RETRACTED ARTICLE: Unaltered cosmic spherules in a 1.4-Gyr-old sandstone from Finland

Micrometeorites—submillimetre-sized particles derived from asteroids and comets1,2,3,4,5—occur in significant quantities in deep sea sediments1,2,4, and the ice sheets of Greenland6,7 and Antarctica8,9. The most abundant micrometeorites are cosmic spherules3, which contain nickel-rich spinels10 that were crystallized and oxidized during atmospheric entry, therefore recording the oxygen content in the uppermost atmosphere10,11,12. But the use of micrometeorites for detecting past changes in the flux of incoming extraterrestrial matter, and as probes of the evolution of the atmosphere, has been hampered by the fact that most objects with depositional ages higher than 0.5 Myr show severe chemical alteration2. Here we report the discovery of unaltered cosmic spherules in a 1.4-Gyr-old13,14,15 sandstone16,17 (red bed) from Finland. From this we infer that red beds, a common lithology in the Earth's history, may contain substantial unbiased populations of fossil micrometeorites. The study of such populations would allow systematic research on variations in the micrometeorite flux from the early Proterozoic era to recent times9 ( a time span of about 2.5 Gyr), and could help to better constrain the time when the atmospheric oxygen content was raised to its present level18,19,20.