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Why is an increase in the amount of oxygen in the atmosphere favorable for life?

Why is an increase in the amount of oxygen in the atmosphere favorable for life?



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I have followed the David Attenborough series on 'First life' and heard, that an increase in the amount of oxygen in the atmosphere, as it took place just before the Cambrium, is generally favorable for life and also enables the evolution of larger organisms. Why is that so? Is it because aerobic respiration had a better conversion rate to energy?


Because it's favorable for the process of aerobic respiration which requires O2 to produce ATP, which is the little power storage unit.

With a bigger concentration of O2, it was possible to produce more ATP, should you evolve in that direction and you needed that constraint to be removed.


Factors Affecting Aerobic Respiration: 8 Factors | Plants

The following eight points will highlight the eight major factors affecting aerobic respiration in Plants.

The eight environmental factors effecting the rate of respiration are: (1) Oxygen Content of the Atmosphere (2) Effect of Temperature (3) Effect of Light (4) Effect of Water Contents (5) Effect of Respirable Material (6) Effect of Carbon Dioxide Concentration (7) Protoplasmic Conditions and (8) Other Factors.

(1) Oxygen Content of the Atmosphere:

The percentage of oxygen in the surrounding atmosphere greatly influence the rate of respiration. But reduction of the oxygen content of the air, however, causes no significant lowering in the respiratory rate until the percentage drops to about 10%. At 5% oxygen definite retardation of respiration occurs.

As shown in the graph (Fig. 7.13), with the increase of oxygen concentration in the atmosphere, the rate of respiration also increases, but this effect is not as accelerating as might be expected. This response of plants and their parts depends upon several factors. The plant tissues which ordinarily have low rate of respiration are not as seriously effected by low concentration of oxygen as those which have higher rate of respiration.

In certain plants, like rice, on removal of oxygen the rate of respiration in terms of total carbon dioxide produced actually increases. This indicates that anaerobic respiration comes into action when oxygen is no longer available and that the plant, if it has to make up for the relative inefficiency of this system, has to respire faster.

(2) Effect of Temperature:

Like most chemical reactions, the rate of respiration is greatly influenced by temperature. Estimation of Q 10 of the process for a rise in temperature from 8° to 18 °C gives a Q 10 of 2, indicating a chemical reaction. If the rise is at a much higher starting temperature, say between 20° and 30°C, then the Q 10 may fall below 2. It should be borne in mind that different plants or plant parts may show considerable variation in regard to optimum temperature for respiration.

In certain cases the rate of respiration increases at lower temperature. E. F. Hopkins (1925) reported that the rate of respiration in white potatoes increases if the temperature in lowered to just above freezing point. This increase in the rate of respiration is primarily due to increase in the quantity of respirable materials (such as soluble carbohydrates) which tend to accumulate in Irish potato at temperature slightly above 0°C.

At temperatures higher than the optimum for respiration, the rate of respiration (in terms of oxygen utilized and CO2 produced) falls due to inter-conversions of respirable materials. For instance, fats may be formed from carbohydrates by a reaction in which carbon dioxide is utilized and oxygen produced. At very high temperatures, the rate of respiration falls significantly and may even come to standstill because of protoplasmic injuries (Fig. 7.14).

(3) Effect of Light:

Light has indirect effects on the rate of respiration. With the increase in light intensity, the temperature of the surrounding atmosphere also increases thus affecting the rate of respiration. Secondly, the quantity of respirable material in the plant largely depends upon the rate of photosynthesis which is directly influenced by light and thirdly, stomata remain open during daylight and hence rapid exchange of gases takes place through them.

(4) Effect of Water Contents:

Over a certain range, water content of the plant tissue greatly influence its rate of respiration. In most of the storage able seeds the moisture content is kept below the point which allows a rapid respiration. With the increase in moisture content, the rate of respiration is likely to go up with the result a rapid loss of viability will occur and at the same time the temperature will also rise and the grain may be spoiled (Fig. 7.15).

Unlike most green tissues, xerophytes, lichens and leafy mosses (Sphagnum species) can be brought to an air-dry condition at low humidity without any apparent loss in their viability.

(5) Effect of Respirable Material:

Amount and kind of respirable material present in the cells greatly effect the rate and course of respiration. It has been shown that plants respire more rapidly after having been exposed to conditions favourable for photosynthesis during which carbohydrates are synthesized. Increase in respiration has also been observed to be associated with increase in soluble sugars.

