Chapter 2: Geological Time

2.1 - Fossils Fossils Fossils provide a window into the past as they provide evidence about the changes that have occurred to the Earth over the 4.5 billion years of existence. A fossil is the preserved evidence in rocs or soils of organisms that once existed on Earth. Fossils may be the whole body, part of the body or traces of its activities.

Fossils. To be preserved as a fossil the organism must decay very slowly. Occurs when the organism is covered in sediment, which then turns to rock. A palaeontology is the study of past life.

Formation of Fossils Animals that are only composed of soft parts such as jellyfish, slugs etc. are rarely found as fossils. Hard objects such as bone, tree bark, shells and teeth are most likely found as fossils. The following process is how fossils occur in sedimentary rock.

Fossil Formation Fossils are found only in sedimentary rock not in igneous or metamorphic rock as the heat and pressure used in formation destroys the organism.

Types of Fossils There are different types of fossils due to the different ways in which they are formed. Types of fossils: - Original fossils - Replacement fossils - Carbon Film fossils - Indirect fossils

Original Fossils Occur when a part of the organism is preserved and its composition is almost the same as when it was alive. Most commonly skeletons, bones, teeth, shells; this is because they are harder to break down and scavengers find them difficult to eat. Common original fossils include: - Sea creatures that had shells, such as mussles and oysters - Verterbrates, because they had teeth and a bony skeleton, such as megafauna – the three meter tall giant kangaroo, sheep sized echidnas, 3 metre long wombats.

Original Fossils

Replacement Fossils A replacement fossil occurs when a part of the organism is chemically changed into another mineral. Commonly occurs when calcium carbonate, from bones and shells, turns into another mineral such as silica.

Carbon film fossils A carbon film fossil occurs when a dead body partially decays and leaves behind a thin black deposit of carbon. The traces of carbon left are often in the shape of the organism that decayed and often show fine details.

Indirect fossils An indirect fossil is not part of the organism but is the preserved remains, such as imprints of the body like footprints, fossilised dung and burrows. Dung and Burrows are sometimes called trace fossils.

Preserving environments The environment in which the organism dies can influence the fossil produced. These preserving environments include: - Permafrost - Amber - Tar - Peat - Dry air

Permafrost Near the artic circle the land is permanently frozen. Provides the perfect environment for fossilisation as the decaying process is stopped due to the temperature being below freezing. This environment is best for original fossils.

Amber Amber is a solid plant sap or gum Insects, spiders or small vertebrates such as lizards and frogs get stuck in the sticky sap which seeps out of trees When it sets it perfectly preserves whatever has become stuck inside it.

Tar Tar pits occur when oil seeps naturally from the ground into the surface. This preservation is rare, but when it is achieved, provides fantastic preservation.

Peat Peat is the partly decomposed remains of plants and is commonly found in swamps and bogs. The peat contains acids making the hard tissue, such as bone, very soft. In Europe ancient human bodies have been found preserved in peat.

Dry Air Extremely dry conditions, reduce the number of bacteria that are able to decay the organism therefore; Dehydrates the soft tissue which fossilises and turns it into a “mummy” (muffication).

Determines the actual age of the fossil 2.2 – dating techniques Dating techniques Palaeontologists use several techniques to decide the age of a fossil. Relative Dating: Compares the age of a fossil or rock with others to determine which is older. Absolute Dating: Determines the actual age of the fossil Stratigraphy Radioactive Dating Fluorine Analysis Tree Rings

Relative Dating Relative dating relies on two factors: Sedimentary rock layers Fossils are the same age as the rocks they are found in.

Layers of sedimentary rock Relative dating Layers of sedimentary rock Sedimentary rock forms in layers called stratum The first layer (base) is the oldest and the top is the youngest.

Layers A3, B1 and C5 contain the same long shell-like fossil species Layers A3, B1 and C5 contain the same long shell-like fossil species. Hence, these layers are the same age Stratigraphy Index fossils within the rock layers can be used to determine the relative ages depending on the rock layer they appear in

A5 and B4 are the same age because they have the same fossil type Site C has four layers about C5. This states these layers are younger than C5 (because C5 is on the bottom). C1 is the youngest layer because its at the top B5 is the oldest layer because it is at the bottom compared to all sites

Index Fossils Relative dating Fossils that can be used to compare the ages of strata in different locations are called index fossils. To be used as an index fossil, the species must: have been fairly widespread in where it lived have lived in a fairly narrow period of time have been abundant (there were many of them) be easy to identify.

Fluorine analysis Relative dating Fluorine analysis compares the amount of fluorine in different bones found in the same rock. Bones absorb the flourine from water over long periods of time This happens slowly therefore the more fluorine the older the bone.

Radioactive dating Absolute dating Radioactive dating is a method that uses the natural rate of decay of radioactive isotopes. The amount of decay allows scientists to estimate how long, before discovery, the rock was laid down. Half life is the time it takes for half of a radioactive sample to decay

Absolute dating

reliable as the amount of carbon-14 remaining is too small to measure. Carbon-dating Carbon dating is most useful when dealing with organic matter (proteins) Carbon dating is useful as far back to less than 40 000 years. Beyond that, however, the method is not very reliable as the amount of carbon-14 remaining is too small to measure. potassium–argon dating potassium–argon dating is most useful in rocks containing volcanic ash

Tree rings Absolute dating A useful method for dating wood. This process involves counting the growth rings in the trunk of the tree. Many trees grow by adding a woody layer around the stem. 1 ring = 1 year

Propose a suitable dating method for each of the following: Question Propose a suitable dating method for each of the following: Showing that two bones were not from the same cave Proving a piece of wood was about 30,000 years old Dating a wooden box found in an Egyptian tomb Dating a primitive stone tool found next to a volcano

Homework 2.2 Unit Review (page 53) Q1,2,3,4,5,6,7,9,10,15