The global carbon cycle and atmospheric CO2 in the Phanerozoic: an approach by the numerical simulation

The global carbon cycle on a geological age consists of an inorganic carbon cycle (continental weathering and metamorphism-volcanism) and organic carbon cycle (oxidative weathering of organic carbon and organic carbon burial). The GEOCARB, one of the global carbon cycle models, calculates these geochemical carbon fluxes and atmospheric CO2 level in the period of Phanerozoic. Important parameters in the GEOCARB are those for the continental uplift, river runoff, evolution of vascular plants, weathering feedback, and CO2 degassings. Seawater strontium isotope ratio and sedimentation rate of terrigenous sediments can be used to estimate the continental uplift parameter. Regarding the vascular plant, more quantitative studies are necessary to elucidate its effect on the global carbon cycle. Concerning the degassing parameter, not only subduction volcanism but also hot spot volcanism and igneous activity in back arc basin should be considered. Runoff is dependent on continental positions and terrestrial temperature, but the interrelationship among them is not fully considered in the GEOCARB. In this respect, another type of the model, namely, the GEOCLIM, may be more appropriate. Volcanic rock weathering and climate sensitivity are also crucial in the global carbon cycle. Volcanic rock weathering might have controlled the atmospheric CO2 level in the Phanerozoic, although the value of the weathering flux has not been constrained. Regarding the climate sensitivity, a short-term feedback (Charney feedback) has been assumed in the GEOCARB. However, a long-term climate feedback, namely, the Earth System Sensitivity (ESS), should be incorporated. According to the recent version of GEOCARB (GEOCARBSULF), most influential parameters on the atmospheric CO2 are those for the climate sensitivity and vascular plants. In this model, atmospheric CO2 level in the Cenozoic is not well consistent with the results of geochemical proxies. This may be due to the insufficient estimation of degassing parameters, and/or a change in the climate sensitivity accompanied with formation of continental ice sheets in the Cenozoic.