The Science of Discworld (Science of Discworld, #1)

by Terry Pratchett

According to the ‘Snowball Earth’ theory, proposed by Paul Hoffman and Daniel Schräg in 1998, ice covered the entire Earth at this time. However, if ice really covered the whole Earth, it should have done more damage than the fossil record indicates. And that’s not the only problem. One key piece of evidence for the Big Freeze is a layer of sedimentary rock that formed just after the glaciers melted and left huge quantities of debris. This layer contains less carbon-13, in proportion to ordinary carbon-12, than normal. Marine photosynthesis converts carbon-12 into carbon dioxide more readily than it does carbon-13, so an excess of carbon-13 is left behind in seawater and in the layers of sediment in it that later turn to rock. So a low ratio of carbon-13 to carbon-12 indicates low biological activity.