YOU WILL NEED YOUR COMPOSITION BOOK ALL PERIOD TODAY.

- Make sure your Origins of Life notes are in your TOC and secured with tape, glue or a staple in your comp book.

- Go ahead and put “Fossil Notes” in your TOC, and under that put “Carbon Dating Lab” in your TOC

Today’s Objective (learning goal)

To identify how fossils are used as evidence in changes within a species.

• About 95 percent of the species that have existed are EXTINCT—they no longer live on Earth.

• The oldest rocks that have been found on Earth formed about 3.8 billion years ago.

• Scientists study fossils to learn about ancient species.

Clues to the Past

• Fossils are evidence of organisms that lived long ago that are preserved in Earth’s rocks.

TYPES OF FOSSILSFossils Types Formation

Trace fossils

Casts

Molds

Petrified fossils

Amber-Preserved

orfrozen fossils

A trace fossil is any indirect evidence

left by an animal and may include a

footprint, a trail, or a burrow.

When minerals in rocks fill a space

left by a decayed organism, they make

a replica, or cast, of the organism.

A mold forms when an organism is

buried in sediment and then decays,

leaving an empty space.

Petrified-minerals sometimes penetrate

and replace the hard parts of an organism

At times, an entire organism was

quickly trapped in ice or tree sap that

hardened into amber.

• Paleontologists, scientists who study ancient life, are like detectives who use fossils to understand events that happened long ago.

• They use fossils to determine the kinds of organisms that lived during the past and sometimes to learn about their behavior.

• For fossils to form, organisms usually have to be buried in mud, sand, or clay soon after they die.

• Most fossils are found in sedimentary rocks. These rocks form at relatively low temperatures and pressures that may prevent damage to the organism.

• Fossils are not usually found in other types of rock because of the ways those rocks form.

• For example, the conditions under which metamorphic rocks form often destroy any fossils that were in the original sedimentary rock.

• Few organisms become fossilized because, without burial, bacteria and fungi immediately decompose their dead bodies.

• Occasionally, however, organisms do become fossils in a process that usually takes many years.

How fossils form

• A Protoceratops drinking at a river falls into the water and

drowns

• Sediments from upstream rapidly cover the body, slowing its decomposition. Minerals from the sediments seep into the body.

• Over time, additional layers of sediment compress the sediments

around the body, forming rock. Minerals eventually replace all the

body’s bone material.

• Earth movements or erosion may expose the

fossil millions of years after it formed.

• Scientists use a variety of methods to determine the age of fossils.

• Relative dating

• This method basically indicates that the fossils found closer to the surface are younger, and the fossils found in deeper strata are more primitive (older). (in undisturbed ground)

• This method does not give a specific age of the fossil or rock.

• Using this principle, scientists can determine relative age and the order of appearance of the species that

are preserved as fossils in the layers.

• To find the specific ages of rocks, scientists use absolute dating, also called Radiometric Dating.

• In this type of dating scientists use radioactive atoms to determine the absolute age of objects.

As specific atoms decay they emit radiation and lose electrons, which causes them to turn into a different

element.

So- these atoms change from one thing into something else over time- and they do this at a constant rate.

• If you know what the unstable atom is (Carbon 14) and you know what it turns into as it decays (Nitrogen 14).

• And you know the rate at which the unstable atom decays/turns into something else

• You can measure the amount of the unstable atom in the rock….and compare that to the amount of the new atom

• This will tell you the age of the sample rock.It takes 5,730 years for ½ of Carbon 14 to

turn into Nitrogen 14

• The half-life of a radioactive atom is the time it takes for half of that atom in a sample to decay and turn into another element.

256 14C atoms How many carbon-14 atoms and how many

nitrogen-14 atoms will this rock have in

5,730 yrs.

After 5730 years

or 1 half-life

128 14C and

128 14N atoms

After 11,460 yrs

or 2 half-lives

64 14C and

192 14N atoms

After 17,190 yrs

or 3 half-lives

32 14C and

224 14N atoms

After 22,920 yrs

or 4 half-lives

16 14C and

240 14N atoms

After 28,650 yrs

or 5 half-lives

8 14C and

248 14N atoms

After 34,380 yrs

or 6 half-lives

4 14C and

252 14N atoms

After 40,110 yrs

or 7 half-lives

2 14C and

254 14N atoms

Because Carbon 14 has such a short half-life, it can only be used to date things that died recently in geologic

history (under 70,000 years ago).

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.

• You can only date organic remains using carbon 14

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:

Most radioactive isotopes have rapid rates of decay (that is, short half-lives) and lose their

radioactivity within a few days or years.

Parent IsotopeStable 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

Question 1

D. land plants

C. reptiles

B. mammals

According to fossil record what organisms have occupied Earth for the longest period of time?

A. prokaryotes

The answer is A. Single-celled prokaryotes have been present on the Earth since the Precambrian period and are still present today.

Question 2Given that volcanoes have erupted since Earth’s early history, why does volcanic rock not contain many fossils?

AnswerLava is subject to high heat and strong pressure changes that prevent fossils from forming in it.

1. Secure your Fossil Notes into your Comp Book on the appropriate page.

2. Once you have that done, turn to the next page and write “Carbon Dating Lab” as your title.

3. Write “Purpose” and copy the purpose down underneath the title.

Carbon Dating Lab

Purpose: to simulate how radioactive isotopes decay over time in rock, and how scientists are able to age them based on this fact.

4. Next….write “Pre-Lab Questions” under your purpose.

5. Write the questions below and COMPLETE answer.

Pre Lab Questions:

1. How do scientists use radioactive isotopes to determine the age of rocks and fossils?

2. What does Carbon 14 turn into as it decays?

3. Explain the “half-life” of an atom.