Scientists can determine if oxygen existed in earth's archean atmosphere by
a. looking at the amount of salt content in rocks.
b. looking for oxidized iron in rocks.
c. determining the lack of oxidation that occurred.
d. determining how many people survived

Scientists can determine if oxygen existed in Earth’s Archean atmosphere by looking for oxidized iron in rock.  In the beginning, there was only a little to no oxygen in the atmosphere. The oxygen that are being produced by cyanobacteria is only enough for the weathering process. There will only enough oxygen that shall remain in the atmosphere when the rocks at the surface are properly and adequately oxidized.  However, as time goes by, evidences said that the amount of oxygen in the Archean atmosphere is actually increasing in a slow steady pace.