Geologic Evidence of Oxygen in the "Primordial" Atmosphere (Sidebar 2)

The question of the existence of oxygen in the earth's atmosphere is relevant to the probability of chemical evolution. If free oxygen (O₂) was present in the atmosphere, then the chemical reactions to form the basic building blocks of even the simplest cell (amino acids, proteins, lipids, etc.) could not occur.

All evolutionary models for the primitive atmosphere begin with an atmosphere devoid of O₂ and consist mainly of N₂, H₂O, and CO₂. The models are generally constructed based on the conditions necessary for chemical evolution to occur rather than on the evidence found in geology. It is necessary to exclude oxygen in their models for two reasons. First, all organic compounds are decomposed quickly in the presence of oxygen. Second, trace quantities of oxygen would inhibit the organic molecules from ever forming. But, the belief that chemical evolution occurred is the "strongest evidence" that the primitive atmosphere was without oxygen.[1]

There are two main sources of free oxygen in the earth's atmosphere: Photodissociation of H₂O, and photosynthesis. The amount of O₂ that can be produced from photodissociation due to ultraviolet light is greatly debated. Some estimates range from 10^-15 all the way to 0.25 of present atmospheric levels. Some of the issues that would effect the amount of photodissociation are volcanic activity (escaping free H₂), ambient temperature, amount of UV light coming from the sun and other stars during the ancient past, and the amount of ozone. The amount of ozone does present quite a conundrum. The presence of oxygen O₂, and hence ozone O₃, would prevent organic molecules from forming, but the presence of ozone is required to protect the newly evolving cells from deadly UV radiation. Assuming, photosynthesizing cells could evolve in this environment, they would supply additional free oxygen until the current level of oxygen was reached.

The evidence from geology regarding original atmospheric composition is anything but conclusive, but does leave the possibility open that there has always been oxygen in the atmosphere. If evidence of O₂ can be found in the oldest mineral deposits, then the likelihood of chemical evolution occurring is reduced to small pockets of anoxic environments that existed in an otherwise oxidizing environment.

Iron Oxides:
Fe₂O₃ (hematite) is an oxidized molecule of iron and is believed to form in an atmosphere containing free oxygen. It has been found in sediments reportedly greater than 2.5 billion years old by evolutionary considerations and in immense hematite deposits as far back as 3.4 billion years ago. The reduced form Fe₃O₄ (magnetite) has been found in recently formed deposits of 400 to 500 million years ago. The fact that all oxidation states of iron have been found in every "age" of deposits leads to the conclusion that both oxidizing and reducing environments have coexisted contemporaneously throughout earth's history, but in separate spaces.

Uranium Oxides:
Mineral deposits of uraninite (UO₂), galena (PbS), pyrite (FeS₂) and gold have been found in supposed 2.5 billion year old sedimentary rocks in South Africa. Uniform well-rounded grains of the minerals are evidence of their being deposited downstream from their original granite source. Is this evidence that the minerals were deposited during a time of a reducing atmosphere free of oxygen?

At first glance, the very idea of being washed downstream to a distance deposition site would seem to indicate that the grains had ample opportunity to be in contact with free O₂ if it existed in the atmosphere during the time it was deposited. However, if the stream was moving very rapidly, the minerals may not have had time to come to equilibrium with the atmosphere before being deposited. This deposition was probably not rapid based on the amount of wear on each grain and the amount of sorting found in the layers. The deposits could also have been transported during glacial periods. There is some evidence of glacial formations in that area of South Africa supposed 2.5 billion years ago. The corresponding lack of contact with the atmosphere and the cold temperatures would have greatly reduced the reaction with O₂ even in the presence of significant levels of O₂ at that time.

Conclusion
There are several reasons to believe the earth's atmosphere has always contained oxygen, while small pockets of anoxic environments coexisted. First, photodissociation of water could have produced up to 10% of current levels of free oxygen. Second, oxidized mineral species of rocks have been dated as old as supposed 3.5 billion years old. Third, the presence of reduced minerals does not necessarily confirm that the environment was anoxic during their formation. Fourth, evidence of oxygen-producing life forms have been found in rocks more than supposed 3.5 billion years old.

After examining the geological evidence, the scientific community is starting to concede that the primitive earth's atmosphere was less reducing than first estimated, and that it may have even been oxidizing. Some chemical evolution experiments have been redone using more neutral (intermediate between reduced and oxidized) atmospheres than the initial experiments. "These experiments have generally yielded products in smaller quantities and less diversity than comparable experiments under more reducing conditions."[2]

References
1. J.C.G. Walker, Evolution of the Atmosphere, Macmillan, NY. 1977. p. 224. Return to Text

2. Thaxton, Charles B., Walter L. Bradley and Roger L. Olsen, The Mystery of Life¹s Origin: Reassessing Current Theories, Philosophy Library, NY. 1984. p.94. Return to Text

Related link: "The Mineral Gallery" at www.galleries.com (collection of mineral descriptions, images, data, and specimens)