COPSE: A new model of biogeochemical cycling over Phanerozoic time

Abstract

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 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 O₂, CO₂, 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 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, O, P and S cycles are necessary for regulating pO₂ while allowing δ¹³C changes of sufficient amplitude to match the record.

The results support a pO₂ dependency of oxidative weathering of reduced carbon and sulfur, which raises early Paleozoic pO₂ 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 pO₂ also contributes to oxygen regulation. An intermediate strength oxygen fire feedback on terrestrial biomass, which gives a pO₂ upper limit of ∼1.6PAL (present atmospheric level) or 30 volume percent, provides the best combined pO₂ and δ¹³C 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 pO₂ (for example, Berner and Canfield, 1989), which contributes to the shuttling of oxygen back and forth between carbonate carbon and gypsum sulfur.

A pO₂ 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 pO₂=0.2–0.6PAL and pCO₂>10PAL, with high oceanic [PO₄^3-] and low [SO₄⁼]. 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 pCO₂ to 3 to 4PAL and a rise in pO₂ to ∼1.5PAL in the Permo-Carboniferous, with temperatures below present, ocean variables nearer present concentrations, and PO₄:NO₃ regulated closer to Redfield ratio. A second O₂ 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.