1. Chronology Building: How to Get a Date
Chapter 5
Chronology Building: How to Get a Date

2. Outline Relative Dating Absolute Dating What Do Dates Mean?
How Old Are the Pyramids?
The Check, Please

3. Dating
Absolute dates are expressed as specific units of scientific measurement—days, years, centuries, or millennia
Relative dates express relationships or comparisons.
Stepped pyramid at Saqqara in Egypt is older than Khufu’s pyramid
Historic settlement of Williamsburg more recent than the pueblos of Chaco Canyon.

4. Relative Dating
The index fossil concept
The law of superposition

5. Index Fossil Concept
The idea that strata containing similar fossil assemblages are of similar age.
This concept enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

6. Law of Superposition
The geological principle that in any pile of sedimentary rocks that have not been disturbed by folding or overturning, each bed is older than the layers above and younger than the layers below.

7. Time Markers
Artifact forms that research shows to be diagnostic of a particular period of time

8. Potsherd Frequencies from Pueblo San Cristobal, New Mexico
Depth
Corrugated ware
Biscuit ware
Type I
Type II
Type III
Total
1st foot
57(36.7)
10 (6.5)
2 (1.3)
81 (52.2)
5 (3.2)
155
2nd foot
116(31.3)
17 (4.6)
2 (.01)
230 (62)
6 (1.6)
371
3rd foot
27 (15.3)
2 (1.1)
10 (5.7)
134 (76.1)
3 (1.7)
176
4th foot
28 (21.3)
4 (3)
6 (4.5)
93 (70.9)
0 (0)
131

9. Nels Nelson’s Excavations at San Cristobal

10. Potsherd Frequencies from Pueblo San Cristobal, New Mexico
Depth
Corrugated ware
Biscuit ware
Type I
Type II
Type III
Total
5th foot
60 (17.3)
15 (4.3)
2 (.01)
268 (77.6)
0 (0)
345
6th foot
75 (18.6)
21 (5.2)
8 (1.9)
297 (73.8)
1(.01)
402
7th foot
53 (23.1)
10 (4.3)
40 (17.5)
126 (55)
229

11. Potsherd Frequencies from Pueblo San Cristobal, New Mexico
Depth
Corrugated ware
Biscuit ware
Type I
Type II
Type III
Total
8th foot
56 (24.6)
2 (.01)
118 (51.9)
51 (22.4)
0 (0)
227
9th foot
93 (45.4)
1 (.01)
107 (52.5)
3 (1.4)
204
10th foot
84 (54.4)
69 (44.8)
154
649 83
364
1,283
15 2,39 4

12. The Next Step: Seriation
A relative dating method.
Orders artifacts based on the assumption that one cultural style slowly replaces an earlier style over time.
With a master seriation diagram, sites can be dated based on their frequency of several artifact styles.

13. Seriation Diagram: Nelson’s San Cristobal Potsherd Frequencies

14. Absolute Dating
Helped to shape archaeology into the science it is today

15. Tree-Ring Dating Also called dendrochronology
Developed by Andrew E. Douglass, an astronomer interested in the effects of sunspots on the earth’s climate.
Trees in temperate and arctic areas are dormant in the winter and burst into activity in the spring.
The result is the formation of concentric growth rings alternating light and dark.

16. Methodology of Tree-Ring Dating
Archaeologist digs up a sample of charcoal or wood with at least 20 rings.
The sample is sent to the appropriate lab with contextual data.
An analyst will cut or sand the sample down so the rings are easily visible, and the widths are then measured individually.
The lab analyst tries to match the sample to the appropriate portion of the regional sequence.

17. Tree-ring Chronology

18. Radiocarbon Dating: Archaeology’s Workhorse
How It Works:
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 is produced in the upper atmosphere, where cosmic radiation creates neutrons that replace one of nitrogen’s protons (14N) to create 14C.

19. Radiocarbon Dating: How It Works
14C oxidizes and forms carbon dioxide, which is dispersed throughout the atmosphere by stratospheric winds.
About 98 percent of all 14C enters the oceans; plants take up the rest through photosynthesis.
From plants, it enters herbivores, and then carnivores. All organic life contains radioactive carbon.

20. Radiocarbon Dating: How It Works
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.
After 5,730 years, Cambridge half-life, half of the14C available in a sample, will have converted to 14N.
Radiocarbon dating is good for organic remains no more than 45,000 years old.

21. Radiocarbon Dating: How It Works
Example:
A sample contains 100 atoms of 14C.
After 5,730 years, 50 of these atoms have decayed into 14N.
After another 5,730 years, half of the remaining C atoms have converted to 14N, leaving only C atoms.
After another 5,730 years, this would be halved again to about 12 14C atoms.

22. Can You Handle Uncertainty?
The dating process of 14C is a statistical process
The number of beta emissions is not constant over a short period, but the rate averages out over the half-life.
Labs measures amount of beta emissions over time and the averages get an age.

23. Are All Organics Created Equal?
Bones and plants are complex.
Photosynthetic pathways
The chemical process through which plants metabolize carbon.
The three pathways discriminate against carbon- 13 differently so that similarly aged plants using different pathways may produce different radiocarbon ages.
Archaeologists should try to identify the kind of plant being dated.

24. The Reservoir Effect Reservoir effect
When organisms take in carbon from a source that is depleted of or enriched in 14C relative to the atmosphere.
Such samples may return ages that are considerably older or younger than they actually are.

