radiocarbon dating Flashcards
1
Q
all carbon atoms have
A
6 protons in the nucleus,
but the nucleus may also contain 6, 7, or 8 neutrons
2
Q
carbon-12
A
Carbon with 6 protons and 6 neutrons is called carbon-12 (12C).
This is a stable nucleus. 99% of all natural carbon is 12C .
3
Q
carbon-13
A
Carbon with 6 protons and 7 neutrons is called carbon-13 (13C).
This is also a stable nucleus. 1% of all natural carbon is 13C .
4
Q
carbon-14
A
Carbon with 6 protons and 8 neutrons is called carbon-14 (14C).
This is an unstable radioactive isotope. About 0.001% of carbon atoms
in the atmosphere is 14C.
5
Q
radioactive carbon
A
Radioactive carbon (14C) is generated in the upper troposphere when a cosmic ray (typically a proton) hits the nucleus of an atom and produces a neutron (among other things) that is then captured by a nitrogen atom (14N) - In the process the 14N becomes 14C, and a H+ ion (a proton) is released
look at ppt
6
Q
Atmospheric neutron intensity and 14C production
A
look at ppt
- 1948- the neutron intensity in the atmosphere increases
- between 0° and 60° this dependence was a ˜ 200-400% effect - depending on altitude .
7
Q
14C in atmosphere
A
14C quickly combines with the oxygen in the atmosphere to form carbon dioxide (CO2).
CO2 diffuses in the atmosphere, is dissolved in the ocean, and is taken up by plants via photosynthesis. Animals eat the plants, and ultimately the radiocarbon is distributed throughout the biosphere.
The ratio of 14C to 12C is ~1.5 parts of 14C to 1012 parts of 12C.
14C/12C ≈1.5 x10-12
8
Q
Carbon present in a plant
A
- Because the carbon present in a plant comes from the atmosphere the ratio of radiocarbon to stable carbon in the plant is virtually the same as that in the atmosphere
- All animals in the food chain, including carnivores, get their carbon indirectly from plant material.
- all living organisms have the same radiocarbon to stable carbon ratio as the atmosphere.
- When the organism (or a tissue) dies absorption of 14C ceases and the amount of 14C gradually decays
look at ppt
9
Q
Decay of 14C
A
- half-life of 14C is ~5,730 years
- concentration in the atmosphere does not reduce as it is constantly produced in the lower stratosphere and upper troposphere
- Radioactive carbon (14C) decays back to nitrogen (14N) emitting an electron (e–) and an antineutrino ( ) with no mass or charge.
- look at ppt
- This radioactivity in living tissue is very weak at about 2.5% of that due to the decay of naturally occurring potassium-40 (40K).
10
Q
Geiger counter and dating
A
- you can’t tell if somebody is alive or not using a Geiger counter; flesh is practically opaque to the radiation. The path length in air is about 22 cm.
- After about ten 14C to 14N half-lives (~57,000 years) there is almost no more 14C left in the tissue.
- By measuring 14C content, you can estimate how long ago the tissue died (providing that it isn’t so old that the 14C level is too low to measure accurately)
- A practical limit for accurate dating is 26,000 years (in other words material that is younger than the Last Glacial Maximum), the last period in the Earth’s climate history during the last glacial period when ice sheets were at their greatest extension.
- you can get less accurate dates up to 43,500 years and, some facilities provide rough dates to ~60,000 years.
look at ppt
11
Q
Conception
A
- 1946 suggests that 14c exists in living matter but stops on death
- Any organic compound has an inbuilt nuclear clock
- 1949 tests on sequoia trees of known dates using tree rings proved radiocarbon dating gave correct result
12
Q
Measuring 14C concentrations- 2 methods
A
- Radiometric: count decay rate of individual atoms in a sample using a gas
- proportional counter (a form of Geiger counter) or a liquid scintillation counter;
- AMS: you do a complete isotopic analysis in an accelerator mass spectrometer
- (AMS).
- Radiometric dating is relatively cheap (about $300/sample), takes about a month to achieve satisfactory statistics, requires about a 100 grams. It is a good method for averaging material composed of material of various ages (lake sediments etc.).
- AMS dating is relatively expensive (about $600/sample or more depending on prep. time needed ), takes about a week, requires only about a gram. It is a good method for dating specific samples, a pine needle for example, when the sample may contain younger extraneous material.
13
Q
AMS
A
look at ppt
14
Q
Radioactivity measurements
A
- Various calibration standards are used for radioactivity measurements.
