Introduction

The main component of the early Earth's atmosphere after the formation of the oceans was CO2 as the second most abundant volatile in the accreting material. On planetary timescales the variation of the CO2 content in the atmosphere is most important in investigating the relation between the evolution of the Sun as a main-sequence star and the stabilization of the Earth's surface temperature. During the history of the Earth the luminosity of the Sun has increased to the present level starting with a 30% lower value. There must be a mechanism that provides a feedback, generating a high concentration of atmospheric CO2 in the past to prevent the Earth from freezing while solar luminosity was low. On the other hand a progressive lowering of CO2 concentration is necessary for an increasing solar luminosity. Such a negative feedback mechanism is provided by the global carbon cycle among the surface reservoirs of carbon (atmosphere, ocean, crust) and the mantle reservoir. The overall chemical reactions for the weathering processes are

CO2 + CaSiO3 $ CaCO3 + SiO2 CO2 + MgSiO3 $ MgCO3 + SiO2

The main idea of this abiotic feedback is the interplay between weathering rate, surface temperature, and atmospheric CO2 pressure. An increase of the luminosity leads to a higher mean global temperature causing an increase in weathering. Then more CO2 is extracted from the atmosphere weakening the greenhouse effect. Overall the temperature is lowered and homeostasis, that is, self-stabilization of the global surface temperature is achieved. Plate tectonics is a necessary condition for closing the carbon cycle. Without spreading and subduction carbonates would be buried on the seafloor and not brought back to the atmosphere via regassing at mid-ocean ridges or andesitic/back-arc volcanism.

During the evolution of the Earth the global carbon cycle has been mediated by the biosphere. The occurrence of microfossils and stromatolites in rocks has shown that life had originated at least 3.5 Gyr ago. Isotopic signatures of 13C/12C suggest the presence of organic carbon in 3.8 Gyr rocks from Greenland. Photosynthetic fixation leads to buildup of organic material consuming CO2 from the atmosphere. On the other hand, organic material is decomposed and CO2 is remobilized:

Carbon isotopes imply that the enzyme Rubisco preferring the lighter isotope C and therefore oxygenic photosynthesis controlled the global distribution of carbon in the atmosphere-ocean system for at least 3.5 Gyr and gave an imprint on the isotopic signature of the carbon reservoirs. Direct evidence of oxygenic photosynthesis is given by the steep rise of oxygen in the atmosphere 2.2 Gyr ago. Life is unable to influence Earth's carbon cycle in the absence of photosynthesis.

The biosphere can in principle be divided into three life forms that appeared on Earth in consecutive order (see Figure 1). First Archaean and prokaryotic bacteria

appeared, second eukaryotic life, and third complex multicellular life. In particular complex multicellular organisms have a strong influence on weathering by amplifying the weathering rates.

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