The stability of the atmosphere in Earth is not a consequence of [[chemical equilibrium]]. [[Oxygen]] is a reactive compound, and should eventually combine with gases and minerals of the Earth's atmosphere and crust. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the [[Great Oxygenation Event]].<ref name=Anabar2007>{{Cite journal| last4 = Arnold| last6 = Creaser| last3 = Lyons| first1 = A. | first2 = Y.| last9 = Scott| last2 = Duan | first3 = T. | first4 = G.| last8 = Gordon | first5 = B. | first10 = J. | first6 = R.| last10 = Garvin | first7 = A.| last11 = Buick | first8 = G. | first11 = R. | first9 = C.| title = A whiff of oxygen before the great oxidation event?| journal = Science| volume = 317| issue = 5846| year = 2007| last7 = Kaufman| pages = 1903–1906| last5 = Kendall| pmid = 17901330| last1 = Anbar | doi = 10.1126/science.1140325|bibcode = 2007Sci...317.1903A }}</ref> Since the start of the [[Cambrian]] period, atmospheric oxygen concentrations have fluctuated between 15% and 35% of atmospheric volume.<ref name=Berner1999>{{Cite journal | The stability of the atmosphere in Earth is not a consequence of [[chemical equilibrium]]. [[Oxygen]] is a reactive compound, and should eventually combine with gases and minerals of the Earth's atmosphere and crust. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the [[Great Oxygenation Event]].<ref name=Anabar2007>{{Cite journal| last4 = Arnold| last6 = Creaser| last3 = Lyons| first1 = A. | first2 = Y.| last9 = Scott| last2 = Duan | first3 = T. | first4 = G.| last8 = Gordon | first5 = B. | first10 = J. | first6 = R.| last10 = Garvin | first7 = A.| last11 = Buick | first8 = G. | first11 = R. | first9 = C.| title = A whiff of oxygen before the great oxidation event?| journal = Science| volume = 317| issue = 5846| year = 2007| last7 = Kaufman| pages = 1903–1906| last5 = Kendall| pmid = 17901330| last1 = Anbar | doi = 10.1126/science.1140325|bibcode = 2007Sci...317.1903A }}</ref> Since the start of the [[Cambrian]] period, atmospheric oxygen concentrations have fluctuated between 15% and 35% of atmospheric volume.<ref name=Berner1999>{{Cite journal |