- A common one currently in use is Oxalic Acid II, which was derived from a crop of 1977 French beet molasses.
- Facilities date this to make sure they all get the same answer.
- Sample preparation (which is a skilled and labour-intensive process ) involves extracting the carbon as CO2, purifying it, and then converting it to an organic compound such as benzene or toluene that’s easy to handle.*
- Methods differ from lab to lab.
15
Q
Materials that have been radiocarbon dated since the inception of the method include:
A
- charcoal, wood, twigs, seeds, peat, pollen, resins
- bones, shells, corals
- hair, leather, blood residues
- lake mud, soil, water
- pottery, wall paintings, fabrics, paper, and parchment. All must have at least some carbon of organic origin.
16
Q
Not everything is easy to date
A
- Bone is mostly hydroxy-apatite a form of calcium phosphate: Ca10 (PO4)6 (OH)2
- The bone probably dates back to the Port Moody Interstade ca. 18,000 BP , but that’s a guess.
17
Q
Simplified version of carbon exchange reservoir
A
- showing percentage of carbon in each reservoir
- and ratio of 14C to 12C as a fraction of the ratio in the atmosphere which is taken as 1.
- e.g. in the oceans there is less14C to 12C than there is in the atmosphere (only 95% as much 14C as would be expected if the ratio was the same as in the atmosphere)
look at ppt
18
Q
Carbon-13 isotope fractionation
A
- Many biochemical processes alter the ratios of 12C, 13C, and 14C.
- Photosynthesis, depletes the amount of 13C compared to 12C by -1.8%.
- 12C is absorbed slightly more easily than 13C, which in turn is more easily absorbed than 14C.
- The carbon in seawater is the reverse. It is enhanced by +0.7%.
- If a sample shows a lower ratio of 13C to 12C than exists in the atmosphere, it is reasonable to expect that the amount of 14C to 12C has also been reduced, making the sample appear older than it actually is.
19
Q
deviations
A
- Standard practice to correct for deviations of 13C to 12C from the norm.
- These are reported as a “delta 13 C” correction (δ13C).
- The δ14C fractionation is taken to be 3 times greater.
The norm for δ13C is -2.5% - measurements published in articles before ca.1990 did NOT make this adjustment,
- These have to be re-evaluated before being compared with more recent measurements.
20
Q
Radiocarbons years
A
- Radiocarbon years are reckoned as “before present”, present being defined as 1950 AD which was when the method was first developed.
- It is important to understand that for various reasons radiocarbon years are not the same as calendar years.
- If a geologist said in 2000 AD, the last ice age ended ca. 11000
- “years ago “what he or she probably meant is 11000 14C BP (11000 radiocarbon years before 1950 AD ). This happens to be 10964 BC (≈13000 calendar years before 2000 AD).
- It is not 9,000 BC.
- Among geologists this difference scarcely matters so long as they are all on the same page, but it is obviously important to historians and archaeologists who have access to other dating methods.
21
Q
Assumptions-
A
- 14c production is constant
- The biosphere and atmosphere have roughly the same 14c concentration
- After death there is no 14c exchange and it is only affected by radioactive decay
22
Q
Some deviations
A
- Glacial effects
- Human activity
- Variations in natural production rate
23
Q
Radiocarbon years –variations in14C generation rates
A
- conventional radiocarbon years BP correspond only approximately to calendar years BP.
- most important is that the rate of generation of 14C in the upper atmosphere has not been and is not constant, but varies slightly from year-to-year, because of variations in cosmic ray intensities from the sun.
- Because the information needed to convert radiocarbon ages to calendar ages is constantly being improved, it was decided to make it a standard that radiocarbon ages and not calendar ages be the prime method of recording results. This has the advantage that thousands of dates published in articles previous to any update do not have to be re-calculated. It is primarily up to the user of the data to make the conversion using the best data available and in whatever way seems appropriate.
24
Q
Variations in production rate major cause of suess wiggles
A
- Han Suess
- Changes in 14C production rate due to solar modulation of cosmic ray flux
- 14C rises by 1% every 20 years and decreases by 1% in every 40 years.
- “wiggles” in 14C production
25
Radiocarbon ages
- radiocarbon ages are about 22% less than calendar ages.
| - Something that is measured to be ten thousand years old, is actually about twelve thousand years old.
26
Radiocarbon years- the actual half-life of 14C
- Another reason for the conventional radiocarbon years BP(RC) age not being the same as the calendar age is that:
- Libby estimated that the half-life of 14C was 5568years.
