Key Beds, Absolute Dating and FOSSILS-

21.4

Index Fossils

Key Beds

Definition: Rock or sediment layers that can

be traced back to a very specific event in

history, and cover a wide geographic area.

http://science.discovery.com/tv-

shows/greatest-discoveries/videos/100-

greatest-discoveries-kt-boundary-

discovery.htm

Absolute Age Dating

Four ways:

Radiometric Dating

Dendochronology

Ice cores

Varves

Radiometric Dating (review)

Certain elements have radioactive properties.

That is, they lose atomic parts from their nucleus

We call this losing of parts radioactive decay

When they lose protons, the element changes to a

whole new element

When they lose neutrons, they become a different

isotope of the same element, with a new atomic mass.

The radioactive decay happens at a steady rate

So we can use these elements to determine how

long ago a rock layer was formed.

Radiometric Dating

(more review) We use half-life calculations, given the amount

of parent material and daughter material in a

sample.

A half-life is the amount of time it takes for half

the element to decay.

Absolute Dating

Dendochronology = the science of studying

tree rings to determine the age of the tree and

seasonal events

Absolute Dating – Ice Cores

Ice Cores – created by drilling hollow tube

through ice up to several kilometers thick

Drilling down through ice gives a record of

seasonal events and atmospheric events.

Ice shows seasonal changes, like tree rings do.

Summer ice has more bubbles and larger crystals

than winter ice does.

Absolute Dating – Ice Cores

Absolute Dating – Varves

Varves: bands of alternating light and dark-

colored sediments of sand, clay and silt

Rates of sedimentary deposits vary by season

(as with ice cores and tree rings)

Help study glacier melting patterns

The Fossil Record and life

The fossil record provides evidence about the history of life on Earth. The fossil record also shows that different groups of organisms have changed over time.

Evolution is the gradual change in species over long periods of time.

When geologists find fossils in rocks, they know that the rocks are about the same age as the fossils. Thus, they can infer that the same fossils found elsewhere are also of the same age.

Radiolarians

Microfossils

Unicellular organisms with hard shells that have populated the oceans since the Cambrian Period

Used by petroleum geologists to determine the age of rocks that might produce oil

Original Preservation

Original Preservation

The picture on the preceding slide is from the La Brea

Tar Pit in California. The soft parts of a mammoth

were preserved in the aforementioned Pit.

Original Preservation: PLANT AND ANIMAL

REMAINS THAT HAVE BEEN ALTERED VERY

LITTLE SINCE THE ORGANISM’S DEATH

(USUALLY SOFT TISSUE DECAYS BUT IN

ORIGINAL PRESERVATION IT DOES NOT)

Original Preservation Example

The insect is completely preserved (hard and soft parts) in

amber.

Altered Hard Parts

The soft portion decays away quickly and the hard portion (bones, shells, cell walls) can become fossils in one of two ways:

1) Mineral replacement:

a) pores in hard parts are filled in with minerals from groundwater

b) groundwater comes into contact with original hard parts mineral and replaces the material with a different mineral

2) Recrystallization:

Original mineral retains the same chemical formula although takes on a crystalline structure for greater long term stability

Molds/Casts

Mold: impression left behind in the sediment

where a shell once was

Cast: filled in mold

Trace Fossils Provide evidence of how an organism lived,

moved and obtained food

Examples: worm trails, footprints, tunneling burrows, gastroliths (rocks in dinosaur stomachs- left) and coprolites (fossilized feces- right)