3Key BedsDefinition: Rock or sediment layers that can be traced back to a very specific event in history, and cover a wide geographic area.
4Absolute Age Dating Four ways: Radiometric Dating Dendrochronology Ice coresVarves
5Radiometric Dating (review) Certain elements have radioactive properties.That is, they lose atomic parts from their nucleusWe call this losing of parts radioactive decayWhen they lose protons, the element changes to a whole new elementWhen they lose neutrons, they become a different isotope of the same element, with a new atomic mass.The radioactive decay happens at a steady rateSo we can use these elements to determine how long ago a rock layer was formed.
6Radiometric Dating (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.
7Absolute DatingDendrochronology = the science of studying tree rings to determine the age of the tree and seasonal events
8Absolute Dating – Ice Cores Ice Cores – created by drilling hollow tube through ice up to several kilometers thickDrilling 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.
10Absolute Dating – Varves Varves: bands of alternating light and dark-colored sediments of sand, clay and siltRates of sedimentary deposits vary by season (as with ice cores and tree rings)Help study glacier melting patterns
11The 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.
12Radiolarians Microfossils Unicellular organisms with hard shells that have populated the oceans since the Cambrian PeriodUsed by petroleum geologists to determine the age of rocks that might produce oil
14Original 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)
15Original Preservation Cont. The insect is completely preserved (hard and soft parts) in amber.
16Altered Hard PartsThe 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 groundwaterb) groundwater comes into contact with original hard parts mineral and replaces the material with a different mineral
172) Recrystallization:Original mineral retains the same chemical formula although takes on a crystalline structure for greater long term stability
18Molds/CastsMold: impression left behind in the sediment where a shell once wasCast: filled in mold
19Trace FossilsProvide evidence of how an organism lived, moved and obtained foodExamples: worm trails, footprints, tunneling burrows, gastroliths (rocks in dinosaur stomachs- left) and coprolites (fossilized feces- right)