Australia: The Land Where Time Began
How Life Controls the
Atmosphere - Bacteria made the world what it is, and keep it
Bacteria are the most important life form in the control of the Biosphere, and always have been since they first arose (Margulis & Sagan). It has been estimated that in ideal growing conditions a single cyanobacterium could theoretically produce all the oxygen in the present-day atmosphere in a few weeks.
Life has been having an effect on the atmosphere since shortly after it evolved, since about 4 billion years ago (4 Ga). It has been suggested that the CO2 levels in the atmosphere of the pre-life earth was about the level that presently exists in the Martian atmosphere, about 93 %, the present level in Earth's atmosphere is 0.03 %. It is believed that at the time the Earth formed the Sun was only about 30 % as bright as the present level, the Earth depending on the high levels of CO2 in the atmosphere to prevent it from freezing once the crust had cooled. The temperature of the earth would then have increased as the Sun heated up until it reached temperatures that would not support life. If the CO2 level in the atmosphere remained unchanged life would have been prevented from gaining a foothold.
The mechanism that lowered the CO2 levels is uncertain, but one suggestion by Vaclav Smil is that it was taken out of the atmosphere by the formation of carbonates. According to this suggestion, as the temperature rose the amount of water evaporation increased. The increasing amounts of water in the atmosphere formed large amounts of carbonate by reacting with the CO2 to form H2CO3, carbonic acid. The calcium silicate of exposed rocks surfaces were weathered by the carbonic acid, resulting in the release of calcium and bicarbonate ions that eventually reached the oceans in runoff from the land where reactions in the oceans led to the carbonate pecipitate, CO2 and water. Large and increasing amounts of carbon were effectively locked up in the sediment. This process can only occur in shallow seas, that were widespread in the early Earth, as below a depth of about 2 km the pressure gets too high for the calcium carbonate to precipitate, as it dissolves at such high pressures. The CO2 is recycled over a very long period, the ocean crust taking much of the carbonate back to the depths of the Earth as the crust is subducted beneath continents, at which point some is scraped off the top of the plate, being buckled up into mountain ranges. The crust that is subducted is eventually melted, then reacts with silica, part being returned to the surface at volcanoes along the plate margins. An example is the Pacific Ring of Fire. The recycling takes many millions of years to complete. The volcanoes along the subduction zones vent large amounts of CO2 to the atmosphere, but carbonate rocks are continually being formed, which takes CO2 out of the atmosphere.
Once life evolved it contributed to the locking up of CO2 in carbonates. In the hot conditions of the early Earth the only life that could survive was the thermophilic Archaea, the earliest ancestors of life on Earth. This was a time of active tectonism, so large quantities of the CO2 were being removed from the atmosphere as the plates were subducted. The temperature eventually dropped enough for other forms of life to survive as the atmospheric CO2 level dropped. It is believed that about 2.8 billion years ago the Sun was about 20 % cooler than at the present, which leads to the belief that it was about 30 % as bright at the time life began. It is believed that the temperature was probably prevented from falling too low for life by the methane produced by the methanogenic sulphur bacteria.
When the temperature dropped low enough, bacteria that photosynthesised by splitting water molecules replaced the Arhaea in the non-extreme environments that became available, such as the oceans. For some time after the photosynthetic bacteria of the oceans began excreting oxygen as a waste product the oxygen was mostly used up in the oxidation of material such as the dissolved iron in the oceans. It was only after most of theses oxidised substances like iron had precipitated out of solution that oxygen began to be released into the atmosphere and began accumulating.
It has been suggested that the activities of these first photosynthisers led to the first ice age, about 2.4-2.3 million years ago (Dr Kasting). According to this suggestion, the oxygen reacted with methane in the atmosphere to produce carbon dioxide and water. Because methane is more than 20 times more efficient as a greenhouse gas than carbon dioxide, replacing it with carbon dioxide reduced the heat trapping ability of the atmosphere, resulting in global cooling. The temperatures are suggested to have continued falling, the spread of glaciers intensifying the cooling by reflecting much of the sunlight back into space, until the carbon dioxide of the atmospheric levels increased enough as a result of greenhouse warming.
Over the next billion years, cyanobacteria continued producing oxygen and depleting the atmospheric carbon dioxide, until about 900 million years ago, when the second ice age began as a result of low levels of carbon dioxide in the atmosphere. This Precambrian ice age continued, with fluctuating intensity, for almost 300 million years. From about 750 to 500 Ma the Neoproterozoic glaciation was the most severe part of the ice age.
It has been suggested that during the coldest part of this global glaciation life may have persisted on the ocean floor around volcanoes and thermal vents until the conditions improved enough for life to flourish in the other parts of the ocean. By that time life had reached the stage of filamentous algae and unicellular protists. It is the scattered nature of these volcanoes and submarine geothermal vents that has been proposed as a source of the genetic diversity that led to the explosion of life forms when the glacial period ended, with each isolated ecosystem around the sources of warmth on the seafloor developing in isolation, producing organisms that evolved in different directions at each hotspot.
