Seven years ago, French geochemists Francois Robert of the National Museum of Natural History in Paris and Marc Chaussidon of the Center for Petrographic and Geochemical Research in Vandoeuvre-lès-Nancy began studying rocks to learn more about the temperature of ancient oceans.

The researchers analysed silicon isotopes in a variety of ancient sediments from Australia, Africa and China. Changes in the silicon isotope composition over time showed that, over the period from 3.5 billion to 0.8 billion years ago, seawater temperatures dropped from about 70 °C to about 20 °C.

Previous analyses of oxygen isotopes have given the same result. But some researchers contend that oxygen isotopes are a poor indicator because hot water is likely to change them. Silicon isotopes, however, should be more resilient, as water contains oxygen but very little silicon.

Recognizing the need for more robust results, Robert and Chaussidon decided to delay publication. Over the next few years, they continued their analysis and improved the technique. They confirmed their results, as reported on page 969. But this is not the final word. Chaussidon says he expects the controversy to continue, and plans further silicon isotope studies.