The research concerns the mechanism leading to rapid global warming, about 600 million years ago, that ended the so-called ‘Snowball Earth’ glacial period. Methane, a strong greenhouse gas, has been implicated as a playing a key role in this instance of rapid climate change. A key piece of evidence supporting this idea are carbonate rocks, deposited when the climate change happened, which have a lot less carbon-13 in them than normal – an indicator that their formation involved methane. We examined some of these carbonates with a new analytical tool that measures the temperature at which they formed. The high temperature we found (~400 deg C) show, when combined with other evidence, that the carbon-13 depleted rocks didn’t form at the end of the Snowball. The implications of this is that a different driver of climate change may have be involved in ending the snowball Earth. We also argue, based on our findings, that changes in ocean chemistry through Earth history influenced the biogeochemcial cycling of methane and have a critical influence on carbon isotope signatures left in the geological record that are often used as evidence of the microbial metabolism of methane. In other words, the clues left in rocks indicating how certain organisms were making a living in ancient Earth history may be different than previously expected.