THE CREATION OF AN OXYGEN ATMOSPHERE.
The previous article looked at what Photosynthesizers did with the Carbon they extracted from the atmosphere and mentioned the release of oxygen into the atmosphere only in passing. This article explores the latter issue more thoroughly. Unfortunately, it is not known which Photosynthesizer played the main role in the creation of an oxygen atmosphere nor is it known what the rate of build up of oxygen in the atmosphere has been over time. If it was known which Photosynthesizer played the main role in creating an oxygen atmosphere this would help to determine when oxygen came to predominate in the atmosphere. There are a number of theories about the emergence of an oxygen atmosphere.
No Oxygen during the Hadean (4.6-3.7 bya).
The Earth started off with only traces of oxygen in the atmosphere.
The Archean (3.8-2.5 bya).
Lovelock believes the archean period marks the first appearance of life on Earth. One commentator believes that Photosynthesizers may have appeared very early in the Earth's history, 3.8bya. Most commentators believe there was little free oxygen in the atmosphere during this period, "The conclusion seems inescapable: that the early Earth lacked oxygen, and that oxygen slowly built up, over at least a billion years."; "Before two billion years ago there was little oxygen in the atmosphere ..."
Cyanobacteria 2.5 bya.
The main Photosynthesizer in the Earth's early history was Cyanobacteria. This anaerobic Bacteria, a prokaryotic life-form, extracted Carbon dioxide from the atmosphere and released oxygen back into the atmosphere, "With the evolution of oxygen releasing metabolism by cyanobacteria came the stromatolites. With the stromatolites come other precambrian evidence for the transition to the oxidizing atmosphere." Cyanobacteria derived the hydrogen needed for Photosynthesis from hydrogen sulphide, "To reduce Carbon dioxide from the air into the hydrogen rich Carbon compounds of cells, microbes needed a source of electrons. An excellent early source of electrons was gaseous hydrogen .." 
Cyanobacteria produced strange
fossil structures called stromatolites, "Stromatolites .. turned out to be the
work of bacterial communities dominated by Photosynthesizing cyanobacteria,
which form felt like microbial mats covering the surface of shallow water sediments.
The microbes secrete a tacky gel that protects them from ultra-violet radiation
and environmental contaminants. But the gel also causes sediment to stick to
the microbes, and when the ensnared sediment grows so thick that it dims the
light, the community creeps sunward and a new microbial mat starts to build,
and then another, and so on. Stromatolites are trace fossils, byproducts of
life."
A number of commentators believe that Cyanobacteria were responsible for creating an oxygen atmosphere, "Prokaryotic microbes (formerly known as blue-green algae, cyanobacteria) were almost certainly responsible for the original transition to the oxygen-containing atmosphere about 2000 million years ago ..."; "By this slow, inexorable process of photosynthesis (by blue-green bacteria) the early atmosphere was modified; gasp by gasp, oxygen was added, and carbon dioxide commensurately reduced. It was life processes that shaped the atmosphere, that paved the way for other more advanced organisms."; "The world became oxygenated enough through the Photosynthesizing mats .. about 2,000 million years ago."; "While we know that oxygen began to accumulate in the Earth's atmosphere around 2 billion years ago, we do not know how much there was. There could have been plenty, as today, or only a little."
Cyanobacteria depositing Iron 2.5-1.8 bya.
It is believed that Cyanobacteria were responsible for laying down vast quantities of iron in banded iron formations (bifs), "Bifs are marine rocks in which layers of iron-rich sediment alternate with iron-poor layers of other minerals." These alternate layers suggest the level of atmospheric oxygen varied over time. When the non-iron layers were being laid down there was some oxygen in the atmosphere, but when iron was being deposited the level of oxygen must have fallen, "But what kind of photosynthesis were the organisms using? A vital marker is the estimated 600 trillion tons of iron ore composing the banded iron formations (bifs) that are today Earth's key commercial source of iron. Only an alien Earth nearly free of molecular oxygen could lay them down. This is because reduced iron (oxygen-free) will dissolve in seawater, but oxidized iron - familiar rust - will not dissove and will precipitate rapidly. Bifs indicate a cycle of activity, in which some possibly seasonal photosynthesizing agency turned up the oxygen supply, and rusted the iron dissolved in the primitive seas. Most of our bifs were deposited between 2.5 and 1.8 billion years ago ..."
Oxygen begins to accumulate 1.5 bya.
