COPSE: A new model of biogeochemical cycling over Phanerozoic time

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, δ¹3C and δ³4S 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 δ¹3C 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 δ³4S variation. They do not support a strong anoxia dependency of the C: P burial ratio of marine organic matter (Van Cappellen and Ingall, 1994, 1996) because this dependency raises early Paleozoic δ¹3C and organic carbon burial rates too high. The dependency of terrestrial primary productivity on *p*O~2~ also contributes to oxygen regulation. An intermediate strength oxygen fire feedback on terrestrial biomass, which gives a *p*O~2~ upper limit of ∼1. 6PAL (present atmospheric level) or 30 volume percent, provides the best combined *p*O~2~ and δ¹3C predictions. Sulfur cycle coupling contributes critically to lowering the Permo-Carboniferous *p*CO~2~ and temperature minimum. The results support an inverse dependency of pyrite sulfur burial on *p*O~2~ (for example, Berner and Canfield, 1989), which contributes to the shuttling of oxygen back and forth between carbonate carbon and gypsum sulfur. A *p*O~2~ dependency of photosynthetic carbon isotope fractionation (Berner and others, 2000; Beerling and others, 2002) is important for producing sufficient magnitude of δ¹3C variation. However, our results do not support an oxygen dependency of sulfur isotope fractionation in pyrite formation (Berner and others, 2000) because it generates unrealistically small variations in δ³4S. In the Early Paleozoic, COPSE predicts *p*O~2~=0. 2--0. 6PAL and *p*CO~2~\>10PAL, with high oceanic ~4~³-^\ and low ~4~^=^\. Land plant evolution caused a 'phase change' in the Earth system by increasing weathering rates and shifting some organic burial to land. This change resulted in a major drop in *p*CO~2~ to 3 to 4PAL and a rise in *p*O~2~ to ∼1. 5PAL in the Permo-Carboniferous, with temperatures below present, ocean variables nearer present concentrations, and PO~4~: NO~3~ regulated closer to Redfield ratio. A second O~2~ peak of similar or slightly greater magnitude appears in the mid-Cretaceous, before a descent towards PAL. Mesozoic CO~2~ is in the range 3 to 7PAL, descending toward PAL in the Cretaceous and Cenozoic.