The Earth's Great Oxidation Event may have ended in a big dip - Ars Technica
The rise of oxygen, which altered the planet's atmosphere and enabled multicellular life, may not have come as two large bursts, as has been widely held, but rather as several "whiffs." That is the provocative conclusion of a study of chromium isotopes in ancient sedimentary rocks published in Nature. A team of geochemists led by Robert Frei of the University of Copenhagen, Denmark, found that oxygen took a few twists and turns before reaching its present level.
via arstechnica.com