Transcript U3A-ClimChange02 7749KB Oct 07 2012

CLIMATE CHANGE
THE GREAT DEBATE
Session 2
EARTH HISTORY
4567 Ma to 2.5 Ma
• No rocks are left that date back to the
beginning of the Earth. Oldest known rocks
are approximately 3800 Ma
• The original atmosphere was probably
composed of carbon dioxide and methane. It
did not contain any oxygen
• Nitrogen and oxygen dominate the Earth’s
atmosphere today and their dominance is
due to biological processes. This has had a
major impact on climate
EVOLUTION OF LIFE
• 3500Ma - Earliest signs of the presence
of life in the form of prokaryotic
organisms, single cells without a
nucleus, including cyanobacteria
capable of photosynthesis. By
producing oxygen they gradually
changed the chemistry of the oceans
and caused the deposition of cherts and
bedded ironstones. These ancient rocks
are found in the Pilbara region of
Western Australia
Bedded ironstones in the Pilbara, Karijini
National Park, Western Australia
Ancient chert deposits at Marble Bar, Pilbara
Region, Western Australia
GREAT OXYGENATION EVENT
• 2450 to 1850 Ma: Photosynthetic
organisms (cyanobacteria) proliferate
in shallow seas. They took in carbon
dioxide and produced enough oxygen
for it to become a component of the
atmosphere. The oxygen reacted with
methane (a strong greenhouse gas) to
form more carbon dioxide (a weak
greenhouse gas).
Stromatolites are colonies of cyanobacteria
capable of photosynthesis and causing deposition
of limestone. Hamelin, Shark Bay, WA
GROWTH OF ATMOSPHERIC OXYGEN
Stage 1 - 3800 to 2450 Ma: No oxygen in the atmosphere
Stage 2 - 3450 to 1850 Ma: Oxygen level low. Most absorbed by rocks and oceans
Stage 3 - 1850 to 850 Ma: Oceans start to contribute oxygen but most absorbed
by rocks on land
Stage 4 - 850 to 470 Ma: Accumulation of oxygen in atmosphere as oceans and
rocks at the surface of the Earth are saturated
Stage 5 - 470 to present: Oxygen production by plants accelerates as plants
invade the land
EVOLUTION OF LIFE
CONTINUES
• 2400 Ma - Evolution from single-celled
prokaryotic life to more complicated
single-celled eukaryotic life where cells
have nuclei that are able to protect
them from the presence of oxygen in
the oceans and atmosphere which
killed off most of the prokaryotic life
forms
Tree of life showing
division into three
major domains
THE FIRST GLACIATION?
• 2400 to 2100 Ma: Huronian Glaciation.
Evidence for this glaciation is found in
Canada. Nothing is known about its
extent but at that time continents are
believed to have been much less
extensive than they are today because
plate tectonics and continental drift
caused them to grow over time
EVOLUTION OF LIFE
CONTINUES
• 1500 to 1000 Ma. First appearance of
multi-celled, soft-bodied animals that
have left little evidence in the
geological record
• 635 to 542 Ma. Ediacaran Period.
Named after a site in the Flinders
Ranges. Many soft-bodied fossils not
closely related to later faunas
EVOLUTION OF LIFE
CONTINUES
• 542 TO 500 Ma. Cambrian Period saw
an explosion of marine life. All animal
phyla now known originated at this
time. Many animals had hard parts
made of chitin, calcium carbonate and
calcium phosphate that were well
preserved as fossils.
• 470 Ma. First appearance of plants on
land followed closely by animals
SNOWBALL EARTH
• Between 750 and 635 Ma there was
intense and widespread glaciation that
affected areas close to the equator. It is
also known as the Neoproterozoic
Glaciation and in Australia as the Sturt
Glaciation
• Tillites of this age are found in the
Flinders Ranges where two major ice
advances have been documented
Bolla Bollana tillite exposed in the Gammon
Ranges, South Australia
MORE GLACIATIONS
• 450 to 420 Ma. Ordovician – Silurian
Glaciation. Ice sheets over Africa and
probably South America
• 360 to 260 Ma. Carboniferous – Permian
Glaciation. Ice sheets over Gondwana Land
• 150 to 130 Ma. Jurassic – Cretaceous
Glaciation. Confined to Antarctica but very
little evidence
• 2.5 Ma to present. Quaternary Glaciation
Ordovician-Silurian Glaciation
Carboniferous-Permian Glaciation
Jurassic-Cretaceous Glaciation
Quaternary Glaciation
Temperature reconstruction for last 540 Ma based
on isotope ratios of fossil shells
WHAT CAUSED ICE AGES?
• No hypothesis fits all of them
• Huronian ice age may have been
caused by reduction of greenhouse
gases (methane and carbon dioxide) in
the atmosphere
• Snowball Earth affected tropical areas
but tilt of the earth may have been
different. A 70° tilt would cause ice to
accumulate near the equator but would
leave polar areas free of ice
WHAT CAUSED ICE AGES?
• Later ice ages may have been triggered by
continental drift moving continents to high
latitudes
• Carbon dioxide in the atmosphere reached
its lowest levels during the Carboniferous Permian and Quaternary Glaciations but was
considerably higher during the other two. It
can be argued that low CO2 levels are a
result rather than a cause of glaciation as
colder oceans absorb more CO2
CATASTROPHIC CLIMATE
CHANGE
• The fossil record for the last 542 Ma
shows five major extinction events as
well as a number of minor ones
• They are attributed to catastrophic
events that changed climate so rapidly
that it did not allow time for many
animals and plants to adapt. A number
of causes have been suggested but two
are particularly likely
FLOOD BASALT ERUPTIONS
• They were very extensive volcanic
eruptions that flooded large areas with
basalt lavas
• They produced large amounts of
volcanic dust, carbon dioxide and
sulphur rich gases. The volcanic dust
would have caused rapid cooling by
reflecting sunlight followed by warming
caused by the addition of greenhouse
gases to the atmosphere
IMPACT EVENTS
• Meteorites larger than one kilometer in
diameter would cause world-wide changes
due to the amount of dust, the triggering of
large scale forest fires and by generating
mega tsunamis if they were to plunge into
the sea
• The last great extinction event, that occurred
65 Ma ago and caused the extinction of the
dinosaurs, may have been caused by an
impact in the sea off Mexico where a 100 km
diameter buried crater has been identified.
Tsunami deposits of this age have been
identified on the south coast of the USA
LATE PERMIAN EXTINCTION
• The largest recorded extinction event
occurred at 251 Ma when 96% of
complex life on Earth was wiped out
• It has been attributed to huge flood
basalt eruptions in Siberia (Siberian
Traps basalts)