CLIMATE CHANGE

THE GREAT DEBATESession 2

EARTH HISTORY4567 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, KarijiniNational Park, Western Australia

Ancient chert deposits at Marble Bar, PilbaraRegion, 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 atmosphereStage 2 - 3450 to 1850 Ma: Oxygen level low. Most absorbed by rocks and oceansStage 3 - 1850 to 850 Ma: Oceans start to contribute oxygen but most absorbed

by rocks on landStage 4 - 850 to 470 Ma: Accumulation of oxygen in atmosphere as oceans and

rocks at the surface of the Earth are saturatedStage 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)