Part 2-Lesson 1. The Environment of the Phanerozoic eon. Conditions for life on Earth.
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The Environment of the Phanerozoic eon
The Earth’s atmosphere began to change around the beginning of the Proterozioc. (Mark this on your timelines) Cyanobacteria were becoming more abundant and adding more and more oxygen to the atmosphere through photosnythesis.
The oxygen cyanobacteria produced reacted with iron dissolved in the oceans and was taken out of the atmosphere and ended up on the bottom of the ocean. (precipitation) This is how BIF’s formed.
Eventually the dissolved iron was used up and oxygen began to accumulate in the atmosphere. As the oxygen accumulated it began to form a very important layer. The ozone layer.
The ozone layer is in the lower part of the stratosphere (10-50km above the surface). This layer blocks out 93-99% ultra violet radiation from the sun.
The energy from the sun causes oxygen molecules to split. The single atoms of oxygen react with other oxygen molecules in a process called photolysis to produce Ozone O3.
This reaction absorbs some of the UV radiation which would otherwise reach the Earth’s surface. How would this affect early life on Earth?
The first organisms evolved in very hostile environments on Earth. Until the ozone layer, ultraviolet radiation from the sun would have made living on land impossible.
As the oxygen concentrations increased, the ozone concentrations increased blocking more and more UV radiation. Organisms were now faced with new living conditions. They could now evolve to live on land and use this new gas (oxygen) to produce energy instead of chemosynthesis.
The ozone layer allowed organisms to evolve and live in terrestrial environments around 500 million years ago, this marks the start of the Phanerozoic Eon. (find this on your timeline)
The Cambrian Explosion
The Cambrian period marks the start of the Phanerozoic Eon. This is when environmental conditions allowed new life to form and evolve and become more complex than it had in the Archaean and Proterozoic.
Fossils around the world provide us with evidence about what these times were like and the organisms that lived during them.
Finding fossils can be easy but determining how old they are is not. Because fossils are found in sedimentary rocks we can get a relative age based on the fact that the layers below are older than those on top. This is called the law of superposition and is the basis of relative dating.
By using the law of superposition and stratigraphic sequences, geologists can construct a geological time scale. This then allows scientists to break the large eons into smaller periods.
To determine the exact age of a rock or fossil geologists use radiometric dating. This method measures the radioactive decay of isotopes. These isotopes come from the minerals that make up rocks. What does this mean for sedimentary rocks?
We know that minerals form in igneous rocks, so the minerals and sediments that make up sedimentary rocks are likely to have formed millions of years before. Radiometric dating therefore cannot be used to date fossils or sedimentary rocks.
Relative dating can give us accurate information for determining the order in which life forms appeared on Earth. Once this order is established, then absolute dating of available igneous rocks provide an exact age for the fossils.
Below is a list of other examples of atoms decaying to become more stable :
In these examples, the unstable atoms on the left are called the parent material and the more stable atoms on the right are called the daughter product or remnant isotope.
The time taken for each of the parent atoms to decay to their daughter products varies from millions of years to minutes.
The time taken for half of the parent material to decay into its daughter product is know as the Half Life of that parent material.
The half-life for each radioactive element remains the constant. For example it takes 5370 years for half a sample of carbon-14 to decay to nitrogen-14
It will take another 5370 years for half the remaining carbon-14 to decay into nitrogen-14. This is known as the second half-life.
This means it has taken 10740 years for ¾ of the original carbon-14 to decay. (two half-lives)
Using the half-lives from radioactive elements to calculate age is known as radiometric dating.
Dating rocks using radioactive elements can indicate a particular time of formation.
When minerals in igneous rocks are first formed after cooling, the amount of each radioactive mineral present at this time is 100%
Immediately after formation these unstable radioactive minerals begin to break down
Think about this:
If you were given two samples of rock, one sample containing 95% of the original radioactive parent element and the other sample had 30% of the same original radioactive parent element, which of these rocks do you think would be oldest? Why?
Radiometric dating methods have enabled scientists to determine the time of mineral formation of some of the oldest rocks on Earth.
The oldest minerals ever dated in Australia are zircon crystals found in quartzite rock at Mt. Narryer in the Murchison region of Western Australia. These minerals are dated at 4.15byo
The correct types of radiometric isotopes have to be used to date Precambrian rocks. Precambrian time starts about 4.7 billion years ago when the Earths crust began to form to the start of the Cambrian about 542 million years ago.
Because the Precambrian dates back to far, uranium/lead isotopes are used to date rocks within this period of time. Naturally occurring uranium contains two radioactive isotopes both with very long half-lives.
Uranium usually occurs as
trace elements in minerals
such as zircon.
The Cambrian Event
The appearance of numerous and varied fossils separates the Precambrian from Cambrian. We know that life existed well before the Cambrian but was not preserved as fossils. During the Cambrian, existing life evolved very rapidly in the oxygen rich environment which allowed more complex evolution to occur.
For example, more complex organisms began reproducing sexually versus asexually which had happened up until this period in time. Sexual reproduction allows individuals to pass on genetic material to their offspring. This mixing of genetic material means that the offspring were not identical to their parents.
Genetic variations happen because of mutations. If such mutation is beneficial to the organism, it can then be passed through the population by sexual reproduction.
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Complete DOT Points 3.4, 3.5, 3.7
Evolution is organisms changing over time. Fossils provide evidence of these changes. Charles Darwin (1809-1882) proposed a theory to explain why evolution occurs.
An observable example of such evolution occurred in the United Kingdom during the industrial revolution. The peppered moth can exist in light and dark colours. During the industrial revolution, trees and forests were covered in soot which gave the darker moth an advantage because it was better able to hide from predators. In just a few generations the majority of the moths were dark.
The four points of Darwin’s theory of evolution are:
Scientists studying organisms today can see similarities between certain species and are able to trace these characteristics to distant relatives.
Today scientists have a number of lines of evidence to support the theory of evolution. This includes:
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Complete ‘To Think About’ page 89-91 HSC Spotlight Text