Dinosaur Extinction: How We Know

The Cretaceous- Paleogene boundary is evidence shown in the rock layers of a mass extinction that occurred approximately 65 million years ago. From the evidence left behind of this event, leave scientists debating theories on how this sudden and significant event in Earth’s history took place, and why it led to the extinction of so many species. From the 5 massive extinctions identified in Earth’s history, the extinction of the dinosaurs of dinosaurs was by far the deadly.

The most evidential theory of why this occurred, was an asteroid larger than Mt Everest, hitting where know today to be the Chicxulub, Gulf of Mexico. The string of events that followed the asteroid’s impact is what lead to the Cretaceous-Paleogene extinction event. It is the leading theory that the extinction of the dinosaurs, also known as the Cretaceous-Paleogene extinction event took place due to an asteroid hitting the earth 65 million years ago. This large mass was supposedly larger than today’s Mt Everest. This impact released a huge amount of energy, vaporising the matter nearby, with rippling shockwaves.

The dinosaurs near the impact were destroyed some buried alive. It is quite believable that a meteorite could hit the Earth at some point since its formation 4.6 billion years ago, but some struggle to understand how such an impact in one area of the planet could wipe out majority of life. It wasn’t so much the initial impact that killed the dinosaurs, but the series of domino effects that followed, which caused ecosystems globally to crumble. The impact of the asteroid caused tsunamis into the coastlines around Mexico, and started fires on land which was carried by winds. Matter was released into the atmosphere, some travelling to the moon and back into space, while some was pulled back into orbit, resulting in a devastating meteor shower all around the planet. The debris re entering the atmosphere was travelling at intense speeds and reaching extreme temperatures. This heat dries out vegetation, leading to firestorms. These meteorites and debris scatter and rain down around the planet for at least 4 days. We know this because of discovered rock identified from Mexico all around the world. A cloud of ash and soot blankets the sky, which resulted in a ‘endless night’, causing cold periods, killing organisms relying on sunlight to photosynthesize. With vegetation dying worldwide, herbivores died of starvation. Domino effects continue as the food chain collapses. With limited herbivores, carnivores suffer also.

The largests carnivores died of first, as they require a larger source of food. This extended period of darkness affected sea life also, and took 3 million years for oceans to recover. 10,000 billion tonnes of carbon was released from the fires. To put in perspective, that is 3000 years worth of our modern carbon release, so you can imagine the significant consequences this would have on the environment. So much so, leading into the various cycles of climate changes over next eras. A lot of the dust cloud was distributed by the winds globally to create the boundary, the boundary has since been affected in the past 65 million years. Tectonic activity such as earthquakes and volcanoes have shifted the boundary, as well as erosion and weathering. We have significant evidence of these events occuring, a combination of stratigraphic evidence and radiometric dating. This geological event is significant because it is one of this main 5 mass extinction events identified in Earth’s history. These events are due to change in environment and help geologists identify time periods and eras. Each time period or era ends in a climate/ weather, landscape change, resulting in a loss of biodiversity.

A new time period begins when fauna begin to adapt to the new environment and new ecosystems are redeveloped. There are 3 main geological eras in earth history; the Paleozoic, Mesozoic, and Cenozoic, complete with sub periods in each. As seen in the diagram, this mass extinction is called the cretaceous-paleogene extinction event because it was the event that ended the cretaceous period and began the Paleogene period (at ‘K-T’ boundary seen at this point in the diagram) and big enough to also begin the Cenozoic Era. The K-T boundary, now referred to the the KPg boundary (K short for Cenozoic, and Pg short for Paleogene), is a very significant piece of evidence of this extinction event. The Kpg boundary is useful because we can use stratigraphy to determine relative terms how long ago the event happened. Other sources of evidence such as fossils and their placement, radiometric dating help determine an accurate estimate about when and what happened during this period. Because both absolute and relative dating need to be used in conjunction to figure out an timeline of earth’s history. To analyse stratigraphic evidence, rocks are organised into several classifications. Rock classifications allow geologists to identify patterns and explanations for what has happened. There are 3 main categories of rock, with varying characteristics: Igneous rock is holey and less dense. This is because the igneous rocks are a product of volcanic activity, and the molten rock cooled with gas bubbles.

Metamorphic rock (root word meaning changing/ transforming) is formed from pressure and heat. Sedimentary rocks are made of sediments compressed together and is a good layer of rock to show fossils (essential evidence of time periods) We know by the category of a rock, what type of geological events it had to go through to end up this way. The colour of a rock can tell us what the rock is made of. A dark colour could mean Basalt rock, and little lighter than Basalt would be Andesite rock, while a paler beige/ white rock could be Rhyolite rock.

