the belief in a primitive reducing atmosphere, ..
Oparin suggested that if the primitive atmosphere was reducing ..
reducing atmosphere of early life were the ..
Did the terrestrial planets ever have primitive atmospheres of anysignificance? For many years, astronomers assumed that Earth's earliestatmosphere was a dense, primitive atmosphere. Hence, the earliest (making amino acids in a test tube filled with methane,ammonia and water) imitating the supposed primeval soup were done in reducingenvironments.
As oxygenic photosynthesis spread through the oceans, everything that could be oxidized by oxygen was, during what is called the (“GOE”), although there may have been multiple dramatic events. The event began as long as three bya and is . The ancient carbon cycle included volcanoes spewing a number of gases into the atmosphere, including hydrogen sulfide, sulfur dioxide, and hydrogen, but carbon dioxide was particularly important. When the continents began forming, carbon dioxide was removed from the atmosphere via water capturing it, , the carbon became combined into calcium carbonate, and plate tectonics subducted the calcium carbonate in the ocean sediments into the crust, which was again released as carbon dioxide in volcanoes.
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used the energy of captured photons to strip electrons from various chemicals. Hydrogen sulfide was an early electron donor. In the early days of photosynthetic life, there was no atmospheric oxygen. Oxygen, as reactive as it is, was deadly to those early bacteria and archaea, damaging their molecules through oxidization. , or the stripping of electrons from life’s molecules, has been a problem since the early days of life on Earth. Oxidative stress is partly responsible for how organisms age, but it can also be beneficial, as organisms use oxidative stress in various ways.
About 2.7 bya, dissolved iron in anoxic oceans seems to have begun reacting with oxygen at the surface, generated by cyanobacteria. The dissolved iron was oxidized from a soluble form to an insoluble one, which then precipitated out of the oceans in those vivid red (the color of rust) layers that we see today and are called ("BIFs"), which became an oxygen sink and kept atmospheric oxygen low. The GOE is widely accepted to have created almost all of the BIFs, but it is not the only BIF-formation hypothesis and there is a great deal of controversy, but life processes are generally considered to be primarily responsible for forming the BIFs. Most iron in the crust is bound in silicates and carbonates, and it takes a great deal of energy to extract the iron from those minerals; the oxides that comprise BIFs are much less energy-intensive to refine, as the iron is so concentrated. Far less ore needs to be melted to get an equivalent amount of iron. BIFs are the source of virtually all iron ore that humans have mined. Life processes almost certainly performed the initial work of refining iron, and humans easily finished the job billions of years later. Copper was not refined by life processes, and copper ore takes twice as much energy to refine as iron ore does.
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In the earliest days of life on Earth, it had to solve the problems of how to reproduce, how to separate itself from its environment, how to acquire raw materials, and how to make the chemical reactions that it needed. But it was confined to those areas where it could take advantage of briefly available potential energy as . The earliest process of skimming energy from energy gradients to power life is called respiration. That earliest respiration is today called because there was virtually no free oxygen in the atmosphere or ocean in those early days. Respiration was life’s first energy cycle. A biological energy cycle begins by harvesting an energy gradient (usually by a proton crossing a membrane or, in photosynthesis, directly capturing photon energy), and the acquired energy powered chemical reactions. The cycle then proceeds in steps, and the reaction products of each step sequentially use a little more energy from the initial capture until the initial energy has been depleted and the cycle’s molecules are returned to their starting point and ready for a fresh influx of energy to repeat the cycle.
Earth's likely began declining during the Hadean Eon, and the also removed methane from the atmosphere (a methane molecule is more than 20 times as effective as a carbon dioxide molecule in absorbing radiation in Earth’s atmosphere), which may have been created by methanogens (), and lasted for 300 million years. There is no scientific consensus regarding the exact dynamics that caused that first ice age (although I consider the above dynamics persuasive and likely relevant), but there is general agreement that it was ultimately due to reduced . That first ice age might have been a “” event, in which Earth’s surface was almost completely covered in ice.
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There is evidence that the atmosphere enveloping the early earth was very different than it is today
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Carbon dioxide, on the other hand, has been generally decreasing as an atmospheric gas for billions of years, and has . The geochemical process is like nitrogen's in that atmospheric water combines with carbon dioxide to form a weak acid, which then falls to Earth in precipitation. But carbon is in the same elemental family as an abundant crustal element: . in crustal compounds and turns into in a process called . Most of Earth’s was probably removed by this process, although the exact mechanisms are in dispute. In all paleoclimate studies, carbon dioxide is a prominent variable, if not prominent variable, for determining Earth’s surface temperature. But perhaps as early as three bya, life became a significant source of carbon removal from the atmosphere, as life forms died and sank to the ocean floor, were subsequently buried by , and further buried them into Earth’s crust and mantle.
