This signal event initiated eukaryotic cell development, land colonization, and species diversification. Perhaps this period rivals differentiation as the most important event in Earth history.
The oxygen built up to today's value only after the colonization of land by green plants, leading to efficient and ubiquitous photosynthesis. The Oxygen Concentration Problem. This is not a trivial question since significantly lower or higher levels would be damaging to life. The Early Ultraviolet Problem The genetic materials of cells DNA is highly susceptible to damage by ultraviolet light at wavelengths near 0.
It is estimated that typical contemporary microorganisms would be killed in a matter of seconds if exposed to the full intensity of solar radiation at these wavelength. Today, of course, such organisms are protected by the atmospheric ozone layer that effectively absorbs light at these short wavelengths, but what happened in the early Earth prior to the significant production of atmospheric oxygen?
There is no problem for the original non-photosynthetic microorganisms that could quite happily have lived in the deep ocean and in muds, well hidden from sunlight. But for the early photosynthetic prokaryotes, it must have been a matter of life and death. It is a classical "chicken and egg" problem. In order to become photosynthetic, early microorganisms must have had access to sunlight, yet they must have also had protection against the UV radiation.
The oceans only provide limited protection. Since water does not absorb very strongly in the ultraviolet a depth of several tens of meters is needed for full UV protection. Perhaps the organisms used a protective layer of the dead bodies of their brethren. Perhaps this is the origin of the stromatolites - algal mats that would have provided adequate protection for those organisms buried a few millimeters in.
Perhaps the early organisms had a protective UV-absorbing case made up of disposable DNA - there is some intriguing evidence of unused modern elaborate repair mechanisms that allow certain cells to repair moderate UV damage to their DNA.
However it was accomplished, we know that natural selection worked in favor of the photosynthetic microorganisms, leading to further diversification. Fluctuations in Oxygen The history of macroscopic life on Earth is divided into three great eras: the Paleozoic, Mesozoic and Cenozoic. Each era is then divided into periods. The latter half of the Paleozoic era, includes the Devonian period, which ended about million years ago, the Carboniferous period, which ended about million years ago, and the Permian period, which ended about million years ago.
According to recently developed geochemical models, oxygen levels are believed to have climbed to a maximum of 35 percent and then dropped to a low of 15 percent during a million-year period that ended in a mass extinction at the end of the Permian.
Such a jump in oxygen would have had dramatic biological consequences by enhancing diffusion-dependent processes such as respiration, allowing insects such as dragonflies, centipedes, scorpions and spiders to grow to very large sizes. Geochemical models indicate that near the close of the Paleozoic era, during the Permian period, global atmospheric oxygen levels dropped to about 15 percent, lower that the current atmospheric level of 21 percent. The Permian period is marked by one of the greatest extinctions of both land and aquatic animals, including the giant dragonflies.
But it is not believed that the drop in oxygen played a significant role in causing the extinction. Some creatures that became specially adapted to living in an oxygen-rich environment, such as the large flying insects and other giant arthropods, however, may have been unable to survive when the oxygen atmosphere underwent dramatic change. Composition of the Present Atmosphere Comparison to Other Planets The overall composition of the earth's atmosphere is summarized below along with a comparison to the atmospheres on Venus and Mars - our closest neighbors.
The variations in concentration from the Earth to Mars and Venus result from the different processes that influenced the development of each atmosphere. While Venus is too warm and Mars is too cold for liquid water the Earth is at just such a distance from the Sun that water was able to form in all three phases, gaseous, liquid and solid. Through condensation the water vapor in our atmosphere was removed over time to form the oceans.
For most atmospheric studies the concentration is expressed as parts per million by volume. That is, in a million units of air how may units would be that species. Carbon dioxide has a concentration of about ppm in the atmosphere i. Greenhouse Gases Click to interactively explore Selective Absorbers. Radiative Properties Objects that absorb all radiation incident upon them are called " blackbody " absorbers. The earth is close to being a black body absorber. Gases, on the other hand, are selective in their absorption characteristics.
While many gases do not absorb radiation at all some selectively absorb only at certain wavelengths. Those gases that are " selective absorbers " of solar energy are the gases we know as " Greenhouse Gases.
Wien's Law states that the wavelength of maximum emission of radiation is inversely proportional to the object's temperature. Using that law we know that the wavelength of maximum emission for the Sun is about 0.
In the activity to the right see where the greenhouse gases absorb relative to those two important wavelengths. Summary We developed a few useful tools for the study of biogeochemical cycles. These include the concepts of the reservoir, fluxes, and equilibria. Atmospheric evolution progressed in four stages, leading to the current situation. Now, scientists at Rensselaer are turning these atmospheric assumptions on their heads with findings that prove the conditions on early Earth were simply not conducive to the formation of this type of atmosphere, but rather to an atmosphere dominated by the more oxygen-rich compounds found within our current atmosphere — including water, carbon dioxide, and sulfur dioxide.
Today, as during the earliest days of the Earth, magma flowing from deep in the Earth contains dissolved gases. When that magma nears the surface, those gases are released into the surrounding air.
These frozen magmas and the elements they contain can be literal milestones in the history of Earth. One important milestone is zircon. Unlike other materials that are destroyed over time by erosion and subduction, certain zircons are nearly as old as the Earth itself. As such, zircons can literally tell the entire history of the planet — if you know the right questions to ask.
Understanding the level of oxidation could spell the difference between nasty swamp gas and the mixture of water vapor and carbon dioxide we are currently so accustomed to, according to study lead author Dustin Trail, a postdoctoral researcher in the Center for Astrobiology.
To do this Trail, Watson, and their colleague, postdoctoral researcher Nicholas Tailby, recreated the formation of zircons in the laboratory at different oxidation levels. As the magma ocean cooled, some compounds would have evaporated out of the molten mix and formed an atmosphere. The gas flowing around the marble behaves as if it were a miniature atmosphere. The researchers then repeated the experiment, altering the composition of the jet of gas by adding and removing different compounds to try to find the likely make-up of the atmosphere of the young Earth.
The oxygen levels in the melted sample changed depending on the composition of the gas.
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