COPSE: A new model of biogeochemical cycling over Phanerozoic time

N. M. Bergman
2004 American Journal of Science  
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 (Berner, , 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
more » ... ractive 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 O 2 , CO 2 , global temperature, ocean composition, ␦ 13 C and ␦ 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 (CO 2 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 ␦ 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 ␦ 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 ␦ 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 ϳ1.6PAL (present atmospheric level) or 30 volume percent, provides the best combined pO 2 and ␦ 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 ␦ 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 ␦ 34 S. In the Early Paleozoic, COPSE predicts pO 2 ‫-2.0؍‬ 0.6PAL and pCO 2 >10PAL, with high oceanic [PO 4 3-] and low [SO 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 pCO 2 to 3 to 4PAL and a rise in pO 2 to ϳ1.5PAL 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. introduction COPSE Model Concept We present a new biogeochemical model to examine possible coupled histories of O 2 , CO 2 and other Phanerozoic Earth system variables. Where possible we have followed earlier models in order to facilitate comparison with earlier work. Thus, at the base of the model are the 'Redfield Revisited' models of Lenton and Watson (2000a, 2000b henceforth LW1 and LW2, respectively), feedback-based descriptions of atmosphere and ocean O 2 and ocean nutrients nitrate and phosphate. Carbon was included by coupling the model with major elements of the geological and geochemical Phanerozoic carbon cycle 'Geocarb' model of Berner (1991, 1994 henceforth B1 and B2, respectively). The model was also extended to include a simple sulfur cycle, based largely on Kump and Garrels (1986) . The C and S cycles are each modeled with one reduced and one oxidized rock reservoir, and a smaller surficial reservoir. The model's C, O, P and S cycles are coupled through terrestrial and marine productivity and through biological and abiological weathering and deposition of C and S, in both reduced and oxidized states. The interactive biota includes marine productivity, dependent on NO 3 and PO 4 , following LW1, and an interactive and evolving terrestrial biota, dependent on O 2 and CO 2 . COPSE (Carbon, Oxygen, Phosphorus, Sulfur and Evolution) is a 'co-evolutionary' model of the Earth for geologic timescales. Besides pCO 2 and pO 2 , the model also gives semi-quantitative predictions of oceanic phosphate, nitrate, sulfate and calcium, and mean global temperature. Significantly, ␦ 13 C and ␦ 34 S records are predicted, rather than used as forcings, (in contrast to Lerman, 1981, 1984; Kump, 1993; Berner and others, 2000) allowing comparison with independent geological data. Tectonic / geological and biogeochemical / evolutionary forcings are included, and feedback loops within the system are studied. Our aim is to develop a 'co-evolutionary' biogeochemical model of the Earth system including evolving biota and geological processes. While the carbon and oxygen cycles are studied thoroughly herein, we acknowledge that further development is necessary. Specifically, future work on the model could include a more comprehensive sulfur cycle, more detailed ocean chemistry, and a more detailed inclusion of paleogeography. Background: Phanerozoic Modeling In a steady state model of the atmosphere-ocean-sediment system, Garrels and Perry (1974) demonstrated that the amount of O 2 and CO 2 cycled through the atmosphere during the past 600Ma has been much greater than the current content of the atmosphere. Garrels and Lerman (1984) similarly concluded that the atmosphereocean system acts more as a medium of transfer than as a reservoir in itself. Both O 2 and CO 2 levels are thought to be controlled by geological and biogeochemical feedbacks on a geological timescale. While the Phanerozoic history of both gases has been studied extensively, they have usually been studied separately. It is, however, apparent that many of the processes important to CO 2 history are also relevant to O 2 . Lerman (1981, 1984) presented a simple sulfur and carbon cycle model, including sulfur reservoirs gypsum (oxidized) and pyrite (reduced), carbon reservoirs carbonate (oxidized) and organic carbon (reduced), and C and S in the over Phanerozoic time
doi:10.2475/ajs.304.5.397 fatcat:ybpsvsaxvbddjd75oqjsa6mucq