Question

What evidence exists for the assumption made by Stanley Miller that the atmosphere of early Earth was reducing and not oxidizing?

According to an Ames Research Center expert on the reducing state of the pre-biotic environment, it is known that volcanic activity currently outgases mostly carbon dioxide and nitrogen, the presence of which in an atmosphere renders it "neutral" or "non-reducing," whereas the presence of oxygen in an atmosphere renders it "oxidizing." If the Earth's early mantle had contained more metallic iron than exists today, then the volcanoes would have outgassed more reducing species, such as methane, carbon monoxide, and hydrogen. Miller assumed that the early Earth’s mantle contained more metallic iron than it does today. But, according to John Delano of SUNY Albany, the amount of metallic iron in the mantle has not changed since as far back as 4 billion years ago. He looked at the distribution of trace elements in volcanic rocks that were formed throughout Earth's history. This distribution depends on the oxidation state of the system when the rock was formed. Since the amount of metallic iron in the early mantle was less than originally believed, the early atmosphere was less "reducing" than originally believed as well.
May 1, 2002