Radiometric Dating The primary method used to determine absolute ages of geologic and some biologic materials.

Recall the basic structure of the atom: a nucleus with protons and neutrons surrounded by shells or orbitals of electrons.

Protons: + charge Electrons: - charge Neutrons: no charge # protons = # electrons # neutrons are different in different isotopes of an element.

Unstable isotopes of certain elements (called parents) radioactively decay to the stable isotopes of other elements (called daughters). This happens in the nucleus by several mechanisms.→ The bottom line is that the number of protons and neutrons in the parent isotope changes as it decays to the daughter. This decay occurs at a precisely determined rate called a half-life.

For example, the parent isotope 238U decays to the daughter isotope 206Pb with a half-life (t1/2) = 4.5 x 109 years.

This means that with the passage of every 4 This means that with the passage of every 4.5 x 109 years, the number of remaining 238U is reduced by 50%: t1/2 = 0 238U = 100 206Pb = 0 t1/2 = 1 238U = 50 206Pb = 50 t1/2 = 2 238U = 25 206Pb = 75

The parent isotope 14C decays to the daughter isotope 14N with a t1/2 = 5730 years.

As sediments gradually accumulate over the years on the floors of the ocean or lakes, younger sediments accumulate on top of older sediments. So in any stack of sediments, the oldest sediments are at the bottom and the youngest are at the top. This is called the Principle of Superposition. Example: Off the East Coast of the U.S. sediments have been accumulating for about 200 million years and are up to 8 km thick. The youngest are accumulating right now. Example: Grand Canyon→ We can determine the relative ages of sediments and sedimentary rocks just by their relative elevations in the stack.

Retrieving the upper few meters of ocean sediments using a sediment corer.

The Joides Resolution can retrieve many kilometers of sediment at one site.

Fossils in sediments aid in correlating rock units and help in relative and absolute age measurements. In marine environments, organisms in the water column or on the sea floor can get incorporated into the sediment layers slowly accumulating on the sea floor. Most species live no more than a few million years, so their fossils are found over relatively short vertical intervals in the sediments cored from the sea floor.

Often assemblages of several fossils with overlapping time intervals can focus the resolution even more.