1. Sedimentary rocks and their depositional environments
How can sedimentary rocks be used to figure out what the environment was in the past?

2. Depositional environments
*How does energy of the environment relate to grain size???

3. Sedimentary environments
Sea level rising, shoreline moves inland transgression
Sea level falling, shoreline moves off land regression
What can cause changes in sea level?
Transgression and regression animation

4. Sea level rising: transgression

5. Transgression

6. How to recognize a transgression
Fining upward

7. Transgression facies
From:

8. Sea level dropping: regression

9. Regression

10. How to recognize a marine regression
Coarsening upward

11. How to recognize a transgression

12. Regressive facies
From:

13. Geologic time: absolute dating
relative dating animations
We’ve talked about rocks and how to tell the relative age of them using Steno’s laws and sedimentary structures.
Radioactive decay of certain elements allows us to date rocks more precisely.

14. Absolute dating Radioative elements are geologic “clocks”
Amount of decay=age of the element and age of rock (have to be careful here!)

15. Isotopes of Hydrogen Hydrogen Deuterium
1 proton, 0 neutrons
Deuterium
1 proton, 1 neutron
Isotope are elements with different atomic masses (neutron #’s differ)
When atoms are unstable they eject particles from their nucleus and become an ISOTOPE!
More on isotopes later

16. Radioactive decay Decay rate in terms of half-life (time)
Half-life: time for 1/2 of all atoms to decay from parent element to daughter element

17. Radioactive Decay Mineral forms with only parent atoms
Through time, the parent decays to form daughter product

18. How to use half-life (example)
Half-life of potassium 40 is 1.3 billion years
After 3.9 billion years,how many half-lives have occurred
What fraction of potassium 40 is left?
1/2 * 1/2 * 1/2 = 1/8

19. All rocks are not created equal for radiometric dating use!
Expensive and may not have good minerals to date
Sedimentary rocks are almost impossible to date…why? (think of what sed rocks are made of)
Best rocks: igneous
Complicated rocks: metamorphic

20. Problems with dating Errors Dating detrital sed rocks
Usually 0.5% of age, as good as 0.1%
E.g billion year rock is +/ million
Dating detrital sed rocks
Is the age found of the sedimentary deposit or igneous formation?
Dating metamorphic rocks
Did isotope ratio survive metamorphism?

21. Common isotopes used to date rocks!
Parent
Daughter
1/2 life
Use
238U
206Pb
4.5 billion
Old rocks
10Ma - 4.6Ga
40K
40Ar
1.3 billion
Young rocks
<10 Ma
14C
14N
5730 years
Young artifacts
100-70,000 years

22. Carbon-14 dating How does Carbon-14 dating work?
Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide.
Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle.
All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion).
At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14.
The change in the Carbon 14 to Carbon 12 ratio is the basis for dating.
The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages).
Assumes that the rate of Carbon 14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years).

23. Carbon-14 Dating For more see this site carbon-14 animations and info
And this site pdf article

24. Structure of the Carbon Atom
Nucleus
Protons (+)
Neutrons (n)
Electrons (-)
# protons: atomic #
# protons + neutrons: atomic mass
Isotope: element with different # neutrons