25. Tree Rings Refine Radiocarbon Dating
Hessel de Vries discovered that the amount of 14C is does not remain constant in the atmosphere measuring tree rings.
De Vries effects
Fluctuations in the calibration curve produced by variations in the atmosphere’s carbon-14 content.
Causes radiocarbon dates to calibrate to more than one calendar age.

26. Accelerator Dating: Taking Radiocarbon to the Limit
Problems with the amount of carbon found in the sample limited some sites.
Accelerator Mass Spectrometry (AMS) reduced the quantity of datable material required.
AMS uses electrostatic tandem accelerator and masspectrometry technology to count the proportion of carbon isotopes in a sample.

27. Trapped Charge Dating
There are three methods for dating trapped charge in artifacts.
Thermoluminescence, Optically Stimulated Luminescence and Electron Resonance.
Dates extend back at least 300,000 years.

28. Trapped Charge Dating: Process
Over time, background gamma radiation sediment causes some electrons of the atoms of certain minerals to move to a different energy state.
Some electrons are “trapped” in atomic imperfections in the minerals’ crystal lattices.
Overtime, more electrons are trapped in this way.

29. Dosimeter
A device to measure the amount of gamma radiation emitted by sediments.
It is normally buried in a stratum for a year to record the annual dose of radiation.
Dosimeters are often a short length of pure copper tubing filled with calcium sulfate.

30. Trapped Charge Dating

31. Trapped Charge Dating Techniques
Thermoluminescence
Used on anything mineral heated to more than 500° C. (ceramics, burnt stone artifacts)
Optically stimulated luminescence
Used to date sediments. The age is the time elapsed between the last time a few moments exposure to sunlight reset the clock to zero and the present.
Electron spin resonance
Can be used to date teeth that are beyond the range of radiocarbon dating.

32. Argon-Argon Dating
Based on the radioactive isotopes decay at certain known rates.
Argon-argon dating is a high-precision method for estimating the relative quantities of argon-39 to argon-40 gas.
Used to date volcanic ashes between 500,000 and several million years due to their extremely long half-lives.
Garniss Curtis used argon-argon dating to estimate the age of Homo erectus in Asia.

33. What Do Dates Mean?
There are many other methods used by archaeologists for dating.
We can never date archaeological sites by simple equvalences.
Show the dated event contemporaneous with a behavioral event of interest.
Old wood problem- old wood was scavenged and reused in a site.

34. How Old Are the Pyramids?
Ages are based on historical documents (i.e. hieroglyphs)
Archaeologists Nakhla and Hawass used Radiocarbon dating to date the pyramids.
Pieces of carbon from fires were caught and trapped in the mortar.
Results revealed the old wood problem for the Old Kingdom dates.

35. The Pyramids at Giza

36. The Check, Please
Archaeology dating methods are bound by cost and budget.
Trapped charge and radiometric dating methods are the most costly.
A visit to the site and other background information is often necessary

37. Summary of Absolute Dating Methods
Technique
Target material
Accuracy
Carbon-14
Any organic material; carbon is the most common.
To 45,000 BP
Thermo-luminescence
Ceramics, burnt stone
Perhaps back to 300,000 years
OSL
Quartz, feldspars in eolian sands

38. Summary of Absolute Dating Methods
Technique
Target material
Accuracy
Electronic spin resonance
Tooth enamel, burned stone tools, corals, shells
10,000 to 300,000 or more years
Argon-argon
Volcanic ash
200,000 to several million years

39. Summary Dating techniques are divided into relative and absolute.
Relative dating includes the use of index fossils and seriation.
Tree-ring dating (dendrochronology) is useful in relatively small regions.
Radiocarbon dating is useful for archaeological sites that are less than 45,000 years old.
Accelerator (AMS) technique allows radiocarbon-dating of minute amounts of carbon.
Trapped charge dating methods date burned stone tools, ceramics, eolian sediments, tooth enamel.
Argon-argon dating is used to date volcanic rock, especially ashes.
Dating techniques tell us nothing about cultural activities.
Documentary evidence provides dates for historical sites.

40. Quick Quiz

41. 1. ________ dates are expressed as specific units of scientific measurement, i.e. days, years, centuries, or millennia; _______ dates express relationships or comparisons.

42. Answer: absolute; relative
Absolute dates are expressed as specific units of scientific measurement—days, years, centuries, or millennia; relative dates express relationships or comparisons.

43. 2. The ___ _____ _____ enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

44. Answer: Index Fossil Concept
The Index Fossil Concept enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

45. The chemical process through which plants metabolize carbon
3. Problems associated with radiocarbon dating include:
The chemical process through which plants metabolize carbon
The Reservoir effect
The De Vries effects
B and C only
A, B, and C.

46. Answer: E
3. Problems associated with radiocarbon dating include the chemical process through which plants metabolize carbon, the Reservoir effect, and the De Vries effects.

47. 5. With Trapped Charge Dating, sites can be dated based on their frequency of several artifact styles.
True
False

48. Answer: B
With a master seriation diagram, sites can be dated based on their frequency of several artifact styles.

49. The radiocarbon dates on Old Kingdom pyramids were from 100 to 400 years older than documented.

50. Answer: A
The explanation was the old wood problem, where Old Kingdom pharaohs recycled old wood to build their pyramids.