Key Beds, Key Beds, Absolute Dating Absolute Dating

and FOSSILS- and FOSSILS- 21.421.4

Index FossilsIndex Fossils

Key BedsKey Beds

Definition: Rock or sediment Definition: Rock or sediment layerslayers that can that can be traced back to a very be traced back to a very specificspecific eventevent in in history, and cover a history, and cover a widewide geographicgeographic area. area.

http://science.discovery.com/tv-shows/greatest-discoveries/videos/100-greatest-discoveries-kt-boundary-discovery.htm

Absolute Age DatingAbsolute Age Dating

Four ways:Four ways: Radiometric DatingRadiometric Dating DendrochronologyDendrochronology Ice coresIce cores VarvesVarves

Radiometric Dating (review)Radiometric Dating (review) Certain elements have radioactive properties.Certain elements have radioactive properties. That is, they lose atomic parts from their That is, they lose atomic parts from their nucleusnucleus

We call this losing of parts We call this losing of parts radioactive decayradioactive decay When they lose When they lose protonsprotons, the element changes to a , the element changes to a

whole whole newnew element element When they lose When they lose neutronsneutrons, they become a different , they become a different

isotopeisotope of the same element, with a new atomic mass. of the same element, with a new atomic mass. The radioactive decay happens at a The radioactive decay happens at a steadysteady rate rate So we can use these So we can use these elementselements to determine how to determine how

long ago a rock layer was formed.long ago a rock layer was formed.

Radiometric Dating (review)Radiometric Dating (review)We use half-life calculations, given the amount We use half-life calculations, given the amount of parent material and daughter material in a of parent material and daughter material in a sample.sample.

A A half-lifehalf-life is the amount of time it takes for is the amount of time it takes for halfhalf the element to decay.the element to decay.

Absolute DatingAbsolute Dating

DendrochronologyDendrochronology = the science of studying = the science of studying treetree ringsrings to determine the age of the tree and to determine the age of the tree and seasonal eventsseasonal events

Absolute Dating – Ice CoresAbsolute Dating – Ice Cores

Ice Cores Ice Cores – created by drilling hollow tube – created by drilling hollow tube through ice up to several kilometers thickthrough ice up to several kilometers thick

Drilling down through ice gives a record of Drilling down through ice gives a record of seasonalseasonal events and events and atmosphericatmospheric events. events.

Ice shows seasonal changes, like tree rings do.Ice shows seasonal changes, like tree rings do. SummerSummer ice has ice has moremore bubbles and bubbles and largerlarger crystals crystals

than winter ice does.than winter ice does.

Absolute Dating – Ice CoresAbsolute Dating – Ice Cores

Absolute Dating – VarvesAbsolute Dating – Varves

VarvesVarves: bands of alternating light and dark-: bands of alternating light and dark-colored colored sedimentssediments of sand, clay and silt of sand, clay and silt

Rates of sedimentary deposits vary by season Rates of sedimentary deposits vary by season (as with ice cores and tree rings)(as with ice cores and tree rings)

Help study Help study glacierglacier melting patterns melting patterns

The Fossil Record and lifeThe Fossil Record and life

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

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

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

RadiolariansRadiolarians MicrofossilsMicrofossils UnicellularUnicellular organisms with hard organisms with hard

shells that have populated the shells that have populated the oceans since the oceans since the CambrianCambrian Period Period

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

Original PreservationOriginal Preservation

Original PreservationOriginal Preservation

The picture on the preceding slide is from the La Brea The picture on the preceding slide is from the La Brea Tar Pit in California. The soft parts of a mammoth Tar Pit in California. The soft parts of a mammoth were preserved in the aforementioned Pit.were preserved in the aforementioned Pit.

Original PreservationOriginal Preservation: PLANT AND ANIMAL : PLANT AND ANIMAL REMAINS THAT HAVE BEEN ALTERED VERY REMAINS THAT HAVE BEEN ALTERED VERY LITTLE SINCE THE ORGANISM’S DEATH LITTLE SINCE THE ORGANISM’S DEATH (USUALLY (USUALLY SOFTSOFT TISSUE DECAYS BUT IN TISSUE DECAYS BUT IN ORIGINAL PRESERVATION IT DOES ORIGINAL PRESERVATION IT DOES NOTNOT))

Original Preservation Cont.Original Preservation Cont.

The insect is completely preserved (hard and soft parts)

in amber.

Altered Hard PartsAltered Hard Parts

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

1) 1) Mineral replacementMineral replacement: : a) pores in hard parts are filled in with a) pores in hard parts are filled in with

minerals from groundwaterminerals from groundwaterb) groundwater comes into contact with b) groundwater comes into contact with original hard parts mineral and replaces original hard parts mineral and replaces

the the material with a different mineralmaterial with a different mineral

2) 2) RecrystallizationRecrystallization::Original mineral retains the same chemical Original mineral retains the same chemical formula although takes on a crystalline formula although takes on a crystalline structure for greater long term stabilitystructure for greater long term stability

Molds/CastsMolds/Casts

MoldMold: impression left behind in the sediment : impression left behind in the sediment where a shell once waswhere a shell once was

CastCast: filled in mold: filled in mold

Trace FossilsTrace Fossils Provide evidence of how an organism Provide evidence of how an organism livedlived, ,

movedmoved and obtained and obtained foodfood Examples: worm trails, footprints, tunneling Examples: worm trails, footprints, tunneling

burrows, gastroliths (rocks in dinosaur stomachs- burrows, gastroliths (rocks in dinosaur stomachs- left) and coprolites (fossilized feces- right)left) and coprolites (fossilized feces- right)