CarbonNitrogen

Pertinent to younger geologic formations, a radiometric method that is often used is one that specifically dates carbon-bearing sediments. Carbon-14 is continuously produced in the Earth's upper atmosphere through the bombardment of nitrogen by neutrons from cosmic rays. The process creates radiocarbon, which then becomes uniformly mixed with the nonradioactive carbon in the atmosphere. This process differs from other methods because it dates the carbon-bearing pieces of evidence directly rather than dating the rock in which it is found and inferring an age. The technique called "radiocarbon dating" has been used extensively since the 1950s.

All plants that live on the Earth extract carbon from the atmosphere for photosynthesis. Animals acquire it from the consumption of plants and other animals. A small portion of that carbon—carbon-14—is radioactive. When the organism dies, it stops exchanging carbon with the atmosphere and starts the decay process. In other words, the clock begins ticking. The carbon-14 parent decays to the nitrogen-14 daughter, which is a gas that escapes. The amount of carbon-14 loss is mea-

Radiocarbon decays at a known rate. Paleontologists are able to determine the age of a fossil by measuring the amount of C-14 it contains.

Fossil

Radiocarbon decays at a known rate. Paleontologists are able to determine the age of a fossil by measuring the amount of C-14 it contains.

A small piece of the fossil is burned and converted to carbon dioxide gas

Fossil

Living organisms absorb C-14 (radiocarbon) during their lifetimes

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A small piece of the fossil is burned and converted to carbon dioxide gas

Unstable C-14

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Living organisms absorb C-14 (radiocarbon) during their lifetimes

C-14 decays into N-14, emitting an electron

Unstable C-14

9 Electron

Nitrogen

A radiation counter records the number of electrons emitted

C-14 decays into N-14, emitting an electron

Nitrogen

A radiation counter records the number of electrons emitted

9 Electron

Radiocarbon decays at a known rate, making it possible to date objects containing carbon. Once an object dies, it stops absorbing carbon-14. The carbon-14 becomes unstable and decays into nitro-gen-14, where it can be measured and its age calculated.

sured by comparing how much carbon-14 there is compared to how much stable isotope of carbon-12, which is not removed by radioactive decay, there is.

Carbon cannot be used to date extremely old objects or events, because its half-life is only 5,730 years. The reasonable limit for which carbon dating can be expected to be reliable is about 30,000 to 50,000 years. Any type of material that contains carbon can be carbon dated, including anything that lived, such as wood, twigs, leaves, charcoal, and bones. Many samples used for carbon dating come from lake environments, which are poor in oxygen. Because of the lack of decay, such samples are well preserved.

One thing that must be kept in mind with any of the radiometric dating techniques is that their precision depends on how carefully the procedure is performed. If a sample being dated becomes contaminated

Isotope Techniques

Uranium to lead (minerals)

1 mill

on to 4.5 billion years on to 4.5 billion years

Rubidium

to strontium (minerals)

60 mill

Potassium

to argon (minerals)

10,000 to 3 billion years

Uranium series disequilibrium (minerals, shell, bone, teeth, coral)

0 to 40C

,000 years

Carbon-14

(minerals, shell, wood.

bone, teeth.water)

0 to 4C

,000 years

Radiation Exposure Te

chniques

Fission track (minerals

natural gl<

iss)

500,000 to 1 b

Ilion years

Thermoluminescence

minerals, natural glass) and optically stir

nulatt

;d luminesc

ence (miner

a Is) o to 50C

,000 years

Electron spin resonanc

e (minerals

tooth enamel, shell, coral)

1

,000 to 1 million years

Other Techniques

Geomagnetic polarity

timescale (r

riinerals)

780,000 to 200 million years

Amino acid racemizat

on (shells, other biocarbonates)

500 to 30C

,000 years

Obsidian hydration (natural glass

500 to 20C

,000 years

Dendrochronology (tree rings)

0 to 12,000 years

Lichenome

try (lichens

100 to 9,000 years

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O o o o o o o o o o o_ o o o o o o" o o o o o o o o o_ o o o

Years ago

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Different radiometric dating methods are only useful over specific time periods. The proper method must be used for the right sample in order to obtain reliable results.

with a material of another age, it can give a false result. The sample can also become naturally contaminated if isotopes mix with the sample after its original creation. Also, it is important that the right technique (half-life) for the right sample type be used.

According to the U.S. Geological Survey (USGS), thousands of materials that have been dated by radiometric techniques are available worldwide and can be used to bracket geologic formations in order to discover more about the Earth's geologic past. As more information is collected and more formations are dated, geologists gain a better understanding of radiometric dating techniques. As they gain more knowledge, the geologic timescale becomes more refined. At present, the geologic timetable represents the current state of knowledge, but as discoveries are made, the table is revised and modified to reflect new information about the Earth's ancient past.

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