Transcript You cannot use this technique to get the actual date of a specimen

Fossils are usually formed when an organism is covered by
sediments that then harden into sandstone, slate, mudstone
Most of the time, organisms die in
or flint.
locations that are not conducive to
fossilization.
Most organisms do not fossilize and
those that do are usually destroyed
by geological processes or they never
surface for examination.
We have only discovered about
250,000 fossilized species. This tells
us that there are many gaps in the
fossil record.
By the 1700s it was accepted that fossils were the
buried remains and impressions of organisms that
had lived in the past.
Biologists generally
recognize two types
of fossils:
• Body Fossils
• Trace Fossils
• Unaltered remains
Unaltered skeletal
material, tar
impregnation, amber
entombment,
refrigeration,
mummification
This category includes those fossils
that have undergone little or no
change in structure and composition.
As a general rule an organism which
lived fairly recently has a greater
probability of being unaltered than a
more ancient one.
• Altered
Remains
As the sediments that entomb fossils
become compressed by the weight of
overlying sediments, they slowly
become rocks. The same processes
that turn sediments into rocks can
alter organic remains .
Permineralization,
dissolution/replacement,
carbonization
A fossil is any trace of past life. A fossil does not have to
be an actual piece of an ancient organism. Many fossils are
bones, shells, or other body parts. However footprints and
other traces are fossils too.
• Mold
• Cast
• Borings and Burrows
• Copralites
• Gastroliths
•Gnawings
Why do we say the “fossil record” is
incomplete?
What types of environments might be best
for becoming a fossil in?
Give an example of unaltered remains.
Give an example of altered remains.
Give three examples of trace fossils.
In what two ways are fossils classified?
• Relative Dating Techniques
Law of superposition
(Nicolaus Steno)
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• Radiometric Dating Techniques
To find the specific age of rocks, scientists
use radiometric dating.
By using the radioactive isotopes present in rocks, and
understanding the rates at which these isotopes decay,
scientists can determine approximate age of the rocks.
Atoms of the same element with differing atomic weights
can be naturally found in the environment, and are called
isotopes. Why do they have a different atomic weight?
Radioactive isotopes are
• The element potassium-40
atoms with unstable nuclei
decays to argon-40 and has a
half-life of approximately 1.3
that break down, or decay,
over time, giving off radiation. billion years
• The element carbon-14
The isotope is eventually
decays more rapidly into
changed into, or replaced by
Nitrogen-14, and has a half-life
another element over time.
of only 5,730 years.
Because Carbon 14 has such a short half-life, it can
only be used to date things that died recently in
geologic history (under 50,000 years ago).
• You can only date organic remains using carbon 14
If you need to date rocky
material from farther back in
our geologic past, you must
use another method, such as
Potassium/Argon, which has a
much longer half-life.
Most radioactive isotopes have rapid rates of
decay (that is, short half-lives) and lose their
radioactivity within a few days or years.
Some isotopes, however, decay slowly, and several of these
are used as geologic clocks. The parent isotopes and
corresponding daughter products most commonly used to
determine the ages of ancient rocks are listed below:
Parent Isotope
Stable Daughter
Product
Currently Accepted
Half-Life Values
Uranium-238
Lead-206
4.5 billion years
Uranium-235
Lead-207
704 million years
Thorium-232
Lead-208
14.0 billion years
Rubidium-87
Strontium-87
48.8 billion years
Potassium-40
Argon-40
1.25 billion years
Samarium-147
Neodymium-143
106 billion years
A half-life is the amount of
time it takes for half of the
radioactive isotope to change
into the daughter material.
How it’s done…
Carbon samples are converted to acetylene gas
by combustion in a vacuum line. The acetylene
gas is then analyzed in a mass spectrometer to
determine its carbon isotope composition.
The half-life of C-14
is approximately
5,600 years. A
specimen is loaded
into a mass
spectrometer, and is
found to have a
ratio of 1:3 (C-14/N14)
How old is the
specimen?
11,200 years
When the specimen is found to have 50% of the original C-14, how old is
it?
5,600 years
The graph below could represent any of the radioactive
isotopes we’ve learned about. As you can see, after about
7 half-lives, there is little, if any of the original parent
isotope left.
How old would a
specimen have to
be before 14C
would be beyond
its useful
potential?
About
50,000
years, give
or take
In lieu of current events project
Watch the short clip on how Lucy became a fossil:
http://www.youtube.com/watch?v=Ca3At5V4Jqg
Your task is to:
• pick any organism (plant or animal) from Earth’s geologic past that
intrigues you. Tell me as soon as you know! First come, first served!
• Write a short story about how your organism lived and died (be
specific).
Tell how your organism became a fossil (be specific about which type of fossil
your organism became, (use academic vocab from lesson), and then was unearthed
in the future by some paleo-scientist.
• You must include a colorful drawing of your organism in its habitat.
(NOT a photo) You must include discussion of an appropriate
radiometric dating technique used to determine the age of your fossil.
Be creative. You may prepare a video presentation, much like the short video clip you
watched in class, where you play a role, perhaps the scientist or the fossil itself… or you
may choose any other electronic media, including claymation.
Use any and all
technologies available to you! Upload it to NetSchool by next Friday.