Abstract
The Cryogenian period (~720–635 Ma) is marked by extensive Snowball Earth glaciations. These have previously been linked to CO2 draw-down, but the severe cold climates of the Cryogenian have never been replicated during the Phanerozoic despite similar, and sometimes more dramatic changes to carbon sinks. Here we quantify the total CO2 input rate, both by measuring the global length of subduction zones in plate tectonic reconstructions, and by sea-level inversion. Our results indicate that degassing rates were anomalously low during the Late Neoproterozoic, roughly doubled by the Early Phanerozoic, and remained comparatively high until the Cenozoic. Our carbon cycle modelling identifies the Cryogenian as a unique period during which low surface temperature was more easily achieved, and shows that the shift towards greater CO2 input rates after the Cryogenian helped prevent severe glaciation during the Phanerozoic. Such a shift appears essential for the development of complex animal life.
Highlights
The Cryogenian period (~720–635 Ma) is marked by extensive Snowball Earth glaciations
In order to replicate extremely low Cryogenian temperatures, biogeochemical models have previously assumed a CO2 degassing rate similar to the present day[5, 11], which when combined with the Neoproterozoic positioning of the continents, and the significantly lower solar flux, results in CO2 less than six times preindustrial atmospheric level (PAL) and a global average surface temperature well below present day
The PALEOMAP Project has produced global plate tectonic reconstructions showing the past configuration of the continents and ocean basins[23], including the location of past plate boundaries[24]
Summary
The Cryogenian period (~720–635 Ma) is marked by extensive Snowball Earth glaciations. In order to replicate extremely low Cryogenian temperatures, biogeochemical models have previously assumed a CO2 degassing rate similar to the present day[5, 11], which when combined with the Neoproterozoic positioning of the continents, and the significantly lower solar flux, results in CO2 less than six times preindustrial atmospheric level (PAL) and a global average surface temperature well below present day. This contrasts with long-term carbon cycle models for the early Paleozoic[10, 12], in which the degassing rate is inferred to be substantially higher. The CO2 degassing rate is not the only control on surface temperature in these models (for example, palaeogeography plays an important role by controlling the efficiency of continental weathering13) but it is clearly a key factor in determining planetary temperature
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