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We present a new model of biogeochemical cycling over Phanerozoic time. This work couples a feedback-based model of atmospheric O~2~ and ocean nutrients (Lenton and Watson, 2000a, 2000b) with a geochemical carbon cycle model (Berner, 1991, 1994), a simple sulfur cycle, and additional components. The resulting COPSE model (Carbon-Oxygen-Phosphorus-Sulfur-Evolution) represents the co-evolution of biotic and abiotic components of the Earth system, in that it couples interactive and evolving terrestrial and marine biota to geochemical and tectonic processes. The model is forced with geological and evolutionary forcings and time-dependent solar insolation. The baseline model succeeds in giving simultaneous predictions of atmospheric O~2~, CO~2~, global temperature, ocean composition, δ^13^C and δ^34^S that are in reasonable agreement with available data and suggested constraints. The behavior of the coupled model is qualitatively different to single cycle models. While atmospheric *p*CO~2~ (CO~2~ partial pressure) predictions are mostly determined by the model forcings and the response of silicate weathering rate to *p*CO~2~ and temperature, multiple negative feedback processes and coupling of the C, O, P and S cycles are necessary for regulating *p*O~2~ while allowing δ^13^C changes of sufficient amplitude to match the record. The results support a *p*O~2~ dependency of oxidative weathering of reduced carbon and sulfur, which raises early Paleozoic *p*O~2~ above the estimated requirement of Cambrian fauna and prevents unrealistically large δ^34^S variation. They do not support a strong anoxia …