Age Dating Fossils and Sediments

Neal Immega

Direct Methods

Dendrochronology (tree rings) – 0 to 12,000 years (beginning of the Pleistocene)

Greenland Ice Core – 0 to 800,000 years (middle of Pleistocene)

Magnetic Reversals – 0 – 100 million years (beginning of the Cretaceous)

Dating by tree rings

Excellent for dry sites that contain preserved wood like American Southwest.

Excellent way to calibrate C14 dating – max is 5%.

Has not been extended to fossil wood.

Ice Cores

Record yearly data back 800,000 years.

Dust, pollen, volcanic ash, gas bubbles

Magnetic Reversals – north is not always north

Cooling igneous rock picks up current magnetic field.

Settling sediments also align

Sea Bottom – polarity stripes

Sediments show magnetic reversals

Magnetic grains settle showing current magnetic field.

Lots of samples are needed in the section.

Stored Energy DatingVery popular with archeologists for pot dating

Thermo luminescence – heat to release energy

Optical Spin Luminescence – light will release energy

Electron spin resonance –

magnetic field will measure

stored energy

Dating using Radioactivity

Potassium/Argon and Argon39/Argon40

Uranium/Lead

Carbon 14 – special case

Fission track

Radioactive Dating

Mass Spectrometry – accelerates ions in a magnetic field.

Different masses follow different paths.

Radioactive parent element > normal daughter element

K40/Ar40 and Ar39/Ar40 looks for argon trapped in the crystal

Drawing by Dr. Bakker

Elapsed time since the crystal cooled.

Drawing by Dr. Bakker

Ar39/Ar40 Correct for weathering by progressive heating

What do you date?

Date every ash layer and lava you can find.Hopefully they will bracket the fossils you want to date – This is Olduvai gorge.

basalt

Drawing by Dr. Bakker

Drawing by Dr. Bakker

Drawing by Dr. Bakker

Uranium/Lead dating - zircons

Uranium fission tracks – etch crystal to show radiation damage

Carbon 14 – continuous input

DNA dating – accumulation of mutations

Estimate time of split between chimps and humans, Eve, dogs/wolves, …

Dating Stratigraphy with Fossils