BGYA02 - Lec3 (text + lec combined notes)

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Biological Sciences
Mary Olaveson

Chapter 21 The History of Life on EarthText BookLecture Slide Combined211How Do Scientists Date Ancient EventsFossils the preserved remains of ancient organismsPaleontologists scientists who study fossils Morphology the body form Age of rocks cannot be determined by just looking at them but the age of rocks can only be determined when comparing one another Strata oldest layers in rocks are known to lie at the bottom and successively higher strata are progressively younger William Smith concluded thatoFossils of similar organisms were found in widely separated places on EarthoCertain organisms were always found in younger rocks than certain other organisms oOrganisms found in higher more recent strata were more similar to modern organisms than were those found in lower more ancient strata RadioisotopesRadioactivity a method of dating rocks discovered at the beginning of the twentieth century HalfLife a successive time interval in which half of the remaining radioactive material of the radioisotope decays either changing into another element or becoming the stable isotope of the same element 14For C production in the upper atmosphere is about equal to its natural decay1412In an organism the ratio of C to C stays constant during its lifetime14When an organism dies it is no longer incorporating C from the environment141412The C that was present in the body decays with no replacement and the ratio of C to C decreasesAs soon as an organism dies it ceases to exchange carbon compounds with its environment Isotopes in a sedimentary rock do not contain reliable information about the date of its formation oSince they are transported overlong distancesand are deposed in another location Butrocks eg lava or volcanic ash that have intruded into layers of sedimentary rock igneouscan be datedOther radioisotopes are used to date older rocksDecay of potassium40 to argon40 is used for the most ancient rocksRadioisotope dating is combined with fossil analysis212 How have Earths Continents and Climates Changed over TimeLithosphere is Earths crust which consists of a number of solid plates each about 40 kilometers thick
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