Evolution of early earths atmosphere, hydrosphere, and biosphere: constraints from ore deposits

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Evolution of Atmosphere and Hydrosphere

Hydrogen that was released by volcanoes but not efficiently lost to space must have accumulated to levels of the order of mb Tian et al. As water was present on Earth before 4. Other atmospheric species that may have been present at this time include: CO and sulphur-bearing species like H 2 S released by volcanoes and possibly methane produced abiotically in hydrothermal vents.

Molecular oxygen was not present, as is indicated by the study of rocks from that era, which contain iron carbonate rather than iron oxide. The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms. The resultant oxygen accumulated in the atmosphere and formed the ozone layer in the upper atmosphere. Multicellular organisms evolved originally in the ancient water bodies. Aided by the absorption of harmful UV radiation in the ozone layer, primitive life forms colonized Earth's surface. Nitrogen on Earth was outgassed during the first hundred million years.

Therefore, the atmospheric pressure was at least 0. If climate regulation via the carbonate-silicate cycle is assumed Walker et al.

Early Earth's atmosphere is still a big mystery because to be conductive to the evolution of life not only significant greenhouse effect must have been present but also its chemical content should have allowed for abiotic synthesis of primordial organic molecules. If there were more CH 4 more potent greenhouse gas and less CO 2 there would be a thick organic haze which would cool the planet accompanied with rapid escape of H 2 to space. If there were more CO 2 and less CH 4 then life would much less likely to occur because of absence of necessary chemical conditions to ensure abiotic formation of organic molecules.

Methane is a trace gas in the present Earth atmosphere about 2 ppm , and its origin is biological except for a small fraction produced in hydrothermal systems. Methanogens are among the most primitive Archaea found in the tree of life, some of which are autotrophic consuming CO 2 and H 2 and others heterotrophic consuming organic molecules. If a biogenic release were equal to the present day, the level of methane would have reached - PAL Present Atmospheric Level in the absence of atmospheric O 2 Pavlov et al.

As today's methanogens can only grow in very limited environments where O 2 is absent and H 2 or organics are present, the production of methane by the biosphere was probably much higher in the early anaerobic environment. Thus, very high levels of methane can be inferred, which lasted for more than 1 Gyr, between the emergence of methanogens probably earlier than 3. Thus, it might be possible that biological methanogens contributed to the oxidation of the atmosphere and lithosphere and enhanced the loss of H 2 , making possible, later, for the rise of oxygen Carling et al.

A naive chemist examining the atmosphere on Earth may be completely surprised that the two most abundant gases are N 2 and O 2. N 2 behaves as a noble gas and it is virtually non-reactive. Geochemists assume that the amount of N 2 in the atmosphere has remained constant since the planet was first formed about 4. Indeed, the turnover time for N 2 in the atmosphere is estimated to be ca 1 Ga Berner By contrast, O 2 , the second most abundant gas in Earth's atmosphere, is highly reactive, and without a continuous source would become rapidly depleted Keeling et al.

This gas exists far from thermodynamic equilibrium with a virtually infinite source of reductant in Earth's mantle. Indeed, high concentrations of gaseous diatomic oxygen are unique to this planet in our Solar System and this feature of our planetary atmosphere has not yet been found on any other planet within approximately 20 parsecs of our Solar System Kasting The presence of high concentrations of the gas in a planetary atmosphere is presently understood to be a virtually irrefutable indication of life on other terrestrial planets.

Geological records have revealed the chemical action of free oxygen after about 2. The reactions of oxygen with the other abundant light elements are almost always exergonic, meaning that, in contrast to N 2 , without a continuous source, free molecular oxygen would be depleted from Earth's atmosphere within a few million years. Indeed, 2. Several reasons could explain this delay. First, the budget reaction of oxygenic photosynthesis also works in the reverse direction, since respiration and oxidation of organic sediments consume oxygen.


GeoChemBio.com/ecology/climate components - biosphere

This rate is balanced by the oxidation of rocks, old sediments, and volcanic gases. The oxidation sinks for O 2 may have been much more efficient on early Earth, partly due to the presence of large amounts of reduced iron in the ocean and the crust Walker Some tectonic processes may have favored the burial of reduced carbon and allowed the rise of O 2 by about 2 Gyr ago Des Marais et al. Another hypothesis has already been mentioned and is linked with the slow oxidation of Earth through the escape of hydrogen to space: in other words,.

There might also be a climatic reason for the delay between the emergence of O 2 - producers and the rise of O 2.

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CH 4 has a very short photochemical lifetime in an O 2 -rich atmosphere, which means that a consequence of a build up of O 2 is a decrease of the CH 4 atmospheric abundance and, thus, a fall of the mean surface temperature that could lead to a global freezing event. Therefore, in a biosphere where CH 4 and O 2 producers both exist, the solar luminosity might be a strong constraint on the timing of the oxygenation Selsis Some authors argue that complex multicellular life can only develop in an oxic environment e.

