For 10 points, identify this diatomic gas, the second-most abundant in our atmosphere, which you need for respiration.
A geological event named after this molecule was preceded by the creation of many banded iron formations. That event led to the Huronian glaciation, when this molecule's predominance led to a reduction of the greenhouse gas methane, leading to an ice age. This compound's abundance has been postulated to explain the size of insects during the Carboniferous. 2.3 billion years ago, a "Great Event," in which this compound's levels rose dramatically, wiped out all the obligate anaerobes on Earth; that event was caused by production of this molecule by photosynthetic cyanobacteria. For 10 points, identify this diatomic gas, the second-most abundant in our atmosphere, which you need for respiration.
A geological event named after this molecule was preceded by the creation of many banded iron formations.
dioxygen [or O2]