User:Origins3fo3ch8/sandbox

Approximately 300 million years ago a supercontinent called Pangaea was the only landmass that existed on the earth and much of it was located in the Southern hemisphere. Around 200 million years ago the supercontinent broke up into two separate landmasses named Laurasia and Gondwana. During this time the location of Antarctica was closer to the equator than it was to the South Pole which resulted in Antarctica having a booming tropical life. This means that it was not always the cold and ice covered landscape that it is today. Gondwana was made up of present day South America, South Africa, India, New Zealand, Australia and Antarctica. Antarctica formed the core of Gondwana. Laurasia consisted of modern day North America, Europe and Asia. Gondwana’s climate was fairly mild and was a host to a large array of flora and fauna. There was also no ice sheet coverage over Gondwana either. Approximately 180 million years ago, in the middle of the Mesozoic era, Gondwana began its process of breaking up into South America, South Africa, India, New Zealand, Australia and Antarctica. Around 50 million years ago Antarctica started to make its way down to the South Pole progressively moving away from Australia and South America and ultimately arriving at its current position.

The locations of modern continents outlined in Pangaea

Overview of Continental drift

All of the landforms that make up the Earth are part of a series of large tectonic plates that have the ability to move very slowly over the Earth’s surface. The mechanism of this movement is widely argued. The origins of these plates are in mid-ocean ridges also known as constructive margins. At the constructive margin new material is made through the action of volcanoes, mainly through volcanic eruptions. When the new material is brought up from the mantle through volcanic activity, it will move to the side of the constructive margin resulting in a process known as sea floor spreading and ultimately leading to the development of a divergent boundary between two tectonic plates. The movement of the tectonic plates allows for different kinds of collisions to take place, thus forming unique boundaries. When two plates that are moving towards each other collide, either both or one will compress and buckle resulting in the development of large mountain ranges such as the Himalayas. If one of the two plates that are colliding is made up with oceanic crust it will sink under the other plate, which results in the production of a subduction zone. The subduction zone is where the crust will be taken back into the mantle and will be melted and taken back into the circulation of the mantle. A convergent boundary is a result of this type of collision. A transform plate boundary is created when plates move past each other and create a large amount of friction. This friction results in the production of earthquakes. The process of continental drift has resulted in the production and breakdown of many land masses over a large time period.

Break down of the Supercontinent Gondwana

The break of Gondwana started approximately 180 million years ago. As the supercontinent began to break apart new ocean crust was being produced which ultimately lead to the development of the Southern Ocean. This new ocean crust is developed when basalt lava erupts at the mid-ocean spreading centers. The basalt eruption allowed for the sea floor to spread at a rate of couple of centimeters a year, which allowed the Southern Ocean to develop in 150 million years approximately.

The supercontinent of Gondwana did not break apart all at once, as different parts departed at different times. The first landmass separation was seen with what is present day Africa and Antarctica. The separation process lasted approximately 10 million years and the earliest signs of this was seen when large quantities of lava was being ejected from Southern Africa and Antarctica around the 180 million year mark and they were fully separated from each other around 150 million years ago. The separation between the other continents of Africa and South America, and between Antarctica, India and Australia took less time comparatively. The last separation was seen in the Cretaceous Period around 96 million years ago, which saw the breakup of Australia and New Zealand from Antarctica.

The Separation Process

Map of Pangaea separation which formed Laurasia and Gondwana.

The break up and spreading of Gondwana has a dramatic effect on the plate tectonic structure of the Earth. The process of how the separation of supercontinents took place is widely debated in the field. There are two ideas in the field that have been proposed to explain the break up mechanism. The first is the passive mechanisms and the second is the active mechanism. In the passive mechanism, also known as the passive mantle flow mechanism, it is believed that through the sinking of the cooling ocean plates causes the continents to diverge away from the mid-ocean spreading ridges. Then the cooling ocean plates are taken up by the subduction zone. In the passive mechanism the plate movement is aided through the assistance of frictional drag force that is cause by the circulation of the mantle convection system. Scientists in the field argue that the mantle convection system does not provide enough force to allow for the separation of landmasses, which is where the second theory comes into place.

The second theory that has been proposed is an active mechanism that is causing the movement of the continents. In this theory the mantle plumes, also known as hotspots are thought to play a role. Mantle plumes can be thought of as a cylinder that rises up from the mantle-core boundary in the Earth and ends off with a large bulbous mushroom appearance and can have a diameter up 2000 km in size. The mantle plumes arise due to thermal disturbances that occur at the mantle-core boundary. The plumes have the ability to actively progress continental movement because it can generate large quantities of basalt magma, which will build up in the bulbous mushroom end. The build up will lead to an increase in pressure overtime and eventually the rupture of the bulbous end that results in basalt lava being released and causing sea floor spreading.

