Archeology: Week 5 Notes Dating Techniques

Dates: Absolute Date: Absolute Date: Most Familiar Most Familiar Specific Units (Days, Years, Centuries) Specific Units (Days, Years, Centuries) Ex: Signing of Declaration of Independence – July 4 th, 1776 Ex: Signing of Declaration of Independence – July 4 th, 1776

Dates: Relative Dates: Relative Dates: Date determined by relative relationships Date determined by relative relationships Ex: Before European Contact Ex: Before European Contact Lumbering Era Lumbering Era Industrial Era Industrial Era

Dating Systems: BC vs. AD: BC vs. AD: Before Christ / Amino Domini (In the Year of Our Lord) Before Christ / Amino Domini (In the Year of Our Lord) BCE vs. CE: BCE vs. CE: Before the Common Era / Common Era Before the Common Era / Common Era BP: BP: Before the Present Before the Present Present = 1950 Present = 1950

Dating Techniques: Index Dating: Index Dating: Based on known artifacts found in the same layer of soil Based on known artifacts found in the same layer of soil Chronology: Chronology: Deeper in the soil the older it is Deeper in the soil the older it is 1 inch of soil = 100 years 1 inch of soil = 100 years

Dendrochronology: Tree-Ring Dating Tree-Ring Dating Developed by A.E. Douglass – Astronomer Developed by A.E. Douglass – Astronomer Each Ring Represents a single year Each Ring Represents a single year Tree Rings vary in size Tree Rings vary in size Preserve information about the environment they grew in Preserve information about the environment they grew in

Dendrochronology: Construction of a “floating chronology” Construction of a “floating chronology” Spanned several centuries Spanned several centuries Was NOT able to correlate to a modern calendar Was NOT able to correlate to a modern calendar Date expressed in R.D. (Relative Date) Date expressed in R.D. (Relative Date)

Dendrochronology: 1929 – Wood beam found in Arizona 1929 – Wood beam found in Arizona Spanned the time gap between relative dating and modern calendar Spanned the time gap between relative dating and modern calendar Dating Sequence: Dating Sequence: Went back to 1284 AD Went back to 1284 AD

Dendrochronology: 1960’s: 1960’s: Archeology work at Betatkin Cave Archeology work at Betatkin Cave Viking Cave Viking Cave Timbered Beams Found Timbered Beams Found Extended Chronology back to 100 AD Extended Chronology back to 100 AD

Dendrochronology: Climate Changes: Climate Changes: Tree Rings: Width Tree Rings: Width Changes in Temperature and Soil Moisture Changes in Temperature and Soil Moisture Ex: Europe’s Mini Ice Age (1320 AD) Ex: Europe’s Mini Ice Age (1320 AD)

Radio Carbon Dating: 1949: Willard F. Libby 1949: Willard F. Libby Physical Chemists Physical Chemists Developed the Technique of Carbon-14 Dating Developed the Technique of Carbon-14 Dating Works on organic Materials younger than 75,000 years Works on organic Materials younger than 75,000 years

Radio Carbon Dating Technique: Technique: Cosmic Radiation produces neutrons and enter the Earth’s atmosphere Cosmic Radiation produces neutrons and enter the Earth’s atmosphere React with Nitrogen (bounces a Proton loose and gained 1 Neutron) React with Nitrogen (bounces a Proton loose and gained 1 Neutron) Forms Heavy Carbon: Isotope of Carbon with 2 extra neutrons = Carbon- 14 Forms Heavy Carbon: Isotope of Carbon with 2 extra neutrons = Carbon- 14

Radio Carbon Dating: Technique: Technique: 2 Extra Neutrons makes the Nucleus of the atom unstable 2 Extra Neutrons makes the Nucleus of the atom unstable Nucleus gradually decays (Radioactivity) Nucleus gradually decays (Radioactivity) Every 5,568 years half of C- 14 in a sample decays Every 5,568 years half of C- 14 in a sample decays 1 Half-Life = 5,568 yrs = ½ sample 1 Half-Life = 5,568 yrs = ½ sample 2 Half-Lives = 11,136 yrs = ¼ sample 2 Half-Lives = 11,136 yrs = ¼ sample 3 Half-Lives = 16,704 yrs – 1/8 sample 3 Half-Lives = 16,704 yrs – 1/8 sample

Radio Carbon Dating: Problems: Problems: Random Process: Random Process: Can’t predict which C-14 molecule will decay Can’t predict which C-14 molecule will decay Can’t be sure it decays at the same rate over long periods of time Can’t be sure it decays at the same rate over long periods of time Atmospheric C-14 has changed over years Atmospheric C-14 has changed over years Cost: Very Expensive Cost: Very Expensive $300 per standard sample $300 per standard sample

Radio Carbon Dating: Problems: Problems: Measured Measured Radiocarbon years - Not Calendar Years Radiocarbon years - Not Calendar Years Must be corrected to reflect calendar years Must be corrected to reflect calendar years Before the Present (BP) – Before 1950 Before the Present (BP) – Before 1950

Radio Carbon Dating: Accuracy: Accuracy: Measured in: Standard Deviation Measured in: Standard Deviation Carbon-14 Date given with a range (± 120 yrs) Carbon-14 Date given with a range (± 120 yrs) 1 sigma = 67% chance that the true date is in this range 1 sigma = 67% chance that the true date is in this range 2 sigma – 95% change that the true date is in this range x 2 = (± 240 yrs) 2 sigma – 95% change that the true date is in this range x 2 = (± 240 yrs) Never 100% certain Never 100% certain Range must always come with Radio Carbon Dating Range must always come with Radio Carbon Dating

Obsidian Hydration: Obsidian – Volcanic Glass Obsidian – Volcanic Glass Used for Tools Used for Tools Irving Friedman and Robert Smith – 1948 Irving Friedman and Robert Smith – 1948 Dry Rock – Contains less than 0.2% water Dry Rock – Contains less than 0.2% water Rock is chipped and exposed to air: Rock is chipped and exposed to air: Water is absorbed = Hydration Water is absorbed = Hydration Absorb water until it reaches = 3.5% water Absorb water until it reaches = 3.5% water

Obsidian Hydration: The degree of hydration can measure how long it has been since the surface was created The degree of hydration can measure how long it has been since the surface was created Equals a thicker Hydration Band Equals a thicker Hydration Band Simple, Cheap, and Rapid Simple, Cheap, and Rapid Test 10 Obsidian samples to 1 C-14 sample Test 10 Obsidian samples to 1 C-14 sample

Obsidian Hydration: Problems: Problems: Rate of Hydration is not uniform throughout the World Rate of Hydration is not uniform throughout the World Based on atmospheric temperature Based on atmospheric temperature Need World Map to correlate dates Need World Map to correlate dates

Obsidian Hydration: Calculation: Calculation: Y = (x² - x) Y = (x² - x) Y = Date in years (BP) Y = Date in years (BP) X = Width of Hydration band in microns X = Width of Hydration band in microns