- Later discovered that the half-life is closer to 5730years,
- Continued to use the "Libby standard" so that the thousands of dates published in articles previously could still be compared to dates going forward.
- The half-life correction is now made when the conventional radiocarbon ages are converted to calendar or calibrated ages.
- “Present" in "before present" (BP) continues to be defined as 1950 AD for the same reason, but also because atomic bomb testing in the 1960s and 1970s artificially raised levels of 14C in the atmosphere.
27
Human activity
- Burning carbon as a fossil fuel increases amount of 14C in atmosphere
- Atomic bomb creates 14C radioisotopes
28
Radioactivity nomenclature
- The conventional radiocarbon age(14C years BP) is a report that conforms to International Standards using:
- • a half-life of 5568 years (the Libby standard);
- • Oxalic Acid I or II as the modern radiocarbon standard;
- • correction for sample isotopic fractionation (δ13C) to -2.5 % relative to
- the ratio of 13C/12C in the carbonate standard VPDB (Vienna Peedee
- Belemnite);
- • 1950 AD as 0 BP;
- • the assumption that 14C reservoirs have remained constant through time.
- Older data often has to be re-interpreted to conform to this standard.
29
Radioactivity nomenclature continued
- The calibrated age(cal. years BP) is the calendar year equivalent of the conventional radiocarbon age.
- Constant improvements to calibration data mean that one person's calendar BP will not be the same as another's.
- Some use 2000 AD as a base, others use 1950 AD.
- Databases used for conversion of 14C years BP ages to calendar years BP are IntCal13 and Marine13. - maintained by an international committee, published in the journal RADIOCARBON and up-dated once every 5 years.
- Information needed for constructing the database comes from analysis of samples that can be independently dated (e.g., counting tree rings, and uranium-thorium analysis of corals and foraminifera
- The corrected radiocarbon age is an intermediate figure, not usually included in final reports.
- It usually means the measured radiocarbon age after correction for δ13C fractionation.
- Some authors use the term to mean the conventional radiocarbon age after correction for carbon storage in reservoirs. This applies only to marine or lacustrine samples.
30
Calibrating carbon-14 dating – Dendochronology
- tree rings from very long-lived trees “bristlecone pines”
- trees only produce one ring per year
- In reality a tree may in fact produce several rings or no rings in a given year depending on environmental factors
- Can determine how old the trees were when they died
- e.g. a very old living tree has 2,500 rings
- innermost rings died 2,500 years ago
- ratio of 14C to 12C in these innermost rings, using the current ratio of 14C to 12C in today’s atmosphere, should give a very direct “age” of these rings that is very near 2,500 years – right? Wrong! The carbon-14 “age” will not match the tree ring “age” very well at all.
- wood from ancient structures with known chronologies can be matched to the tree-ring data (a technique called cross-dating), and the age of the wood can thereby be determined precisely.
- Adequate moisture and a long growing season result in a wide ring, while a drought year may result in a very narrow one.
- alternating poor and favourable conditions, such as mid-summer droughts, can result in several rings forming in a given year.
- Some tree-species present "missing rings", missing rings are rare in oak and elm trees
- oak and pine in central Europe extends back 12,460 years
- bristlecone pine in the Southwest USA extends back 8500 years
31
radiocarbon years- variations in reservoirs (global)
- Lakes and oceans act as reservoirs of carbon.
- The carbon content of a small lake may be only 20 years older than the carbon on the atmosphere, but in the ocean, the carbon may be many hundreds of years older. This will result in marine shells and foraminifera appearing to be older than they actually are.
- The average age of carbon in the surface water of the world's oceans is about 400 radiocarbon years, and this is quite constant in places like the eastern Atlantic Ocean.
- Prior to 2004, it used to be standard practice to subtract 400 years from the conventional radiocarbon age of marine samples. This is called R(t), the pre-industrial "global reservoir correction". Not needed if use Marine13.
32
Glacial effects
- CO2 solubility is temperature dependant
33
Radiocarbon years- more variations in reservoirs (local)
- The local reservoir correction, "constant" was far from constant during the late-Pleistocene/early-Holocene transition.
- the age of the carbon in the ocean on the west coast depends on deep-water circulation patterns in the Pacific, and these were different during the ice age.
- Comparisons between the radiocarbon ages of wood and shell found at the same location in ice-age deposits have shown differences in excess of 1000 years. One paper records a total reservoir correction (R(t) + ΔR) that for younger samples would be 790 radiocarbon years, as 1250 years at the time of de-glaciation.
