Comparative Analysis of the Intensity and Directionality of Modern Weathering Processes in the Northern and Southern Humid Zones (Based on Hydrogeochemical Data)

COMPARATIVE ANALYSIS OF THE INTENSITY

AND DIRECTION OF MODERN WEATHERING PROCESSES

IN THE NORTHERN AND SOUTHERN HUMID ZONES

(based on hydrogeochemical data)*

S. L. SHVARTSEV

(Presented by the scientific seminar of the Department of Hydrogeology
and the Problem Geological Laboratory)

The question of the causes of the different direction of weathering processes in the northern and southern humid zones, despite numerous publications, remains controversial. Quite recently, a discussion on this matter unfolded in the pages of the journal Geochimica et Cosmochimica Acta (Curtis, 1970; Brydon and Mckeague, 1971). To explain the observed differences in the character of weathering crusts, such factors are usually invoked as the magnitude of mean annual temperatures, the amount of precipitation, the intensity of removal of aluminum and silicon, the role of organic substances in the migration of elements, and so on.

As regards the intensity of modern processes of chemical weathering in these two zones, this question remains poorly covered in the literature. At best, comparisons are made between the magnitudes of the mechanical and chemical components of weathering products, calculated from the volume of removal of chemical elements by rivers in dissolved and suspended forms. On the basis of such comparisons, as a rule, a conclusion is drawn about the insignificant scale of chemical weathering in northern regions.

Meanwhile, knowledge of the scale and direction of processes of modern chemical weathering is important not only for understanding the conditions of formation of ancient weathering crusts, but also for solving many related problems of lithology, hydrogeology, geochemistry, soil science, etc.

Weathering processes are usually studied by means of a comparative analysis of weathering rocks and the products of their chemical alteration—weathering crusts. However, for these same purposes another approach may also be applied, based on the study of the volume of elements removed by groundwater and on comparison of the ratios of elements in waters with the composition of the rocks being leached. This method, which may be called hydrogeochemical, was formulated by B. B. Polynov (1956) in the form of the well-known proposition on the different mobility of elements in waters and was later developed by A. I. Perelman (1966). In research practice, however, it is used extremely rarely, and all its possibilities and advantages are far from sufficiently utilized. The advantages of this method consist in the fact that it makes it possible to assess, with sufficient objectivity, not only the scale but also the direction of modern weathering processes in different climatic zones, i.e. to study—

* Reported in Moscow at the conference on weathering crusts, October 24, 1973.

…to address precisely those questions that escape the attention of researchers in the traditional approach to the study of weathering crusts.

Let us apply the hydrogeochemical method to solve the tasks posed above; to this end we shall successively examine the features of the geochemistry of groundwater in two regions that differ sharply in climatic terms, where we have conducted the corresponding investigations over a period of several years. These are the northwestern part of the Siberian Platform and the western part of the African Platform.

Geochemistry of Groundwater in the Igarka–Norilsk Region

The region studied encompasses the western part of the Central Siberian Plateau, with absolute elevations of up to 500 m, and belongs to the transitional tundra–taiga zone. In its characteristic features the climate of the region is transitional between sharply continental and purely polar, with mean annual temperatures from \(-7\) to \(-12^\circ\)C. The average amount of precipitation, most of which falls in the summer period, is 350–450 mm/year.

The short summer and low temperatures cause low evaporation and, consequently, high values of runoff coefficients. Although most of the runoff occurs in the form of surface runoff and the coefficient of groundwater runoff is only 10–15, more rarely 20–25%, in absolute terms the magnitude of groundwater runoff, according to our calculations, reaches 4.0 in the Norilsk region, and in other areas 2.5–3.2 l/sec·km².

The formation of active runoff is promoted by the presence of permafrost, whose thickness in the Norilsk region reaches 400 m, and in the area of the town of Igarka 60 m. Taliks are developed along river valleys. The temperature of the frozen rocks reaches \(-6.7^\circ\)C; the thickness of the active layer is 0.5–3.0 m. Most groundwater runoff is associated with the active layer, the groundwater of which we shall consider further below. It is also important to emphasize that the thawing of permafrost throughout the entire summer season leads to increased moisture content of the active layer even in well-drained areas.

The diversity of geological conditions in the region under consideration largely determines the diversity in the composition of suprapermafrost waters, which requires subdividing the latter into corresponding complexes. We carried out such a subdivision taking into account the genetic type and lithological composition of the rocks (Table 1). The same table also summarizes the main factual material on the composition of waters from various rock complexes. As can be seen from the data presented, the influence of the type of water-bearing rocks on the composition of the waters is fairly great, especially where carbonate and carbonate–gypsum-bearing deposits are present; these contribute to a sharp increase in pH, and in the content of calcium, magnesium, sulfate, and bicarbonate ions. At the same time, the contents of silica and aluminum remain insignificant, which indicates the presence here of ordinary dissolution of rocks, somewhat complicated by the weathering of the silicate minerals present. Such dissolution, from the standpoint of questions concerning the formation of weathering crusts, is not of great interest, and therefore below we shall consider only the waters of the last three complexes.