(6) Effect of Carbon Dioxide Concentration:

The rate of respiration is normally not affected by increase of carbon dioxide concentration in the surrounding atmosphere up to 19%, but as the concentration increases from 10% to 80%, a progressive decrease in respiration occurs.

Specific response to higher CO2 concentration varies with the particular kind of tissue and plant. The effect of CO2 concentration is more significant when the temperature and oxygen supply are low. At a very high concentration of CO2 the plant tissues are injured or even killed.

(7) Protoplasmic Conditions:

The young growing tissues which have greater amount of protoplasm as compare to older tissues, show higher rate of respiration. Their higher rate of respiration support the meristematic activities of the cells by supplying large amount of energy. The degree of hydration of the protoplasm in the cells affects the rate, and mechanical injury to plant tissues will accelerate respiration.

(8) Other Factors:

Various chemicals, such as cyanides, azides and fluorides, have been reported to possess respiration retarding properties through their effect on respiratory enzymes. Respiration rate may likely be accelerated by low concentrations of the compounds like ethylene, carbon monoxide, chloroform and ether.

Chlorides of various minerals, like sodium, potassium, calcium and magnesium have pronounced effect on the rate of respiration. Monovalent chlorides, like KCl and NaCl, increase the rate of respiration while the divalent chlorides, such as MgCl2 and CaCl2 greatly decrease it. Steward and Preston (1941) found cations to depress respiration and photosynthesis.


Rise in oxygen levels links to ancient explosion of life, researchers find

Oxygen has provided a breath of fresh air to the study of the Earth's evolution some 400-plus million years ago.

A team of researchers, including a faculty member and postdoctoral fellow from Washington University in St. Louis, found that oxygen levels appear to increase at about the same time as a three-fold increase in biodiversity during the Ordovician Period, between 445 and 485 million years ago, according to a study published Nov. 20 in Nature Geoscience.

"This oxygenation is supported by two approaches that are mostly independent from each other, using different sets of geochemical records and predicting the same amount of oxygenation occurred at roughly the same time as diversification," said Cole Edwards, the principal investigator of a study conducted when he was a postdoctoral fellow in the lab under the paper's senior author, David Fike, associate professor in Earth and Planetary Sciences in Arts & Sciences. The other authors are Matthew Saltzman of Ohio State University and Dana Royer of Wesleyan University in Connecticut.

"We made another link between biodiversification and oxygen levels, but this time during the Ordovician where near-modern levels of oxygen were reached about 455 million years ago," said Edwards, assistant professor in geological and environmental sciences at Appalachian State in Boone, N.C. "It should be stressed that this was probably not the only reason why diversification occurred at that time. It is likely that other changes—such as ocean cooling, increased nutrient supply to the oceans and predation pressures—worked together to allow animal life to diversify for millions of years."

This explosion of diversity, recognized as the Great Ordovician Biodiversification Event, brought about the rise of various marine life, tremendous change across species families and types, as well as changes to the Earth, starting at the bottom of the ocean floors. Asteroid impacts were among the many disruptions studied as the reasons for such an explosion of change. Edwards, Fike and others wanted to continue to probe the link between oxygen levels in the ocean-atmosphere and diversity levels of animals through deep time.

Estimating such oxygen levels is particularly difficult: There is no way to directly measure the composition of ancient atmospheres or oceans. Time machines exist only in fiction.

Using geochemical proxies, high-resolution data and chemical signatures preserved in carbonate rocks formed from seawater, the researchers were able to identify an oxygen increase during the Middle and Late Ordovician periods—and a rapid rise, at that. They cite a nearly 80-percent increase in oxygen levels where oxygen constituted about 14 percent of the atmosphere during the Darriwilian Stage (Middle Ordovician 460-465 million years ago) and increased to as high as 24 percent of the atmosphere by the mid-Katian (Late Ordovician 450-455 million years ago).

"This study suggests that atmospheric oxygen levels did not reach and maintain modern levels for millions of years after the Cambrian explosion, which is traditionally viewed as the time when the ocean-atmosphere was oxygenated," Edwards said. "In this research, we show that the oxygenation of the atmosphere and shallow ocean took millions of years, and only when shallow seas became progressively oxygenated were the major pulses of diversification able to take place."

