Atmospheric oxygen may have been present earlier than expected

Science News : Pinpointing when oxygen was first present in Earth’s atmosphere is significant in determining the development of photosynthetic life. A new discovery may have moved that date to 3 billion years ago, 300 million to 400 million years earlier than previously believed. Sean Crowe (University of British Columbia), Lasse Døssing (University of Southern Denmark), and colleagues were examining rock samples collected 1 km underground in South Africa. They were using a recently developed technique that measures the ratio of chromium-52 and chromium-53. Although both isotopes are stable, oxidized ⁵³Cr is more soluble in water than oxidized ⁵²Cr is. Consequently, more ⁵³Cr than ⁵²Cr is washed away from soil, and rocks formed from the soil are deficient in the heavier isotope. Crowe and Døssing found that the 3-billion-year-old rock samples had unexpectedly low levels of ⁵³Cr. They also found elevated levels of ⁵³Cr in nearby ocean sediments where the chromium could have been deposited. Chromium ratio measurements are still a new technique, so further evidence is required to confirm the presence of oxygen because other gases could have caused the oxidization.