Planet Earth/4b. Oxygen in the Atmosphere

How Earth’s Atmosphere became enriched in Oxygen
Classified as a lithophile element, the vast majority of oxygen on Earth is found in rocks, particularly in the form of SiO2 and other silicate minerals and carbonate minerals. During the early history of Earth most oxygen in the atmosphere was bonded to carbon (CO2), sulfur (SO2) or nitrogen (NO2). However, today free oxygen (O2) accounts for 20.95% of the atmosphere. Without oxygen in today’s atmosphere you would be unable to breathe the air and die quickly.

The origin of oxygen on Earth is one of the great stories of the interconnection of Earth’s atmosphere with planetary life. Oxygen in the atmosphere arose during a long period called the Archean (4.0 to 2.5 billion years ago), when life first appeared and diversified on the planet.

Early microscopic single-celled lifeforms on Earth utilized the primordial atmospheric gasses for respiration, principally CO2, SO2 and NO2. These primitive lifeforms are called the Archaea, or archaebacteria, from the Greek arkhaios meaning primitive. Scientists refer to an environment lacking free oxygen, as Anoxic, which literally means without oxygen. Hypoxia, meaning an environment with low levels of oxygen, while Euxinic means an environment that is both low in oxygen and has a high amount of hydrogen sulfide (H2S). These types of atmospheres were common during the Archean Eon.

Three major types of archaebacteria lifeforms existed during the Archean, and represent different groups of microbial single-celled organisms, all of which still live today in anoxic environments. None of these early archaebacteria had the capacity to photosynthesize, and instead relied on chemosynthesis, the synthesis of organic compounds by living organisms using energy derived from reactions involving inorganic chemicals only, typically in the absence of sunlight.

Methanogenesis-based life forms
Methanogenesis-based life forms take advantage of carbon dioxide (CO2), by using it to produce methane CH4 and CO2, through a complex series of chemical reactions in the absence of oxygen. Methanogenesis requires some source of carbohydrates (larger organic molecules containing carbon, oxygen and hydrogen) as well as hydrogen, but these organisms produce methane (CH4) particularly in sediments on sea floor in the dark and deep regions of the oceans. Today they are also found in the guts of many animals.

Sulfate-reducing life forms
Sulfate-reducing life forms take advantage of sulfur in the form of sulfur dioxide (SO2), by using it to produce hydrogen sulfide (H2S). Sulfate-reducing life forms require a source of carbon, often in the form of methane (CH4) or other organic molecules, as well as sources of sulfur, typically near volcanic vents.

Nitrogen-reducing life forms
Nitrogen reducing life forms take advantage of nitrogen in the form of nitrogen dioxide (NO2) by using it to produce ammonia (NH4). Nitrogen-reducing life forms also require a source of carbon, often in the form of methane (CH4) or other organic molecules.

All three types of life-forms exhibit anaerobic respiration, or respiration that does not involve free oxygen. In fact, these organisms produce gasses that combust or burn in the presence of oxygen, and hence oxidize to release energy. Both methane (CH4) and hydrogen sulfide (H2S) are flammable gasses and are abundant in modern anoxic environments rich in organic carbon, such as in sewer systems and underground oil and gas reservoirs.