The chemical signatures that served as proxies for dissolved inorganic carbon included data from geologic settings ranging from the Great Basin in the western United States, to the northern and eastern U.S., to Canada and its Maritimes, as well as Argentina in the Southern Hemisphere and Estonia in the Eastern Hemisphere. Nevada, Utah, Oklahoma, Missouri (New London north and Highway MM south of St. Louis), Iowa, Ohio, West Virginia and Pennsylvania were among the data points across the U.S.

The researchers concluded that it remained unclear whether the increased oxygenation had a direct effect on animal life, or even if it had a passive effect by, say, expanding the oxygen-rich ecospace. So it is difficult to resolve if temperature, increased oxygenation or something else served as the driver for biodiversification. But the findings showed that oxygen certainly was spiking during the times of some of the greatest change.

"Oxygen and animal life have always been linked, but most of the focus has been on how animals came to be," said Saltzman, professor and school director of Earth Sciences at Ohio State. "Our work suggests that oxygen may have been just as important in understanding how animals came to be so diverse and abundant."


Proterozoic Era

This is the era of many interesting events in the Earth’s history. There were changes and developments everywhere in the Earth. Unlike the other ancient eras the Proterozoic Era contains good evidence of fossils, mainly of archaeans and bacteria. These evidences are the proofs that living organisms were in abundance in this era which lasted from 2.5 billion years to 543 million years ago.

Along with these two major living organisms existence of eukaryotic cells are also known by the fossils of the same. At the mid of the Proterozoic Era there was an increase in the atmospheric oxygen. Though this improved level of oxygen in the air cause catastrophe for the bacteria but it helped the eukaryotic cells to grow profusely. Multicellular algae are included in this group and finally the first animals appeared at the end period of the Proterozoic Era.

Division of the Proterozoic Era

This era is divided into 4 major periods and each of these periods has special features which contribute to understand the Proterozoic Era. The four divisions of the Proterozoic Era are the Siderian Period, the Rhyacian Period, the Orosirian Period, and the Statherian Period.

Stratigraphy of the Proterozoic Era

As stated above, there was in an increase in the level of oxygen in the atmosphere. It is surprising to know that the Earth was hit by pollution crisis for the first time in this era. Evidences of increased level of atmospheric oxygen in this era are enough like the red beds which contain metal oxides, fossil soils which contained iron oxides. According to the researchers, the levels of oxygen was less than the present level by 1% at the time archaean but this level increased by 15% at least 1.8 billion years ago and still now the level is rising.

It is really strange and contradictory to define this rising of oxygen as a pollution crisis. It is strange because it is the cause of degradation of different organic compounds and contradictory because oxygen is the major requirement of our lives and at the same time, it is the cause of destruction of some other life forms.

Even today, oxygen is used to kill several protists and bacteria. It is also interesting to know that to render oxygen harmless for them organisms had to undergone several biochemical methods like oxidative respiration. This method has the benefit to produce a large amount of energy for cells like eukaryotes.

Life during the Proterozoic Era

Proterozoic Era is the time of the stramolites. These bacteria continued to grow profusely throughout the Proterozoic Era and contributed richly to understand the Proterozoic life. These bacteria have been transformed into a kind of rock called stromatolitic chert where the exclusive microfossils of their microbes have been preserved.

Decline of Stramolites

They started declining before 700 million years after reigning in the Proterozoic Era. A common theory states their mass destruction due to the herbivorous eukaryotes, which evolved during this time and mainly fed on these growing stramolites. Though they are now counted in rare fossils but they can be found in some restricted habitats of saline and shallow water, like the Shark Bay of Australia.

The fossils of Proterozoic Era also include a type of macroscopic organism which was believed to exist before 2.1billion years. Also, from other types of fossils, carbon films, it can be said that life at the end of the Proterozoic Era was mainly in the form of multicellular organisms.

These organisms were like circles, leaves, or ribbon and dark compressions, and small and grew profusely in the Neoproterozoic. Some of these organisms also resemble seaweeds representing eukaryotic algae. It has been proved by evidences that green algae and reec algae appeared before 1 billion years ago approximately, which is the time of the Proterozoic era.

Localities in the Proterozoic era

The possible localities of the Proterozoic era include White Sea, Nopah Range, Mistaken Point in Newfoundland, Ediacara Hills, and Bitter Springs Formation. Each of these places have major contributions in understanding the life and other features of the Proterozoic era.

Bitter Springs Formation:

Fossils of eukaryotic cells are known to discover from this place of the late Proterozoic dolomite, which is in the central Australia.
Ediacara Hills – This site is known as the fossil bed of some the oldest known animals. These fossils were discovered in 1946, in this locality of Australia.

