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Lecture 2

BIOC50H3 Lecture Notes - Lecture 2: Zion National Park, Geologic Time Scale, Radiocarbon Dating


Department
Biological Sciences
Course Code
BIOC50H3
Professor
Jason Weir
Lecture
2

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Lecture Two
Geological strata
- Strata (singular: stratum):
o They are layers of rock or soil with internally consistent characteristics that
distinguish them from other layers
o They are each laid down in an event (such as a flood that lays down layer of
sediment at the bottom of the lake; volcanic activity; turbodites (underwater
avalanches)
o A lot of different processes can help with the formation of a strata
- Different levels of strata
o Useful for the history of the planet
o Use it for identifying different time periods
Each strata formed at a different time period
o Oldest strata is at the bottom and youngest at the top
This cannot be the case when the Earth has buckled
- Divide geological periods based on the strata
o Eras
Then can be subdivided to periods
o Memorize the geological column
Earth history divided into different eras…and then periods
Geological column is not complete anywhere in the different world; how do you get the entire
sequence of the geological column
- If you go to the four state regions of Utah, Colorado, New Mexico and Arizona…there is
a lot of canons
- Each of the different canyon shows us a series of strata but they don’t have the entire
sequence
- Erosion can get rid of strata
- In Zion National Park at Utah, we find that the strata there starts at the Permian period
and they go up to the middle of the Jurassic period
- In Bryce canyon, this series of strata go from just above the Jurassic to the period of time
- Putting it all together, we can piece the different history
- Often how we define strata composition…through fossil materials
An introduction to the geological column
- Fossils
o The next major tool used to understand the history of life on Earth
o They represent an organism that in some aspect has preserved over time
o In order for fossil to form, dead organic material need to be buried rapidly so they
don’t decay

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Followed by the process of mineralization; the original organic material
gets replaced chemically by time…it’s the chemical replacement that is
preserved over time
o Another way that fossils can be formed
Insects that get into amber tree pitch (it’s antiseptic so it has anti-bacterial
properties)
An insect that gets stuck in a tree pitch gets preserved from
bacterial decay
The tree pitch gets fossilized itself
- Fossils and the geological column together form a 4D record of the history of life
o There are 3 spatial dimensions in the strata and then the 4th dimension indicate the
time
o Within a stratum, there isn’t a sequence of time (it’s laid out in one event such as
turbidite)
Where we get the time is between the stratum
To date the stratum, we rate it through radioactive age dating
- Radioactive age dating:
o Absolute ages of rocks in the geological column are measured using naturally
occurring radioactive elements in the rocks (isotopes)
o Radioactive elements may come in several isotopes
o Isotopes are defined by the number of neutrons in the nucleus
o The sum of the protons and neutrons is referred to as the mass number
Uranium 235 has 143 neutrons and Uranium 238 has 146 neutrons
These are two different isotopes of uranium
Each of these isotopes of U decays to a different isotope of lead
o The amount of time it takes to decay is called the half- life
o Half-life: amount of time needed for one half of the atoms of the atoms of the
radioactive element to decay
The amount of time required for half of the actual material to disappear
Example: Uranium 235: It takes 704 million years for half of the original
Uranium 235 to decay into lead…if you wait another 704 million years,
another half of that material will disappear
o Some isotopes have half-lives as rapid as 1/millionth of a second
Very unstable
o Some isotopes like Rubidium have very stable isotopes
o If we’re wanting to date fossil and organic matter, we need to use something that
decays as fast as them
Example: C14 decays to N14 in about 6000 years (half-life of 6000)
Since carbon is incorporated in living tissues/ organisms and we know it’s
ratio….we can look at a sample and see how much C14 is left in the
sample to determine how old the sample is
C14 dating is useful for dating bone
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C14 dating is only good for the last 50,000 years…more than that, there is
not enough C14 to tell you the age
The ratio of living tissue incorporates C12 to C14 is constant through
time: We can test that assumption: By looking at a known chronology:
Bristlecone pine (oldest individuals)
Taking a core through their trunk and count the rings…every ring
represents a year of birth
We can look at the pines/ logs on the floor
o We can match up the pattern of the growth ring across
different logs that are dead on the group
What to do: Take a tissue sample and C14 of each ring and then
count them
Graph: Red line is the actual relationship
C14 dating is good for dating bones, ashes and wood but not good
for fossils that go back 50,000
o To date strata/ geological column…it’s better to use
potassium argon dating and uranium lead dating are
commonly used to date layers made up of igneous rock
o Igneous rocks form when molten lava solidifies
o At the moment of solidification, the isotopes in the lava
become fixed in the igneous rock
o We look at how much potassium has distingerated into
argon OR how much uranium has disintegrated into lead
and we use these methods to data the igneous rock
Looking at the geological column…however not all layers are igneous.
Mostly are sedimentary layers made up of sediments
We can’t date the sedimentary layers ONLY the igneous rocks
Therefore, by dating igneous layers in the geological column, we
can place upper and lower bound ages on non-igneous layers
between them
o Why can’t we apply the same dating methods into sedimentary layers through
potassium argon?
- Most fossils are found in sedimentary rocks. Why might radioactive dating not work well
on sedimentary rock?
o Sedimentary rocks are made of things like sand and gravel and silk that we have
washed in through a flood; they compressed together to form a rock; the
components could’ve formed a million years ago before the actual sedimentary
layer for found
o Using radioactive age dating, you would date the actual formation of rock and not
the actual sedimentary layer
o If you used radioactive dating, you would date when the rock formed a long time
ago
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