In this case, what first of all stands out is the identity of the composition of the waters, despite the difference in lithological composition and structural features of the water-bearing rocks. This is expressed both in the magnitude of their total mineralization and in the content of all components. Such a situation is explained by the similarity of landscape-geochemical conditions.

formation of these waters and by the uniform character of the weathering processes that develop. Under such conditions, the composition of the waters is made up of the composition of the atmospheric precipitation falling there, as well as ions borrowed from the enclosing rocks and synthesized from water and carbon dioxide (the ion \(\mathrm{HCO_3^-}\)). Knowing the ratio of all the listed constituents of the water and its discharge per unit time, one can calculate the volume of chemical elements removed from the rocks and, consequently, the minimum scales of modern chemical weathering. Calculations carried out by us in this way (Table 2) showed that the volume of elements removed from the rocks is \(0.186\ \text{g/sec}\cdot\text{km}^2\), or \(5.85\ \text{t/year}\cdot\text{km}^2\), which is close to the values of the modulus of subsurface chemical runoff calculated by F. A. Makarenko and V. P. Zverev (1970) for the tundra and taiga zones, respectively \(0.144\) and \(0.389\ \text{g/sec}\cdot\text{km}^2\).

Turning to clarification of the directionality of weathering processes in the northern climatic zone, it is important to note the character of the change in the composition of suprapermafrost waters in the direction from watersheds toward river valleys. Generalization of the enormous factual material in this regard revealed a curious regularity, consisting in the fact that, as groundwater moves downslope, the values of pH and total mineralization increase, while the partial pressure of carbon dioxide decreases and, what is very important, so does the content of silica (Table 3). No clear regularities have been found in the behavior of aluminum and potassium, although most often their contents also decrease somewhat in the direction of groundwater movement.

This behavior of silicon, aluminum, and potassium, which at first glance is unusual, is explained by the fact that the highest concentrations of these elements are associated with watershed spaces, or, more precisely, with soil waters, which are the most acidic and least mineralized. This, in turn, indicates a connection between the sources of these elements and the soil horizons. This situation is not specific only to the region we studied, but is general for the entire northern humid zone; it consists in the fact that, as shown in the generalizing work of V. A. Kovda (1973), soil solutions are always richer in silica than subsurface groundwaters. The same is also characteristic of potassium.

The decrease in the content of silica and potassium in subsurface waters compared with soil waters is a fact of fundamental significance, since it indicates that the geochemistry of these elements in the hypergenesis zone is determined above all by the intensity of their biological cycle. It is precisely the soils, continuously replenishing their stock of elements through the annual litterfall, that supply infiltrating atmospheric precipitation primarily with biogenic elements, which include silicon and potassium. Moving then downward through the profile, these waters do not simply leach the enclosing rocks; interacting with them, they lead to the formation of secondary minerals, above all clay minerals. The synthesis of clays from the elements of an aqueous solution in soil and subsoil horizons has been described in many works, a list of which may be found in the already mentioned work of V. A. Kovda (1973), as well as in J. Millot (1968). It is clay formation that explains the decrease in the contents of silicon, potassium, and certain other elements in waters after they have passed through the soil layer.

In order to answer the next question—namely, in the formation of which clay minerals the silica and potassium of the waters participate—let us consider the stability-ratio diagram of the principal potassium minerals, with data on groundwaters plotted on it (Fig. 1).

In constructing this diagram we used data, pri-

Table 1

Average chemical composition of groundwaters of the active water-exchange zone of the Siberian Platform, mg/l

Table 2

Calculation of the output of chemical elements from the rocks of the weathering crust

* The groundwater module is taken as equal to 3.0 l/sec·km².
** According to V. V. Votintsev (1954), for the precipitation of Cisbaikalia.

Table 3

Chemical composition of groundwaters within massifs of the principal rocks, mg/l (Severnaya River basin)

Fig. 1. Stability relations of certain phases in the system \(K_2O—Al_2O_3—SiO_2—H_2O\) at \(25^\circ C\) and 1 atm, with plotted data on water composition. Average water composition: 1 — principal effusive rocks; 2 — principal intrusive rocks; 3 — sandy–coal-bearing deposits (northwest of the Siberian Platform); 4 — granite–gneisses of the Ivory Coast; 5 — magmatic rocks of varied composition in the western part of Guinea. A — field of distribution of groundwaters of the northwest of the Siberian Platform; B — the same for Guinea and the Ivory Coast.

