 photo credit: Eliot Hoyt This cut and polished sample of two billion year-old banded iron formation, from Ontario, Canada, tells the story of how the Earth's atmosphere has changed over time. This type of rock, consisting of layers of iron-rich minerals alternating with layers of chert, may have formed on the ocean floor by precipitation from sea water that was very poor in oxygen. This rock tells us that the Earth's atmosphere and oceans were very poor in oxygen two billion years ago, as opposed to today. The evolution of photosynthetic life on Earth has increased the amount of oxygen in the atmosphere and oceans to today's levels. |
