**2. Two geological concepts supporting natural redox carbon cycle hypothesis. The suggested mechanism of the cycle functioning**

Two known geological concepts form the basis for the natural redox carbon cycle hypothesis. They are plate tectonics and orogenic cycles. The plate tectonics concept [6–8], or mobilism theory, asserts that the lithospheric plates, covering the entire Earth's surface, are in permanent motion. The motion is believed to occur due to convection of magma in the asthenosphere. Some researchers think that magma convection is a result of the impact of celestial bodies with the Earth's motion around the Sun [9]. Though the plate motion is an experimental fact, the real reason for the motion is still arbitrary. The motion is similar to the movement of an escalator. In some places of the Earth, in the zone of the mid-Atlantic ridge, where the crust is most subtle, magma erupts onto the surface and, coming into the contact with ocean water, hardens to form a new plate. It pushes other plates, causing their movement. In other places of the Earth (Wadati-Benioff–Zavaritsky zone) the plates, moving toward each other, collide. One of them, bending and moving down under the other, is absorbed by magma. The area where the collisions occur is called the subduction zone.

The orogenic cycle's concept was developed, in particular, by Rutten [2]. He studied the spatiotemporal distribution of sedimentary strata and concluded that the intensity of geolog‐ ical processes on the Earth over its geological history was unequal. There were relatively short periods, named orogenic periods, and the subsequent relatively extended periods of quiet development of the crust, named geosynclynal periods. The geosynclynal and the orogenic periods both constitute the orogenic cycle. According to Rutten's estimate, the duration of orogenic periods is about 50 million years, while that of geosynclynal periods amounts to several hundreds of millions years (up to 500 million years).

Orogenic periods are characterized by intensive mountain buildup and active volcanism, accompanied by volcanic eruptions and the entry of large masses of igneous rocks onto the Earth's surface. Geosynclynal periods correspond to the time of quiet Earth crust development and slow volcanic activity. It is the time for the accumulation of sediments and photosynthesis.

I took from Rutten his idea on orogenic cycle, and from the plate tectonic concept, I adopted the idea about plates' movement and plates' collisions. Combining both ideas and assuming that plates' movement was uneven, I have developed the following model.

In orogenic periods of the cycle, plates move rapidly and their collisions are frequent. Great quantities of volatile products, including CO2 and H2S, go onto the Earth surface from the subduction zone. In geosynclynal periods, the plates move slowly and the collisions are rare. Photosynthesis and weathering become the dominant processes. The collisions with the participation of the continental plates, bearing the sedimentary rocks with carbon in the form of carbonates and organic matter, are most interesting from the point of view of the natural redox carbon cycle.

During its life span, a continental plate accumulates sedimentary rocks with the buried organic matter and carbonates. When a plate descends and reaches the subduction zone, the rocks under high temperatures and great pressures are destroyed, but before they are absorbed by magma, the following reactions occur:

$$\begin{aligned} \text{MeCO}\_3 + \text{SiO}\_2 &\rightarrow \text{MeSiO}\_3 + \text{CO}\_2\\ \text{MeCO}\_3 &\rightarrow \text{MeO} + \text{CO}\_2 \end{aligned} \tag{1}$$

"Me" designates Ca2+ or Mg2+ cations.

These transformations do not change the redox state of carbon, and carbon transfer can be considered as a constant increment of the oxidative pool which does not affect carbon turnover.

The burial of organic matter and its transformation represent a different case. In thermochem‐ ical sulfate reduction, the organic matter reacts with evaporated sulfates according to the equation:

$$2\text{ SO}\_4^{2-} + 2\text{(CH}\_2\text{O)} \Leftrightarrow 2\text{CO}\_2 + 2\text{H}\_2\text{O} + \text{S}^{2-} \tag{2}$$

The resultant CO2, together with sulfides, is transferred from the subduction zones onto the Earth surface. This final step of oxidation completes the transfer of the reduced carbon into the oxidative forms. The above processes occur in the orogenic period.

hardens to form a new plate. It pushes other plates, causing their movement. In other places of the Earth (Wadati-Benioff–Zavaritsky zone) the plates, moving toward each other, collide. One of them, bending and moving down under the other, is absorbed by magma. The area

The orogenic cycle's concept was developed, in particular, by Rutten [2]. He studied the spatiotemporal distribution of sedimentary strata and concluded that the intensity of geolog‐ ical processes on the Earth over its geological history was unequal. There were relatively short periods, named orogenic periods, and the subsequent relatively extended periods of quiet development of the crust, named geosynclynal periods. The geosynclynal and the orogenic periods both constitute the orogenic cycle. According to Rutten's estimate, the duration of orogenic periods is about 50 million years, while that of geosynclynal periods amounts to