The data cited in the work of R. M. Garrels and C. L. Christ (1968), as well as our unpublished data on the calculation of equilibria of hydromica with aqueous solutions, obtained during long-term storage of groundwaters from the basin of the Kureika River, from which hydromica precipitated, were used. This made it possible for us to show on the accompanying diagram the field of stability of hydromica, which R. M. Garrels and C. L. Christ had not done. The positions of the stability fields of potassium mica and feldspar are given by us without changes, according to the work of the already mentioned authors.

A study of the diagram presented (Fig. 1) shows that the field of distribution of groundwaters of the three complexes under consideration almost completely falls within the field of stability of hydromica. Within the latter are also located the points reflecting the average composition of the groundwaters studied by us. All this makes it possible to conclude that modern weathering processes in the climatic zone under consideration lead above all to the formation of hydromicas, less often of montmorillonite, whose stability field is not shown on the diagram, and very rarely of kaolinite and gibbsite.

The data presented agree well with known facts concerning the wide distribution of hydromicas in northern regions, including soils, weathering crusts, and even groundwaters, which contain these minerals in a finely colloidal state. Hydromicas are formed both from primary minerals during the restructuring of their crystal lattice and directly from waters as a result of synthesis processes. Modern kaolinite and gibbsite, in strict accordance with the character of the diagram obtained, are encountered extremely rarely, but their formation under these conditions is not contraindicated.

Geochemistry of Groundwaters of West Africa

This is considered using the example of the territory of Guinea, where we carried out investigations (Shvartsev, 1971, 1972), and the Ivory Coast (Rougérie, 1960; Tardy, 1969). This region is characterized by typically tro—

... with a tropical and equatorial-tropical climate, with two sharply expressed seasons of the year: dry and wet. The amount of atmospheric precipitation exceeds 1500 and reaches 4500 mm. Mean annual temperatures range between 24–28°C. The magnitude of underground runoff reaches 20 l/sec km² (Shvartsev, 1971). In landscape terms, tropical forests and high-grass savanna predominate. The geological conditions were described by B. M. Mikhailov (1969).

A characteristic feature of the groundwaters of this region is their increased acidity and reduced total mineralization (Table 4), which is due to intense water exchange and the high biological activity of the landscape.

By analogy with the method already set forth above, from the composition of the waters and the value of the underground-runoff modulus we determined the intensity of the modern chemical weathering of rocks, which amounted to 0.603 g/sec km², or 19.0 t/year km², for Guinea and, respectively, 0.258 g/sec km², or 8.1 t/year km². The difference in the figures obtained for the two regions is apparently explained by the fact that, in the waters of Guinea, sodium and potassium were determined from the difference between the sums of cations and anions, which, under conditions of absence of neutrality, leads to an overestimation of the results. Therefore the removal of elements in the territory of Guinea should be reduced by approximately half and, thus, the amount of removal should be taken as 0.3 g/sec km², which will be 9.5 t/year km². Consequently, the intensity of element removal in the zone of active water exchange in the territory of West Africa at present is 8–10 t/year km².

To avoid misunderstandings, it should be emphasized that we are speaking only of the very upper, most thoroughly leached zone of interfluve areas, and not of the entire river basin. In the latter case, as detailed studies by G. Ruzheri have shown, chemical runoff increases as a result of the destruction of more soluble rocks, drainage of formations of river valleys, under-channel deposits, the participation of deep runoff, etc.

To elucidate the direction of modern processes of rock weathering in this climatic zone, according to the method already adopted, let us plot the data on water composition on the already known diagram (Fig. 1).

The picture thus obtained—of the considerably lower position not only of the points of the average composition of the water, but also of the entire field of groundwaters in the stability region of kaolinite and gibbsite—really reflects the character of modern weathering in the direction of the predominant formation of these compounds. At the same time, despite the fact that the points of the average composition of the water are located in the stability region of kaolinite, a considerable part of them nevertheless also falls within the region of gibbsite development. Let us consider where the waters that are in equilibrium with this latter mineral are formed.

Analysis of the materials from these positions showed that groundwaters of recharge areas under conditions of the most active water exchange (interfluve spaces of the hilly part of Guinea) are characterized by minimal concentrations of silica, which shifts the thermodynamic equilibrium of the water–rock system toward the formation of gibbsite (Shvartsev, 1972). As groundwater moves toward river valleys, the silica contents in the waters increase somewhat, thereby shifting the position of the points of water composition into the stability region of kaolinite. This regularity agrees well with the natural facts of the predominant distribution of kaolinite on hill slopes and in river valleys, and of gibbsite in the most thoroughly leached interfluve areas. Consequently, under tropical conditions, in contrast to northern regions, the content of silica in groundwaters, as they move from recharge areas to discharge areas, does not decrease—

Table 4

Results of calculating the intensity of chemical weathering of rocks in the territory of Guinea and the Ivory Coast

decreases, but increases. This conclusion is important for understanding the mechanism of weathering processes as a whole, and therefore we draw the readers’ attention to this fact.

