Three-step modernization of the ocean: Modeling of carbon cycles and the revolution of ecological systems in the Ediacaran/Cambrian periods

Abstract

Important ecological changes of the Earth (oxidization of the atmosphere and the ocean) increase in nutrient supply due to the break-up of the super continent (Rodinia) and the appearance of multi-cellular organisms (macroscopic algae and metazoan) took place in the Ediacaran period, priming the Cambrian explosion. The strong perturbations in carbon cycles in the ocean are recorded as excursions in carbonate and organic carbon isotope ratio (δ13Ccarb and δ13Corg) from the Ediacaran through early Cambrian periods. The Ediacaran–early Cambrian sediment records of δ13Ccarb and δ13Corg, obtained from the drill-core samples in Three Gorges in South China, are compared with the results of numerical simulation of a simple one-zone model of the carbon cycle of the ocean, which has two reservoirs (i.e., dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). The fluxes from the reservoirs are assumed to be proportional to the mass of the carbon reservoirs. We constructed a model, referred to here as the Best Fit Model (BFM), which reproduce δ13Ccarb and δ13Corg records in the Ediacaran–early Cambrian period noted above. BFM reveals that the Shuram excursion is related to three major changes in the carbon cycle or the global ecological system of the Earth: (1) an increase in the coefficient of remineralization by a factor of ca. 100, possibly corresponding to a change in the dominant metabolism from anaerobic respiration to aerobic respiration, (2) an increase of carbon fractionation index from 25‰ to 33‰, possibly corresponding to the change in the primary producer from rock-living cyanobacteria to free-living macro algae, and (3) an increase in the coefficient of the organic carbon burial by a factor of ca. 100, possibly corresponding to the onset of a biological pump driven by the flourishing metazoan and zooplankton. The former two changes took place at the start of the Shuram excursion, while the third occurred at the end of the Shuram excursion. The other two excursions are explained by the tentative decrease in primary production due to cold periods, which correspond to the Gaskiers (ca. 580 Ma) and Bikonor (ca. 542 Ma) glaciations.