LEC 103 - Global Climate Change 1.1

Past climate
1. Relative dating
  1. Uses principles of stratigraphy and the study of fossils to determine the relative age of rocks and sediments
2. Absolute dating
  1. Radiometric dating techniques were developed to determine the absolute ages
    1. Dates in years of rocks and sediments
  2. Isotopic dating
    1. Atoms of radioactive elements spontaneously disintegrate to form atoms of a different element, liberating energy in the process
      1. The reason that radioactive decay offers a dependable means of keeping time is...
        The number of radioactive atoms in a mineral declines at a fixed rate over time
        The decay rate does not vary with the change in T or P which typically accompany geological processes
        The isotopic age of rock = time since the isotopic clock was reset
        When the isotopes were locked into the minerals e.g. crystallisation from melt or during metamorphism
    2. Radioactive parent elements decompose to form daughter elements
  3. Types of dating
    1. Radiocarbon, 14C, Dating
      1. All atoms of the same element have the same atomic number. Different isotopes of an element have the same no. of protons but different no. of neutrons
        C isotopes: 6,7,and 8 neutrons i.e. atomic masses of 12,13,14
        12C and 13C are stable and do not disintegrate. 14C is generated in Earth's atmosphere; 14C in the atmosphere remains constant
      2. Growing organism incorpoates C into tissues at the ratio found in the atompshere
        Death; C is not longer absorbed - 14 C in tissue decays
      3. Half of the radioactivity will be lost after 5730 years.
        By measuring the amount of 14C remaining in the fossil the date at which death occurred can e determined
    2. Rubidium strontium dating
    3. Argon argon dating
    4. Uranium series dating
3. Rocks and sediments and fossils
  1. Observations of rocks/sediments/fossils provide information on past climate. this coupled with their ages, implies climate chnage
4. Landforms
  1. Glacial Cirques
    1. Lowest cirque flor of the group of contemporaneous cirques = snowline
      1. Ancient snowline can be compared with present dat snowline, and temperature changes calculated
  2. U-shaped vallys
  3. Sand DUnes
    1. Large continental sand dunes only develop where precipitation is <100 mm a-1
    2. If precipitation >100mm a-1, vegetation cover reduces sand movement and encourages soil development
    3. Fossil and dunes currently sound in areas of high rainfall suggest that rainfall has increase since dune formation
5. Permafrost
  1. Permanently frozen ground at or below freezing point of water for two or more years where only superficial layer may thaw during summer
  2. Former mean annual temperatures can be inferred from distribution of ancient permafrost features
  3. Covers 25% of earths land surface
  4. Features include
    1. Ice wedges, ice mounds, patterned ground
Varves
1. Regular alternations in lake sediment layers
  1. Couplets/pairs represent annual seasonal deposition
    1. Fine grained layers = autumn/winter. Coarse grained layers = spring/summer
  2. Characteristics of an individual varve layer may provide 1) an indicator of the climate at the time of deposition and 2) a date
Palynology
1. Study of pollen - organic miscrofossils, 5-500 micrometers in size
2. Some sediments contain pollen grains derived from local/regional vegetation
  1. Analysis of abundance and type of pollen grains in a particular horizon provides a picture of vegetation which thrived at the time of deposition
Beetles
1. Used for climate change studies for rocks deposited in last 22 ka, although beetles have existed the Lower Permian
Diatoms
1. Diatoms are aquatic microscopic unicellular algae
2. They absorb nutrients and Co2 from the surrounding water and are commonly preserved in lake sediments
3. Use to establish...
  1. Past lake quality
    1. Past shorelines
  2. Interface of fresh and saline envrionments
    1. E.g. salinity, nutrients, temperature
Deep sea cores
1. Sea floor offers more continuous stratigraphic record than terrestrial sections
2. Cores indicate a series of cold and warm episodes related to glacials and interglacials for last 2.6 Ma
3. Indicator
  1. 18O/16O of foraminifera
    1. Temperature, ice volume
  2. Coarse debris
    1. Iceberg rafting
  3. Fluvial sediments
    1. River inputs
4. Foraminifera are microscopic aquatic organisms with a CaCo3 shell
  1. They are especially sensitive to water temperature and salinity

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LEC 103 - Global Climate Change 1.1

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