Global Climate Change and Weathering during the Phanerozoic

Abstract

 Weathering is deeply related to global climate change. In the carbon cycle, silicate weathering, especially volcanic rock weathering, transfers carbon in the atmosphere (as CO₂) to the lithosphere, and oxidative weathering of organic matter releases carbon (as CO₂) from the biosphere to the atmosphere. Moreover, as an indirect effect of weathering on climate change, negative feedback in the climate system, which results from the dependence of weathering rate on temperature and evolution of terrestrial plants, is crucial. It has stabilized the long-term global climate throughout the Phanerozoic. Weathering rate is controlled by several geochemical external factors: tectonic forces such as lithology, continental uplift, and continental drift (paleogeography); climate forces such as temperature, runoff, and glaciations; and, biological forces such as terrestrial plant evolution. Regarding biological forces, accelerated weathering assisted by ectomycorrhizal fungi (EM fungi) and arbuscular mycorrhizal fungi (AM fungi), as well as vascular plants of gymnosperms and angiosperm, are emphasized. Variations of global weathering in the geological past are estimated using experimental approaches, such as isotope analysis (e.g., ⁸⁷Sr/⁸⁶Sr, ¹⁸⁷Os/¹⁸⁶Os, δ⁷Li), and theoretical approaches, such as numerical simulations (e.g., carbon cycle model). Each is used differently according the purpose of a study. Based on these estimates, geological past climate changes in the Phanerozoic are found to be closely related to weathering. For example, on the order of magnitude of 10⁷ years, changes in weathering patterns due to continental drift (paleogeography) have resulted in variations of atmospheric CO₂, hence climate change. On the order of magnitude of 10⁶ years, it is suggested that a decrease in atmospheric CO₂ from the mid- to late Cretaceous was caused by enhanced weathering according to terrestrial plant evolution and that variations of atmospheric CO₂ in the late Cenozoic were regulated by weathering directly or indirectly influenced by continental uplift. Additionally, contributions of weathering to global climate change involved in oceanic anoxic events in the Mesozoic have been investigated.