Microfossils in the mid-Neoproterozoic Chuar Group in the Grand Canyon provide evidence that the massive species die-off of the time actually occurred 16 million years before the "Snowball Earth" glaciations of 726 - 635 million years ago.
A team from UC Santa Barbara led by geoscience professor Susannah Porter report evidence for abundant bacterial blooms and eutrophication in the Chuar sediments, in an article published in Nature Geoscience ("Palaeontology: Extinction before the snowball" - pp386 - 387, doi:10.1038/ngeo533).
The researchers say high levels of organic matter can deplete oxygen levels in the water creating widespread "dead zones." They also report finding "evidence for extreme anoxia in association with the bacterial blooms." [right, Chuar Group in the Grand Canyon. Credit UCSB]