Discussion of the Results and Principal Conclusions

The factual material presented makes it possible, as it seems to us, to understand the reasons for the differing direction of weathering processes in the northern and southern humid zones. First of all, this concerns the intensity of modern weathering. As follows from the figures given above, the difference in the values of this indicator (5 t/year for northern regions and 10 for tropical ones) is not as great as is commonly believed. Of course, these figures cannot be taken as averages for the climatic zones under consideration; nevertheless, they evidently correctly reflect the order of magnitude of the observed phenomenon. A certain underestimation of the volume of elements removed is possible only because some of the components released during weathering already participate, within the soil horizon, in the formation of clays. The intensity of formation of the latter is considerably greater in tropical regions, which affects the composition of the waters being formed and, consequently, the scale of removal of elements from the weathering profile.

The direction of weathering processes, as follows from the diagram presented (Fig. 1), is determined mainly by the character of the environment: at identical or similar concentrations of silica and potassium in solution, kaolinite and gibbsite form under more acidic conditions (the tropics), whereas hydromica and montmorillonite form under more alkaline conditions (northern regions). Thus, the whole matter lies in different pH values. It is therefore important to understand why and under what conditions differences arise in the character of the environment.

It is known that during weathering there is a release of alkali and alkaline-earth elements; their accumulation in groundwater leads to an increase in its alkalinity. The opposite effect, acidification, is associated with the formation of carbonic acid through the interaction of atmospheric and biogenic carbon dioxide with water. The ratio of these two oppositely directed processes leads to definite pH values in groundwater. Under tropical conditions, where the biological productivity of landscapes is considerably higher than in northern regions (Perelman, 1966), more favorable conditions are created for the acidification of groundwater, whereas in northern regions they are created for its alkalization.

However, the decisive factor in determining the pH of waters is still not the volume of annual production of living matter, but, first, its relation to the scale of weathering and, second, the direction of transformation of the organic matter accumulating in soils. If in the taiga zone, as also, incidentally, in the tropical zone of humid forests, a large part of the organic matter is subjected to humification processes, then in the zone of tropical savannas it undergoes mineralization. One reason for this situation is that in the zone of northern forests (and in the humid tropics) leaf fall is confined to the wet season of the year, whereas in savannas it is confined to the dry season. The high temperatures in the latter lead to the rapid “burning” of organic substances, the consequence of which is a sharp increase in the partial pressure of carbon dioxide in soils and, through them, in groundwater. This circumstance is the cause of the formation of acidic waters in savannas (Shvartsev, 1972; Bronevoi et al., 1970).

The mineralization of organic substances in savannas leads to the rapid release of silica and other mobile elements, which

which, under conditions of active water exchange and an acidic environment, are rapidly leached from the weathering profile. Only relatively immobile aluminum and iron accumulate in the soils. The humification of organic substances in northern regions leads to their temporary preservation and, consequently, accumulation. More alkaline conditions promote the formation of hydromica and montmorillonite. Less active washing of soils by atmospheric precipitation promotes the enrichment of waters with biogenic elements already at the first stages of their formation, which is a decisive factor in determining their subsequent behavior. In areas where soils are absent or only slightly developed, kaolinite or even gibbsite may form.

Thus, soils play a most important role in enriching groundwater with chemical elements at the first stage of its formation, and especially in creating a definite character of the environment. In this sense, soils are a kind of “potential-setting” system which, through water, directs and regulates the intensity and orientation of the processes of chemical weathering of the underlying rocks. In other words, the nature of the geochemical environment determines the orientation of weathering processes (including the non-soil formation of secondary minerals produced as a result of restructuring of the crystal lattice of primary minerals), and not the reverse, as is commonly assumed. Undoubtedly, these two processes are interrelated and mutually conditioned, i.e., a change in one of them leads to a change in the other. However, the development of the first stages of weathering is determined by the environment, whose character is formed within the soil horizon. In this sense the role of the environment is leading, and its influence extends throughout the entire weathering profile.

From the foregoing several important conclusions may be drawn. First, the difference in climatic conditions in the northern and southern humid zones is manifested in weathering products not directly, but through biogenic factors. In this sense all weathering products, and above all clays, have a biogenic genesis. In this connection one cannot fail to recall one of the last works of B. B. Polynov (1953), in which he demonstrated the biogenic character of clays. Second, the geochemical setting that exists at present within tropical savannas promotes the development of the lateritic process on a considerable scale. Third, there is a close relationship between the composition of waters and the character of weathering, which makes it possible to use the former as a reliable indicator of the orientation of modern weathering.

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