Lecture Topic 4: Dating Fossils
Lecture Topic 4: Dating Fossils Bio 113
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This 5 page Class Notes was uploaded by Sky Holmes on Thursday September 10, 2015. The Class Notes belongs to Bio 113 at Arizona State University taught by Udo Savalli in Spring 2015. Since its upload, it has received 21 views. For similar materials see Dinosaurs in Science at Arizona State University.
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Date Created: 09/10/15
Lecture Topic 4 98 15 Dating Fossils 0 To understand the evolution of extinct animals we need to know their age 0 Relative Dating determining the sequence in time in which fossils occurred 0 Absolute Dating determining the actual age or duration in years 0 Both methods are part of the field of stratigraphystudy of the age of geological strata Relative Dating 0 Relative dating depends on the law of superposition o The oldest strata layers of sedimentary rocks will be at the bottom newest at the top 0 Law of superposition o Igneous intrusions lava ows can be more complicated 0 Complications to Law of Superposition o Mixing of material from different strata o Lithostratigraphydating based on relative positions of rock layers 0 Only applicable to fossils in the same general area 0 Biostratigraphy correlates layers of rocks at different locations by using indicator fossils Absolute Dating 0 Chronostratigraphy to arrange the sequence of deposition and the time of deposition of all rocks within a geological region and eventually the entire geologic record of the Earth 0 Atoms are composed of 3 kinds of elementary particles Protons neutrons and electrons Radioactive Decay 0 Element type of atom determined by number of protons atomic number I E g Potassium K has 19 protons I E g Argon Ar has 18 protons 0 Number of neutrons can vary without affecting atom s chemical properties except weight 0 Atomic Weight of protons of neutrons I Isotopes are atoms with different atomic weight ie different number of neutrons 0 Isotopes can be stable persist definitely or unstable 0 Unstable isotopes release energy and particles form their nucleus radioactive decay o The rate at which an isotope decays is predictable and constant 0 If a particular atom undergoes decay is at any instant is random with some probability 0 This means that the more atoms present the more atoms are undergoing decay 0 Thus rate of decay is described as the halflife of the atoms the time it takes half of the atoms to decay o The relative number of parent vs daughter isotopes can be used to estimate the age of the mineral o The start of the radioisotope clock begins when the mineral cools and is solidi ed from its molten state in magma or lava O 0 At this point atoms are trapped in the mineral and both parent and daughter atoms can be measured Only igneous and some metamorphic can be dated 0 The most common isotope techniques 0 Radiocarbon lt60000 years applies to organic material involving a decay of Carbon14 to Nitrogen14 with a halflife of 5730 yrs UraniumLead anything greater than 1 million yrs Involving Uranium238 to Lead206 and Uranium235 to Lead207 with a halflife of 45 and 7 billion yrs by Uraniumthorium applies to shells and bone lt700000 yrs Involving Uranium 234 to Thorium230 with a halflife of 45 by Potassiumargon gt100000 yrs Involving Potassium40 to Argon40 with a half life of 13 by Rubidiumstrontium gt100 ma million years involving Rubidium87 to Strontium86 with a halflife of 48 by 0 Most sedimentary rocks cannot be dated since the minerals were formed long before they were eroded and redeposited as sediments O Exceptions are volcanic ash volcanic mud ows and other volcanic particles in sediments 0 Some techniques Carbon14 and Uraniumthorium can be used on organic remains but only effective for recent remains lt700000 yrs Old 0 Age of sedimentary layers is given as a range determined by closest datable layers above and below the sedimentary rock Stratigraphic Sequences o The sequence of various fossils allowed geologists to name various time periods represented by the rock strata 0 First region to be studied was named then other regions with same fossils given same name 0 E g fossilbearing strata in J ura Mts FrenchSwiss border were called Jurassic 0 Name now represents a period of time that includes those strata 0 Geological time is subdivided into differentlength categories 0 Eon I Era 0 Period 339 Epoch 0 Age or Stage gt Local Strata Formations etc The Four Eons o Hadean Hidden Eon 4638 bya o No known life 0 Archean Ancient Eon 3825 bya 0 Only simple bacteria no oxygen o Proterozoic Earlier life Eon 2500542 mya 0 Simple mostly singlecelled organisms Oxygen present 0 Phanerozoic Visible Life Eon 542 myapresent 0 Complex diverse animal life Eras of the Phanerozoic o Paleozoic ancient life Era 542 251 mya o Diversification of animals including fishes o Invasion of land by plants animals 0 First amphibians reptiles insects 0 Mesozoic middle life Era 25165 mya 0 Origin and extinction of dinosaurs 0 Origin of birds mammals owering plants 0 Cenozoic new life Era 65 myapresent o Diversification of birds mammals 0 Origin of humans