Climate Change: Vital Signs of the Planet: Evidence
Deep-ocean currents, , do not seem to have existed during supercontinental times, and atmospheric oxygen was likely only a few percent at most when the Cryogenian Period began. Canfield’s ocean-oxygenation evidence partly came from testing sulfur isotopes. As with , , and other elements, life prefers the lighter isotope of sulfur, and are two stable isotopes that can be easily tested in sediments. Canfield proposed that in pre-Cryogenian oceanic depths, , which are among Earth’s earliest life forms and produce as its waste product, abounded. Hydrogen sulfide gives rotten eggs their distinctive aroma and is highly toxic to plants and animals, as it . Hydrogen sulfide would react with dissolved iron to form iron pyrite and settle out in the ocean floor, just as the iron oxide did that formed the BIFs. The sulfate-reducing bacteria will enrich the sulfur-32/34 ratio by 3% and did so before the Cryogenian, but the Ediacaran iron pyrite sediments showed a 5% enrichment. A persuasive explanation is recycling sulfur in the oceanic ecosystem, which can only happen in the presence of oxygen.
The Transcension Hypothesis, John M. Smart, 2011
Precambrian deposits in the Dominion and Witwatersrand Reefs, SouthAfrica; Serra de Jacobina, Brasil; and Blind River, Ontario, districts contain ancientconglomerates and sands cemented to a very hard rock carrying grains of uraninite (UO2),pyrite and ilmenite (FeTiO3). The origin of these gold-uranium blanket ores hasbeen controversial for more than half a century. In 1958 Ramdohr suggested they could beconsidered as sediments laid down under an anoxygenic atmosphere and presented evidencefor repeated cycles of weathering-erosion-transportation-sedimentation, indicating thatthese beds must have been in repeated contact with the contemporaneous atmosphere. Thereduced state of the pyrite and uraninite (the stable form of uranium under present-dayatmospheric conditions is UO3) was taken as an indication of an anoxygenicatmosphere at the time of deposition (32, 23). Davidson, on the other hand, maintains thatthe uranium and pyrite ore bodies have been formed within the earth's crust long after themother rock sediments were laid down, by infiltration of hydrothermal metal-bearingsolutions, and that they cannot be explained in terms of abnormal atmospheric conditions(24, 28). Even Rutten, an enthusiastic supporter of the reducing atmosphere hypothesis,admits that the deposits show extensive effects of hydrothermal processes (33).
If, however, it could finally be proved that the uraninite and pyritedeposits are sedimentary in origin, there is no need to invoke an anoxygenic atmosphere toexplain their formation. The thermodynamically stable uranium oxide under an atmosphere ofoxygen is UO3, and consequently UO2 converts to this, but the rateof conversion depends strongly on the physical form of the uraninite. Conversion is rapidfor finely divided or powdered UO2, but granular or compacted material isstable indefinitely (34) and it is even used, fabricated into rods, as fuel elements fornuclear reactors. Grains of uraninite would be expected to remain unchanged duringerosion, transportation and deposition even under the present oxygenic atmosphere. Zeschkehas shown that uraninite is transported as minute grains by the Indus River in Pakistan atthe present day (35). Furthermore, in the Mozaan rocks of Swaziland, depositedconcurrently with the Witwatersrand strata, and in the Lorrain sandstones of Ontario,almost contemporary with the Blind River conglomerates, heavy mineral assemblages,completely of modern aspect, are widely developed (24). This indicates that theuranium-pyrite ores, which are themselves strikingly similar to recent deposits except forthe presence of uraninite and pyrite grains (36), cannot be explained in terms of abnormalatmospheric conditions.
Many of the Precambrian mineral deposits are associated withcarbonaceous matter. Uraninite and pyrite are frequently found in association withthucolite, a carbonaceous mineral, and kerogen, which is graphitized organic matterresembling medium- to high-grade coal (37, 38). Pyrite is deposited in present-dayenvironments, such as the Black Sea, by the action of bacterial sulphate-reducers (39).Micro-organisms are known to play an important role in the deposition and concentration ofmany minerals. The presence of carbonaceous matter in Precambrian ore deposits hasgenerated interest in the possibility that they owe their formation to oxygen-producing oroxygen-utilising micro-organisms (40). According to Koen the uranium was fixed andconcentrated by biological action (41), and it has been observed that the gold-uraniumreefs of the carbon seam type of the Kaapvall Craton are confined to environments expectedto favour algal growth (42). Schidlowski, on the other hand, has advocated an origin forthe thucolite involving migration of biogenic compounds into the conglomerates where theyare exposed to radiation from the uranium and undergo condensation and polymerizationreactions, finally solidifying as carbonaceous material around individual uraninite grains(37). Doubt has been cast on this hypothesis by observations on Precambrian thucoliteoccurrences in Australia which showed that the substance does not have the propertiesexpected of a radiolytic polymer (43).
Obviously no final conclusions can be drawn at present, but there mustbe a strong presumption that photosynthetic organisms are involved, particularly forpyrite. Further support for this view comes from the isolation of chemical fossilsincluding porphyrins from the carbonaceous material (see below).
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