Earth Systems 3209 Unit 2: Absolute Dating

Absolute Dating - pg Is finding the exact age of a mineral, rock, fossil, landform or finding exactly when a geological event occurred. 3 Ways 1.Tree Growth Rings 2.Varves - Glacial deposits 3.Radiometric dating ● ALL PRODUCES NUMBERS!

Tree Growth Rings 1 Growth Ring = 1 Year In places where seasonal changes occur (e.g. Newfoundland), plants add on a new growth ring each year. By simply counting the growth rings in a tree, one can calculate its age.

How old are these trees?

7 years 12 years

Varves Are seasonal deposits of sediment that show alternating layers of clay and sand. Dark fine layer is deposited in fall/winter when lake is frozen and humus settles out of water. (clay) Light coarse layer is deposited in spring/summer due to abundant meltwater carrying large sediment loads (sand) 1 layer of light and dark (2 sediment layers) = 1 year

Question! How old would a glacial deposit be if it had 308 sedimentary layers?

Question! How old would a glacial deposit be if it had 308 sedimentary layers? 1 varve equals two layers -> 154 varves so… 154 years old!

Question! How old is this section of glacial deposit?

Question! How old is this section of glacial deposit? 11 varves – so 11 years old

Radiometric Dating Method of calculating the absolute age of minerals, rocks and fossils that contain radioactive isotopes

Radiometric Dating - subatomic particles

Radiometric Dating - isotope Variations of an element that have different mass numbers The same number of protons but different number of neutrons.

Radiometric Dating - isotope Some are stable, others unstable and will emit particles/energy to get stable. the emission is called radioactivity

Radioactive Isotopes Are unstable isotopes The nucleus breaks down so the original isotope called the “Parent Material” decays into a new stable isotope called the “Daughter Product”. The rate at which the nucleus breaks down is called “Half-Life”.

Radiometric Dating - half life Is the time required for ½ or 50% of the nuclei in a sample to decay. Is exponential Is a constant rate that is unaffected by changes in the environment.

Comparing Parent Material to Daughter Product 0 years represents when rock,mineral or fossil formed and contains 100% of parent material. Half–life rates are known for each radioactive isotope. After the first half life, 50% or ½ the parent nuclei have decayed into a stable daughter product. 1:1, 50%:50%, 1/2 : 1/2

# OF HALF- LIVES % OF PARENT MATERIAL % OF DAUGHTER PRODUCT Ratio of Parent:daughter Fractions 0 100% 100:1:0 1/ :11/ :31/ :71/ :151/ :311/ :631/64

Half-Life Rates pg. 231 ParentDaughterHalf-Life Carbon-14Nitrogen y Uranium-235Lead by Uranium-238Lead by Potassium-40Argon by Rubidium-87Strontium-8747 by *don't need to memorize half lives

Radiometric Dating Using U-238/Pb207

Radiometric Dating Age = Number of half-lives X length of half-life

Radioactivity Problem These questions could make reference to the radioactive parent isotope in; ● Fraction Form (ex. 1/16 th ) ● Percent Form (ex. 25%) ● Amount in Grams (360 grams)

Problem Type #1: Fraction of parent material remaining Given the half-life of U-235 is 0.7 billion years, determine the age of a sample of U-235 if 1/16 of the starting material remains. Given: Half-life = 0.7 billion years Fraction of parent (U-235) remaining = 1/16 ● You must first find out how many half-lives have passed if 1/16 of the parent (U-235) remains. Radioactivity Problems

Problem Type #1: Fraction of parent material remaining Given the half-life of U-235 is 0.7 billion years, determine the age of a sample of U-235 if 1/16 of the starting material remains. Given: Half-life = 0.7 billion years Fraction of parent (U-235) remaining = 1/16 ● You must first find out how many half-lives have passed if 1/16 of the parent (U-235) remains. Age = # of Half-lives x Time for 1 Half-life Age = ( 4 ) (0.7 Billion years) Age = 2.8 Billion years Radioactivity Problems

Problem Type #2: Mass of parent material remaining 1200 g of a radioactive element has decayed to produce 150 g of the element. If the half-life of the mineral is 0.40 billion years, what is the age of the sample? Given: 1200 grams decays to 150 grams & Half-life = 0.4 Billion years You must first find out how many half-lives have passed when 1200 grams decays to form 150 grams Radioactivity Problems

Problem Type #2: Mass of parent material remaining 1200 g of a radioactive element has decayed to produce 150 g of the element. If the half-life of the mineral is 0.40 billion years, what is the age of the sample? Given: 1200 grams decays to 150 grams & Half-life = 0.4 Billion years Age = # of Half-lives x Time for 1 Half-life 1200 g 600 g 300 g 150 g 3 Half lives Age = ( 3 ) (0.4 Billion years) Age = 1.2 Billion years Radioactivity Problems

Problem Type #3: Ratio of parent to daughter A radioactive isotope y has a half life of 20,000 years. If the ratio of radioactive parent to stable daughter in a rock sample is 1:7, how old is the rock? Given: Ratio of Parent to daughter is 1:7 & Half-life = 20,000 years Radioactivity Problems

Problem Type #3: Ratio of parent to daughter A radioactive isotope y has a half life of 20,000 years. If the ratio of radioactive parent to stable daughter in a rock sample is 1:7, how old is the rock? Given: Ratio of Parent to daughter is 1:7 & Half-life = 20,000 years Age = # of Half-lives x Time for 1 Half-life Age = ( 3 ) (20,000 years) Age = 60,000 years Radioactivity Problems Ratio parent to daughter 1:7 Out of 8 parts 1 is parent material Fraction = 1/8 3 half lives

Sources of Error pg Addition or loss of either the parent material or daughter product will produce an inaccurate date. ex. Contamination by other radioactive isotopes. ex. Chemical weathering on surface of sample or erosion by water. ex. Ar-40 is easily lost because it is a gas.

2. Cannot be used to date sedimentary rocks directly since the age of each sediment predates when it was deposited. Radiometric dating determines how long a go liquid rock cooled to a solid. (Best used to date igneous and metamorphic rocks)

3. Metamorphism resets the radioactive clock by adding hydrothermal fluids or loss of parent material and daughter product. 4. Appropriate application of certain parent-daughter pairs. ex. C-14 can only be used to recent date fossils (50,000 y) HL is short. ex. U-238 half life is too slow to date Cenozoic rocks. Not enough decay has occurred

Group work Practice questions