Geological records have provided us with only qualitative information about the presence or absence of oxygen in the atmosphere.

During the Phanerozoic from - Myr to now , models based on the chemical and isotopic composition of sedimentary rocks allow us to trace back the evolution of the level of O 2 and show that it has varied roughly between 0. The principal cause of this enhanced level was the rise of large vascular land plants and the consequent increased global burial of organic matter. Higher levels of O 2 are consistent with the presence of Permo-Carboniferous Myr ago giant insects.

The abundance of O 3 was more certainly affected by changes in the trace gases content. We do not know for sure whether, between the rise of O 2 and the beginning of the Phanerozoic, O 3 provided a UV shield for land life, but it can be inferred that it was present when the first lichens colonized the lands during the Ordovician - Myr ago. Larger image.

Evolution of Early Earth's Atmosphere, Hydrosphere, and Biosphere - Constraints from Ore Deposits

The atmosphere has apparently been oxygenated since the 'Great Oxidation Event' ca 2. However, geological and geochemical evidence from older sedimentary rocks indicates that oxygenic photosynthesis evolved well before this oxygenation event. Fluid-inclusion oils in ca 2. Mo and Re abundances and sulphur isotope systematics of slightly older 2. As early as 2. Even at 3. Hence, the hypothesis that oxygenic photosynthesis evolved well before the atmosphere became permanently oxygenated seems well supported.

The rise of atmospheric O 2 was a milestone in the history of life.

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  8. Although O 2 itself is not a climate-active gas, its appearance would have removed a methane greenhouse present on the early Earth and potentially led to dramatic cooling. Moreover, by fundamentally altering the biogeochemical cycles of C, N, S and Fe, its rise first in the atmosphere and later in the oceans would also have had important indirect effects on Earth's climate. Around 2. Of all the biochemical inventions in the history of life, the machinery to oxidize water — photosystem II — using sunlight is surely one of the grandest.

    Not only did the ability to use water as a fuel provide early cyanobacteria with the advantage of an almost limitless supply of energy, but the production of O 2 as a waste product also profoundly changed the composition of the world's oceans, continents and atmosphere. The rise of atmospheric O 2 also had important indirect effects on Earth's climate.

    Excerpts from the 2007 workshop on teaching about the early earth

    An anoxic atmosphere on the early Earth would likely have contained significant amounts of methane, which is a potent greenhouse gas. Increasing pO 2 would have removed this methane greenhouse, possibly triggering dramatic cooling. Indeed, a series of near-global glaciations are believed to have occurred at about the same time that atmospheric O 2 first rose significantly. The prevailing hypothesis, based largely on structural studies of the reaction centers, is that oxygenic photosynthesis resulted from lateral gene transfer between a purple non-sulphur bacterium with a quinone-based reaction center and a green sulphur bacterium with an iron—sulphur-based reaction centre, giving rise to a chimeric organism.

    However, while both the electron transfer processes and amino acid sequences of the two reaction center complexes are significantly different, the structural homology between them is strikingly similar, suggesting that they may have evolved from a single ancestor in one organism via gene duplication events, followed by divergent evolution Blankenship et al.

    Whatever process led to oxygenic photosynthesis, this energy transduction machine is undoubtedly the most complex in nature. In extant cyanobacteria, well over genes are required for the construction of the protein scaffolds as well as the enzymes required for biosynthesis of the prosthetic groups Shi et al. Consequently, oxygenic photosynthesis is, unlike any other core prokaryotic metabolic pathway, completely isolated to cyanobacteria. The production and consumption of oxygen are almost always very closely balanced on local scales. These results strongly suggest that photosynthesis and respiration are extremely tightly coupled.

    On longer geological time scales, however, there are net changes in atmospheric oxygen.

    Dr. (PhD ZA) Jens Gutzmer | TU Bergakademie Freiberg

    A net accumulation of oxygen in the atmosphere requires a net sink for reductants; that is, there must be an imbalance between oxygen production and its biological and abiological consumption Holland Indeed, the very presence of oxygen in the atmosphere implies a permanent sink for reductants hydrogen atoms. The primary biological sink for the reductants generated by oxygenic photoautotrophs is carbon dioxide, leading to the formation of reduced organic carbon Berner Indeed, the counterpart of the story of O is, in large measure, the formation and sequestration of C—H bonds.

    Mixing of these two reservoirs would, purely from a thermodynamic perspective, consume both pools, leading to the formation of water and inorganic carbon. Hence, a permanent reservoir for organic matter is imperative if oxygen is to accumulate from water splitting coupled to carbon fixation.

    Similar processes must have operated in the Late Archaean and Early Proterozoic oceans ca 2. Grazing pressure on these organisms was almost certainly nil; there were no metazoan grazers.