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D3: Human Evolution. D.3.1: outline the method for dating rocks and fossils using radioisotopes, with reference to 14 C and 40 K D.3.2: Define half-life.

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Presentation on theme: "D3: Human Evolution. D.3.1: outline the method for dating rocks and fossils using radioisotopes, with reference to 14 C and 40 K D.3.2: Define half-life."— Presentation transcript:

1 D3: Human Evolution

2 D.3.1: outline the method for dating rocks and fossils using radioisotopes, with reference to 14 C and 40 K D.3.2: Define half-life D.3.3: Deduce the approximate age of materials based on a simple decay curve for a radioisotope Pages Campbell

3 The Fossil Record Fossils are any form of preserved remains from a living organism  Like: bones in rock, insects in amber, frozen mammoths The fossil record is the ordered array in which fossils appear within layers of sedimentary rocks Paleontologists collect and determine fossils  How do we determine their age?  Contributions and limitations of the fossil record

4 How are fossils formed? If a dead organism gets buried in sedimentary silt, it will decay slowly and leave a place in the surrounding silt They become solid, and fill the exact gap the organism left behind (like a cast) The silt around this solidifies, becoming sedimentary rock In the rock you find a different looking stone  Looks like the dead organism  fossil Very few circumstances that fossils are formed  This is why the fossil record is incomplete  Most individuals do not leave a fossil after death  / /

5 Age of fossils We can use isotopes to determine the age of fossils Isotopes are …  Atoms which have a mass different from most other atoms  Different number of neutrons  Unstable; will spontaneously change into one or more atoms of other elements, often emitting radiation

6 Half life This change takes time Radioactive decay:  During a fixed interval (depending on the isotope), half of the amount present will decay  As a result, at the end of the period, the radioactivity will be half of what is was before  This is called half-life of the isotope  Half-lives vary from fractions of seconds to thousand of years Half-life of 14 C is 5730 years

7 So to repeat … Half life: the number of years it takes for 50% of the original sample to decay  Unaffected by temperature, pressure and other environmental factors  radioactive_decay3.html radioactive_decay3.html

8 Carbon dating Method used to determine the age of organic material Involves radioactive 14 C  A normal atom of carbon is referred to as 12 C 6 protons, 6 electrons, 6 neutrons  14 C has 6 protons, 6 elections, 8 neutrons Unstable; spontaneously change to 14 N Emits radiation

9 Carbon dating 14 C is present in small amounts on Earth All living organisms 14 C in the same proportion as is found in the atmosphere  14 C is emitted by the sun  photosynthesis (enters food chain) After an organism dies, the process of incorporating new carbon into the body stops After 5730 years, the amount of 14 C present in the remains is half of what is was at the time of death After 11,460 years, it would be ¼

10 Carbon dating Accurate and useful for “young fossils” (20,000-50,000 years old) Older material, another isotopes should be chosen (with a longer half-life)  40 K Will decay to form 40 Ar (argon) Half life: 1300 million years  238 Uranium (volcanoes)

11 How can we use a graph to figure this out? At time 0, the fraction of 14 C is 1 (the original amount present – the organism just died) When only ½ the original amount of 14 C is present a time equal to the half-life of the isotope has passed  5730 years When only of the original amount of 14 C is present, 3 half lives have passed  The specimen died years ago

12 Review: A paleontologist estimates that when a particular rock formed, it contained 12 mg of the radioisotope potassium-40. The rock now contains 3 mg of potassium-40. The half-life of potassium-40 is 1.3 billion years. About how old is the rock?

13 Answer 2.6 billion years  Passed through 2 half lives

14 Review: Your measurements indicated that a fossilized skull you unearthed has a carbon-14 to carbon-12 ratio of about 1/16 that of the atmosphere. What is the approximate age of the skull?

15 Answer: ~22,920 years  4 half-lives  5730 years

16 Absolutely hilarious … The Simpsons Take on Evolution and Creationism ution-vs-creationism-simpsons-style/ ution-vs-creationism-simpsons-style/  Just click on Ned Flanders


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