FOSSILS, RELATIVE & ABSOLUTE DATING

FOSSILS Traces and preserved remains of ancient life found within rock layers Fossils show: Biodiversity How species have changed over time Correlation between rock layers from around the world Relative ages to particular strata Evidence for the geological time scale Traces are footprints, droppings, or any other type of evidnece an organism might leave behind How fossils form: Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.

PALEONTOLOGY the study of fossils  remains of ancient life Body fossils vs. trace fossils Body = remain of organism, like bones; Trace = evidence of organism, like footprints Scientific dating Absolute dating (gives age in years) Radiometric / radioactive dating (isotopes) Relative dating (gives age before, after, during) Observation of rock layers Radiometric dating: use the natural radioactivity of certain elements found in rocks to help determine their absolute age- the use of half-lifes to determine the absolute age of a sample. In radioactive dating, scientists calculate the age of a sample based on the remaining radioactive isotopes. Radioactive elments decay into nonradioactive elements at a steady rate which is measured in a unit called half-life.

Ways to tell the age of a rock Absolute Dating: All you need is a tiny sample of material (mineral, bone) no larger than a grain of rice. Gives us the true “age” of a fossil or rock Mainly organic tissue or igneous crystals Measure the amount of unstable isotopes that have “decayed” to figure out age

Ways to tell the age of a rock Relative Dating: Places events in geologic history in the proper order. The basis for the geologic time scale Mainly Sedimentary Rocks Does not provide a true “age”

RELATIVE DATING & AGE Relative Dating: putting rocks and geological events in correct chronological order Relative Age: how old something is in comparison to something else HOW? Use of sedimentary rocks Use of fossils Study of strata

INDEX FOSSIL Fossil that defines and identifies geologic periods; often in only one layer of rock Easily recognizable Short-lived (found only in a few layers of rock worldwide) Wide distribution (geographic range)

Ex/ INDEX FOSSIL: AMMONITE Ammonite fossils are found worldwide, but they existed for only a very specific period of time this means ammonites are found in very specific layers of rock when an index fossil is found, the age of the rocks it is preserved can be determined Traces are footprints, droppings, or any other type of evidnece an organism might leave behind How fossils form: Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.

LAW OF HORIZONTALITY Sediments are originally deposited in horizontal layers Folds or inclines: layers must have been deformed after they were deposited.

LAW OF SUPERPOSITION For undisturbed rocks, the oldest layer is on the bottom and the youngest is on top (Supai is oldest)

What kind of rocks are these fossils in? Which layer is oldest? Which layer is youngest? How do you know?

LAW OF SUCCESSION Fossils are found in a predictable sequence Fossils in rock B are older then fossils in rock A

LAW OF INCLUSIONS If a rock body (Rock B) contained fragments of another rock body (Rock A), then Rock B must be younger than the fragments of rock it contained

GEOLOGIC TIME SCALE a series of time intervals that divides Earth’s history Each layer of rock represents specific interval of time Index fossils help determine specific period Time periods divided by specific events like mass extinctions

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay). Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods. Marie Curie Ernest Rutherford 15

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION The number of protons (the atomic number) is fixed for any element and is unique for each element but the number of neutrons in atoms of different elements can vary. Atoms of an element having different numbers of neutrons are referred to as the isotopes (of that element). Figure 3.3: Schematic representation of the isotopes of carbon. Carbon has an atomic number of 6 and an atomic mass number of 12, 13, or 14, depending on the number of neutrons in its nucleus. M&W4 Fig. 3.3; M&W5 Fig. 3.4 16 16

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION Radioactive decay occurs when an isotope of one element is transformed into a different element by changes in the nucleus. There are three different decay mechanisms: “Parent” “Daughter” Figure 17.18: Three types of radioactive decay. (a) Alpha decay, in which an unstable parent nucleus emits 2 protons and 2 neutrons. (b) Beta decay, in which an electron is emitted from the nucleus. (c) Electron capture, in which a proton captures an electron and is thereby converted to a neutron. M&W4 Fig. 17.18; M&W5 Fig. 17.18 17

How can we tell age based on the number of parent isotopes? Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.

The Half-Life of C14 is 5,730 years. C14 is an isotope of carbon that forms from Nitrogen in the atmosphere. Living things consume this radioactive carbon. Once dead, no new carbon is absorbed, and C14 turns back into Nitrogen. The Half-Life of C14 is 5,730 years. This method works best for fossils younger than 50,000 years. Why? (end) M&W4 Fig. 17.24; M&W5 Fig. 17.24

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION For radioactivity dating we use igneous rocks and minerals. The clock starts when radioactive atoms that are present in the magma get incorporated in the crystalline structure of certain minerals in the rocks. The crystals containing the parent atoms form and so we then have a “container” with parents that can begin decaying to form daughters. We can then use measure the parent-daughter ratio. This is our “atomic clock” that records the time since the rock crystallized. Figure 17.21: (a) Magma contains both radioactive and stable atoms. (b) As magma cools and begins to crystallize, some radioactive atoms are incorporated into certain minerals because they are the right size and can fit into the crystal structure. Therefore, at the time of crystallization, the mineral will contain 100% radioactive parent atoms and 0% stable daughter atoms. (c) After one half-life, 50% of the radioactive parent atoms will have decayed to stable daughter atoms. M&W4 Fig. 17.21; M&W5 Fig. 17.21 20

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION To the oldest materials ever dated by the radioactive method are found in the Jack Hills of western Australia and are tiny zircon grains contained in sandstones and conglomerates. The zircons are 4.4 billion years old. The very remote “outback” of western Australia--the Jack Hills Scanning electron microscope image of a Jack Hills zircon. Scale bar is 0.1 mm 21