LESSON 10: Radiometric Dating (ABSOLUTE DATING)

What are Isotopes? Same number of PROTONS Different number of NEUTRONS C12 C14

Why are some Isotopes ‘unstable’? Elements are composed of one or more naturally occurring isotopes, which are normally stable. Some elements, however, have unstable (radioactive) isotopes Unstable atoms are radioactive because their nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves. Radioactive isotopes decay so slow that a fraction still remains since they were created (examples: uranium, potassium) About 339 isotopes occur naturally on Earth, of which about 79% are stable

What Isotopes are used for Dating Fossils? Technique Range (in yrs) Daughter Mol. Half life What they are Used for: lead 210 Pb210 1 – 150 Bismuth 210 22.3 year lake and ocean sediments, glacial ice carbon 14 C14 1 – 60,000 Nitrogen 14 5,730 yrs previously living things Uranium 235 U235 1 – 400,000 Lead 207 704 million yrs bone, teeth, coral, shells, eggs Potassium 40 K40 10,000 – 3 billion Argon 40 1.25 billion minerals, igneous rocks Uranium 238 U238 1 million - 4.5 billion Lead 206 4.5 billion yrs Rubidium 87 60 million – 4.5 billion Strontium 87 48.8 billion yrs

What is Radiometric Decay? Naturally-occurring radioactive materials break down into other materials at known rates. Radioactive parent elements decay to stable daughter elements. Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements. Each radioactive isotope has its own unique half-life time it takes for half of the radioactive element to decay

RADIOACTIVE DECAY GRAPH

What is Carbon Dating? Method that uses the naturally occurring 14C isotope to determine the age of carbonaceous materials up to about 70,000 years old. One of the frequent uses of the technique is to date organic remains from archaeological sites. Plants fix atmospheric carbon during photosynthesis, so the level of C14 in living plants and animals equals the level of C14 in the atmosphere.

How does Carbon Dating Work? Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide. Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle. All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion). At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14. The change in the Carbon 14 to Carbon 12 ratio is the basis for dating. The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old.

How does Radiometric Dating work? Carbon-14 50% 5,730 years Carbon-14 25% 11460 years Carbon -14 12.5% 17190 years Carbon-14 100% 0 years 1000 particles 500 particles 250 particles 125 particles

What is the Oldest Dated Rock? Currently, the oldest known rocks in the world come from the Acasta area of Canada's Northwest Territories. They have been dated using the isotope U-Pb. The oldest rocks dated around 3900 million years old or roughly 3.9 billion years. (why not 4.5 billion years?)

Let’s Practice Carbon-14 half-life is 5,732 years A fossil has 50% carbon-14 and 50% Nitrogen-14 How old is the fossil? 5732 years old A fossil has 25% carbon-14 and 75% Nitrogen-14 11464 years old

Let’s Practice Some More: An ancient Eocene fossil fish Priscacara liops from Green River formation in Utah was found in a sedimentary layer of rock that contained 50 parent molecules (C14 ) and 950 daughter molecules (N14 ). How old is this fossil?

ANSWER: C14 Decays at a rate of ~ 5700 yrs per half life. There were a total of 1000 atoms originally (50 C14 + 950 N14 ) After 1 half-life there would have been 500, 2nd 250, 3rd 125, 4th 62.5, 5th 31.25, etc. The number of parent atoms falls between the 4th and 5th half-life, so…. The Fossil is approximately 22,800 years old Can you explain how I got this answer?