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Radiometric and Trapped-Charge Dating R.A. Varney Paleoresearch Institute. Presentation Format Basic Chemistry for Radiometric Dating Types of Radiometric Dating for Archaeology Primer on Radiocarbon and Radiocarbon Dating Potassium-Argon Dating Argon-Argon Dating
Relative dating is great for determining the sequence of events within a culture, but to compare cultures to other cultures or to place events within calendar time you need to have a method of absolutely dating an archaeological context.
This calls for absolute dating.
All of the nuclei pictured here are the element Hydrogen, because they all contain a single proton, but each is a different isotope of Hydrogen, because they contain different numbers of neutrons.
Number of Neutrons and Protons This is how the isotope if identified
Number of Protons This is how the element if defined
Radioactive decay does not happen all at once. A certain percentage of the quantity of the radioisotope will decay over a period of time. The time that it takes for half of the original mass of the radioisotope to decay is called the half-life of the isotope. If the original mass is 1000 grams, at one half-life in time there will be 500 grams of the radioactive material remaining. At two half-lives, there will be 250 grams remaining.
Carbon 14 is formed when cosmic radiation in the upper atmosphere excites a neutron, causing the neutron to impact a Nitrogen 14 atom and dislodges a proton forming carbon 14.
Carbon 14 is created most intensely at altitudes from 30,000 to 50, 000 feet and at high latitudes. This is a plasma model of the magnet fields of the earth showing the relatively less protected polar areas where gamma radiation is able to most easily interact with the Nitrogen in the atmosphere and change the Nitrogen 14 to Carbon 14.
Radiocarbon dating was developed in 1949 by Willard Libby, one of the developers of the atomic bomb who wanted to explore peaceful uses of radiation after his involvement in the bomb. In 1960, Dr. Libby received the Nobel prize for his radiocarbon research.
The Graphite sample is placed on a sample wheel, where a Cesium ion beam turns the sample into atoms.
Radiocarbon dates returned from the lab need to be calibrated to calendar years.
Radiocarbon creation in the atmosphere has varied a little through time, due to changes in the levels of gamma radiation based variations in the earths magnetic field and the frequency and intensity of solar storms. These variations in the quantity of radiocarbon in the atmosphere are called Seuss Squiggles.
Radiocarbon mixing in the atmosphere and old carbon influx into the atmosphere has varied a little through time primarily due to changes in climate ie. Rock weathering and exposure after glacial periods
This is a normal curve, the curve is the same shape on either side of the mean or center point of the distribution.
Radiocarbon dates from the lab are usually returned in one standard deviation.
This is read as zero plus or minus one half (0 +/- 1/2)to two standard deviations
The result from the counter or accelerator will be reported as a number( the mean:7630) and a range (Standard Deviation or the sigma: ±15).
This is the current radiocarbon calibration curve going back to about 14800 radiocarbon years.
This is INTCAL 2004 calibration curve for the past ~14000 Radiocarbon years
Radiocarbon age calibration is the process of drawing a line from the 14C determination to the curve and then dropping a line to the calendar year. Sounds simple right?
Example: 14C determination of 12500 RCYBP becomes calendar year 15000 BP.
When the sample is prepared for dating the rock is split in a potassium and an argon sub-sample. If the rock is not completely homogeneous the ratios of argon and potassium in the two sub-samples may not be the same and the date will not be accurate.
Argon/ Argon dating prevents this problem by measuring the same sample for both isotopes.
This is the complex Uranium Lead series. All of these daughter products are radioactive except Lead 206 (Pb 206) in the lower left corner. The half-life of each isotope is given in the number below the isotope.Several components of this decay series are used for dating.