Key Beds, Absolute Dating and FOSSILS- 21.4

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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.

Absolute Age Dating Four ways: Radiometric Dating Dendrochronology
Ice cores Varves

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.

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 Dendrochronology = 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.

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 Cont.
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)