Absolute Dating

Absolute Dating Methods Uses Radioactive Decay Sequences 1. Radioactive Isotopes change (decay) into daughter isotopes at known rates. 2. Rates vary with the types of isotopes: Uranium, Carbon, etc. 3. Act as an “atomic clock”. We see the clock at the end of its cycle—like watching a stopwatch count down. 4. Allows for the assignment of Numerical Dates to Rocks and Fossils.

What is radiation and what does it do?

Radiometric dating Uses continuous decay to measure time since rock formed Only possible since late 1890’s -- radioactivity discovered in 1896 As minerals crystallize in magma; they trap atoms of radioactive isotopes in their crystal structures Radioactive isotopes will decay immediately and continuously As time passes, rock contains less parent and more daughter

Chemistry Review: What is a Radioactive Isotope? Atom model # protrons = atomic #, defines the element electrons nucleus protrons neutrons # neutrons can vary: “isotopes”

Radioactive isotopes 1. have nuclei that spontaneously decay 2. emit or capture subatomic particles 3. parent: decaying radioactive isotope 4. daughter: decay daughter parent daughter loss or gain 5. The loss or gain of neutron converts parent to daughter of same element 6. The loss or gain of proton changes parent into entirely new daughter

particle has 2 neutrons and 2 protons U238 Th234 92 protons 90 protons 3 primary ways of decay alpha decay (Z ≥ 58) particle has 2 neutrons and 2 protons U238 Th234 92 protons 90 protons beta decay (n0 = p+ + e-) breakdown of neutron into an electron and a proton and loss of the electron to leave a proton (result is gain of one proton) K40 Ca40 19 protons 20 protons electron capture (e- + p+ = n0) capture of an electron by a proton and change of proton to neutron (result is loss of proton) K40 Ar40 19 protons 18 protons

What about the RATE? (i.e. How Fast does an element decay?) To use the rate to determine age, we must understand the concept of half-life.

Half-life Amount of time it takes for half the atoms of the parent isotope to decay Regardless of isotope, the ratio of parent to daughter atoms is predictable at each half-life.

Rate of Decay t All atoms are parent isotope or some 1 3 All atoms are parent isotope or some known ratio of parent to daughter 1 half-life period has elapsed, half of the material has changed to a daughter isotope (6 parent: 6 daughter) 2 2 half-lives elapsed, half of the parent remaining is transformed into a daughter isotope (3 parent: 9 daughter) 3 half-lives elapsed, half of the parent isotope (1.5 parent: 10.5 daughter) We would see the rock at this point.

Radioactive Decay Rates are Exponential! (not Linear…) Exponential decay: never goes to zero

Radioactive Isotopes analogous to sand in an hour glass As more sand flows out, you have less “parent” and more “daughter” sand. Only here each set of sand is a different element. 100 Parent Parent % parent remaining Daughter Daughter 50 25 13 time----------->

Five Common Radioactive Isotope Pairs Five Radioactive Isotope Pairs Effective Dating Range Minerals and Isotopes Half-Life of Parent (Years) Rocks That Can Parent Daughter (Years) Be Dated Uranium 238 Lead 206 4.5 billion 10 million to Zircon 4.6 billion Uraninite Uranium 235 Lead 207 704 million Muscovite Thorium 232 Lead 208 14 billion 48.8 billion Biotite Potassium feldspar Rubidium 87 Strontium 87 4.6 billion 10 million to Whole metamorphic 4.6 billion or igneous rock Potassium 40 Argon 40 1.3 billion 100,000 to Glauconite 4.6 billion Muscovite Biotite Hornblende Whole volcanic rock

Example: Uranium 238 decay to Lead 206 (stable)

Most common Radioactive dating systems •1. uranium-thorium-lead dating (previous example) U-238, U-235, Th-232 each of these decays through a series of steps to Pb U-238 to Pb-206 half-life = 4.5 by U-235 to Pb-207 half-life = 713 my Th-232 to Pb-208 half-life = 14.1 my •2. potassium-argon dating …argon is a gas--may escape (ages too young--daughter missing) K-40 to Ar-40 half-life = 1.3 by •3. rubidium-strontium dating Rb-87 to Sr-87 half-life = 47 by

4. What about carbon dating?

Radiocarbon Dating Carbon-14 dating is based on the ratio of C-14 to C-12 in an organic sample. Living things take in three isotopes of carbon Carbon 12 and 13 are stable, but carbon 14 is not Carbon 14 has a half-life of 5730 years When the organism dies, the “clock” starts. (C-14 begins to decay to N-14) Valid only for samples that are less than 70,000 years old (not useful for most rocks) Method can be validated by cross-checking with ice cores and tree rings

Carbon 14 Cycle

Correlation: Cross-checking dating techniques How do we “check” our readings and correlate from different sources? Absolute Dating Techniques are cross-checked using: 1. Ice Cores—up to 70,000 years old 2. Dendrochronology—about 14,000 years (recent samples only) 3. Stratigraphy—rock layers of known formation rates and types

Ice Cores?

Some Ice core drill sites in Greenland:

How are ice cores sampled? Samples are examined using various light sources for particle types and ice dynamics

What do we get from ice cores? 1. Climate information: Paleoclimatology. Temperature changes Precipitation changes Air Particulates counts/types—including carbon, dust, bacteria, algae 2. Atmospheric Composition (Paleoatmospheric studies)—what gases were in the atmosphere in the past and at what levels? 3. Solar Activity (i.e. by the algae, bacteria and particulate types)

More recent events? Use Dendrochronology

Tree Ring Dating Method

Cross-checking: Stratigraphy of Vertical Layers Stratigraphy—large undisturbed layers of rocks/fossils are laid down in sequential order. (Law of Superposition) Some Layers of iron-bearing igneous rocks can be correlated by their magnetic properties based on the Earth’s periodic reversal of magnetic poles! (like Seafloor Spreading)

What about Horizontal or Diagonal section layers? We use Walther's Law • The vertical sequence is repeated by the horizontal sequence - walking from A to B to C to the Coast you would encounter the rocks that would be encountered by drilling a core into the earth at any point (A, B, or C)

Basic Geochronological assumptions 1. Decay rates are constant through geological time * Good reasons to believe that this is correct from nuclear physics * Measurements of decay sequences in ancient supernovae yield the same values as modern lab measurements 2. The system is closed to adding or subtracting of parent/daughter isotopes * Isotopic system and type of mineral are important. * Careful procedure is essential to correct analysis The type of rock used for dating is well known. * Igneous rocks are most reliable for absolute dating. * Metamorphism may cause loss of daughter products * Sedimentary rocks will give ages of the source rocks