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The world is heading for an environmental catastrophe. This essay will focus on the belief of some leading scientists and environmentalists that we are in the process of a sixth mass extinction due to global warming and its acidifying effects on the world’s oceans. Firstly, it will focus on the previous five mass extinctions, from the first one 434 million years ago that wiped out 60 percent of all genera according to fossil records; to the last one at the end of the cretaceous period, 65 million years ago – famous for wiping out the dinosaurs. Then it will introduce the reason some scientists now agree was a leading factor in all five previous mass extinctions – cyanobacteria, commonly known as blue-green algae. It will then go into more detail on cyanobacteria, what they are, how they work and why they are significant contributors to mass extinctions and why scientists believe they will be responsible for a sixth mass extinction. The beneficial properties of cyanobacteria will be discussed also. It will then explain what stromatolites are and why they were the focus of scientific studies. A brief summary of the hypothesis of John Rodgers and James Castle, of Clemson University, and the conclusion they reached following their two year study will be given. Finally, the article will discuss Thermohaline Circulation also known as the Global Oceanic Conveyor Belt. Its purpose and a description of how it operates will demonstrate its importance in keeping the world alive and the consequences of rising global temperature.
Mass extinctions are nothing new; the average longevity of any species in the geological record is ten million years (Botkin, Keller, 2003). Approximately, 99 percent of all species that have ever existed have become extinct. It is known that there have been at least five mass extinction events in the last 540 million years. The first we know about was at the end of the Ordivian period, 434 million years ago. According to fossil records 60 percent of all genera were wiped out. At the end of the Devonian period, 360 million years ago, a mass extinction of reefs occurred. At that time life on earth was sparse, there were no dinosaurs and plants had only just begun to put their roots in soil, however the reefs were abundant with life until the waters receded leaving a “perfectly preserved coral corpse” in the far north of Western Australia; this place is a haven for geological researchers (Ward, 2010). The third mass extinction event occurred around 251 million years ago at the end of the Permian period. It is estimated that 80 – 95 percent of all marine species became extinct. During the Triassic period, 205 million years ago, 50 percent of all marine life and 80 percent land quadrupeds were wiped out. The most famous, and recent, of all mass extinctions was the ‘death of the dinosaurs’ 65 million years ago at the end of the Cretaceous period. Virtually, no land animal survived. Plants and tropical marine life decimated and oceans flooded over 40 percent of continents. The global temperature at this time was 5ºC – 15ºC warmer than today’s and sea levels were 200m higher than current levels (Miles, 2003) “Each mass extinction event corresponds with periods of quickly changing atmospheric CO²” (Veron, 2008). Scientists believed for many years that these extinctions were the result of meteor strikes, asteroid impacts or massive volcanic eruptions that would change the atmosphere temporarily, but suddenly, by the distribution of pollutants into the atmosphere or dust clouds drowning out sunlight, however, in the last ten years ideas are changing. Scientists say “current environmental conditions show significant similarities to times when previous mass extinctions occurred and warned that high levels of toxic algae are increasing” (Alleyne, 2009).
Scientific research suggests that cyanobacteria were a significant factor in all five previous mass extinctions. Cyanobacteria are often referred to as blue-green algae. They obtain their energy from the sun. They were the first microbial life form to produce oxygen. Oxygen is a bi-product of the photosynthesis process created by solar energy absorption. They are therefore responsible for life on our planet (Siegal, 2003). Cyanobacteria are probably the oldest surviving living species on earth; fossil records show their existence up to 3.5 billion years ago. Cyanobacteria are an incredibly important part of the life cycles of many aquatic plants and animals; they are responsible for much of the ocean's marine nitrogen cycle. Found naturally in surface waters, they thrive in warm, stagnant water where there is little or no oxygen. Rises in temperature in both the water and the atmosphere encourage the growth of Cyanobacteria. “Warmer weather has created longer growing seasons, enabling cyanobacteria to grow in Northern waters previously too cold for their survival” (Pearl, 2008). They reproduce and ‘blooms’ occur; when this happens, fish and other aquatic animals and plants have little chance of survival (Pearl, 2008). The toxins they produce, normally in small amounts, increase as the bloom grows, poisoning the atmosphere. The bloom growth gradually crowds the surface of the water, preventing natural light from reaching the plants below, which then die and subsequently leave fish with no food so they starve. When the algae die, they sink to the bottom of the ocean and decompose, causing extensive oxygen depletion. If global temperatures continue to rise, more algal blooms will be produced.
The presence of cyanobacteria prior to the first mass extinction event has been proven in the examination of stromatolites. Stromatolites are basically mounds of mud and silt that have been preserved (Burkhalter, 2003). They are also referred to as ‘algal mounds’ and ‘microbial mats.’ They accumulate when cyanobacteria produce a layer of sticky film that traps the mud and silt. Gradually, the layers build up in a process that can take thousands of years. A fossilized stromatolite has flaky layers and mineral deposits are commonly found between the layers providing clues to their age and the environmental conditions at their formation (Reid, 2008). Stromotalites were once only found in fossilized form, but the discovery of a living colony in Shark Bay, Australia caused intense interest in the geological research world. The area has been designated a world heritage site because of its uniqueness, (Smith, 2010). Modern marine stromatolites are living examples of one of the Earth’s oldest ecosystems; fossil and geological records show their existence over 3.4 billion years ago.
