A COMPARISON OF THE NEOPROTEROZOIC AND PALEOZOIC CARBON CYCLES

Recent compilations of high resolution δ¹³C_carb curves for both the Neoproterozoic and the Paleozoic illustrate several important similarities and differences between the global carbon cycle of the two Eras. Although the maximum amplitude of positive excursions (>+8‰) is similar, their duration is an order of magnitude longer during the Neoproterozoic. Both the stratigraphic thickness (>1km) and geochronometric duration (>50Ma) of Neoproterozoic excursions dwarf Paleozoic examples, which are typically tens of meters thick and last a few million years or less. Neoproterozoic positive excursions occur between extensive glaciations, which are marked by pronounced negative excursions to low values (<-5‰) not found in the Paleozoic. Although some early interpretations of Paleozoic positive excursions suggested a coincidence with glacial conditions, it is becoming increasingly clear that Paleozoic positive δ¹³C_carb excursions share a connection to greenhouse (or interglacial) conditions that is similar to the Neoproterozoic.

The similar climatic relations but diminishing duration of Paleozoic excursions as compared to the Neoproterozoic is here interpreted to reflect the establishment of thermohaline circulation that was sufficiently well-oxygenated to prevent prolonged C_org burial. In this model, increased C_org burial in an anoxic/euxinic deep ocean during greenhouse conditions is attenuated by global cooling that drives increased oxidative ventilation of the deep ocean. Prior to the buildup of sufficient pO₂ that could fully oxygenate the deep oceans consistently, this oceanographic component of the global carbon cycle would have been ineffective as a negative feedback on global cooling. The lack of this oceanographic component would have promoted the influence of the silicate weathering-CO₂ system over the global carbon cycle and global climate of the early earth. This is consistent with current models that attribute Neoproterozoic glaciations to prolonged periods of organic carbon burial and global cooling, which was reversed by CO₂ buildup as ice sheets expanded to cover exposed silicates. This assumes that downwelling deep water in the Neoproterozoic did not contain sufficient oxygen either because of low pO₂ or because of continued warm saline bottom water production in tropical regions. In the Paleozoic, prolonged periods of organic carbon burial were no longer possible due to the ability to sustain oxidative thermohaline ventilation of the deep ocean, which also prevented the recurrence of snowball-scale events.