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, CO2, global temperature, ocean composition, delta(13)C and delta(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 pCO(2) (CO2 partial pressure) predictions are mostly determined by the model forcings and the response of silicate weathering rate to pCO(2) and temperature, multiple negative feedback processes and coupling of the C, O, P and S cycles are necessary for regulating pO(2) while allowing delta(13)C changes of sufficient amplitude to match the record. The results support a pO(2) dependency of oxidative weathering of reduced carbon and sulfur, which raises early Paleozoic pO(2) above the estimated requirement of Cambrian fauna and prevents unrealistically large delta(34)S 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 delta(13)C and organic carbon burial rates too high. The dependency of terrestrial primary productivity on pO(2) also contributes to oxygen regulation. An intermediate strength oxygen fire feedback on terrestrial biomass, which gives a pO(2), upper limit of similar to1.6PAL (present atmospheric level) or 30 volume percent, provides the best combined pO(2) and delta(13)C predictions. Sulfur cycle coupling contributes critically to lowering the Permo-Carboniferous pCO(2) and temperature minimum. The results support an inverse dependency of pyrite sulfur burial on pO(2) (for example, Berner and Canfield, 1989), which contributes to the shuttling of oxygen back and forth between carbonate carbon and gypsum sulfur. A pO(2) dependency of photosynthetic carbon isotope fractionation (Berner and others, 2000; Beerling and others, 2002) is important for producing sufficient magnitude of delta(13)C 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 delta(34)S. In the Early Paleozoic, COPSE predicts pO(2)=0.2-0.6PAL and pCO(2)>10PAL, with high oceanic [PO43-] and low [SO4=]. Land plant evolution caused a 'phase change' 4 4 in the Earth system by increasing weathering rates and shifting some organic burial to land. This change resulted in a major drop in pCO(2) to 3 to 4PAL and a rise in pO(2) to similar to1.5PAL in the Pen-no-Carboniferous, with temperatures below present, ocean variables nearer present concentrations, and PO4:NO3 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 CO2 is in the range 3 to 7PAL, descending toward PAL in the Cretaceous and Cenozoic.