The Advent of Photosynthesis
During the Archean, a new group of organisms arose that would dramatically change the planet’s atmosphere, these are called the cyanobacteria. As the first single-celled organism able to photosynthesis, cyanobacteria convert carbon dioxide (CO2) into free oxygen (O2). This allows microbial organisms to acquire carbon directly from atmospheric air or ocean water. Photosynthesis, however, required the use of sunlight or photons, which prevents these organisms living permanently in the dark. They would grow into large “algal” blooms seasonally on the surface of the oceans based on the availability of sunlight. Able to live in both oxygen-rich and anoxic environments, they flourished. The oldest macro-fossils on Earth are fossilized “algal” mats called stromatolites, which are composed of thin layers of calcium carbonate secreted by cyanobacteria growing in shallow ocean waters. These layers of calcium carbonate are preserved as bands in the rocks, as some of Earth’s oldest fossils. Microscopically, cyanobacteria grow in thin threads, encased in calcium carbonate. With burial, cyanobacteria accelerated the decrease of carbon dioxide from the atmosphere, as more and more carbon was sequestrated into the rock-record as limestone, and other organic matter was buried over time. The first appearance of free oxygen in ocean waters led to a fifth group of organisms to evolve, the iron-oxidizing bacteria, which use iron (Fe). Iron-oxidizing bacteria can use either iron-oxide Fe2O3 (in the absence of oxygen) or iron-hydroxide Fe(OH)2 (in the presence of oxygen). In the presence of small amounts of oxygen, these iron-oxidizing bacteria would produce solid iron-oxide molecules, which would accumulate on the ocean floor, as red-bands of hematite (Fe2O3). Once the limited supply of oxygen was used up by the iron-oxidizing bacteria, cyanobacteria would take over, resulting in the deposition of siderite, an iron-carbonate mineral (FeCO3). Seasonal cycles of “algal” blooms of cyanobacteria followed by iron-oxidizing bacteria would result in yearly layers (technically called varves or bands) in the rock record, oscillating between hematite and siderite. These oscillations were enhanced by seasonal temperatures, as warm ocean water holds less oxygen than colder ocean waters, hence the hematite bands would be deposited during the colder winters when the ocean was more enriched in oxygen. These bands of iron minerals are common throughout the Archean, and are called Banded Iron Formations (BIFs). Banded Iron Formations form some of the world’s most valuable iron-ore deposits, particularly in the “rust-belt” of North America (Michigan, Wisconsin, Illinois, and around the Great Lakes). These regions are places where Archean aged rocks predominate, preserving thick layers of these iron-bearing minerals.

The Great Oxidation Crisis
Around 2.5 to 2.4 billion years ago, cyanobacteria quickly rose as the most dominant form of life on the planet. The ability to convert carbon dioxide (CO2) into free oxygen (O2) was a major advantage, since carbon dioxide was still plentiful in the atmosphere and dissolved in shallow waters. This also meant that free oxygen (O2) was quickly rising in the Earth’s atmosphere and oceans, and quickly outpacing the amount of oxygen used by iron-oxidizing bacteria. With cyanobacteria unchecked, photosynthesis resulted in massive increases in atmospheric free oxygen (O2). This crisis resulted in the profound change in the Earth’s atmosphere toward a modern oxygen-rich atmosphere, resulting in the loss of many anoxic forms of life that previously flourished on the planet. The Great Oxidation Crisis was the first time a single type of life form would alter the planet in a very dramatic way and cause major climatic changes to the planet. The Banded Iron Formations disappeared, and a new period is recognized around 2.4 billion years ago, the Proterozoic Era.

The Ozone Layer
An oxygen-rich atmosphere in the Proterozoic resulted for the first time the formation of the ozone layer in the Earth’s atmosphere. Ozone is where three oxygen atoms are bonded together (O3), rather than just two (O2). This results from two of the oxygen atoms sharing a double covalent bond and one of these oxygen atoms sharing a coordinate covalent bond with another oxygen atom. This makes ozone highly reactive and corrosive as it easily breaks to form a single ionized atom of oxygen (O-2) which quickly bonds to other atoms. Oxygen gas (O2) is much more stable as it is made up of two oxygen atoms joined together by a double covalent bond. Ozone has a pungent smell, and is highly toxic because it easily oxidizes both plant and animal tissue. Ozone is one of the most common air pollutes in oil and gas fields, as well as large cities, and a major factor in air quality indexes.

Most ozone, however, is found high up in the Earth’s atmosphere, where it forms the ozone layer between 17 and 50 kilometers above the surface of the Earth, with highest concentration of ozone about 25 kilometers in altitude. The ozone is created at these heights in the atmosphere through the complex interaction with Ultra-Violet (UV) electromagnetic radiation from the sun. Both oxygen and ozone block Ultra-Violet (UV) light from the sun, acting as a sun-block for the entirety of the planet. Oxygen absorbs ultraviolet rays with wavelengths between 240 and 160 nanometers, this radiation results in breaking oxygen bonds, and results in the formation of ozone. Ozone can further absorb ultraviolet rays with wavelength between 200 and 315 nanometers, and most radiation smaller than 200 nanometers are absorbed by nitrogen and oxygen, resulting in oxygen and ozone blocking more incoming electromagnetic radiation in the form of high-energy UV light.

