I hadn't looked in on the Cryogene in the last couple years; let's try again.
Last October they were looking at "ooids", laminated pearls but without the oyster. Each layer has data, like tree-rings (or indeed like pearls I guess). Back then, they were looking at "organic" carbon, the carbon in living tissue; even if it's plankton or paramecial tissue, as one might expect before the Ediacaran from 635 Mya on. They didn't find much. So it wasn't plants locking up the carbon.
Recently other scientists were looking at the climate patterns - at least during the Sturtian patch 720–660 Mya, from Garvellach. Now they know: climate was happening. A lot of that ice melted 660 Mya, so... yeah. Before the next cold snap, which they call Marinoan (650–635 Mya).
The article hints that Garvellach snapshots a mere 3 ky span, in this 60000 ky Sturtian; but... it's something. Maybe they can finetune more exactly when the sample was laid down, like with ooids.
They can say definitively, during the Sturtian at some point anyway, that some water did peep out from time to time, like 15%... of the ocean. The land at the time is assumed also covered in the white stuff (and quite dead), although Antarctican summer vacationers may ponder Dry Valleys.