1. Quaternary Environments Dating Methods I

3. Accuracy Versus Precision
Precision means that the samples have low amount of error associated with the dating
Accuracy means that the samples are dated to the true age of the sample
We strive for both accuracy and precision in dating techniques

4. Accuracy Versus Precision

5. Relative Versus Absolute Dating
Relative Dating
Principle of Superposition
Absolute Dating
Provides solid chronological dates (within error bars) that are related to a calendar year scale

6. Methods Radioisotopic Methods Paleomagnetic Methods
Based on rate of atomic disintegration
Paleomagnetic Methods
Relies on past reversals of the Earth’s magnetic field
Organic and Inorganic Chemical Methods
Based on time-dependent chemical changes in a sample
Biological Methods
Based on the growth of an organism

7. Using Radioactivity in Dating
Radiometric dating
Useful radioactive isotopes for providing ages
87Rb/87Sr – 47.0 billion years
232Th/208Pb – 12.1 billion years
238U/206Pb – billion years
40K/40Ar – billion years
235U/207Pb – 713 million years
14C/14N – 5,730 years (5,570 Libby years)

8. Sources of Error A closed system is required
To avoid potential problems, only fresh, unweathered samples should be used

9. Using Radioactivity in Dating
Reviewing basic atomic structure
Nucleus
Protons – positively charged particles with mass
Neutrons – neutral particles with mass
Electrons – negatively charged particles that orbit the nucleus

10. Using Radioactivity in Dating
Reviewing basic atomic structure
Atomic number
An element’s identifying number
Equal to the number of protons in the atom’s nucleus
Mass number
Sum of the number of protons and neutrons in an atom’s nucleus
Identifies an isotope

11. Using Radioactivity in Dating
Reviewing basic atomic structure
Isotope
Variant of the same parent atom
Differs in the number of neutrons
Results in a different mass number than the parent atom

12. Using Radioactivity in Dating
Spontaneous changes (decay) in the structure of atomic nuclei
Types of radioactive decay
Alpha emission
Emission of 2 protons and 2 neutrons (an alpha particle)
Mass number is reduced by 4 and the atomic number is lowered by 2

13. Using Radioactivity in Dating
Types of radioactive decay
Beta emission
An electron (beta particle) is ejected from the nucleus
Mass number remains unchanged and the atomic number increases by 1

14. Using Radioactivity in Dating
Types of radioactive decay
Electron capture
An electron is captured by the nucleus
The electron combines with a proton to form a neutron
Mass number remains unchanged and the atomic number decreases by 1

15. Common Types of Radioactive Decay

16. Using Radioactivity in Dating
Parent – an unstable radioactive isotope
Daughter product – the isotopes resulting from the decay of a parent
Half-life – the time required for one-half of the radioactive nuclei in a sample to decay

17. Using Radioactivity in Dating
Radiometric dating
Principle of radioactive dating
The percentage of radioactive atoms that decay during one half-life is always the same (50 percent)
However, the actual number of atoms that decay continually decreases
Comparing the ratio of parent to daughter yields the age of the sample

18. Radioactive decay curve

19. Radiocarbon Dating Dating with 14C
Half-life of 5730 years
Used to date very recent events
14C is produced in the upper atmosphere
Useful tool for anthropologists, archeologists, and geologists who study very recent Earth history

21. Sources of Error in 14C Dating
Problems of Sample Selection and Contamination
Young carbon effects
Old carbon effects
Variation in 14C content in the ocean reservoir
Fractionation Effects

22. 14C Age of Sea Water

23. Radiocarbon Variation Over the Last 2000 years

24. Calibration Curve Showing Departure

25. Calibrated Curve

26. Radiocarbon Plateaus

27. 14C Bomb Spike

28. Potassium Argon Dating (40K/40Ar)
Instrumental in dating sea-floor basalts and providing the timing of magnetic reversals
Used in dating lava flows
Also for dating metamorphic events

29. Potassium Argon Dating (40K/40Ar)
39K and 41K, Stable
40K unstable and 0.012% of all potassium
40K decays to 40Ca and 40Ar
Ca is common in rocks and is therefore not useful in dating
Measure the amount of 40Ar in the lab and the amount of 40K is also measured from the sample