Mistaken Point, Newfoundland – Some of the mysterious fossils of the Proterozoic era were discovered from this coast of Newfoundland.

Nopah Range:

This region of the Southern California is known for the oldest sedimentary rocks, which date back to 1.5 billion years ago. The rocks are rich with the deposits of stromatolites.

White Sea:

This site is an active research center to find some of the Vendian fauna.

Geology of the Proterozoic era

This is also the era of significant geological changes. It is in this era that continents began to widen when the stable continents shelved and moved through the processes of plate tectonic. Erosion and deposition rapidly proceeded over those continents which were then devoid of plants. Thick beds of sandstone of pure quartz began to form rapidly. Also, formation of the banded iron beds which started in the Archean era continued throughout the Proterozoic Era. An interesting fact about these formations is that they were consisted of alternating layers of iron and quartz and are considered as the prime sources of iron in the world. These types of banded formations did not take place after the Proterozoic Era.


Why is an increase in the amount of oxygen in the atmosphere favorable for life? - Biology

An adequate supply of dissolved oxygen gas is essential for the survival of aquatic organisms. A deficiency in this area is a sign of an unhealthy river. There are a variety of factors affecting levels of dissolved oxygen. The atmosphere is a major source of dissolved oxygen in river water. Waves and tumbling water mix atmospheric oxygen with river water. Oxygen is also produced by rooted aquatic plants and algae as a product of photosynthesis.

There are physical factors that can lessen the amount of oxygen dissolved in the Cuyahoga. High temperatures, which may result from high turbidity, from the return of industrially used water to the river (the phenomenon of thermal pollution), or from dry periods, decrease the amount of gases that can be dissolved in water. Dry periods also decrease flow which reduces the amount of oxygen churned into the water.

In the navigation channel near the mouth of the Cuyahoga River, the river is dredged regularly to maintain sufficient depth for boats. This extra depth slows the river which hampers its mixing action. The navigation channel has particularly low dissolved oxygen levels.

Bacteria which decompose plant material and animal waste consume dissolved oxygen, thus decreasing the quantity available to support life. Ironically, it is life in the form of plants and algae that grow uncontrolled due to fertilizer that leads to the masses of decaying plant matter.

Too much dissolved oxygen is not healthy, either. Extremely high levels of dissolved oxygen usually result from photosynthesis by a large amount of plants. Great uncontrolled plant growth, especially algal blooms, is often the result of fertilizer runoff. This phenomenon is called cultural eutrophication.

Dissolved oxygen levels in sections of the river in which plants are the major contributor of oxygen fall sharply at night because photosynthesis ceases.

In the Cuyahoga, dissolved oxygen levels in one study of fourteen sites ranged from 1.5 to 90 percent saturation, with an average of 13.2 percent. 100 percent saturation is most desirable.

The pH of river water is the measure of how acidic or basic the water is on a scale of 0-14. It is a measure of hydrogen ion concentration. U.S. natural water falls between 6.5 and 8.5 on this scale with 7.0 being neutral. The optimum pH for river water is around 7.4. Water's acidity can be increased by acid rain but is kept in check by the buffer limestone. Extremes in pH can make a river inhospitable to life. Low pH is especially harmful to immature fish and insects. Acidic water also speeds the leaching of heavy metals harmful to fish.

The Cuyahoga River had a measured pH ranging from 6.0 to 8.0 in fourteen tests of a range of locations in September 1991. The lower values present a problem for most organisms with the exception of bacteria, which can survive pH's as low as 2.0. A pH of 8.0 should be sufficient to support most river life with the possible exception of snails, clams, and mussels, which usually prefer a slightly higher pH. The average pH in the study was 6.9, a value that is only sufficiently basic for bacteria, carp, suckers, catfish, and some insects.

Ready to test water quality

Turbidity

Turbidity is the condition resulting from suspended solids in the water, including silts, clays, industrial wastes, sewage and plankton. Such particles absorb heat in the sunlight, thus raising water temperature, which in turn lowers dissolved oxygen levels. They also prevent sunlight from reaching plants below the surface. This decreases the rate of photosynthesis, so less oxygen is produced by plants. Turbidity may harm fish and their larvae. It is caused by soil erosion, excess nutrients, various wastes and pollutants, and the action of bottom feeding organisms which stir sediments up into the water.