Recently researchers and scientist in the field are also proposing a model that includes both the active and the passive mechanism united in one process. The hypothesis states that the mantle convection current produces tension force on the continents when both sides of the continent are in the subduction zone. This tensional force is taken advantage of by the mantle plumes. When new mantle plumes arise where the tension is high, the eruption of a plume will lead to the weakening of the continent and ultimately the separation and production of a new ocean floor. Therefore leading to the production of two separate continents.

Evidence of Continental Drift in Antarctica

The purple line going through Southern Africa, India and Antarctica show the regions where Lystrosaurus fossils were discovered.

In 1969, the fossilized remains of species from the genus Lystrosaurus were discovered in a region of Antarctica known as Coalsack Bluff. This was a significant discovery for paleontologists and geologists as it was yet another piece of evidence for the theory of continental drift. Species belonging to Lystrosaurus lived during the Lower Triassic period on the supercontinent Gondwana. When Gondwana eventually split up, it led to the formation of the southern continents such as Antarctica, South Africa and India. The fossils discovered in Coalsack Bluff were very similar to fossils seen in South Africa, with the species Lystraosaurus murrayi being most prominent on both continents. Further excavations of the region also revealed the skeletal remains of another species, Thrinaxodon liorhinus, a species identical to the fossilized structures found in South Africa.

This begs the question: How can two continents in different parts of the planet contain fossils of two of the same species? After all, Africa and Antarctica share very different climates, vegetation and biodiversity. The answer lies in the fact that these two regions were once a part of the same continent. It is highly unlikely that the Lystrosaurus and Thrinaxodon species could have survived in modern day Antarctica and South Africa as the two regions contain extremely different climates. It is also unlikely that they could have evolved separately in these two continents through a process known as convergent evolution because this form of evolution usually results in two species resembling each other as a result to living in regions with similar conditions and similar selection pressures being placed on them.

The excavations were studied and Lystrosaurus and Thrinaxodon were found to have survived during the Lower Triassic era when Southern Africa and Antarctica were joined together in the supercontinent Gondwanaland. By joining the pieces together, these discoveries can be seen as further evidence of continental drift, and more importantly as clear evidence for the movement of Antarctica from a warm, species-rich environment into its current location where conditions are dry and cold with very little biodiversity.

Antarctica's Change from Warm to Cold Temperatures

As stated above Antarctica’s climate was not as cold as it is today. It has been shown to have a climate that was more tropical in nature, which is a complete opposite of the cold wasteland that its is now. What lead to this complete change in climate for Antarctica? Through the process of plate tectonics it has been shown that Antarctica departed from Gondwana and moved towards the south poles. This southern movement did somewhat contribute to the development of the colder climate and allowed for ice sheet accumulation to take place. However, it was not the sole cause for the change in climate. The major contributors to the cooling of Antarctica were the changes in the carbon concentrations of atmosphere and the changes in the ocean currents that resulted due to the movement of Antarctica. The cooling of the Antarctica is also a story of how the Earth as a whole began its cooling process.

Carbon Cycle

Carbon is a fairly abundant element on Earth and it is constantly being cycled from the atmosphere to the Earth over millions of years. The atmospheric form of carbon comes as carbon dioxide (CO₂). Carbon dioxide is a greenhouse gas that can trap heat on Earth, which would affect the climate. The carbon cycle in the simplified form starts when volcanic activity causes a release of carbon dioxide into the atmosphere. This large amount of carbon release into the atmosphere will interact with water vapor (H₂O), which will lead to the production of acidic rain also known as carbonic acid (H₂CO₃). When the acidic rainfall occurs on the Earth it will lead the degradation of the rock also known as chemical weathering. For example, if the acid rain interacts with Silicate rock (CaSiO₃) the chemical reaction leads to the production of Calcium Carbonate (CaCO₃). The ions and calcium carbonate will be transported to the ocean where plankton will incorporate it into their shells. Theses plankton will eventually become part of the sediment and therefore returning the carbon back to the Earth and completing the cycle. The Spreading Rate hypothesis and the Uplift Weathering hypothesis have been proposed to explain how changes in the carbon cycle have resulted in the cooling of Antarctica and the world in turn.