30. Potassium Argon Dating (40K/40Ar)
Long half-life makes this useful over long time scales but not really usable at less than 100,000 years
Optimal time range is around 30 ma and up to 1 ba rocks can be dated
Dating is done on sanidine, plagioclase, biotite, hornblende, and olivine in volcanic rocks and glauconite, feldspar, and sylvite in sedimentary rocks

31. Problems of 40K/40Ar Assumptions Checks Problem
No Ar was left in the rock at formation
System has remained closed since formation
Checks
The ratio of 36Ar to 40Ar is known in the atmosphere and can be measured in the rock to determine atmospheric contamination
Problem
Loss of Ar due to diffusion, recrystallization, solution, and chemical reactions

32. 40Ar/39Ar Dating
A problem with 40K/40Ar dating is that K and Ar are measured at different places in the rock
This can be solved by irradiating the samples and converting 39K to 39Ar
With the known ratio of 40K to 39K, the amount of 40K can be calculated from the same lattice structure as the 39Ar

33. Uranium Series Dating
238U and 235U have a decay process that cascades through a series of elements
Each decay stage can be used as a dating tool
Thermal Ionization Mass Spectrometry (TIMS) allows very accurate estimates from small samples
U series are useful in dating corals and speleothems
Mollusks seem to be an open system in relation to U and are not generally conducive to U series dating

35. Problems of U Series Dating
Assumes the initial 230Th/234U, 234U/238U, and 231Pa/235U ratios
Likely in ocean sediments but more in flux in the atmosphere
Assumes a closed system

36. Luminescence Dating Principles and Applications
Light emitted from a mineral crystal (usually quartz or feldspars) when exposed to heat or light
The light emitted is related to the amount of ionizing radiation that the sample has been exposed to from sediment
The clock is set to zero by heating or optical bleaching
Therefore Loess and fluvial sediments make good candidates for luminescence dating

37. Luminescence Thermoluminescence (TL Dating)
When the light is emitted as a result of thermal hearting
First published Wintle and Huntley 1979
Optical and Infrared Stimulated Luminescence (OSL and IRSL Dating)
Visible or infrared energy emitted in response to radiation

38. Problems in TL Dating
Assumes that the relationship between the radiation dose and the resulting luminescence is a linear relationship; not always the case
<5,000 yrs the rate of electron accumulation is slow, possibly needing to exceed a threshold
Some minerals may reach saturation >300,000 yrs
Anomalous Fading – Minerals do not hold the electrons beyond a few weeks
Variations in environmental dose; related to moisture content for one example

39. Optical and Infrared Stimulated Luminescence (OSL and IRSL Dating)
Zero in the modern sediments
Sensitive to light bleaching setting the system to zero
Multiple measurements are possible because short stimulation to the light source does not deplete the potential luminescence

41. Fission Track Dating
Uranium will decay through fission, splitting the nucleus and shooting the two halves into the mineral
The results are fission tracks from 10-20μm in length
Some glassy minerals will loose their fission tracks through heating, setting the clock to zero
Different minerals have different annealing temperatures

42. Fission Track Dating
The samples are polished and etched with a chemical that brings out the tracks
The tracks are counted, then the sample is heated, annealing the tracks. Then the sample is irradiated with a slow neutron beam and the tracks from the fission of 235U are counted
The number of induced tracks is proportional to the amount of 238U in the sample
The known fission rate of 238U is used to calculate the age of the sample

43. Fission Track Dating
Can be used in apatite, micas, sphene, and zircons
Can also be used in rocks such as volcanic ash, obsidian, basalts, granites, tuffs, and carbonatites
Ranges from 103 to 108 years
The error associated with this technique is hard to determine and is seldom reported
Repeat measures are the best, but are seldom undertaken because of time constraints

47. Dendrochronology Temperate trees produce annual rings.
The trees are recording all of the environmental variables that affect tree growth.
Can be used to date objects with annual resolution back 10,000 years in the best circumstances.

48. Neutron capture (A) and Beta emission (B)