- Further research might provide interesting data on changes in ocean circulation in the north Pacific at that time as well as better accuracy.
34
The dating principle
- During its life, a plant or animal is exchanging carbon with its surroundings, so the carbon it contains will have the same proportion of 14C as the atmosphere.
- Once it dies, it ceases to acquire 14C, but the 14C will continue to decay, and so the ratio of 14C to 12C in its remains will gradually decrease.
- By looking at the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing fairly precisely.
- Because 14C decays at a known rate, the proportion of radiocarbon can be used to determine how long it has been since a given sample stopped exchanging carbon – the older the sample, the less 14C will be left
- The equation governing the decay of a radioactive isotope is
- Nt = Noe-λt
35
A formula to calculate how old a sample is by carbon-14 dating is:
- t = [ ln (Nf/No) / (-0.693) ] x t1/2
- where ln is the natural logarithm, Nf/No is the percent of carbon-14 in the sample compared to the amount in living tissue, and t1/2 is the half-life of carbon-14 (5,700 years).
- So, if you had a fossil that had 10 percent carbon-14 compared to a living sample, then that fossil would be:
- t = [ ln (0.10) / (-0.693) ] x 5,700 years
- t = [ (-2.303) / (-0.693) ] x 5,700 years
- t = [ 3.323 ] x 5,700 years
- t = 18,940 years old
- ( no adjustment of calibration has been made for simplicity)
36
Calculation
Obtain ratios of:
- 14C to 12C in the sample
- and in a modern carbon standard (takes account of fossil fuel effect of 1950)
- Ratio between sample and standard is called the “fraction modern”
- Adjust fraction modern to take account of isotopic fractionation
- Calculate “radiocarbon age”
37
The principle of carbon-14 dating applies to other isotopes as well
- Potassium-40 is another radioactive element naturally found in your body and has a half-life of 1.3 billion years.
- Other useful radioisotopes for radioactive dating include:
- Uranium -235 (half-life = 704 million years),
- Uranium -238 (half-life = 4.5 billion years),
- Thorium-232 (half-life = 14 billion years)
- and Rubidium-87 (half-life = 49 billion years).
38
Further complications arise due to:
- contamination- e.g. soil or conservation work becomes incorporated into the sample resulting in an admixture of carbon with a different radiocarbon content;
- chemical pre-treatment is used to remove contaminants
Hemisphere effect
- southern hemisphere has a lower 14C/12C ratio than the northern hemisphere
Volcanic eruptions
- ejects large amounts of carbon into air
- geologic origin hence no 14C
- ratio of 14C/12C in area of volcano is lower.
Geologic effects
- Fresh water passing through limestone (calcium carbonate) or humus has C with no 14C thus lowering ratio
39
What can be dated?
- must once have been part of a living organism.
- radiocarbon date tells us when the organism was alive (not when the material was used).
- better to date a properly identified single entity (such as a cereal grain or an identified bone) rather than a mixture of unidentified organic remains.
look at ppt
40
Forensic applications: Applying Carbon-14 Dating to Recent Human Remains
- Measuring carbon-14 levels in human tissue could help forensic scientists determine age and year of death in cases involving unidentified human remains.
- Traditional radiocarbon dating is applied to organic remains between 500 and 50,000 years old
- From 1955 to 1963, atmospheric radiocarbon levels almost doubled due to above-ground nuclear weapons testing
- For teeth formed after 1965, enamel radiocarbon content predicted year of birth within 1.5 years
- tooth enamel radiocarbon content is determined by the atmospheric level at the time the tooth was formed giving the year of birth.
- Radiocarbon levels in teeth formed before then contained less radiocarbon than expected, so when applied to teeth formed during that period, the method was less precise.
- year of death is determined by radiocarbon levels in soft tissues, which unlike tooth enamel, are constantly made during life. Thus, their radiocarbon levels mirror those in the changing environment.
- soft tissue radiocarbon content is transferred to, and preserved in, the pupal cases of insects whose larvae feed on these tissues
- Barring any future nuclear detonations, this method should continue to be useful for year-of-birth determinations for people born during the next 10 or 20 years.
- Everyone born after that would be expected to have the same level of carbon-14 that prevailed before the nuclear testing era.
- Radiocarbon dating is an extremely useful technique for determining the ages of geological materials (that have some organic-derived carbon in them), and it is highly applicable to the study of Quaternary materials (that are younger than 50 ka).
- But, interpretation of radiocarbon data can be quite complex, and several factors need to be taken into account to understand what the results actually mean.