Orogenic periods are characterized by intensive mountain buildup and active volcanism, accompanied by volcanic eruptions and the entry of large masses of igneous rocks onto the Earth's surface. Geosynclynal periods correspond to the time of quiet Earth crust development and slow volcanic activity. It is the time for the accumulation of sediments and photosynthesis. I took from Rutten his idea on orogenic cycle, and from the plate tectonic concept, I adopted the idea about plates' movement and plates' collisions. Combining both ideas and assuming

In orogenic periods of the cycle, plates move rapidly and their collisions are frequent. Great quantities of volatile products, including CO2 and H2S, go onto the Earth surface from the subduction zone. In geosynclynal periods, the plates move slowly and the collisions are rare. Photosynthesis and weathering become the dominant processes. The collisions with the participation of the continental plates, bearing the sedimentary rocks with carbon in the form of carbonates and organic matter, are most interesting from the point of view of the natural

During its life span, a continental plate accumulates sedimentary rocks with the buried organic matter and carbonates. When a plate descends and reaches the subduction zone, the rocks under high temperatures and great pressures are destroyed, but before they are absorbed by

3 2 32

These transformations do not change the redox state of carbon, and carbon transfer can be considered as a constant increment of the oxidative pool which does not affect carbon turnover. The burial of organic matter and its transformation represent a different case. In thermochem‐ ical sulfate reduction, the organic matter reacts with evaporated sulfates according to the

® + (1)

+® +

3 2 MeCO SiO MeSiO CO

MeCO MeO CO

where the collisions occur is called the subduction zone.

56 Applied Photosynthesis - New Progress

several hundreds of millions years (up to 500 million years).

redox carbon cycle.

equation:

magma, the following reactions occur:

"Me" designates Ca2+ or Mg2+ cations.

that plates' movement was uneven, I have developed the following model.

On the Earth surface, due to chemical exchange reactions and in accordance with thermody‐ namics laws, CO2 is redistributed in the atmosphere and hydrosphere, composing the common "carbon dioxide–bicarbonate–carbonate" system:

$$\text{CO}\_2\text{(gas)} \Leftrightarrow \text{CO}\_2\text{(dissolved)} \Leftrightarrow \text{H}\_2\text{CO}\_3 \Leftrightarrow \text{HCO}\_3^- \Leftrightarrow \text{CO}\_3^{2-} \tag{3}$$

This system is close to equilibrium since the rate of chemical exchange is much greater than the rate of geological processes. The observed differences in carbon isotope composition of the atmospheric CO2 (δ<sup>13</sup>С ≈ −7‰) and of the carbonate species dissolved in seawater (δ<sup>13</sup>С ≈ 0‰) evidence for the state close to equilibrium. In fact, the difference is about 5–7 ‰ [10, 11], corresponding to the thermodynamic (equilibrium) values of isotope separation coefficients α(СО2/СО<sup>3</sup> 2−) and α(СО2/НСО<sup>3</sup> <sup>−</sup> ), which are equal to 1.005–1.008 [10, 12, 13] and typical to Earth surface temperatures (0–30°С).

Under the action of sunlight, photosynthesizing organisms absorb CO2 and water and convert the oxidized forms of carbon into the reduced ones, producing "living matter." After the conversion of the buried "living matter" into the sedimentary organic matter, the latter undergoes oxidation. Then all the processes repeat. This sequence of transformations forms a close loop.

All the above are depicted in Figure 2. In orogenic periods, shown as filled triangles, CO2 concentration in the system should increase abruptly because of frequent collisions of litho‐ spheric plates when sedimentary rock masses fall in subduction zone and are destroyed with CO2 evolution. The entry of CO2 into the "atmosphere–hydrosphere" system leads to consid‐ erable growth of all oxidized carbon species in the system. Photosynthesis is stimulated by high СО2 concentrations achievable in the orogenic period.

**Figure 2.** The scheme of the putative changes of CO2 and O2 in atmosphere and organic matter in sedimentary rocks in the course of orogenic cycles. Note that the variations of CO2 and O2 are in anti-phase, while the variations of O2 and organic matter are in phase.

At the same time, the atmospheric O2 concentration decreases since it is utilized in the oxidation of the reduced igneous rocks and reduced sulfur species, lifting from the subduction zones onto the Earth surface.

In the following relatively extended geosynclynal period, the rate of photosynthetic consump‐ tion of CO2 becomes greater than its emission from the subduction zones. It results in the depletion of the oxidative pool of carbon in the "atmosphere–hydrosphere" system. Onthe contrary the O2 concentration, due to photosynthesis, increases, achieving maximum concen‐ tration by the end of the geosynclynal period. The curve describing organic matter accumu‐ lation in sedimentary rocks in accordance with the global photosynthesis reaction CO2 + H2O → hv CH2O+O2 coincides with the curve of O2 concentration, since the burial rate of organic matter changes in parallel with the growth of atmospheric oxygen concentration. Note that the atmospheric CO2 appears as a substrate, while the oxygen and the assimilated organic matter (CH2O in this approximation) are the products of the global photosynthesis reaction.

