What it was
A ribbon of seaweed reaches toward the light beneath an icy sky. The sea below can stay liquid and still be too dark for it. Thick ice cuts off the sunlight it needs to make food.
Seaweed is a familiar name for large algae. Their cells have inner parts, like the cells in your body. They use photosynthesis, catching sunlight to help make food. The show's seaweed stands for large algae; its cut-paper shape does not name one known fossil species.
When and where
During Snowball Earth, ice spread over much of the ocean's surface while water stayed liquid below. Algae needed places where light could reach the water. A hot vent on a dark seafloor could feed some microbes, but it could not replace sunlight for seaweed.
Fossils from southern China preserve large algae in coastal rocks from near the end of the Marinoan freeze. That was the later major freeze in the chapter. The fossils point to water that life could live in during part of that frozen time.
How we know
Look at the stone around a fossil as well as the fossil itself. Fine, dark mud layers sit among rocks linked to glaciers. Together they place the algae within the freeze. A loose fossil without its surrounding layers would tell a thinner story.
Scientists also test steranes, chemical leftovers linked to molecules in cell skins. Changes in their amounts point to algae spreading between the major freezes. These tiny chemical clues add to what the larger fossils show.
Still a mystery
Scientists still argue about how much open water remained. Thin ice could pass some light; thick, snowy ice would block much more. The Chinese fossils give us a coastal refuge. They cannot show every shore on a frozen planet.