Radiometric dating
Using radioactive isotopes, igneous and metamorphic rocks to determine the age of fossils, rocks, and minerals.
- Parent isotopes of elements undergo radioactive decay and release energy to become a more stable daughter atom.
- Rate of decay is steady, allowing scientists to calculate age from amount of radioactive isotopes remaining in sample
Methods of measuring radioactivity
- SHRIMP (Sensitive High Resolution Ion Microprobe): dates very resilient grains of zircon. Can identify the oldest rocks on Earth e.g. 4.4 billion year old zircon grains from north-west WA
- Fission track dating: use electron microscopes to see the tracks left by decaying uranium atoms in the surface of grains as they release particles and energy
- Thermoluminescence: measuring the amount of radiation trapped in a rock using heat or laser light. The longer the crystal has been buried = the more radiation it has accumulated = the brighter the light
CO2 analysis
More carbon dioxide in the atmosphere: warmer earth (greenhouse effect, CO2 traps solar radiation)
- CO2 bubbles trapped in polar ice indicate concentration of CO2 in atmosphere in ancient times → can infer past climate of earth
- Warming/cooling of climate directly impacts types of plants and animals that exist in a period
O2 analysis
Use ratio of oxygen isotopes in ice core samples to reconstruct water temepratures on ancient earth
- Ratio of O-18 and O-16 in carbonate samples measured. Relationship between ratio and temperature is inverse: higher ratio of O-18 : O-16 = lower temperatures when carbonate was formed
Case study: climate change in Australia from continental drift
Intially Australia was warm and humid when it was still connected to Gondwana
- Fossil evidence of soft-shelled insects from the Miocene: tropical rainforest habitats dominated Australia
132-96 million years ago Australia separated from Gondwana
- Climate became more arid as continent drifted north → tropical rainforests became limited to pockets of land
- Plant and animal life evolved to contend with harsher climate of arid Australia
Sclerophyll adaptation as climate changed
- Tough leaves contain toxic chemicals
- Roots clumped together for efficiency in obtaining nutrients from nutrient-deficient soils
- E.g. eucalyptus and acacia
- Spread of eucalyptus and acacia coincided with increase in charcoal deposits in fossil records of around 20 million years ago → more frequent fires from hotter and drier climate = selection pressure against rainforest trees, favours sclerophyll species which can withstand dry climates
- Sclerophyll radiated into ecological niches vacated by rainforest plants
Kangaroo adaptation as climate changed
- Decline of rainforests caused a decline in arboreal mammals that lived in trees
- Open grasslands became more prominent, favouring the evolution of kangaroos
- 25 mya kangaroo ancestors lived in the rainforest. Had possum-like features, climbed trees and ran through the forest floor on all fours. Most similar to: musky rat kangaroo
- 15-20 mya kangaroos took over woodlands and grasslands. Adapted to bipedal locomotion to move more quickly through open woodlands and grasslands.
- Teeth have higher tooth crowns and replace themselves naturally by migrating rear teeth forward = built for wear and tear → grazers adapted to eating grass and sclerophyll plants
- Hopping: efficient method of transport
Procoptodon goliah
- Kangaroo-like megafauna that coexisted with modern kangaroos, now extinct
- Up to 3m high, couldn’t hop because body weight too heavy (240kg)
- Bulky, cube-shaped molars and tough jaw muscles, indiciating a propensity for eating woody vegetation.
Localised extinction
- Population of a species ceases to exist in an area of its former range but other populations of the species still exist elsewhere
- Caused by selection pressure in one habitat that does not occur in other areas
- Fragmentatin of koala population across QLD, NSW and VIC has caused local extinctions to occur more frequently. Isolated populations due to land clearing and road construction.
- Lack of population migration prevents mixing of genes between populations → less genetic diversity → reduced ability to withstand environmental changes
Origin of Plant in Australia
Three ways
- Some were already on the continent when it split from Gondwana millions of years ago
- Some dispersed from South-East Asia to Australia
- Introduced species
Origin of Animals in Australia
Four ways
- Some are considered ‘original residents’ that were on the continent when it split from Gondwana.
- Asian ‘immigrant’ species that arrived when sea levels were low, both 15 mya and again 40 000-30 000 years ago
- Some were introduced by immigrant traders or late Aboriginals four thousand years ago
- Some were introduced by European immigrants beginning two hundred years ago
