For 10 points, name this element which makes up ozone.
An increase in this element's concentration in the Earth's past led to a sharp decrease in the concentration of nickel. Banded iron formations arose during the "Crisis" caused by this element, which led to the Huronian ice age and a possible Snowball Earth. When nitrates or phosphates enter water, this element's concentration decreases due to eutrophication. Radical chlorine atoms can abstract an atom of this element in the stratosphere, which limits the ability to absorb UV-B light. A mass extinction of anaerobes occurred when this element was first produced by photosynthetic cyanobacteria 2.4 billion years ago. For 10 points, name this element which makes up ozone.
An increase in this element's concentration in the Earth's past led to a sharp decrease in the concentration of nickel.
oxygen [or O2]