ON THE INITIATION OF A SNOWBALL EARTH

The reasons why the Earth was susceptible to a Snowball climate in the Neoproterozoic remain a mystery. The convergence of continents at low-latitudes may have been a contributing factor. When all the continents are at tropical latitudes, at least two interesting perturbations of the carbon cycle may result. First, most of the ocean area would exist in polar regions with no land mass available to supply dust. This would limit biological productivity in the high latitude oceans, focusing most of the productivity into smaller, partially restricted ocean basins in low latitudes. Organic carbon burial would be enhanced by input of sediment from large tropical rivers. This organic burial could be sustained through more efficient recycling of phosphate under anoxic conditions. This may explain the envelope of high d13C values through most of the Neoproterozoic. In addition, the low latitude continents would allow for lower carbon dioxide levels by removing the glacial barrier to chemical weathering. In the current continental configuration, as carbon dioxide emissions from volcanic sources decrease, ice caps on high latitude continents decrease the rate of chemical weathering, stabilizing the amount of carbon dioxide in the atmosphere. If all the continents were at low latitudes, no such "safety switch" would exist, and carbon dioxide could reach lower levels. This may explain why the snowball glaciations occurred at this time, and not in the Phanerozoic, and not in the 1.5 billion years preceding. Finally, we propose that the drop in d13C in sediments immediately underlying the glaciation may involve an unusual interaction between atmospheric methane.and the silicate weathering feedback on atmospheric CO2. If methane were ever dominant over CO2 as a greenhouse gas, the short residence time of methane would make the Earth's climate unstable, susceptible to a crash if the methane source were to stop for any reason.