How did we get all that free oxygen to burn?

WHEN DID EARTH GET A 20.8% OXYGEN-RICH ATMOSPHERE?

It is generally believed that life on Earth probably wouldn’t have developed if the early atmosphere had been oxygen rich. Photosynthesis bacteria were surely not the first living organisms, but the history of life in the period that preceded their appearance is still obscure. What little information can be inferred about early earth is consistent with the idea that the environment was then largely anoxic (without oxygen). One tentative line of evidence rests on the assumption that among organisms living today those that are simplest in structure and in biochemistry are probably the most closely related to the earliest forms of life. Those simplest organisms are bacteria of the clostridal and methanogenic type, and they are all obligate anaerobes.
Somewhat later such bacteria gave rise to the first organisms capable of aerobic photosynthesis, the precursors of modern cynaobacteria. For the anaerobic photosynthetic bacteria the molecular oxygen released by this mutant strain was a toxin, and as a result the aerobic photosynthesizers were able to supplant the anaerobic one in the upper portions of the mat communities. The anerobic species became adapted to the lower parts of the mat, where there is less light but also a lower concentration of oxygen.
The anaerobic nature of bacterial photosynthesis seems to present a paradox: photosynthetic organisms thrive where light is abundant, but such environments are also generally ones having a high concentration of oxygen, which poisons bacterial photosynthesis. These contradictory needs can be explained if it is assumed that anaerobic photosynthesis evolved among primitive bacteria early in the Precambrian, when the atmosphere was essentially anoxic. The photosynthesizers could thus have lived in mat-like communities in shallow water and in full sunlight.
The several groups of photosynthetic bacteria differ from one another in their pigmentation, but they are alike in one important respect: unlike the photosynthesis of cyanobacteria and eukaryotes, all bacterial photosynthesis is a totally anaerobic process. Oxygen is not given off as a byproduct of the reaction, and the photosynthesis cannot proceed in the presence of oxygen. Whereas oxygen appears to be a requirement of green plants for the synthesis of chlorophyll, oxygen inhibits the synthesis of bacteriochlorophylls.
It is argued that oxygen must have been freely available by the time the first eukaryotic cells appeared, probably 1,400 to 1,500 million years ago. Hence, the proliferation of cyanobacteria that released the oxygen must have take place earlier in the Precambrian. How much earlier remains a question. The best available evidence bearing on this issue comes from the study of sedimentary minerals, some of which may have been influenced by the concentration of free oxygen at the time they were deposited. In recent years a number of workers have investigated this possibility, most notably Preston E. Cloud, Jr., of the University of California at Santa Barbara and the U. S. Geologic Survey.
One mineral of significance in this argument is uraninite (UO2), which is found in several deposits that were laid down in Precambrian streambeds. In the presence of oxygen, grains of uraninite are readily oxidized to U3O8 and are thereby dissolved. David E. Grandstaff of Temple University has shown that streambed deposits of the mineral probably could not have accumulated if the concentration of oxygen was greater than about 1 percent. Uraninite-bearing deposits of this type are found in deposits older than about two billion years but not in younger strata, suggesting that the transition in oxygen concentration may have come at about that time.
The most intriguing mineral evidence for the date of the oxygen transition comes from another kind of iron-rich deposit; the banded iron formation. These deposits include some tens of billions of tons of iron in the form of oxides embedded in a silica-rich matrix; they are the world’s chief economic reserves of iron. A major fraction of them was deposited within a comparatively brief period of a few hundred-million years beginning some what earlier than two billion years ago. That would have been a time when the earth was cooling after the planet building phase.
A transition in oxygen concentration could explain the major episode of iron sedimentation through the following hypothetical sequence of events. In a primitive, anoxic ocean, iron existed in the ferrous state (that is, with a valence of +2) and in that form was soluble in seawater. With the development of aerobic photosynthesis small concentrations of oxygen began diffusing into the upper portions of the ocean, where it reacted with the dissolved iron. The iron was thereby converted to the ferric form (with a valence of +3) and as a result hydrous ferric oxides were precipitated and accumulated with silica to form rusty layers on the ocean floor. As the process continued virtually al the dissolved iron in the ocean basins was precipitated: in a matter of a few hundred million years as the world’s oceans rusted. Could this have been a time when our solar system entered an area of space with a salt cloud? Does the Oort cloud and Kippier belt of the Sirius system contain salt?
In my book, Cosmological Ice Ages I propose that our solar system was captured by Sirius binary system at about that time 700-million years ago thereby imparting additional ultraviolet light to earth which would release more oxygen into the atmosphere with increased photosynthesis. It would have taken the power of a White Dwarf star to break through early earth’s thousand-pound per square inch-thick atmosphere to get oxygen producing plants to grow. During the Precambrian the sun didn’t burn nearly as hot as it does today. Any suggestion that our sun is solely responsible for all the biological-deposited layers on earth isn’t taking into consideration the higher atmospheric pressures and the fact that Earth had previously been in an Ice Age for over one billion years.
Fossil stromatolites first became abundant in sediments deposited about 2,300 million years ago, shortly before the major episode of iron-ore deposition. It is therefore possible that the first widespread appearance of stromatolites might mark the origin and the earliest diversification of oxygen-producing cyanobacteria. Even at that early date the cynaobacteria would probably have released oxygen at a high rate, but for several hundred million years the iron dissolved in the oceans would have served as a buffer for the oxygen concentration of the atmosphere, reacting with the gas and precipitating it as ferric oxides almost as quickly as it was generated.
One thing the scientist may have missed here is the fact that iron and dust from space during the Precambrian planet-building phase near our sun’s birthplace in Orion may account for some of the dissolved iron in the oceans. After our sun was captured by the Sirius trinary (multiple star-system) it passed through several oort clouds that may have imparted additional iron and salt to fertilize Earth’s oceans. The salt would have sped up the oxidation of the iron. Scientist aren’t sure where all the salt came from on earth and our capture by the Sirius and Procyon multiple star cluster would explain it.
Sirius B was a six-solar-mass star before it shrunk down into a white dwarf of 1.5 solar masses. That means there was 4.5 solar masses of iron and other material injected into the surrounding oort cloud of Sirius A. In addition Procyon B, currently 10.4 light years away also injected considerable iron into the neighborhood. Of course none of these stellar explosions could have happened while our sun was in the neighborhood otherwise we wouldn’t be here.
Only when our solar system traveled away from its birthplace in Orion and the oceans had been swept free of unoxidized iron and similar material would the concentration of oxygen in the atmosphere have begun to rise toward modern levels.
Much still remains uncertain regarding the evidence from the fossil record. Modern biochemistry from geology and mineralogy make possible a tentative outline for the history of Precambrian life.
Much is also uncertain about the fossil record of human evolution as well. After the mapping of the human genome scientists noticed large segments of human DNA that seemed totally unrelated to the development of a human—that is, until they started to compare these segments with other animals. They experienced the most astounding thing—the shock of a lifetime. Segments of the so-called “junk DNA” were identical to pig, cow, horse and even bacterial DNA. Humans obviously evolved on this planet and are related to most every animal on the planet including bacteria. Without bacteria we couldn’t digest our food. We carry around billions of bacteria in order to live. We share a symbiotic relationship with most everything on Earth—especially the diatoms which at the major producer of free oxygen.