UNALTERED COSMIC SPHERULES IN A 1.4-GYR-OLD SANDSTONE FROM FINLAND

Micrometeorites-submillimetre-sized particles derived from asteroids a nd comets(1-5)-occur in significant quantities in deep sea sediments(1 ,2,4), and the ice sheets of Greenland(6,7) and Antarctica(8,9). The m ost abundant micrometeorites are cosmic spherules(3), which contain ni ckel-rich spinels(10) that were crystallized and oxidized during atmos pheric entry, therefore recording the oxygen content in the uppermost atmosphere(10-12). But the use of micrometeorites for detecting past c hanges 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 Mpr show severe c hemical alteration(2). Here we report the discovery of unaltered cosmi c spherules in a 1.4-Gyr-old(13-15) sandstone(16,17) (red bed) from Fi nland. From this we infer that red beds, a common lithology in the Ear th's history, map contain substantial unbiased populations of fossil m icrometeorites, The study of such populations would allow systematic r esearch on variations in the micrometeorite flux from the early Proter ozoic era to recent times(9) (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 level(18-20)