In the Cuyahoga, the average turbidity from a study of twelve sites was 24.9 Nephelometer Turbidity Units (NTU), with a range of 60 units. A value of 24.9 indicates that a device called a Secchi disk can be seen underwater up to a depth of ten to twelve inches. An extreme recorded value of 60 NTU indicates water that is relatively clear to a depth of five inches, while at the other extreme, a value of zero NTU corresponds to water with visibility to five feet, which is the maximum depth that can be measured with this turbidity test.

Temperature

Temperature impacts the rates of metabolism and growth of aquatic organisms, rate of plants' photosynthesis, solubility of oxygen in river water, and organisms' sensitivity to disease, parasites, and toxic materials. At a higher temperature, plants grow and die faster, leaving behind matter that requires oxygen for decomposition.

The temperature of the Cuyahoga as tested in September 1991 did not exceed 20 C or 68 F and thus did not create a climate for many fish diseases. It often exceeded 13 C or 55 F, creating a climate right for many fish, plants, insect nymphs and some fish diseases. Temperatures were recorded below this value, reducing plant life and fish diseases as well as indicating uninhabitable water for salmon. In the Cuyahoga, temperature changes radically in the spring and autumn. As a result, fish that are not indigenous to the region and not yet adapted for these shifts often die.

(Source: Cuyahoga River Water Quality Monitoring Program, Cleveland State University)


Changes in Earth's crust caused oxygen to fill the atmosphere

Matthijs Smit of the University of British Columbia examines ancient rocks from the deep crust in Norway during the summer of 2017. Credit: Matthijs Smit

Scientists have long wondered how Earth's atmosphere filled with oxygen. UBC geologist Matthijs Smit and research partner Klaus Mezger may have found the answer in continental rocks that are billions of years old.

"Oxygenation was waiting to happen," said Smit. "All it may have needed was for the continents to mature."

Earth's early atmosphere and oceans were devoid of free oxygen, even though tiny cyanobacteria were producing the gas as a byproduct of photosynthesis. Free oxygen is oxygen that isn't combined with other elements such as carbon or nitrogen, and aerobic organisms need it to live. A change occurred about three billion years ago, when small regions containing free oxygen began to appear in the oceans. Then, about 2.4 billion years ago, oxygen in the atmosphere suddenly increased by about 10,000 times in just 200 million years. This period, known as the Great Oxidation Event, changed chemical reactions on the surface of the Earth completely.

Smit, a professor in UBC's department of earth, ocean & atmospheric sciences, and colleague, professor Klaus Mezger of the University of Bern, were aware that the composition of continents also changed during this period. They set out to find a link, looking closely at records detailing the geochemistry of shales and igneous rock types from around the world—more than 48,000 rocks dating back billions of years.

"It turned out that a staggering change occurred in the composition of continents at the same time free oxygen was starting to accumulate in the oceans," Smit said.

Before oxygenation, continents were composed of rocks rich in magnesium and low in silica - similar to what can be found today in places like Iceland and the Faroe Islands. But more importantly, those rocks contained a mineral called olivine. When olivine comes into contact with water, it initiates chemical reactions that consume oxygen and lock it up. That is likely what happened to the oxygen produced by cyanobacteria early in Earth's history.

However, as the continental crust evolved to a composition more like today's, olivine virtually disappeared. Without that mineral to react with water and consume oxygen, the gas was finally allowed to accumulate. Oceans eventually became saturated, and oxygen crossed into the atmosphere.

"It really appears to have been the starting point for life diversification as we know it," Smit said. "After that change, the Earth became much more habitable and suitable for the evolution of complex life, but that needed some trigger mechanism, and that's what we may have found."

As for what caused the composition of continents to change, that is the subject of ongoing study. Smit notes that modern plate tectonics began at around the same time, and many scientists theorize that there is a connection.


The Origin of Oxygen in Earth's Atmosphere

It's hard to keep oxygen molecules around, despite the fact that it's the third-most abundant element in the universe, forged in the superhot, superdense core of stars. That's because oxygen wants to react it can form compounds with nearly every other element on the periodic table. So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff?

The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. In fact, all the plants on Earth incorporate symbiotic cyanobacteria (known as chloroplasts) to do their photosynthesis for them down to this day.

For some untold eons prior to the evolution of these cyanobacteria, during the Archean eon, more primitive microbes lived the real old-fashioned way: anaerobically. These ancient organisms&mdashand their "extremophile" descendants today&mdashthrived in the absence of oxygen, relying on sulfate for their energy needs.