Thus the model claims the periodic filling/depletion of "atmosphere–hydrosphere" system with CO2 and counter-phase parallel changes of O2. Accordingly, the periodic strengthening of CO2 assimilation and weakening of photorespiration should take place. In parallel with atmospheric O2 concentration changes, the accumulation of organic matter occurs in sedi‐ ments, since they are both the products of global photosynthesis. It means the periodic intensification of organic matter accumulation in sediments should take place.

Some important notes concerning carbon isotope fractionation in photosynthesis based on recent findings [5, 14] should be taken into account. The periodic depletion of carbon oxidative pool result in the appearance of carbon isotope Raleigh effect which affect the carbon isotope composition of sedimentary carbonates and organic matter displaying gradual 13C enrichment with the extent of depletion. It allows examining orogenic cycles by means of the analysis of isotope composition of carbonate and organic carbon.

CONCENTRATION

58 Applied Photosynthesis - New Progress

organic matter are in phase.

onto the Earth surface.

hv

CO2 + H2O →

Geosynclynal period

CO2

O2

Corg

Orogenic period

GEOLOGIC TIME

**Figure 2.** The scheme of the putative changes of CO2 and O2 in atmosphere and organic matter in sedimentary rocks in the course of orogenic cycles. Note that the variations of CO2 and O2 are in anti-phase, while the variations of O2 and

At the same time, the atmospheric O2 concentration decreases since it is utilized in the oxidation of the reduced igneous rocks and reduced sulfur species, lifting from the subduction zones

In the following relatively extended geosynclynal period, the rate of photosynthetic consump‐ tion of CO2 becomes greater than its emission from the subduction zones. It results in the depletion of the oxidative pool of carbon in the "atmosphere–hydrosphere" system. Onthe contrary the O2 concentration, due to photosynthesis, increases, achieving maximum concen‐ tration by the end of the geosynclynal period. The curve describing organic matter accumu‐ lation in sedimentary rocks in accordance with the global photosynthesis reaction

organic matter changes in parallel with the growth of atmospheric oxygen concentration. Note that the atmospheric CO2 appears as a substrate, while the oxygen and the assimilated organic matter (CH2O in this approximation) are the products of the global photosynthesis reaction.

Thus the model claims the periodic filling/depletion of "atmosphere–hydrosphere" system with CO2 and counter-phase parallel changes of O2. Accordingly, the periodic strengthening of CO2 assimilation and weakening of photorespiration should take place. In parallel with atmospheric O2 concentration changes, the accumulation of organic matter occurs in sedi‐

CH2O+O2 coincides with the curve of O2 concentration, since the burial rate of

Following the actualism principle, one can take organic carbon as analog of "living" matter in the past, and coeval carbonates as analog of CO2 , corresponding to that time. Then the isotopic difference between organic matter and carbonates should be regarded as analog of 13C carbon isotope discrimination in modern plants. We denoted the above isotopic differences, after Popp et al. [15] and Hayes et al. [16] as *ε* parameter. If so, in the orogenic period and at the beginning of the geosynclynal period when the CO2 concentration as well as the CO2/O2 concentration ratio in the environment are maximal, the 12C enrichment of the organic carbon and *ε* parameter should be maximal too.

By the end of geosynclynal period, when the CO2 and CO2/O2 concentration ratios are the lowest (photorespiration is maximal), the buried organic carbon should be the most enriched in 13C (additional to Raleigh effect). Accordingly, the isotopic difference between organic matter and coeval carbonates should be minimal (Figure 3).

**Figure 3.** The scheme of putative changes of carbon isotope discrimination in photosynthesis *ε* in the course of geologi‐ cal time; *ε* is equal to carbon isotope difference between carbon isotope composition of sedimentary carbonates and coeval organic matter. The isotope discrimination decreases steadily with each subsequent cycle.

The validity of the assertion can be seen from the measurements of the buried organic and coeval carbonates of different age [16]. They revealed noticeable differences in *ε* parameter. In the Neoproterozoic era, from 1000 Ma to 541 Ma, the isotope discrimination was found to be greater than 32 ‰. In the period from Cambrian to Jurassic, *ε* changed to 28‰, and then in the period from Cretaceous to Late Cenozoic it was less than 28‰. They also noticed the successive growth of atmospheric O2 concentration from the Neoproterozoic to Late Cenozoic.

Thus the isotope technique provides an immensely effective and delicate tool for the orogenic cycle studies. This claim is supported by the fact that isotope ratios of organic carbon and coeval carbonates are the main and widely used factual materials in common geological studies.
