FROM VOLCANIC WINTER TO SNOWBALL EARTH: AN ALTERNATIVE EXPLANATION FOR NEOPROTEROZOIC BIOSPHERE STRESS

The Biosphere was clearly affected by Neoproterozoic Snowball Earth. Warming due to increased greenhouse gasses (CO2, CH4) may have ended Neoproterozoic ice ages, but it is not understood what caused global cooling. A wide range of possibilities have been advanced, from intense weathering to true polar wander to intergalactic molecular clouds. One possibility that has not been considered is that cooling was triggered by a major increase in explosive volcanism. Neoproterozoic igneous activity affected all of the continents but was especially intense in Gondwana and Eurasia: S. America, Africa, Arabia, central Asia, and S. China. Volcanism in especially W Gondwana was arc-like, with abundant evidence of explosive volcanism. Plinian eruptions inject enough ash and aerosols into the stratosphere to significantly decrease incoming solar radiation and trigger cooling. A protracted increase in explosive volcanism in Neoproterozoic time would have produced a stratospheric reflective blanket to solar radiation. This would have caused cooling analogous to Rampino's “Volcanic Winter” but on a much greater scale, resulting in cooling, glaciation and runaway albedo increase. Such an interpretation is favored by indications that production rate of juvenile arc crust and presumably explosive volcanism in the Arabian-Nubian Shield alone was comparable to the total occurring today around the Pacific Ocean, and that ~630 Ma Marinoan glaciation occurred at the time of peak igneous activity in the East African Orogen. The link between ~720 Ma volcanism in the ANS and the Sturtian glaciation is less clear; this may be partly due to the fact that Sturtian-age igneous rocks are mostly metavolcanics and deformed tonalites, less well known than enriched Marinoan volcanics and plutons. ~720 Ma is about the time of a strong melting event in the ANS. A strong global mantle input to the hydrosphere is shown by low 87Sr/86Sr of Neoproterozoic seawater, reflecting the predominance of juvenile arc systems. Cooling due to stratospheric reflectivity may have been accompanied by cooling due to CO2 drawdown by weathering of extensive areas covered by lavas and tuffs. Both explosive volcanism cooling and weathering cooling effects would be maximized when continents and Andean-type arcs lay at low latitudes.