Global Geochemical Cycling

The global matter cycling consists of a more or less periodical process of migration and transformation of chemical agents in nature. In part, it is initiated by abiotic factors and proceeds without participation of biological objects. Such a pure geochemical cycle is sometimes called as the big or geological turnover and characterized by global scale and extremely long course. It demonstrates abilities of even abiotic matter to primitive self-organization.

Calcium Cycling

For example, the global abiotic cycle of calcium includes the following main stages: natural destruction (denudation, erosion, etc.) of limestone; generation of soluble calcium salts (bicarbonates, etc.) and their dissolution in water; transportation of the salts by rivers to oceans (present discharge is about 5 x 1011 kg of calcium per year); sedimentation of calcium-containing substances; meta-morphization of sediments, limestone formation; sea regression, return of limestone to the land. In this way the cycle is terminated, but the repetition is not complete: the type of limestone can essentially change; for example, Paleozoic limestone is richer in carbonic magnesium than younger rocks. Biological components can hasten some steps of the cycle: rock destruction and, especially, accumulation of calcium from oceanic water and its sedimentation. However, even with regard to biotic effects, the period of this type of calcium circulation is estimated to be 108 years.

Water Cycling

Much more intensive abiotic cycling is typical for water. Contrary to the calcium circulation, where the main work is done by the radiogenic energy of the Earth (sea regression) and only partially by the solar energy (rock destruction), for the water turnover the solar energy is the constitutive driving force.

Abiotic water cycling is extremely important for forming the global biogeochemical turnover, playing the role of its background (see Water Cycle). The main flows, forming the water cycling, are represented in Figure 1. Theoretically the cycle can be divided into five separate cyclic processes: I -water circulation under the ocean; II - circulation under the land; III - water interchange between the ocean and the land; IV - groundwater cycling in the lithosphere; and V - the big water cycling through the Earth mantle.

Cycle I is initiated by evaporation from ocean surface (flow 1). The intensity of the flows in Figure 1 is represented in 1015gyr_1 (109tyr_1; flow 1 has a maximum value - 384. Then the main part of the water (347) returns back to the ocean with precipitations (flow 2). The last value (347 x 1018gyr_1) can be used as a characteristic of the total intensity of cycle II. In the cycle an additional loop (flow 3) can be noted. Arctic and Antarctic ice can accumulate part of incoming water and then return it to the cycling in corresponding conditions. This loop can be considered as a damping structure, which reflects self-organizational abilities of abiotic systems.

The intensity of cycle II is much smaller, mainly because the aquatic surface in the land (lakes, rivers, marshes, etc.) is not so big. The intensity of precipitations (flow 6) and evaporation (flow 4) is equal to 102 and 17,



Figure 1 The global water cycling.

respectively. In Figure 1, one biogenic flow (flow 5) is also represented. It illustrates the importance of terrestrial plants even for the mainly abiotic water cycling. It is the process of respiration, in the course of which plants accumulate from soil and give back to the atmosphere some part of the precipitation. The intensity of flow 5 (which equals 48) is much bigger than the intensity of the passive evaporation. Totally the intensity of cycle II can be estimated as 55 x 1018gyr~\ In the terrestrial water cycling, the role of damping device is played by mountainous glaciers (flow 7).

Between the ocean and land water circulation, there are essential links, which form cycle III. An essential part of evaporated water is transported by winds from the ocean to the land and in the opposite direction. Because the evaporation of the ocean is essentially bigger, we can generally talk about flow 8, directed from the ocean to the land. Then the water returns to the oceans with rivers (flow 9) and groundwater (flow 10), extracting solubles from lithospheric rocks on the way. The general intensity of cycle III is about 37 x io18gyr_1.

Some part of the precipitated water can be involved in more large-scale processes. Cycle IV is formed by groundwater, which leaks to the lower stratum of the Earth crust, concentrates under the influence of pressure and temperature, and returns to the surface as thermal waters (flow 11).

Cycle V through the upper mantle is called big or geological cycling in the narrow sense of this term. Water can be bound by sedimentary rocks, migrate together with them to deeper in the Earth, accumulate to the magma, and return to the surface in the course of volcanic eruptions (flow 12). The intensity of cycles IV and V is very low in accordance with other cycles mentioned.

Although the water cycling is the fastest abiotic turnover, its influence on general geochemical situation is very gradual. Thus, the period of total abiotic renewal of oceanic water is estimated to be 106 years (whereas the biological renewal lasts about 2800 years).

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