It has been proposed that the ice age of about 300 Ma, in the Late Carboniferous to Early Permian, may have been caused by the rapid rise of vascular land plants in the Early Carboniferous (Vaclav Smil). According to this proposal, about 350 Ma the very large amounts of plant material, that was to become the Northern Hemisphere coal deposits of the present, were accumulating in swamps, which took large amounts of CO2 out of the atmosphere, triggering the cooling that produced the ice age. According to Mary White, basing her view on data from Gondwana, even allowing that the formation of the coal played a part in the cooling, the mechanisms were more complex than that alone. According to White, during the ice age Australia was near the South Pole, and glaciers covered about half of the continent. There were no coal deposits formed in the Carboniferous in Australia. The present burning of the fossil fuel that had sequestered the atmospheric CO2 leading to the cooling of the atmosphere is reversing the process, once more heating the atmosphere to temperatures with unknown and unpleasant consequences.
At the close of the ice age Glossopteris flora flourished all across Gondwana. It was then that the widespread coal measures of the Permian were formed, locking up CO2 on a similar scale to that of the Northern Hemisphere deposits. In the Late Permian the Australian climate was sub-tropical, and the vegetation flourished into the Triassic. White asks the question, if only coal formation determined the conditions leading to the ice age, why didn't the ice return or persist. During the Cretaceous, greenhouse conditions returned - the South Pole was sub-tropical in the Late Cretaceous. White suggests that the tectonic activity of the formation of the supercontinent at this time led to the increased atmospheric CO2 levels, the levels dropping as the tectonic activity decreased.
White claims that, "when the whole of the carbon cycle through time is reviewed, it is obvious that it is Life that has kept some sort of equilibrium in the carbon dioxide levels in the Biosphere".
The role of marine ecosystems
In the oceans, carbon dioxide is readily exchanged between the mixed and surface layers and the atmosphere, but not with the water of the deeper, stable layers, that comprise about 98 % of the water in the oceans, where the temperature usually remains between 2o and 4o C. Below the mixed layer most carbon exchange takes place as a result of the deep currents that move the deep water around the globe. In places where the nutrient-rich deep waters upwell through the carbon dioxide-rich surface water there are large phytoplankton blooms at these times. Such upwelling areas in the central and eastern Pacific support up to half of the total amount of plankton production, and are the largest source of carbon dioxide in the oceans. The phytoplankton in the surface layers consume vast quantities of carbon dioxide, but 90% of the carbon products circulate within the mixed layer, the remaining 10 % being precipitated as carbonate and settling to the seafloor in the form of dead organisms and detritus. The recycled 90 % of the carbon dioxide results from the respiration of organisms that eat the phytoplankton. Some escapes to the atmosphere, but much of it used up by the phytoplankton.
The total biomass of the phytoplankton is about 0.5 % of the biomass on land, but they consume at least half as much CO2 as the terrestrial photosynthesis. In the polar oceans the summer sunlight can allow photosynthesis for more than 18 hrs per day, making the polar oceans carbon dioxide sinks. Most of the carbon in the oceans is isolated from the terrestrial ecosystems. In the sediments of the ocean there is about 50 % more than the total in land plants and soils.
It has been estimated that without the functioning ecosystems in the oceans the amount of carbon dioxide in the atmosphere would treble within 1000 years (Vaclav Smil).
Census of Marine Life
The census began in 2000 with 2500 scientists from 80 countries taking part in the project that is to complete in 2010. Many new discoveries have already been reported, one of the most extraordinary is the discovery off the west coast of South America of a continuous bacterial mat about the size of the country of Greece. It is now estimated that the oceans contain between 50 and90 % of the total biomass of the Earth, much of it comprised of microbes such as bacteria and Achaea. As a result of the census the biodiversity of microbes has been increased 100-fold, with an estimate for the number of microbial species now being possibly about 1 billion.
A surprising finding is that many of the microbes have been found in relatively small numbers, the reason for the large number of microbial species that have been found to have only small numbers of individuals has been speculated about, but it is not known at the present why such a situation exists. Suggestions for their existence have been that they each provide some essential substance for the ecosystems, or that they are a reserve of biodiversity ready to expend in numbers when environmental changes require a different suite of microbes to fill the available niches. One thing there is no doubt about is that microbes rule the world. They play a pivotal role in recycling everything that is recycled. It is also known that they are responsible for the accumulation of many minerals into ore bodies that are now mined. So they have had a huge impact on all parts of 'Gaia', the distribution of minerals in the crust, recycling the gaseous components of the atmosphere and powering the food webs of the Earth.
In another part of the census, scientists working on Australia's Great Barrier Reef have found a single sponge that is host to nearly 3,000 different categories of bacteria. The latest report states that microbes make up 90% of the biomass of the oceans. The census has produced evidence confirming that microbes are essential to the functioning of ecosystems in all parts of the Earth, making them the basis of most life systems.
Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
|Author: M. H. Monroe Email: email@example.com Sources & Further reading|