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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