12 From www.learner.org/courses/envsci/ Unit 1 Geologists do not find iron formations after about 1.8 billion years ago, once oxygen levels in the atmosphere and ocean were high enough to remove almost all of the dissolved iron. However, iron formations are found once again around 700 million years ago within snowball glacial deposits. The explanation appears to be that during a Snowball Earth episode sea ice formed over most of the ocean's surface, making it difficult for oxygen to mix into the water.From Unit 1
14 From www.learner.org/courses/envsci/ Unit 1 Our present oxygen atmosphere (21 %) developed after the last snowball Earth around 500 million years ago. Oxygen first became abundant on Earth about 2 billion years ago but was only 1% of our atmosphere.From Unit 1
15 StromatolitesStromatolites: layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as blue-green algae). Stromatolites provide some of the most ancient records of life on Earth.
16 Cambrian explosionBetween about 570 and 530 million years ago, when a burst of diversification occurred, with the eventual appearance of the lineages of almost all animals living today.