Geoscientists have corroborated evidence that oxygen production began in Earth's oceans at least 100 million years before the Great Oxidation Event (GOE). The researchers analyzed 2.5 billion-year-old black shales, which revealed that episodes of hydrogen sulfide accumulation in the oxygen-free deep ocean occurred nearly 100 million years before the GOE. Scientists have long believed that the early ocean was characterized by high amounts of dissolved iron.