An Evolving Atmosphere
Part of Hall of Planet Earth.
About 1.7 billion years ago, banded iron formations — sedimentary rocks consisting of iron-rich layers alternating with iron-poor ones — stopped forming. By this time, photosynthesis had supplied enough oxygen to entirely deplete the oceans of their iron. With no more iron available to remove the oxygen, the gas slowly began to accumulate in the atmosphere, increasing to perhaps 2 percent near the end of the Precambrian (560 million years ago), and eventually to its present level of 20 percent.
Rocks Reveal Conditions of Early Earth
Clues to the conditions on the early Earth are found in rocks like the Huronian Supergroup, a set of sedimentary rock formations, up to 12 kilometers thick, laid down between 2.5 and 2.2 billion years ago. The rocks are exposed just north of Lake Huron, Canada. Many geologists believe that this sequence of rocks records a change from an oxygen-free to an oxygen-bearing atmosphere.