2D3: Human EvolutionD.3.1: outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40KD.3.2: Define half-lifeD.3.3: Deduce the approximate age of materials based on a simple decay curve for a radioisotopePages Campbell
3The Fossil RecordFossils are any form of preserved remains from a living organismLike: bones in rock, insects in amber, frozen mammothsThe fossil record is the ordered array in which fossils appear within layers of sedimentary rocksPaleontologists collect and determine fossilsHow do we determine their age?Contributions and limitations of the fossil record
4How are fossils formed?If a dead organism gets buried in sedimentary silt, it will decay slowly and leave a place in the surrounding siltThey become solid, and fill the exact gap the organism left behind (like a cast)The silt around this solidifies, becoming sedimentary rockIn the rock you find a different looking stoneLooks like the dead organism fossilVery few circumstances that fossils are formedThis is why the fossil record is incompleteMost individuals do not leave a fossil after death
5Age of fossils We can use isotopes to determine the age of fossils Isotopes are …Atoms which have a mass different from most other atomsDifferent number of neutronsUnstable; will spontaneously change into one or more atoms of other elements, often emitting radiation
6Half life This change takes time Radioactive decay: During a fixed interval (depending on the isotope), half of the amount present will decayAs a result, at the end of the period, the radioactivity will be half of what is was beforeThis is called half-life of the isotopeHalf-lives vary from fractions of seconds to thousand of yearsHalf-life of14C is 5730 years
7So to repeat …Half life: the number of years it takes for 50% of the original sample to decayUnaffected by temperature, pressure and other environmental factors
8Carbon dating Method used to determine the age of organic material Involves radioactive 14CA normal atom of carbon is referred to as 12C6 protons, 6 electrons, 6 neutrons14C has 6 protons, 6 elections, 8 neutronsUnstable; spontaneously change to 14NEmits radiation
9Carbon dating 14C is present in small amounts on Earth All living organisms 14C in the same proportion as is found in the atmosphere14C is emitted by the sun photosynthesis (enters food chain)After an organism dies, the process of incorporating new carbon into the body stopsAfter 5730 years, the amount of 14C present in the remains is half of what is was at the time of deathAfter 11,460 years, it would be ¼
10Carbon datingAccurate and useful for “young fossils” (20,000-50,000 years old)Older material, another isotopes should be chosen (with a longer half-life)40KWill decay to form 40Ar (argon)Half life: 1300 million years238Uranium (volcanoes)
11How can we use a graph to figure this out? At time 0, the fraction of 14C is 1 (the original amount present – the organism just died)When only ½ the original amount of 14C is present a time equal to the half-life of the isotope has passed5730 yearsWhen only of the original amount of 14C is present, 3 half lives have passedThe specimen died years ago
12Review: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?
13Answer2.6 billion yearsPassed through 2 half lives
14Review: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?