Plants that survive solely on C3 fixation (C3 plants) tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate, carbon dioxide concentrations are around 200 ppm or higher, and ground water is plentiful. The C3 plants, originating during Mesozoic and Paleozoic eras, predate the C4 plants and still represent approximately 95% of Earth's plant biomass. C3 plants lose 97% of the water taken up through their roots to transpiration.[1] Examples include rice and barley.
C3 plants cannot grow in hot areas because RuBisCO incorporates more oxygen into RuBP as temperatures increase. This leads to photorespiration, which leads to a net loss of carbon and nitrogen from the plant and can, therefore, limit growth. In dry areas, C3 plants shut their stomata to reduce water loss, but this stops CO2 from entering the leaves and, therefore, reduces the concentration of CO2 in the leaves. This lowers the CO2:O2 ratio and, therefore, also increases photorespiration. C4 and CAM plants have adaptations that allow them to survive in hot and dry areas, and they can, therefore, outcompete C3 plants.
Therefore, the d13C carbon value of the paleosoils depends largely on the type of plant that grew on them. It is less when the C3 plants were dominant and higher when those of type C4 proliferated. The study of the variations of d13C in the continental paleosoils can give an indication of the type of plant, C3 or C4, that dominated in specific periods.
Indirectly, the value d13C of the paleosoils can also indicate the evolution of the concentration of atmospheric CO2. It so happens, that with elevated concentrations of CO2, the type C3 plants are favoured with respect to plants of type C4, since the C3 pathway plants require less energy to carry out photosynthesis. On the contrary, when the CO2 concentration is low, the C42’s increase since they possess a mechanism of concentrating CO2 that favours them. So, the lower the d13C in the paleosoil, the higher the probability that the CO2 concentrations were high, and vice versa.
