Stretching the envelope of past surface environments: Neoproterozoic glacial lakes from Svalbard.

The oxygen isotope composition of terrestrial sulfate is affected measurably by many Earth-surface processes. During the Neoproterozoic, severe "snowball" glaciations would have had an extreme impact on the biosphere and the atmosphere. Here, we report that sulfate extracted from carbonate lenses within a Neoproterozoic glacial diamictite suite from Svalbard, with an age of approximately 635 million years ago, falls well outside the currently known natural range of triple oxygen isotope compositions and indicates that the atmosphere had either an exceptionally high atmospheric carbon dioxide concentration or an utterly unfamiliar oxygen cycle during deposition of the diamictites.