The Environment of the Phanerozoic eon Part 2-Lesson 1
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Conditions for life on Earth The Earths 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.
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Conditions for life on Earth 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 BIFs formed.
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Conditions for life on Earth 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.
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Conditions for life on Earth 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.
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Conditions for life on Earth 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.
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Conditions for life on Earth This reaction absorbs some of the
UV radiation which would otherwise reach the Earths surface. How
would this affect early life on Earth?
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Conditions for 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.
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Conditions for life on Earth 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.
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Conditions for life on Earth 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)
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Homework Read page 71 HSC Spotlight Text Update electronic
vocab list Complete To Think About SET 2 pg 73 HSC Spotlight Text
Complete DOT Points 2.1-2.4 pg 33-34
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The Cambrian Explosion Part 2-Lesson 2
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Dating 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.
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Dating the Cambrian Explosion Fossils around the world provide
us with evidence about what these times were like and the organisms
that lived during them.
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Dating the Cambrian Explosion 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.
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Dating the Cambrian Explosion 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.
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Activity Read pages 11-12 A snapshot handout from the TAFE
Environments Through Time Module Underline key words and phrases
Complete the Dating Game Activity on page 13
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Dating the Cambrian Explosion 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?
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Dating the Cambrian Explosion 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.
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Dating the Cambrian Explosion 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.
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Homework Read pages 75 HSC Spotlight Text Update Electronic
Vocabulary Complete DOT Point 3.1
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Radiometric Dating Part 2-Lesson 3
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Radiometric Dating Relative dating is based on position in a
strategraphic column as well as a comparison of the lithology of
the rocks and their fossils. To determine an absolute date of a
rock or fossil, scientists use radiometric dating.
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Radiometric Dating RD is based on the radioactive decay of
isotopes to measure age. Radioactive decay happens when the
unstable parent isotope breaks down into a more stable daughter
isotope.
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Radiation and Decay Below is a list of other examples of atoms
decaying to become more stable : Uranium-235 to lead-207
Thorium-232 to lead 208 Rubidium-87 to strontium-87 Potassium-40 to
argon-40 Carbon-14 to nitrogen-14 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.
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Determining ages from half-lives The time taken for each of the
parent atoms to decay to their daughter products varies from
millions of years to minutes. KEY CONCEPT: The time taken for half
of the parent material to decay into its daughter product is know
as the Half Life o f that parent material.
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Determining ages from half-lives 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
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Determining ages from half-lives 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 of the original carbon-14 to decay. (two
half-lives)
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Determining ages from half-lives This diagram shows the
relationship between half-life and amount of radioactive parent
material
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Dating Rocks KEY CONCEPT: 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.
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Dating Rocks 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
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Dating Rocks 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?
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Dating the oldest rocks in Australia 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
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Dating the oldest rocks in Australia
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Methods Used to Date Precambrian Rocks 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.
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Methods Used to Date Precambrian Rocks 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.
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Homework Read pages 76-77 HSC Spotlight Text Update electronic
vocabulary Complete Activity 2.7 pg 77 HSC Spotlight Text Complete
DOT Point 3.3
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The Cambrian Event Part 2-Lesson 4
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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.
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The Cambrian Event 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.
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The Cambrian Event 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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The Cambrian Event If a new population is separated from the
original population by geographical isolation or other isolating
mechanism, over many generations they can evolve into a new
species.
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The Cambrian Event Organisms with hard parts and shells did not
exist before the Cambrian period (Precambrian). A well known fossil
site for these soft bodied organisms is in Ediacara, South
Australia. This site is very important geologically because fossils
of these organisms are very rare. Spriggina Dickinsonia
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The Cambrian Event The first hard-bodied organisms had an
advantage over their soft bodied competitors. They could use this
to hide from predators and for protection against environmental
elements. Do you think it allowed them to colonise new
environments?
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The Cambrian Event These organisms with hard parts have a much
greater chance of being preserved as a fossil than those that were
soft. The most famous fossil site for Cambrian organisms is the
Burgess Shale in British Colombia.
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The Cambrian Event Man of the late Cambrian organisms such as
brachiopods and trilobites had very specialised armour to protect
them from predators. These organisms also evolved long spines which
allowed them to burry themselves in the sand to prevent being
washed around by currents.
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The Cambrian Event These hard bodied organisms became more
abundant and still exist today. Examples include molluscs and
crustaceans.
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Activity Complete activity 2.8 and 2.9 pg 79 HSC Spotlight Text
in class.
Science Skills Activity Read and discuss the bottom of page 79
Complete To Think About pg 80-81 Complete any outstanding
homework
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Evolution Part 2-Lesson 6
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The theory of evolution Evolution is organisms changing over
time. Fossils provide evidence of these changes. Charles Darwin
(1809-1882) proposed a theory to explain why evolution occurs.
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The theory of evolution Darwin believed that evolution occurred
because of natural selection. Certain traits passed on from
generation to generation make certain organisms more likely to
survive and reproduce. Those better suited to survive will do just
that, and those who arent, die and do not pass on their genetics to
others.
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The theory of evolution 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.
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The theory of evolution The four points of Darwins theory of
evolution are: 1. Genetic variation can occur in any population and
individuals within a species are not identical. 2. Offspring do not
always survive to adults. As they die, the characteristics they
posses are not passed on. 3. The offspring that do survive and
reproduce pass on favourable genetic variations as they are well
adapted to the environment. 4. These favourable genetic variations
are passed on to offspring and become more common in the
population.
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The theory of evolution Scientists studying organisms today can
see similarities between certain species and are able to trace
these characteristics to distant relatives.
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The theory of evolution Today scientists have a number of lines
of evidence to support the theory of evolution. This includes:
Anatomy Physiology Biogeography Embryonic development
Biochemistry
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Activity Read and discuss pages 83-88 HSC Spotlight Text
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Homework Update electronic vocabulary Complete To Think About
page 89-91 HSC Spotlight Text