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 O2 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 coevolution 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 timedependent solar insolation. The baseline model succeeds in giving simultaneous predictions of atmospheric O2, CO2, global temperature, ocean composition, (delta)13C and (delta)34S 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 pCO2 (CO2 partial pressure) predictions are mostly determined by the model forcings and the response of silicate weathering rate to pCO2 and temperature, multiple negative feedback processes and coupling of the C, 0, P and S cycles are necessary for regulating p02 while allowing (delta)13C changes of sufficient amplitude to match the record. The results support a p02 dependency of oxidative weathering of reduced carbon and sulfur, which raises early Paleozoic p02 above the estimated requirement of Cambrian fauna and prevents unrealistically large (delta)34S 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)13C and organic carbon burial rates too high. The dependency of terrestrial primary productivity on p02 also contributes to oxygen regulation. An intermediate strength oxygen fire feedback on terrestrial biomass, which gives a p02 upper limit of ~ 1.6PAL (present atmospheric level) or 30 volume percent, provides the best combined pO 2 and (delta)13C predictions. Sulfur cycle coupling contributes critically to lowering the Permo-Carboniferous pCO2 and temperature minimum. The results support an inverse dependency of pyrite sulfur burial on p02 (for example, Berner and Canfield, 1989), which contributes to the shuttling of oxygen back and forth between carbonate carbon and gypsum sulfur. A p02 dependency of photosynthetic carbon isotope fractionation (Bemer and others, 2000; Beerling and others, 2002) is important for producing sufficient magnitude of (delta)13C variation. However, our results do not support an oxygen dependency of sulfur isotope fractionation in pyrite formation (Bemer and others, 2000) because it generates unrealistically small variations in (delta)34S. In the Early Paleozoic, COPSE predicts pO2=0.2-0.6PAL and pCO2>1OPAL, with high oceanic [PO^3- 4] and low [SO4 ]. Land plant evolution caused a ʹphase changeʹ in the Earth system by increasing weathering rates and shifting some organic burial to