evidence for Oxygen in the early earth's atmosphere 

Did the early earth have more oxygen than today? Why is there so much oxygen in the crust? Why is there so much nitrogen in the earth’s atmosphere today? These are questions that must be asked to understand the origin and formation of the earth. Many have postulated that the atmosphere of the early earth had more oxygen than today. Fossils of large animals including dinosaurs have been found in the past and scientists speculate that a higher oxygen level in the past would allow for longer lifespans. The large amount of nitrogen in today’s atmosphere must also be explained, possibly from volcanic gases or from some other source inside the earth. In addition to making up 21 percent of the earth's atmosphere today, oxygen makes up 46 percent of the earth's crust by mass and 89 percent of seawater on earth. A recent study has shown that air bubbles have been found trapped inside salt crystals in pre-Flood sedimentary rock layers in west central Australia (known as Precambrian layers according to mainstream scientists).  These air bubbles and their composition were compared to modern air bubbles inside salt crystals and showed that the ancient bubbles had more than half the oxygen percentage (10.9 percent) of the modern atmosphere. This discovery gives strong evidence for a pre-Flood atmospheric oxygen level much higher than previously thought and much closer to today's level. Evolutionists believe that the early earth had very little oxygen (less than 10 percent of today's amount) and was composed primarily of ammonia, carbon dioxide, methane, and water vapor. The Miller-Urey experiment used these gases to simulate early earth conditions in hopes of proving spontaneous chemical reactions that would spur the naturalistic formation of life. However, that experiment was only successful in producing certain organic molecules and could not explain how these molecules could combine together to form proteins and the basic tenants of the living cell. The Australia research gives evidence for large oxygen levels in the early Earth's atmosphere and that complex life including animals could have existed on Earth earlier than previously proposed. Creationists use the evidence of large percentages of oxygen in the ancient past to give evidence for a much shorter time span for the age of the Earth. According to the creationist model, suitable oxygen levels were were installed during the creation week, which would have allowed complex life to exist near the beginning of the formation of the Earth. Billions of years were not needed to produce the adequate oxygen levels needed to support the complex life of today that would allow animals and humans to survive, according to the creationist model. In addition, scientists have examined the chemical composition of early Earth rocks and sedimentary strata, including Precambrian sedimentary layers and much evidence exists that these rocks and sediments of the early Earth formed during the existence of an atmosphere abundant in oxygen, despite the assumptions of mainstream evolutionist scientists.
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Nigel J.F. Blamey et al., “Paradigm Shift in Determining Neoproterozoic Atmospheric Oxygen,” Geology 44 (2016): 651–654.
Erich Dimroth and Michael M. Kimberley, “Precambrian Atmospheric Oxygen: Evidence in the Sedimentary Distributions of Carbon, Sulfur, Uranium, and Iron,” Canadian Journal of Earth Sciences 13 (1976): 1161–1185.