But roughly 2.45 billion years ago, the isotopic ratio of sulfur transformed, indicating that for the first time oxygen was becoming a significant component of Earth's atmosphere, according to a 2000 paper in Science. At roughly the same time (and for eons thereafter), oxidized iron began to appear in ancient soils and bands of iron were deposited on the seafloor, a product of reactions with oxygen in the seawater.

"What it looks like is that oxygen was first produced somewhere around 2.7 billion to 2.8 billon years ago. It took up residence in atmosphere around 2.45 billion years ago," says geochemist Dick Holland, a visiting scholar at the University of Pennsylvania. "It looks as if there's a significant time interval between the appearance of oxygen-producing organisms and the actual oxygenation of the atmosphere."

So a date and a culprit can be fixed for what scientists refer to as the Great Oxidation Event, but mysteries remain. What occurred 2.45 billion years ago that enabled cyanobacteria to take over? What were oxygen levels at that time? Why did it take another one billion years&mdashdubbed the "boring billion" by scientists&mdashfor oxygen levels to rise high enough to enable the evolution of animals?

Most important, how did the amount of atmospheric oxygen reach its present level? "It's not that easy why it should balance at 21 percent rather than 10 or 40 percent," notes geoscientist James Kasting of Pennsylvania State University. "We don't understand the modern oxygen control system that well."

Climate, volcanism, plate tectonics all played a key role in regulating the oxygen level during various time periods. Yet no one has come up with a rock-solid test to determine the precise oxygen content of the atmosphere at any given time from the geologic record. But one thing is clear&mdashthe origins of oxygen in Earth's atmosphere derive from one thing: life.


Helping Hands

There are many small things you can do in your daily life to help protect the Earth. Image by Olivier Bresmal.

Although the Earth’s climate is changing, there is still time to help. One way to help could be to reduce CO2 emissions. Energy sources which produce GHGs are very important to our society but making little changes to reduce the amount we each use can make things better. To reduce the energy you use, you could:

  • Turn off the lights when you leave a room.
  • Use energy-efficient light bulbs
  • Unplug electronics that are not in use
  • Reduce car use (use public transportation, bike, or walk)

Every little bit can help and if enough people take small steps, they can add up to a huge leap.


Rise in oxygen levels links to ancient explosion of life, researchers find

Oxygen has provided a breath of fresh air to the study of the Earth's evolution some 400-plus million years ago.

A team of researchers, including a faculty member and postdoctoral fellow from Washington University in St. Louis, found that oxygen levels appear to increase at about the same time as a three-fold increase in biodiversity during the Ordovician Period, between 445 and 485 million years ago, according to a study published Nov. 20 in Nature Geoscience.

"This oxygenation is supported by two approaches that are mostly independent from each other, using different sets of geochemical records and predicting the same amount of oxygenation occurred at roughly the same time as diversification," said Cole Edwards, the principal investigator of a study conducted when he was a postdoctoral fellow in the lab under the paper's senior author, David Fike, associate professor in Earth and Planetary Sciences in Arts & Sciences. The other authors are Matthew Saltzman of Ohio State University and Dana Royer of Wesleyan University in Connecticut.

"We made another link between biodiversification and oxygen levels, but this time during the Ordovician where near-modern levels of oxygen were reached about 455 million years ago," said Edwards, assistant professor in geological and environmental sciences at Appalachian State in Boone, N.C. "It should be stressed that this was probably not the only reason why diversification occurred at that time. It is likely that other changes -- such as ocean cooling, increased nutrient supply to the oceans and predation pressures -- worked together to allow animal life to diversify for millions of years."

This explosion of diversity, recognized as the Great Ordovician Biodiversification Event, brought about the rise of various marine life, tremendous change across species families and types, as well as changes to the Earth, starting at the bottom of the ocean floors. Asteroid impacts were among the many disruptions studied as the reasons for such an explosion of change. Edwards, Fike and others wanted to continue to probe the link between oxygen levels in the ocean-atmosphere and diversity levels of animals through deep time.

Estimating such oxygen levels is particularly difficult: There is no way to directly measure the composition of ancient atmospheres or oceans. Time machines exist only in fiction.