It was only when all the iron had been deposited on the ocean floor that it was possible for oxygen to build up in the atmosphere, "Once the reduced iron ran out, there was no other chemical sink big enough to hold the continuing biological surge of oxygen and this gas built up, first dissolved in water and then escaping into the atmosphere." This suggests the build up of oxygen couldn't have started until after 1.8 billion years when there was much less iron to be deposited. Some commentators argue there was a significant level of oxygen in the atmosphere one and a half billion years ago, "By 1500 Ma ago, atmospheric oxygen may have risen to 15% of its present level."
Bacteria could have removed all the Carbon from the Atmosphere.
Loveock has argued that even in the early part of the Earth's history Bacteria were so plentiful they could have removed all the Carbon from the atmosphere within a few million years, "The growth of Photosynthesizers would have reduced the atmospheric abundance of this gas (CO2) and consequently the protective warmth of the gaseous greenhouse. A biosphere one hundredth as active as the one today could have removed all of the Carbon dioxide in a few million years." However, in other places he seems to indicate the opposite, "Unicellular life was hardly vigorous enough to bury the larger quantities of Carbon needed to sustain a high level of oxygen by counteracting its rapid removal by reaction with the rocks." The reason that Cyanobacteria couldn't extract all Carbon from the atmosphere was because other Bacteria, such as fermenters, released Carbon back into the atmosphere. There has never been a time when Cyanobacteria were able to outpace
Photosynthesizing Protoctists emerge 1bya.
Bacterial Photosynthesizers which derived hydrogen from hydrogen sulphide eventually helped to create more complex Photosynthesizers deriving hydrogen from water. They were Protoctists, the first eukaryotic forms of life on Earth. Whilst Bacteria do not have a nucleated cell, Protista do, "Protoctists - all the eukaryotes that are neither Animals, plant, nor fungi .. nearly all phytoplankton are protoctists ..."; "Familiar protocists include amoebae, euglenas, ciliates, diatoms, red seaweeds, and all other algae, slime molds, and water molds. Unfamiliar protoctists have strange names: foraminifera, heliozoa, ellobiopsids and xenophyophores." Many types of Protoctist acquired the ability to carry out Photosynthesis. It is believed that Protoctists began to emerge about 1 billion years ago .. "a burst of evolution that gave rise to groups that include red and brown algae, chromophytes (such as diatoms and yellow-brown algae), green algae, fungi, and animals. Estimates based on the "molecular countback" system suggest that this surge took place 1 billion years ago, and the fossil record seems to confirm this."Prokaryotic Photosynthesizers were anaerobic and would have been poisoned by oxygen. It has been argued that it wasn't until the emergence of eukaryotic Photosynthesizers capable of protecting themselves against oxygen that water could be used - however, this explanation doesn't explain how prokaryotic Photosynthesizers managed to survive when they extracted oxygen from Carbon dioxide. It is not known whether all Photosynthesizing Protista used water to provide hydrogen or whether this capability was acquired only by later types.
The Causes of Oxygen Accumulation in the Atmophere: Part I.
Some commentators believe the rise in the level of oxygen in the atmosphere was caused by Cyanobacteria. Others believe it was various Photosynthesizing Protoctists. Alternatively, it could have been the combined efforts of both - Cyanobacteria and Protista. There was an overlap between the heyday of Cyanobacteria and the emergence of Photosynthesizing protoctista. Cyanobacteria had been around since at least 2.5 bya, peaked about 1.5 bya and started to die out close to the Cambrian period, "Since about 1.5 billion years ago they (cyanobacteria) had flourished in tropical seas almost everywhere. Their steepening decline after about 680my ago ..."The fourth possibility is that it was only when Protoctists started using water to obtain hydrogen in Photosynthesis that oxygen began to accumulate in the atmosphere, "But now, as water replaced hydrogen sulphide as the largest reserve of electrons for photosynthesis, oxygen began to build up in the atmosphere."
Shell-forming Protoctists in the Cambrian (570-510 mya).
Some commentators believe that an oxygen atmosphere emerged only when Photosynthesizing Protoctists started developing shells. In the cambrian period, Protoctists used the Carbon acquired through Photosynthesis to grow shells. When the micro-organisms died the shells were deposited on the sea-floor and were eventually crushed into rock layers. This brought about the long term burial of Carbon thereby allowing the accumulation of oxygen in the atmosphere. Without the permanent burial of Carbon on the ocean beds there was always the likelihood that oxygen would once again react with atmospheric Carbon. Hsu believes this happened in the cambrian period 570-510mya, "The Cambrian explosion was an event when skeletal eukaryotes usurped the function of prokaryotes in removing greenhouse CO2 through CaCO3 precipitation."
Marine Plants - 439 mya.