With oxygen’s ability to prevent incoming UV sunlight to reach the surface of the planet, oxygen had a major effect on Earth’s climate. Acting like a large absorbing cover, oxygen blocked high energy UV light, and as a consequence Earth’s climate began to drastically cool down. Colder oceans increased the absorption of oxygen into the colder water, resulting in well oxygenated oceans during this period in Earth’s history.

A new group of single-celled organisms arose to take advantage of increased oxygen levels, by developing aerobic respiration, using oxygen O2 as well as complex organic compounds of carbon, and respiring carbon dioxide (CO2). These organisms had to consume other organisms in order to find sources of carbon (and other vital elements), allowing them to grow and reproduce. Because oxygen levels likely varied greatly, these single-celled organisms could also use a less-efficient method of respiration in the absence of oxygen, called anaerobic respiration. When this happens, waste products such as lactic acid or ethanol are also produced in addition to carbon dioxide. Alcohol fermentation uses yeasts which convert sugars using anaerobic respiration to produce alcoholic beverages containing ethanol and carbon dioxide. Yeasts and other more complex single-celled organisms began to appear on Earth during this time.

Single celled organisms became more complex by incorporating bacteria (Prokaryotes), either as chloroplast that could photosynthesize within the cell or mitochondria that could perform aerobic respiration within the cell. These larger more complex single-cellular lifeforms are called the Eukaryotes and would give rise to today’s multicellular plants and animals.

An equilibrium or balance between carbon dioxide consuming/oxygen producing organisms and oxygen consuming/carbon dioxide producing organisms existed for billions of years, but the climate on Earth was becoming cooler than any time in its history. More and more of the carbon dioxide was being used by these organisms, while oxygen was quickly becoming a dominant gas within the Earth’s atmosphere, blocking more of the sun’s high energy UV light. Carbon was continually being buried either as organic carbon molecules or calcium carbonate, as these single-celled organisms died. This resulted in the sequestration or removal of carbon from the atmosphere for long periods of times.

The Cryogenian and the Snowball Earth
About 720 million years ago, the amount of carbon dioxide in the atmosphere had dropped to such low levels that ice sheets began to form. Sea ice expanded out of the polar regions toward the equator. This was the beginning of the end of the Proterozoic, as the expansion of the sea ice reflected more and more of the sun’s rays into space with its much higher albedo. A tipping point was reached, in this well oxygenated world, where ice came to cover more and more of the Earth’s surface. This was a positive feedback as expanding ice cooled the Earth by raising its albedo, and resulting in runaway climate change. Eventually, according to the work and research of Paul Hoffman, the entire Earth was covered in ice. An ice-covered world or snowball Earth effectively killed off many of the photosynthesizing lifeforms living in the shallow ocean waters, as these areas were covered in ice preventing sunlight penetration. Like the ice-covered moon of Jupiter, Europa, Earth was now a frozen ice planet. These great glacial events are known as the Sturtian, Marinoan and Gaskiers glacial events, which lasted between 720 and 580 million years ago. From space, Earth would appear unhabituated and covered in snow and ice.

The oxygen-rich atmosphere was effectively cut off from the life-forms that would also draw down the oxygen and produce carbon dioxide. Life on Earth would have ended sometime during this point in its history, if it were not for the active volcanic eruptions which continue to happen on Earth’s surface, re-releasing buried carbon back into the atmosphere as carbon dioxide. It is startling to note that if carbon dioxide had been completely removed from the atmosphere, photosynthesizing life, including all plants would be unable to live on Earth and without the input of gasses from volcanic eruptions, Earth would still likely be a frozen nearly life-less planet today. Levels of carbon dioxide slowly increased in the atmosphere (an important green-house gas) and these volcanic eruptions slowly thawed the Earth from its frozen state and the oceans became ice free. Life survived, resulting in the first appearance of multicellular life forms, and the first colonies of cells, with the advent of jelly-fish and sponge-like animals and the first colonial corals found in the Ediacaran, the last moments of the Proterozoic and the early diversification of multicellular plants and animals in a new era, the era of multicellular-life, the Phanerozoic.

Today, carbon dioxide is a small component of the atmosphere, making up less than 0.04% of the atmosphere, but carbon dioxide is rising dramatically just in the last hundred years, to levels above 0.07% in many regions of the world, nearly doubling the amount of carbon dioxide in the Earth’s atmosphere in a single human lifespan. A new climatic crisis is facing the world today, one driving by rising global temperatures and rising carbon dioxide in the atmosphere.