Relative Dating vs Absolute Dating seriation was the most important technique for ordering Prehistory for several decades still couldn t provide a range of absolute dates Absolute Dating date expressed in units of time specify a discrete interval of time an age range of years centuries or millennia dendrochronology A E Douglass 1929 University of Arizona by 1930 s later SW cultures had accurate chronology radiocarbon dating cid 127 Willard Libby 1949 University of Chicago specific discrete span of time could now be associated with specific sites artifacts larger patterns of cultural changes allowed archaeologists to begin tackling more interesting questions about processes of cultural change Dendrochronology 1929 A E Douglass Principle of cross dating the ring patterns match and overlap back into time compare rings from samples at sites to master sequences Cross dating cid 127 matching ring growth patterns across many samples permits identification of exact year of formation of each ring Radiocarbon Dating 14C W Libby 1949 used to date organic remains things that were once alive range 300 45 000 years old based on relative amount of 14C to 12C 12C 98 89 13C 1 11 14C 0 000000000010 there are three principal isotopes of carbon 12C 13C and 14C only one 14C atom exists for every trillion atoms of 12C in living material 14C a radioactive carbon isotope is formed in the upper atmosphere Carbon is fixed during respiration and incorporated into all living things when organism dies 14C decays back to 14N at known rate absolute dating radiocarbon dating as long as an organism is alive the amount of 14C in it remains in equilibrium with the atmosphere once the organism dies the amount of 14C in its body begins to decrease radioactive isotopes decay over time half life the time required for half of the carbon 14 available in an organic sample to decay back to Nitrogen half life 5 730 years Problems need to reference a fixed point in time so B P refers to years before 1950 the year radiocarbon dating was first used basic assumptions of carbon dating amount of carbon has remained stable same as it was in the past but that isn t true DeVries effects fluctuations in the calibration curve produced by variations in the atmosphere s carbon 14 content need a calibration curve to convert radiocarbon years to calendar years can make certain radiocarbon dates match more than one group of calendar dates on a curve De Vries Effects Wiggles discovered by Hessel De vries first dated wood using tree rings then carbon 14 dated found some significant variations wiggles in calibration curve due to variation in 14C in atmosphere caused by variations in solar radiation solar flares sun spot activity Other issues different photosynthetic pathways in plants differences in relative amounts of 14C to 12C easily accounted for during calibration reservoir effect oceans and inorganic C deposits retain old C limestone dolomite so marine organisms shells tend to date older than really are cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127