https://phys.org/news/2018-06-extreme-fluctuations-oxygen-gradual-cambrian.html

Wei explains that the goal was to reconstruct a continuous record of global marine oxygen levels from the late Ediacaran (latest pre-Cambrian) into the early Cambrian. To do that, the team, from Yale and Nanjing University in China, measured changes in uranium isotope ratios that reflect changing marine oxygen levels, as recorded in carbonates deposited during that interval, from China.

The results revealed huge global swings from anoxic-completely depleted in oxygen to oxygenated conditions over periods of two to ten million years. Such big ups and downs in oxygen, the scientists suggest, could have destabilized ecosystems, fragmented habitats, and triggered an explosion of changing life forms.