The classifications of these 3 types of rock is because of the different amounts magnesium and iron and how much silica. Generally the more silica is the lighter the rock will appear. We can also discover things from rocks by looking at their composition, for example: the crystal size. We can often see crystals in metamorphic rocks and sometimes even igneous rocks. If the crystals are small, we know the rock cooled quickly( example of obsidian - when water meets lava. If there are larger crystals we know the rock cooled down over a long period of time - days up to years (ie Copper sulfate homemade crystals - takes about 2 weeks). There are alot of inferences you can make by studying rocks. Stratigraphy is significant because it shows geologists a history of events by looking at the rock layers. This method works based on the law of superposition, meaning the old rocks are further below, and the newest rocks are closer to the surface. The benefits of using stratigraphy is by looking at the rock layers we know in what order events happened.

The drawbacks of using only stratigraphy is there are no numeric ages, we only know in what order events happened. A numeric piece of evidence that helps us understand when and how this extinction is by using radiometric dating. To understand how this works we need to look at the chemical composition of fossil or rock samples. Radiometric dating is used to analyse the decay of samples, by figuring out how many half lifes it has gone through. There are several types of radiometric dating. Selecting the most appropriate method for the senorio and/or using several methods of radiometric dating to accurately estimate the age of something. Radiometric dating can and has been used to find the ages of both natural and man-made materials. This diagram shows example of mass-energy theory and equation E=mc^2 is relevant when looking at nuclear fission event. Nuclear fission happens when the nucleus is too large and unstable causing it to split. Breaking down into smaller nuclei.

The Electrostatic force is larger than the strong nuclear force. In other words, the strong nuclear force (attractive) is overcome by the (repulsive) electrostatic force, causing the nucleus to split therefore releasing energy. The original mass is not equal to the total masses after the fission event due to energy being released. The number of nucleons will be conserved but the mass will be different. Mass-energy equivalence means mass is just concentrated energy, that can be released and transferred. This concept is important when understanding how nuclear physics is applied when trying to use radiometric dating to find the age of objects. Radioactive decay means that the nuclei of some isotopes are unstable and can split up or ‘decay’ and release radiation.

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Binding energy is the amount of energy that is needs to split nucleons from its nucleus, and mass defect is a nucleus’ mass minus the total mass of the nucleons. Binding energy and mass defect explain why energy is needed to split nucleons from its nucleus. Binding energy and and the mass defect are equal to each other.

Binding energy = Mass defect x C^2 E = mc^2

Energy is mass, and if the nucleons have a greater mass due to their increase in energy, the nucleus becomes too large causing fission to occur. Main methods of radiometric dating are carbon dating, Potassium-argon dating, and Uranium-Lead dating.

The equation for radioactive decay is D = Do +N(e^λt - 1)

Expanded expression to describe symbols: No. atoms of the daughter isotope in sample = No. of atoms of the daughter isotope in original composition + number of atoms of the parent isotope in sample (e^decay constant x age of sample - 1)

This knowledge can help us to date materials, the way we choose which radiometric dating method is best to used for each situation is by how many half lifes the material has been through. The more half lifes it has been through, the harder it is to get an accurate age so we choose different elements to use in radiometric dating. But what is a half-life? Half life is a statistical process and is never completely accurate, it is used to help scientists guess the around about ages of a material such as fossils and rocks. It is the amount of time it takes for a radioactive material to decay to point where only half of its original content is remaining.

The half of the element that ‘disappeared’ has experienced nuclear fission to form different elements. Carbon- 14 contains a half life of 5,730 years which implies that if you're taking one gram of Carbon -14, half it'll decay in 5,730 years. Or, if you leave Lead - 192 to undergo a half life, it will decay into part Lead-192 and part Thallium and Mercury. Every half life the element undergoes, the smaller the original sample element is in ratio to its by products. By looking at this ratio/ looking at the objects chemical composition, we can determine how many half lives the element has been through, therefore determining its age. This diagram shows that decay is exponential. The more half lives the thing has been through the harder it is to determine with certainty the exact age of anything (as it is only a statistical process).

Each half life only half of the original sample will remain ie, 1 half life = 50% left, 2 half lives = 25%, 3 half lives = 12.5%, 4 half lives = 6.25% … and so on so forth. Until the percentage gets so small it is hard to get an accurate result, this is when a more appropriate method of radiometric dating is to be used - an element with a longer half life. If you used more than one radiometric method to date a sample, you can compare results to conclude a likely range of the samples age (estimate).

The age of the Earth was determined by measuring lead content in a sample (product of uranium) U-Pb dating. This method was used because it had a longer half life than carbon for example. Depending on the situation, someone may choose Uranium isotopes over carbon isotopes to determine the age of a fossil (or artifact/ other material) because Uranium has a longer half life. So it is more appropriate for dating much older artifacts. The timeline of events is estimated by using both stratigraphy and several dating methods in conjunction estimate as accurately as possible. Cross-correlation of evidence from both Absolute and Relative dating methods is what allows geologists to make an accurate estimate of ages. It is these clues from what rocks look like and the order of the rock layers we can use as geological evidence to discover what happened and create a timeline of events. There is combination of sufficient evidence to support the leading theory of Cretaceous-Paleogene extinction event. From the stratigraphic evidence, analysing and dating rocks and fossils. Further proving that no, the Government did not place Dinosaur fossils to trick us. An asteroid really did begin the extinction of the Dinosaurs 65 million years ago.