Spreading Rate Hypothesis

The scientists that proposed the Spreading Rate Hypothesis were Robert Berner, Antonio Lasaga, and Robert Garrels. It’s also called the BLaG hypothesis based off the last names of researchers that proposed it. This theory states that the climate change that was experienced by Antarctica and in turn the world in the last 100 million years or so was due to the changes in carbon dioxide concentrations in the atmosphere and oceans by the activity of plate tectonics. Majority of the carbon dioxide that is being released from the earth are by volcano’s that are located at the divergent and convergent plate boundaries. Changes in the rate of seafloor spreading will result in the changes in volcanic activity and therefore changes in the rate of CO₂ delivery to the atmosphere, which results in a change in the climate. For example, if there were a decrease in the rate of seafloor spreading that would result in a decrease in the rate of volcanic activity. The decreased volcanic activity would mean that less CO₂ is being pumped back into the atmosphere and this would mean a lower greenhouse effect and in turn lower climate temperature. The researchers hypothesized a decrease in the rate of seafloor spreading occurred and that resulted in the cooling of Antarctica and the world.

Uplift Weathering Hypothesis

The main concept of the Uplift Weathering Hypothesis is that chemical weathering is the driving force behind the climate cooling that was experienced. The activity and the rate of chemical weathering are mainly dependent on the availability of fresh rocks and mineral surfaces that are exposed. The activity of tectonic processes such as, development of steep slopes and mass erosion would lead to an increase in fragmentation of rocks and minerals increasing the vulnerability to chemical weathering. This increased chemical weathering activity would lead to an increased rate of CO2 removal from the atmosphere. The decreased CO2 concentration would mean a weaker greenhouse effect and ultimately cooler temperatures. This process can be seen with the rise of the Tibetan Plateau due to the collision between the India and Asia. As the plateau rose it created large amounts of rock for chemical weathering to take place and that acted as a sponge to absorb all large amount of CO2 leading to global cooling of Antarctica and the Earth.

The black line represents the Antarctic Circumpolar Current which moves in an eastward direction.

Ocean Currents

Continental drift has also affected the ocean currents which in turn have altered temperatures in Antarctica. When the supercontinents existed millions of years ago, there was only one large ocean called the Panthalassa Ocean. Today there are four officially recognized oceans: Pacific, Atlantic, Indian and Arctic. Ocean currents are of great importance as it has a huge impact on the climate of the planet. Ocean currents transfer heat around the planet with warmer waters moving towards the poles and heating the regions along the way while cooler, dense waters near the poles sink and move towards the equators in order to maintain global temperatures.

As the continent of Antarctica separated from Gondwanaland and moved southwards, it became an island at the South Pole that was surrounded by oceans on all sides. This led to the development of the Antarctic Circumpolar Current which is a large ocean current that surrounds Antarctica. This current connects to the Pacific, Atlantic and Indian Oceans and travels eastward. As a result it transfers all the heat being sent down towards Antarctica in an easterly direction and prevents the warm currents from reaching Antarctica. The led to the extreme cold temperatures seen in Antarctica today.

Effect on Human Vision

Cooling of the poles had an impact on how humans perceived the world visually. To understand how it affected humans would require turning to the closest primate relatives of the human species known as the Old World monkeys, which includes: gorillas, chimps and orangutans. All can perceive the same colours as humans because have Trichromatic colour vision similar to humans. Trichromatic colour vision is characterized by three retinal photopigments that respond to peak wavelengths of 430 nm (blue), 535 nm (green), and 562 nm (red) approximately. The ability of an animal to distinguish and process stimuli from its surrounding is paramount of the survival and reproductive successes; this is where trichromatic colour vision was beneficial for the Old World monkeys. Trichromatic colour vision gave the primates the ability to discriminate between red-green colours.

Research that was conducted by Dominy and associates showed that this unique characteristic was advantageous to them because it allowed for the detection of ripe fruits and younger leaves. Dominy and colleagues also identified that the leaves the primates were eating contained a higher amount of proteins and free amino acids, in addition to being softer to chew compared to other leaves that were around the primates. These characteristics first began to arise around 40 to 30 million years ago, coinciding with with the climate change seen in Antarctica and the world. This characteristic became very valuable when the climate started to change with Antarctica and the world. As the world started to cool plants responded to the cooling and there would have been a decrease in plant availability as well. Therefore animals found the trichromatic colour vision very valuable and were able to identify more nutritious leaves. This most likely resulted in better survival rates. The cooling was one of the selection pressures that maintained trichromatic characteristics in primates.

In many ways, continental drift led to many changes in the Earth’s climate. The movement of Antarctica, for instance, led to specific changes in global temperatures which in turn became one of many factors that affected evolution on a grand scale. The resulting evolutionary pressures allowed speciation to occur and eventually led to the development of the homo sapien race.