**6. Irregular periodicity in organic carbon accumulation in sediments is the reason for uneven stratigraphic distribution of sediments rich in organic matter and oils**

It stems from the model's logic that there should be uneven rate of organic matter accumulation in sediments as a result of CO2 variations in the course of orogenic cycles. In the orogenic periods of the repeated cycles, the "atmosphere–hydrosphere" system of the Earth is filled with CO2, coming from the subduction zone. In the subsequent geosynclynal periods, due to a dominant role of photosynthesis, the system is gradually depleted in the oxidized carbon species. It means that the photosynthesis rate is maximal at the beginning of the cycle and reaches its minimum by the end. The opposite scenario could be expected for organic matter accumulation in sediments. The maximum of organic matter accumulation should take place by the end of the cycle, when most of the CO2, that entered the "atmosphere–hydrosphere" system in the orogenic period, after some transformation is converted into organic matter. As a result, the uneven stratigraphic distribution of sediments rich in organic matter appears. The other consequence of the uneven stratigraphic distribution is the irregular distribution of the oils generated by these sediments.

Though the kinetics of global photosynthesis and organic matter accumulation is unknown, we assumed that the mentioned kinetics is described by the simplest first-order equation. It corresponds to proportional, in-phase of O2 and buried organic carbon variations (Figure 5), and anti-phase atmospheric CO2 and O2 variations (see Figure 1 from [25]). The corresponding curves, depicting the behavior of CO2 and O2 in the atmosphere and organic matter in sedimentary rocks obtained in climatic and depositional models, are in full agreement with the changes expected from the proposed carbon cycle model.

**Figure 5.** The in-phase changes of oxygen content in the atmosphere and burial organic matter rates in the sedimentary rocks in the Phanerozoic. The shaded zone for oxygen designates the zone of possible errors based on sensitivity analy‐ sis [29].

In fact, Bazhenova and Sokolov [28], examining the stratigraphic distribution of Domanic oil source rocks, revealed that these sediments rich in organic matter were present in different continents at the same stratigraphic levels. They are found practically in all systems of the Phanerozoic and of the Precambrian. The stratigraphic levels, where domanicoids were fixed, are Ediacaran–Cambrian, Devonian−Carboniferous, and Late Jurassic–Early Cretaceous. Such nonuniform stratigraphic distribution can be explained in the frames of the natural redox carbon cycle dynamics.

The formation of sediments rich in organic matter is likely bound to the transitions from one cycle to another when there was a change of aerobic conditions to anoxic causing mass extinction of living organisms. Their biomass is a probable source of organic material. The repeatability of orogenic cycles determines the appearance of domanicoids at different stratigraphic levels in the Precambrian and the Phanerozoic.

Though the periods, indicated by Bazhenova and Sokolov [28], are rather conditional, they are known to include glacial periods. During the Ediacaran period, there were Varanger (660 to 635 Ma) and Gaskiers (590 to 575 Ma) glaciations; in Devonian, there was the Andean-Saharan (ca. 440 Ma) and then Permo-Carboniferous (320 Ma) glaciations; in Late Jurassic–Early Cretaceous period, traces of glaciations were not found with certainty, but there was a great extinction (66 Ma), which was caused by a bolide impact. Besides this, the indicated time was characterized by high oxygen concentration likely associated with glaciations and land life development. It should be stressed that Bazhenova and Sokolov [28] marked that periods of organic matter accumulation were followed by rifting process which in accordance with our model corresponds to the orogenic period, occurring behind the cooling time.

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64 Applied Photosynthesis - New Progress

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**Figure 5.** The in-phase changes of oxygen content in the atmosphere and burial organic matter rates in the sedimentary rocks in the Phanerozoic. The shaded zone for oxygen designates the zone of possible errors based on sensitivity analy‐

In fact, Bazhenova and Sokolov [28], examining the stratigraphic distribution of Domanic oil source rocks, revealed that these sediments rich in organic matter were present in different continents at the same stratigraphic levels. They are found practically in all systems of the Phanerozoic and of the Precambrian. The stratigraphic levels, where domanicoids were fixed, are Ediacaran–Cambrian, Devonian−Carboniferous, and Late Jurassic–Early Cretaceous. Such nonuniform stratigraphic distribution can be explained in the frames of the natural redox

The formation of sediments rich in organic matter is likely bound to the transitions from one cycle to another when there was a change of aerobic conditions to anoxic causing mass

–600 –500 –400 –300

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sis [29].

carbon cycle dynamics.

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It is natural to consider that Domanic oil source rocks as well as other rocks rich in organic matter ("black" shales) are oil kitchen. Therefore, the periods of oil source rock – formation should be related to oil generation, and one could expect that the oil field discoveries are mostly related with this time with high probability. This assertion is supported by the available data. Figure 6 illustrates the stratigraphic distribution of the discovered oil fields in the world [30] and the distribution of oil fields in the former USSR [31]. The latter is presented as a ratio of number of oil fields for a given period to the total number of oil fields [Figure 6b]. As it can be seen, in both cases the distributions are very similar and irregular. Though the comparison is not very strict, Figure 6b, unlike Figure 6a, does not take into account the sizes of oil fields, nevertheless the comparison is still reasonable. The peaks in both distributions correspond to each other; the number of the peaks is the same. According to both distributions, the beginning of oil generation falls in time limits from 600 to 500 million years ago. This is in agreement with the considerable growth of oxygen concentration in the atmosphere. As it was noted, oxygen concentration is an indicator of organic matter accumulation in sediments. According to some data [32, 33], one can accept that in most of the Precambrian, oxygen concentration was less than 1%, and only by the end of the Precambrian it started to increase significantly [34]. It is logical to assume that photosynthesis needs a prolonged time to accumulate the amount of organic matter in sediments sufficient to produce hydrocarbons capable to form oil fields. One can follow it by tracing the O2 concentration in the atmosphere.

**Figure 6.** Comparison of distribution of explored extractable oil fields in the largest world reservoirs (Vishemirsky, Kontorovich, 1997) [30] (a) and revealed oil fields (% of the total number of oil fields) of the former USSR (Korchagin, 2001) [31] (b) with stratigraphic subdivisions.
