On the Physical Sense of 13C Values of Carbonate and Organic Matter of Sedimentary Rocks

Abstract

EditorialCarbon isotope composition (δ 13 С) of carbonates and organic matter in sedimentary rocks is the most commonly used parameter in geological, paleontological, paleoclimatic and other studies of the history of the Earth. The difference of them ε=(δ 13 Сorg.matter -δ 13 Сcarb) has the meaning of 13 С discrimination in photosynthesis in the past [1], but only in the cases where samples of organic matter and carbonates are coeval and taken from the same facies. Often in organic matter studies the changes its carbon isotope composition are interpreted as if they were the results of 13 С isotope discrimination in the past photosynthesis [2,3]. It should be emphasized that we focus on the global photosynthesis that has many features typical to ordinary photosynthesizing organisms [1].All photosynthesizing protobionts prior to photorespiration appearance had the only function -to assimilate carbon dioxide. This function was followed by carbon isotope effect resulting in 12 С enrichment of biomass. That is why the organic carbon of the ancient sedimentary rocks was unusually "light" [4]. In the course of photosynthesis evolution with the accumulation of O 2 in the atmosphere photorespiration has appeared and photosynthesis began to include both processes. They were accompanied with carbon isotope fractionation having opposite signs. Such organisms belong to C-3 type and were dominant until Miocene when a decrease of CO 2 concentration in the 'atmosphere -hydrosphere" system led to emergence of organisms of C-4 type with different pathway of CO 2 assimilation [5], which was followed by different carbon isotope effect.Let's examine the factors influencing δ 13 С isotope discrimination in photosynthesis to understand how Δ 13 С discrimination varied in the past and what's the sense has the difference of carbon isotope composition of carbonates and organic carbon.As said, CO 2 assimilation leads to the enrichment of biomass in 12 С, while photorespiration is accompanied by the enrichment of biomass in 13 С. Both processes contribute to isotope composition of total biomass. The summary isotope composition of biomass depends on CO 2 /O 2 concentration ratio in the environment. The increase of oxygen concentration intensifies photorespiration and strengthens 13 С the enrichment of biomass. The growth of carbon dioxide stimulates assimilation and 12 С accumulation in biomass [6]. Figure 1 illustrates the way how CO 2 /O 2 concentration ratio affects the carbon isotope composition of biomass of C-3 organisms. Carbon dioxide enters the cell and fixed by RuBP acceptor is involved into Calvin cycle. The fixation is followed by carbon isotope effect resulting in 12 C enrichment of the assimilated carbon. Then a part of this pool is used to provide glycolate cycle, where photorespiration occurs. In glycine dehydrogenase reaction of the cycle a part of the carbon flow is subjected to oxidative decarboxylation and is evolved from the cell.The other part of the carbon flow is used for photo respiratory pool creation. Carbon isoto...