Climate, paleoecology and abrupt change during the Late Proterozoic: A consideration of causes and effects

This chapter examines the influence of the biosphere on the initiation, and termination of, the glaciations of the late Proterozoic. Recent considerations suggest that the biosphere controlled the timing of the onset of glaciation and also controlled the timing of the end of glaciation. Massive carbonate accumulation and giant stromatolites of the Late Proterozoic, combined with major blooms of phytoplankton, led to significant drops in the carbon dioxide content of the atmosphere, and forced climate from greenhouse to icehouse conditions. Cryoconites and hyperscums, each with a distinctively adapted cryophilic microbiota, developed during the Proterozoic ice ages and may have been a factor in melting the ice. The Proterozoic Tindir Group, Alaska provides evidence for such a cryophilic microbiota. Only by invoking the activity of such organisms can we explain the rapidity of deglaciation. A propensity to accumulate massive carbonates was present before the glaciation as well as after the deposition of the cap carbonates. Substrate disturbance by burrowing metazoa after the ice ages disrupted the microbial mat component of Proterozoic carbonate sequestration. Stromatolites after the glaciation tend to have porous, clotted and thrombolitic textures instead of evenly laminated textures and would therefore be less effective at retaining carbon dioxide (as carbonate and organic matter) and keeping it out of marine circulation. Newly emergent, burrowing metazoa of the Late Proterozoic eventually halted the development of ice-age inducing conditions, and may have prevented even worse glaciations by releasing hydrocarbons sequestered in seafloor sediment.