The next set of Photosynthesizers were Plants, "The ancestors of all land plants are green algae. These photosynthesizers had certainly adapted to freshwater rivers, lakes, and ponds before the end of the ordovician (439mya), and perhaps much earlier. Freshwaters contained the carbon dioxide they needed, as well as phosphates and nitrates essential for constructing necessary biomolecules." Protista were the first to start using water in Photosynthesis then followed by Plants, "The essential difference is that in Plant Photosynthesis the hydrogen is obtained, not from hydrogen sulphide, but from the photolysis (breakdown using light energy) of water." It is possible that marine Plants were responsible for the build up of oxygen in the atmosphere. At the very least, they contributed to this build up.
Edicaran Faunas.
The emergence of eukaryotic Protista was followed by the emergence of edicaran faunas, "These are soft bodied ancestors of a wide range of animals that started secreting skeletons around the Cambrian-Precambrian boundary. Many of them were filter feeders and scavengers of the sea bottom; others were floating micro-carnivores. They include primitive arthropods, sea-pens, jellyfish, worms and sea-urchins." Although they were not Photosynthesizers they would have absorbed Carbon and thereby allowed oxygen to remain in the atmosphere.
Terrestrial Plants; The Greening of the Earth in the Devonian (409-363mya).
The greening of the Earth's terrestrial landmasses started in the silurian (439-409my ago) and was completed in the devonian period (409-363mybp). This boosted the scale of Photosynthesis and could have been responsible for initiating or boosting the accumulation of oxygen in the atmosphere.
The Rise of Oxygen in the Carboniferous (363-290mya)/Permian (290-245mya).
It is believed there must have been an increase in atmospheric oxygen during the carboniferous/permian periods when the first Trees began to decay into swamps. Vast quantities of Carbon were buried and eventually turned into coal deposits, "The rise is caused by the removal of organic carbon buried in the coal levels that built up during much of the permian just as they had done in the late carboniferous."
The Decline of Oxygen during the Permian-Triassic (245-208mya).
There was a sharp decline in the level of atmospheric oxygen at the permian-triassic boundary as a result of coal seams being exposed to the air and releasing Carbon into the atmosphere, "Tony hallam and paul wignall believe that falling sea levels in the permian exposed huge areas of land to chemical weathering. In particular they think that the organic carbon previosuly buried in coal deposits reacted with atmospheric oxygen to produce a great increase in the volume of carbon dioxide, and of course to reduce the volume of free oxygen. Oxygen isotopes measured in the same samples also show an abrupt peak in the late permian followed by the same sharp decline just before the P-Tr boundary and lasting well into the triassic. It has been estimated that the level of atmospheric oxygen fell from a normal value of about 30% to as little as 15%. With so much less oxygen left in the air, the supply to the sea was inevitably reduced, and this is recorded in deposits such as black shales and pyrites, which can only be formed in the absence of oxygen.
The Rise of Atmospheric Oxygen due to Large Plants and Animals.
Lovelock believes the concentration of atmospheric oxygen remained at 1% for most of the Earth's early history and increased dramatically only with the arrival of large Trees and Animals, "The proterozoic period .. is poorly understood. Scientists know that for the most part it was a world of micro-organisms like the archean. Unicellular life was hardly vigorous enough to bury the larger quantities of Carbon needed to sustain a high level of oxygen by counteracting its rapid removal by reaction with the rocks. I think that oxygen did not increase much above 1% until the evolution of large plants and Animals." The first large Plants started to appear in the carboniferous period (363-290mya) whilst the first large Animals could be the dinosaurs in the triassic.
The High Level of Atmospheric Oxygen.
Lovelock and margulis believe that the level of atmospheric oxygen has remained high for the last few hundred million years. .. "it is highly unlikely that current concentrations of oxygen have fallen much below their present values in some hundreds of millions of years." Margulis believes that microbes stabilized the level of oxygen in the atmosphere. If oxygen declined too much the more complex Animals would have died off, whilst if it rose too much there would have been global Forest fires, "Thus the quantity of oxygen in the atmosphere must have remained relatively constant since the time that air-breathing Animals have been living in Forests - which has been over 300 million years."
The Causes of Oxygen Accumulation in the Atmosphere: Part II.
In part one above it was suggested that four factors have been suggested could be responsible for the rise of an oxygen atmosphere. There are four more factors:-- The fifth is when Photosynthesizing Protoctists started developing shells;
- the sixth is the emergence of marine Plants;
- the seventh is the emergence of terrestrial Plants;
- the eighth is the emergence of large Trees;
- The ninth is the emergence of large Trees and large Animals.
