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9.3 Environments through time: 2. The Phanerozoic eon

Syllabus reference: (October 2002 version)
2. The environment of the Phanerozoic eon	*Students learn to:* outline the chemical relationship between ozone and oxygen explain the relationship between changes in oxygen concentrations and the development of the ozone layer describe the role of ozone in filtering ultraviolet radiation and the importance of this for life that developed during the Phanerozoic eon	*Students:* analyse information from secondary sources to identify the major era subdivisions used to describe the Phanerozoic and describe the general differences in life forms in each era

Extract from Earth and Environmental Science Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit:23Jul08]

Prior Learning: Preliminary Module 8.2 (subsection 4)
Science Stages 4–5 syllabus: Outcome 4.9 (content: 4.9.4 – the atmosphere), Outcome 5.9 (content: 5.9.4 – natural events)

Background: The Phanerozoic eon marks the profusion of macroscopic life forms as evidenced by the fossil record. For the Earth to support life, there had to be a major change in the Earth's atmosphere from that of Archaean and Hadean eras. The cyanobacteria helped to provide the increasing atmospheric oxygen concentration and were thought to be able to tolerate the high levels of ultra-violet radiation but the newly emerging life forms needed some protection from the ultra-violet radiation. Without the formation of the ozone layer, life as we know it would not have evolved.

outline the chemical relationship between ozone and oxygen

Oxygen is a gaseous compound of two oxygen atoms (diatomic) and comprises approximately 20% by volume of air in the troposphere.

Oxygen in the atmosphere is generated by the photosynthetic process of plants and some other living things.
Ozone is a gaseous compound of three oxygen atoms and comprises a small volume (approximately 2.5 parts per million) of the stratosphere. It comprises only about 0.02 ppm of the volume of the troposphere.

It is generated from oxygen molecules in the presence of solar ultra-violet (UV) radiation. It forms a thin layer in the upper atmosphere and absorbs ultraviolet radiation.
Ozone in the stratosphere is produced from oxygen molecules that have been dissociated (formed into ions) by UV light. UV light with a wavelength less than 240 nm is absorbed by oxygen molecules to form oxygen radicals. The radicals combine with oxygen molecules to form ozone molecules. Optimal conditions for these reactions occur at about 50 kilometres above the Earth's surface.

Ozone production can be represented by the following chemical equation.

Three molecules of oxygen react to form two molecules of ozone.

UV light can also cause ozone to be converted to oxygen (see below).

explain the relationship between changes in oxygen concentrations and the development of the ozone layer

Aquatic anaerobic organisms, such as cyanobacteria, began producing molecular oxygen (O₂) from about 3500 million years ago. However, until about 2000 million years ago, the oxygen was used up in precipitating sediments, such as the banded iron formations. This process stopped around 1700 million years ago.
From about 2000 million years ago, molecular oxygen began to accumulate in the atmosphere. It was only as oxygen levels built up high in the atmosphere that ozone could be produced from it.
The amount of ozone built up over time to form a thin layer that protected the Earth's surface from the harmful ultra-violet (UV) radiation from the Sun.
life on land is thus possible because of the high concentration of oxygen in the atmosphere and the UV protection afforded by the ozone layer.

The ozone layer formation Professor B. Shakhashiri's Science is fun, Wisconsin initiative for science literacy, USA

describe the role of ozone in filtering ultraviolet radiation and the importance of this for life that developed during the Phanerozoic eon

Ozone molecules absorb ultra-violet (UV) radiation, producing oxygen in the process.
This process absorbs UV radiation in the wavelength range from 200 – 300 nm. This range of wavelengths is the range that is most harmful to living things. Thus, the ozone layer protects living things at the Earth's surface from lethal UV radiation.
During Precambrian times all life was anaerobic. The cyanobacteria are thought to have been able to tolerate the high levels of UV radiation. Cyanobacteria depended on high levels of carbon dioxide (CO₂) in the atmosphere for their photosynthesis. Any other forms of life were restricted to the oceans where they were protected from solar UV radiation.
As oxygen formed, ozone production increased in the upper atmosphere, thus shielding and protecting the Earth's surface from ultraviolet radiation. The accumulation of oxygen allowed the emergence of aerobic life forms during the Proterozoic and the production of an ozone layer allowed the emergence of life forms on land at the beginning of the Phanerozoic eon.

analyse information from secondary sources to identifythe major era subdivisions used to describe the Phanerozoic and describe the general differences in life forms in each era

For this syllabus point, you will need a source of information to analyse, such as the Enchanted Learning web site's Geologic Time Scale (Long version )

Analyse the information from your source, by making distinguishing generalisations about the types of living things that existed in each era. It may assist your analysis to record information in a table like the following.

Time	Era name	General life forms present
65 mya to the present	Cenozoic	"Age of Mammals"Most animal phyla represented. Large mammals, primates and birds existed. The flowering plants became abundant.

Additional information

The following is a summary of some interpretations from the fossil record:

The beginning of the Paleozoic era is characterised by the apparent "explosion" in diversity of animal life and almost all living animal phyla appeared within a few million years.
During the middle of the Paleozoic era, animals, fungi and plants colonised the land and flying insects evolved.
At the end of the Paleozoic the largest mass extinction (almost 90%) of all marine animal species occurred.
During the Mesozoic era the world's fauna changed drastically. Dinosaurs came and went. The birds that evolved continued on after the Mesozoic.
Ferns, cycads and other unusual vegetation dominated the terrestrial vegetation and the gymnosperms made their first appearance. At the end of the Mesozoic the angiosperms emerged.
During the Cenozoic era, more modern animal forms evolved. Mammals became dominant, with a number reaching giant sizes. Angiosperms became dominant amongst the plants.