKIN, O. A. and BRAZHNIKOVA, L. V., K izucheniju stoka rastvorennykh veschestv
s zemnoy poverkhnosti. On the problem of dissolved matters discharge from the
earth's surface. Report of the Academy of Sciences of the USSR, 1960, 131, No. 4.
(3) ALEKIN, O.A. and BRAZHNIKOVA, L.V., Stok rastvorennykh veschestv s territorii
SSSR. Dissolved matters discharge from the territory of the USSR. Moscow, ed.
" N a u k a " , 1964.
4
( ) CLARKE, F.W., The date of geochemistry. U.S. Geol. Survey Bull, 770, Washington,
1924.
(5) LIVINGSTONE, D.A., Data of Geochemistry. Sixth Edition, Chapter g. Chemical
Composition of Rivers and Lakes. Geological survey professional paper 440-g.
Washington, 1963.
(6) LOPATIN, G.V., Sediments of the USSR rivers — Nanosy rek SSSR •— Moscow,
Geografgiz, 1955.
(7) MURRAY, J., On the total annual rainfall on the land of the globe and relation of
the annual discharge of rivers. Scottich. Geog. Mag., vol. 3, Edinburgh, 1877,
p. 65.
8
( ) PENK, A., Morphologie der Erdoberflache. t.l.Stuttgart, verlag von 1. Engelhorn,
1894.
(9) POLDERVAART, A., Chemistry of the earth's crust. Geol. Soc. Am. Spec. Paper 62,
119-144, 1955.
(10) READE, T. M., Chemical denudation in relation to geological time. Proc. Liverpool
Geol. Soc, 1876-1877, 3.
41