Geologist, James Castle, and ecotoxicologist, John Rodgers of Clemson University, spent two years examining and analysing data from ancient stromatolite structures. Their studies found evidence that cyanobacteria were present in sufficient quantities to kill of numerous plants and animals. Their study focused mainly on cyanobacteria because of its predominance in producing toxins in modern environments; however, they do note that other types of algae produce toxins that can cause death to higher organisms. It is difficult to prove that these other algae played a part in previous mass extinction events due to the lack of fossils to examine. Cyanobacteria are much more available, both in living and fossil form, to study. In order to test their theory the scientists studied chronological distribution of cyanobacteria at times of mass extinctions and the effects of toxins produced by modern cyanobacteria. Their research data shows that cyanobacteria produce quantities and types of toxins sufficient to cause mass extinctions. They conclude their paper, entitled “Hypothesis for the role of toxin producing algae in Phanerozoic mass extinctions, based on the evidence from the geological record and modern environments,” by stating that they anticipate further increases in blooms with “more frequent and persistent production of potent toxins” and that rising global temperatures will lead to an increase in extinction rates. They say their hypothesis, “gives us cause for concern and underscore the importance of careful and strategic monitoring as we move into an era of global climate change”(Rodgers J, Castle J, 2003). Their published hypothesis is available to view online, please refer to bibliography for details.
The Earth’s temperature is regulated by the oceans. They drive the weather and climate pattern around the globe. They do this by the use of currents; surface currents redistribute heat around the globe. Ocean circulation is a complex circulation comprised of different oceanic currents, which create an underlying transport pattern. Water cycles from surface currents to deep water currents then back to the surface again, scientists liken this process to a giant conveyor belt; they call this Meridional Overturning Circulation (Schiele, 2010). Wind, the rotation of the Earth and water density drives this circulatory system. Temperature affects water density, the colder and saltier the water, the denser it becomes. As water becomes denser it sinks. Cold water holds more oxygen so as the cold water sinks it delivers oxygen to the depths of the ocean. This density driven circulation is called thermohaline circulation. Without this density driven process, deep water currents would not be created and the Global Oceanic Conveyor Belt would halt. Mass extinction would occur if the conveyor stopped, and the way to ‘stop’ the conveyor is to simply remove the difference in temperature (Ward 2010). If the conveyor stops the ocean becomes still, water becomes warmer and holds less oxygen. cyanobacteria would thrive and eventually overtake the oceans. As the cyanobacteria die and sink to the bottom of the ocean and decompose, hydrogen sulphide is produced which then rises in huge bubbles to the surface and bursts into the atmosphere, poisoning land animals and plants.
In conclusion, scientific studies show that the world is indeed heading for an environmental catastrophe, due to rising global temperature. If there is no difference between the temperature at the poles and the temperature in the tropics thermohaline circulation would cease, causing a massive chain reaction ultimately ending the majority of life on the planet. The oceans would be purple in colour and the sky green. The only life forms surviving would be microbial – as Pearl puts it, “it’s ironic, without cyanobacteria we wouldn’t be here. Animal life needs the oxygen the algae produce. These algae that were first on the scene will probably be the last to go.”
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Botkin, D, Keller, E, 2003. Environmental Science: Earth As A Living Planet. 4th ed. London: Wiley & Sons [pg 142]
Burkhalter, R. 2003. Sam Noble Museum of Natural History, Oklahoma University. [ONLINE] Available at: http://www.snomnh.ou.edu/collections-research/cr-sub/invertpaleo/common_fossils_of_ok/paleobot/stromato.htm. [Accessed 29 January 11].
Castle J, Rodgers J, 2009, Hypothesis for the role of toxin producing algae in the Phanerozoic mass extinctions, based on the evidence from the geological record and modern environments [ONLINE] www.clemson.edu/mediarelations/files/articles/2009/2336_295_mass_extinctions.pdf
[Accessed 29 January 2011]
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Schiele, E. 2010. Ocean Motion: Impact: Ocean Conveyor Belt. [ONLINE] Available at: http://www.oceanmotion.org/html/impact/conveyor.htm. [Accessed 05 October 2010].
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Smith S. 2010. Wisegeek – What is a Stromatolite. [ONLINE] Available at: http://wisegeek.com/what_is_a_stromatolite. [Accessed 29 January 11].
Veron, J.E.N, 2008. Mass extinctions and ocean acidification: biological constraints on geological dilemmas. Coral Reefs, [ONLINE]. Vol 27, Available at: http://adsabs.harvard.edu/abs/2008CorRe..27..459V [Accessed 06 October 2010].
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