Using geochemical proxies, high-resolution data and chemical signatures preserved in carbonate rocks formed from seawater, the researchers were able to identify an oxygen increase during the Middle and Late Ordovician periods -- and a rapid rise, at that. They cite a nearly 80-percent increase in oxygen levels where oxygen constituted about 14 percent of the atmosphere during the Darriwilian Stage (Middle Ordovician 460-465 million years ago) and increased to as high as 24 percent of the atmosphere by the mid-Katian (Late Ordovician 450-455 million years ago).

"This study suggests that atmospheric oxygen levels did not reach and maintain modern levels for millions of years after the Cambrian explosion, which is traditionally viewed as the time when the ocean-atmosphere was oxygenated," Edwards said. "In this research, we show that the oxygenation of the atmosphere and shallow ocean took millions of years, and only when shallow seas became progressively oxygenated were the major pulses of diversification able to take place."

The chemical signatures that served as proxies for dissolved inorganic carbon included data from geologic settings ranging from the Great Basin in the western United States, to the northern and eastern U.S., to Canada and its Maritimes, as well as Argentina in the Southern Hemisphere and Estonia in the Eastern Hemisphere. Nevada, Utah, Oklahoma, Missouri (New London north and Highway MM south of St. Louis), Iowa, Ohio, West Virginia and Pennsylvania were among the data points across the U.S.

The researchers concluded that it remained unclear whether the increased oxygenation had a direct effect on animal life, or even if it had a passive effect by, say, expanding the oxygen-rich ecospace. So it is difficult to resolve if temperature, increased oxygenation or something else served as the driver for biodiversification. But the findings showed that oxygen certainly was spiking during the times of some of the greatest change.

"Oxygen and animal life have always been linked, but most of the focus has been on how animals came to be," said Saltzman, professor and school director of Earth Sciences at Ohio State. "Our work suggests that oxygen may have been just as important in understanding how animals came to be so diverse and abundant."


Pressure in the atmosphere

Figure 4: The graph on the left shows how pressure changes with altitude in Earth's atmosphere. The mountain profile shown in the lower left represents Mt. Everest, the point of highest elevation on Earth's surface. The image on the right is a representation of the density of gas molecules in the atmosphere, with the layers of the atmosphere labeled.

Atmospheric pressure can be imagined as the weight of the overlying column of air. Unlike temperature, pressure decreases exponentially with altitude. Traces of the atmosphere can be detected as far as 500 km above Earth's surface, but 80 percent of the atmosphere's mass is contained within the 18 km closest to the surface. Atmospheric pressure is generally measured in millibars (mb) this unit of measurement is equivalent to 1 gram per centimeter squared (1 g/cm 2 ). Other units are occasionally used, such as bars, atmospheres, or millimeters of mercury. The correspondence between these units is shown in the table below.

Table 3: Correspondence of atmospheric measurement units.

At sea level, pressure ranges from about 960 to 1,050 mb, with an average of 1,013 mb. At the top of Mt. Everest, pressure is as low as 300 mb. Because gas pressure is related to density, this low pressure means that there are approximately one-third as many gas molecules inhaled per breath on top of Mt. Everest as at sea level – which is why climbers experience ever more severe shortness of breath the higher they go, as less oxygen is inhaled with every breath.

Though other planets host atmospheres, the presence of free oxygen and water vapor makes our atmosphere unique as far as we know. These components both encouraged and protected life on Earth as it developed, not only by providing oxygen for respiration, but by shielding organisms from harmful UV rays and by incinerating small meteors before they hit the surface. Additionally, the composition and structure of this unique resource are important keys to understanding circulation in the atmosphere, biogeochemical cycling of nutrients, short-term local weather patterns, and long-term global climate changes.

Summary

Earth's atmosphere contains many components that can be measured in different ways. This module describes these different components and shows how temperature and pressure change with altitude. The scientific developments that led to an understanding of these concepts are discussed.

Key Concepts

Earth's atmosphere is made up of a combination of gases. The major components of nitrogen, oxygen, and argon remain constant over time and space, while trace components like CO2 and water vapor vary considerably over both space and time.

The atmosphere is divided into the thermosphere, mesosphere, stratosphere, and troposphere, and the boundaries between these layers are defined by changes in temperature gradients.

Pressure decreases exponentially with altitude in the atmosphere.

Our knowledge about the atmosphere has developed based on data from a variety of sources, including direct measurements from balloons and aircraft as well as remote measurements from satellites.


Watch the video: Luftdruck, barometrische Höhenformel (August 2022).