What does the existence of iron-rich rocks during the Precambrian indicate about the evolution of Earth‘s atmosphere?
the precambrian represents a VERY VERY large chunk of time. it is everything before about 540 million years ago. if earth is 4.6 billion years old, thats means the pC lasted about 4.1 billion years. there was not always abundant oxygen on earth. in fact, earth was very anoxic, and oxygen was a nasty, poisonous byproduct to the earliest entities of life. about 2 billion years ago, halfway through the pC, the first photosynthetic bacteria start pumping the O2 out into the ocean. iron in the seawater quickly reacted with any new oxygen to form that characteristic rust-colored iron oxide, Fe2O3. the iron minerals quickly fell out of suspension, causing the notorious banded iron formations (BIFS) however, since there was a limited amount of iron and an exponentially increasing amount of free oxygen, eventually that chemical sink was used up and O2 was able to accumulate. organisms at the time either had to adapt to being tolerant of oxygen, die off, or take refuge (e.g. the organisms that now live by deep ocean thermal vents). so, for your answer, we really only see iron-rich rocks during the later half of the precambrian. it documents a time where the oxygen produced was being locked up into iron-oxide minerals instead of accumulating in the ocean (and thus the atmosphere because of mixing). it was only once all the iron was oxidized that the O2 crisis set in and free oxygen began to accumulate.
