DYNAMICS OF THE NEOPROTEROZOIC CARBON CYCLE

The Neoproterozoic (1000-543 Ma) record of the isotopic composition of inorganic carbon exhibits extraordinarily large fluctuations. The isotopic fractionation between carbonate and marine organic carbon varies similarly. However the latter changes are less unusual, and are also found, for example, in the Cenozoic (0-65 Ma). How, then, could the isotopic composition of inorganic carbon have varied so wildly in the Neoproterozoic but not the Cenozoic?

Such differences may be explained by a dynamical model of the long-term carbon cycle consisting of two coupled reactive reservoirs, of inorganic and organic carbon, respectively. To no surprise, the model shows that the Cenozoic changes in the carbon isotopic record may be understood as systematic "quasistatic" changes from one geochemical steady state to another. Just as definitively, however, the model reveals that the Neoproterozoic carbon cycle varied dynamically, far from steady state.

The lack of a steady state in the Neoproterozoic is attributed to an unusually large oceanic reservoir of dissolved and suspended organic carbon. The most significant of the Neoproterozoic negative carbon-isotopic excursions would have resulted from increased remineralization of this reservoir, possibly initiated by extensive global cooling associated with these events. The existence of such a large organic reservoir may have derived from the peculiarities of a global carbon cycle largely free of macroscopic animals and the mechanisms they introduced for exporting organic matter to the deep sea. It would also have suppressed oxygen concentrations below the photic zone. Support for both of these hypotheses is found by analysis of the terminal Neoproterozoic event, at the Precambrian-Cambrian boundary. This event qualifies as quasistatic, therefore signalling the organic reservoir's diminution to a size consistent with the Phanerozoic norm. The coincidence of this change with the Cambrian radiation of multicellular life and the resulting reorganization of global biogeochemical cycles could not have been an accident.