A BIOGEOCHEMICAL EARTH SYSTEM MODEL FOR THE PHANEROZOIC (PAST 550 MYR)

Although the history of O₂ and CO₂ in the atmosphere has been studied extensively, each gas has generally been studied separately. A new biogeochemical model of the Earth system will be presented that attempts to synthesise the different approaches of existing models. The model incorporates geological forcings and biogeochemical processes, including ocean nutrients, interactive marine and terrestrial biota, atmospheric CO₂ and O₂, mean global temperature, and d¹³C. As the key carbon cycle processes and biogeochemical feedbacks controlling atmospheric O₂ and CO₂ concentrations are to some degree coupled, the model predicts a coupled O₂ and CO₂ history. The work represents the beginning of a 'co-evolutionary' model of the Earth for geologic timescales.

Results demonstrate the major role played by life in controlling O₂ and CO₂, through a combination of primary productivity and burial and weathering of organic matter. The evolution and spread of vascular land plants drives a fundamental transition in the state of the Earth system, including an order of magnitude drop in CO₂ levels accompanied by a ~25% rise in O₂ levels. Thereafter, the dependency of primary productivity on atmospheric partial pressures of CO₂ and O₂ exerts a strong control on O₂ levels. Phosphorous weathering patterns and organic matter C/P burial ratios also strongly influence O₂ levels. In contrast to previous models, Carboniferous CO₂ levels are never predicted to fall as low as at present. Marine nitrate and phosphate are tightly coupled close to Redfield ratio, but dissolved oxygen remains uncoupled from the other nutrients. Changes in tectonic uplift and solar luminosity also provide significant forcing. For example, the recent uplift of the Himalayas and the Tibetan Plateau may be shifting the system towards a new steady state, with oxygen sinks currently stronger than sources.