A widespread biotic turnover in a tropical ocean 740 million years ago seems to be linked to an increase in surface nutrients an idea that rules out Snowball Earth. The study online in Nature Geoscience shows that the biotic turnover in which a diverse assemblage of plankton was replaced by a bacterial bloom preceded the onset of low-latitude glaciation associated with Snowball Earth by 16 million years.
Susannah Porter and colleagues reconstructed the timing of the plankton shift using samples from rocks exposed in the Grand Canyon of the United States. The researchers sampled a particular unit of rocks with uniquely high sedimentation rates for their age, which allowed them to create an unprecedented temporal reconstruction of the events leading up to the local extinction.
Their analyses show that the evidence for a rise in surface nutrients and the extinction appears millions of years before the first evidence of glaciers appeared in the rocks, ruling out Snowball Earth as the cause of this particular event.