4/4/17. Dating Methods. Dating in Archaeology. These things date to 2500 B.C. (but there are no completion dates written anywhere on or near them)

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1 Dating in Archaeology These things date to 2500 B.C. (but there are no completion dates written anywhere on or near them) Dating Methods One of the biggest questions in archaeology is WHEN? Successfully dating archaeological finds requires: A secure contet Closed environment (no contamination) 1

2 Dating Terminology Relative Dating Placing events in chronological order but without assigning numerical values. Eample: Julius Caesar died before today s class. This was the only way archaeologists measured time in prehistory until the 20 th century. Relative Dating Seriation ordering sites chronologically based on the presence of dominant styles Based on the notion that some styles follow a pattern: they appear, become popular, and then fade away Music trends over time CDs VINYL TAPES 2

3 San Cristobal Potsherd Frequencies Seriation Diagram: Illumination in Pennsylvania Pot Type C Pot Type A Pot Type B 3

4 Flourine Dating Relative Dating Methods Fluorine Dating measures the accumulation of fluorine levels in bone. The greater the amount of fluorine in bones from the same area, the older they are Fluorine Dating Fluorine Time 4

5 Piltdown Man Human skull Orangutan jaw Absolute Dating Dating Terminology Absolute Dating Giving a calendar date to an event or events Eample: Julius Caesar died on March 15, 44 B.C. Today s class occurred on A.D March 22, 2017 Calendar dates use a CULTURAL REFERENCE: BC/AD BCE before common era BP (bp) before present (1950) 5

6 Hydrogen bomb testing massively altered the chemical content of the atmosphere The Target Event is the event in which archaeologists are interested. E.g. when was the village abandoned? The Dated Event is the event that a technique actually provides a date for. E.g. when did the wood in the buildings in the village die? 6

7 Target event vs. Dated event There can be (and often is) an unknown and significant time-span between these two events. Precision vs Accuracy Precision: The ability of a measurement to be consistently reproduced (measurements are close to EACH OTHER) You could determine the volume of one gumball (G) and the volume of the glass jar (J) Then J = G where () is the number of gumballs that will fit in the machine Precision vs Accuracy Accuracy: The ability of a measurement to match the actual value of the quantity being measured. (when you count it, you get the correct number- on average) 7

8 Dendrochronology the use of annual growth rings in trees to assign calendar ages to ancient wood. Summer growth light) Winter growth 8

9 Master Chronology Dendrochronology Very precise (every time you count the rings you get the same number) Mostly accurate (there can be a big gap between when a tree is cut down and when a house is built) Bristlecone pines can live for 6000 years 9

10 Radiocarbon dating This is an atom! Protons (positive charge) Neutrons (neutral charge) Electrons (negative charge) 10

11 isotopes Variants of a particular element with different numbers of neutrons All isotopes of an element have the same number of protons Carbon C Protons (always 6) + Neutrons (usually 6) Electrons (6) Carbon C Protons (always 6) + Electrons (6) Neutrons (in this case 8) 11

12 COSMIC RADIATION + neutron 14 N 14 C Proton emitted 14 6C is unstable Unstable isotopes tend to shake themselves apart. This is called radioactivity Beta Decay One neutron will crack and break into a proton and an electron So Carbon-14 decays into Nitrogen-14 12

13 99 % 12 6C 1 % 14 6C After death Half life of carbon-14 Half-life years 2 Half-lives - 11,460 years 13

14 Half life of carbon years 11,460 years 17,190 years 22,920 years Radiocarbon Decay Radiocarbon dating Dating Range Radiocarbon 0 bp 50,000 bp Accuracy good at determining age of death Precision only as good as the counter 14

15 14 Measuring 6C is like counting popcorn pops You can determine the AVERAGE number of atoms that are present at any one time But you cannot precisely measure the eact quantity Radiocarbon Dating: Error Terms Standard Deviation E.g years before present ± 150 There s always a catch Assumptions: 1. The decay rate of 14 C is constant 2. The rate of production of 14 C in the atmosphere is constant 3. No contamination 4. Carbon Fractionation does not occur 5. Reservoir Effect does not occur 15

16 Violations The decay rate is an average and is therefore an estimate Calibration Violations 14 C isotopes are not produced at a constant rate over time. 16

17 Calibration eample: /- 150 Contamination is common Violations Violations Fractionation does occur 17

18 Violations Plants take in carbon through photosynthesis: But some plants preferentially take in lighter carbon in this case 12 6C Fractionation does occur Violations Reservoir Effect is hard to estimate in some places Last 1/3 SUN MON TUES WED THUR FRI SAT 31 Dating (again) 07 First cultures ETHNO #3 due 14 Pleistocene Etinctions 21 Lab #2 02 Richard III Quiz #3 09 Peopling of the Americas 16 First Cities 23 NO CLASS Thanksgiving Holiday 28 Egyptians 30 Collapse QUIZ #4 18

19 Not quite done with dating!!!!! Shroud of Turin Fabric discovered in the 14 th century that was purported to be Christ s Burial Shroud. When viewed under special conditions, the body and face of a man appears. If real, then the shroud should be ~2000 years old Negative image of shroud Radiocarbon dated to AD 1335 ± 75 ( ). Shroud was partially burned in 14 th century. Would that make a difference to the age? 19

20 Number of measurements Years AD Calibrating Radiocarbon Dates: Calibration Curve AMS vs. Conventional Dating In Conventional radiocarbon dating, a Geiger counter registers the number of electrons emitted by decaying 14 C isotopes during beta decay advantages: cheap ($100 per sample), easy to do disadvantages: more inaccurate, requires large samples Accelerator mass spectrometry (AMS) counts the proportion of 14 C isotopes directly advantages: more accurate, requires only small samples disadvantages: epensive ($750 / sample), requires special equipment 20

21 Luminescence Dating LUMINESCENCE: Any emission of light that occurs at low temperatures. A phenomenon ehibited by many crystals, particularly quartz and feldspars. These are the two most common minerals on Earth Luminescence Dating: Principles 1. Energy absorbed from ionizing radiation (ά, β, γ, and cosmic rays) stimulates electrons to move thru the crystalline lattice. 2. Some electrons are trapped in imperfections, or traps, within the lattice. 3. Electrons continue to get trapped in these imperfections through time. Luminescence Dating: Principles 4. Electrons will be released when eposed to energy from heat or even sunlight this is luminescence 5. Buried crystals will continue to store electrons until they are eposed to energy 21

22 Luminescence Accumulation Bleaching Luminescence Signal Natural Signal : Time since deposition or firing Mineral Formation Archaeological Zeroing Time Measurement in Lab Luminescence Dating: Applications Fired Ceramics: The manufacture of pottery zeros the electrons in the traps. This is Thermo-luminescence (TL) Optically Stimulated Luminescence Buried Soils: Soils that are eposed to light are constantly having their traps emptied. Once they are buried, the electrons in the traps begin to accumulate from natural, or background, radioactivity. 22

23 Luminescence Dating: Black Sea Flood Modern Black Sea Black Sea Mediterranean Sea Black Sea Flood: Sea Level Rise 23

24 Black Sea Flood: Dating with OSL Bleaching Event? Sediments eposed to light during flooding. OSL of buried sediments associated with flooddeposited layers. Marine, Post-Flood Flood deposit Freshwater, Pre-Flood Black Sea Sediments Black Sea Flood = Noah s Flood? 24

25 Dating Conclusions Most common absolute methods Dendrochronology very precise, only works in arid environments Radiocarbon not very precise, commonly used because organic material is common Luminescence etremely accurate and precise, useful for dating rocks, pottery, and dirt 25