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Here's what the paper says. A Snowball Earth would accumulate incoming space dust on the ice, loaded with iridium. When it melted, there would be an outwash of all the iridium all at once, which would give an iridium spike in the sediments. Indeed, such a spike occurs after the two main major glaciations linked with the Snowball hypothesis.
I don't see why a Slushball ice sheet on the tropical continents wouldn't also accumulate iridium that would also outwash to give an iridium spike: in other words, I don't see how this study differentiates between Snowball and Slushball. The samples were taken at sites that were along the edge of the tropical continent. So let's do a thought experiment. Antarctica today has been accumulating space dust for 30 m.y. or so. If it melts, as it must some day, won't it produce an iridium spike? Would that imply that Antarctica had been part of a Snowball Earth? I think the answers to these two questions are Yes, and No, respectively. So I'm not going to alter my piece on Slushball Earth (yet).
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