**3. Isotope data support the idea on organic carbon oxidation in subduction zone during orogenic period**

The oxidation of organic carbon in subduction zone coupled with sulfate reduction during orogenic period is one of the critical points of the model that requires proof. According to [17], the natural sulfur cycle, like the carbon cycle, consists of the oxidative (sulfate) and reductive (sulfide) branches. To substantiate the above assertion, let us address the data from the work of Mackenzie and Piggot [17]. Temporal curves in Figure 4 demonstrate synchronous isotopic variations of carbon and sulfur of marine carbonates and gypsum (sulfates) in time.

**Figure 4.** Coupling of global carbon and sulfur cycles. Synchronous variations of the curves of carbon isotope composi‐ tion of carbonates (a) and sulfur isotope composition of sulfate sulfur in evaporates (b) for the last 700 million years in the geologic history of the Earth [17].

Such synchronism prompts itself that both cycles are somehow bound. Each curve has two differently directed humps. Next to them there are inscriptions, made by authors, to indicate minerals of sedimentary rocks that were mostly spread at the corresponding periods. Carbo‐ nates and pyrites correspond to the humps in lower parts of the curves. Organic matter and gypsum correspond to the humps in the upper part of the curves.

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

The oxidation of organic carbon in subduction zone coupled with sulfate reduction during orogenic period is one of the critical points of the model that requires proof. According to [17], the natural sulfur cycle, like the carbon cycle, consists of the oxidative (sulfate) and reductive (sulfide) branches. To substantiate the above assertion, let us address the data from the work of Mackenzie and Piggot [17]. Temporal curves in Figure 4 demonstrate synchronous isotopic

Δ13C, ‰ Δ34S, ‰

**Figure 4.** Coupling of global carbon and sulfur cycles. Synchronous variations of the curves of carbon isotope composi‐ tion of carbonates (a) and sulfur isotope composition of sulfate sulfur in evaporates (b) for the last 700 million years in

+3 +2 +1 0 –1 –2 10 15 20 25 30 35

carbon Gypsum

**a b**

Pyrite

Pε

C

O

C P

O S

J K T

T1

variations of carbon and sulfur of marine carbonates and gypsum (sulfates) in time.

Organic

**3. Isotope data support the idea on organic carbon oxidation in**

**subduction zone during orogenic period**

Carbonates

700

the geologic history of the Earth [17].

600

500

400

300

200

100

0

GEOLOGICAL TIME, Ma

studies.

60 Applied Photosynthesis - New Progress

If we compare the above substances with the substrates and the products of sulfate reduction, it is easy to see that the substances corresponding to the lower humps on the curves coincide with the reaction products, and the substances corresponding to the upper humps on the curves coincide with the reaction substrates. The analysis of the isotopic changes of carbon and sulfur proves that the coincidence is not accidental.

By analyzing the dynamics of carbon and sulfur isotopic variations resulting from the curves on Figure 4, we should first note that the thermochemical sulfate reduction is followed by sulfur isotope fractionation [18, 19]. Then, due to the periodic character of the reaction, the substrate is depleted. The depletion is followed by Raleigh effect. The more the reaction proceeds and the more the substrate pool is depleted, the greater the residual substrate (gypsum) is enriched with a "heavy" sulfur isotope 34S. As it follows from the analysis of the lower part of the curves, the enrichment of gypsum with 34S is accompanied with the enrich‐ ment of carbonates with a "light" carbon isotope 12C. Both traits evidence in favor of high extent of sulfate conversion. Indeed, in the case of high extent of sulfate conversion, another reaction product CO2 should also be produced in a considerable amount. Thus the CO2 inherits "light" carbon isotope composition from organic matter. Hence, when "light" CO2 enters marine "carbon dioxide–carbonate" system with carbon enriched in 13C, it makes carbon in the system "lighter" due to chemical isotope exchange.

A quite opposite scenario can be deduced from the analysis of the upper parts of the curves. The 32S enrichment of gypsum evidences that the extent of sulfate conversion is low. Hence small amounts of "light" CO2 are produced, and marine carbonates become "heavier" as compared with the previous case. Thus, the coupled isotopic changes of carbon and sulfur of marine carbonates and gypsum, in addition to chemical arguments, give firm proofs that they represent the results of sulfate reduction process occurring in the subduction zones.

An indirect argument in favor of sulfate reduction in subduction zone was the great abundance of sulfide-oxidizing bacteria in the Precambrian. It evidences for significant inflow of the reduced sulfur forms (sulfides and hydrogen sulfide) onto the Earth surface. This was favored by low oxygen concentration in the Earth's atmosphere at that time. The sulfide-oxidizing bacteria were so widely disseminated that gave grounds for Hayes et al. [16] to conclude that these bacteria were the main source of organic matter in rocks in the Proterozoic.

Thus, the coupled isotopic changes of carbon and sulfur of marine carbonates and gypsum, in addition to chemical and paleontological arguments, give firm proofs that they represent the results of sulfate reduction process occurring in the subduction zones.
