Quaternary Geology

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  • Evidence for reconstructing climate change. - evidence for glacial/interglacial periods
    • Sea Level Change
      • Growth and decay of ice sheets and glaciers can cause global sea level fluctuations of up to 180m over a glacial/ inter-glacial cycle. Changes in sea level are caused in 2 ways.
        • 1. Glacio-Eustatic change (global) - Sea level change caused by the removal or addition of ocean water in response to the growth or decay of glaciers/ ice sheets.
          • During glacials Moisture is taken out of the Earth's short term hydrological cycle through the growth of ice sheets and locked up as ice. At the same time, continued evaporation reduces global sea levels
            • In contrast, during interglacials global warming causes the ice sheets to melt and the water to return back to the oceans, raising sea levels
        • 2. Glacio- Isostatic - In areas covered by Ice sheets, the land is depressed relative to the sea level.  The weight of the ice pushes the continental crust down into the mantle beneath.
          • When the ice is removed by the melting of ice, the land rebounds by isostatic uplift, causing the sea level to fall relative to the land.
          • The continntal crust can be pushed down because the Asthenonsphere is plastically deformed
    • Oxygen Isotope analysis of micro-organisms buried in sediment
      • Micro organisms such as foraminifera secrete CaCO3 from the seawater to form their protective shells.
        • By measuring the isotopic composition of oxygen in CaCO3 using a mass spectrometer, geologists can see the O18/O16 ratio of seawater at the time the creature was living. To do this, a sediment core is drilled into the ocean floor sediment
          • LOW O18/6 = INTERGLACIAL                  -This means O18/016 ratios are balanced
            • When sea water evaporates, natural fractionation  occurs because the lighter H2O16 molecules evaporate much  more easily than the more dense H2O18 molecules.
              • Therefore, the moisture that fuelled the growth of ice sheets during glacials was relatively enriched with less dense H2O16 molecules which became trapped in ice.
                • As a result, the remaining seawater became depleted in H2O16 molecules. This is shown by HIGH H2O18 O18/O16 molecules
                  • However, the melting of ice during interglacials causes liberates the H2O16 back into the sewater, decreasing the O18/O16 ratio
          • HIGH O18/16 = GLACIAL     -This means that there is a higher ratio O18 compared to O6
            • When sea water evaporates, natural fractionation  occurs because the lighter H2O16 molecules evaporate much  more easily than the more dense H2O18 molecules.
              • Therefore, the moisture that fuelled the growth of ice sheets during glacials was relatively enriched with less dense H2O16 molecules which became trapped in ice.
                • As a result, the remaining seawater became depleted in H2O16 molecules. This is shown by HIGH H2O18 O18/O16 molecules
                  • However, the melting of ice during interglacials causes liberates the H2O16 back into the sewater, decreasing the O18/O16 ratio
    • Ice Cores         - Is a core sample extracted from a polar ice sheet or glacier
      • 1. Can be accurately dated because it is accurate to a single year. It follows a natural timescale of annual summer and winter layers
        • Summer Layers- Thick due to greater snowfall, LIGHT and COARSELY CRYSTALLINE
        • WINTER LAYERS - Thin due to less snowfall, DARKER
          • Deeper layers are destroyed by the plastic deformation of ice
            • Summer Layers- Thick due to greater snowfall, LIGHT and COARSELY CRYSTALLINE
      • 2. Ice cores have air bubbles which provide a precise record of the composition of atmosphere at the time the air bubbles were trapped by snowfall
      • CLIMATIC INFO DERIVED FROM ICE CORES
        • 1. AIR TEMP + OCEAN VOLUME
          • Ice formed during glacials contains less O18
        • 7. DESERT EXTENT
          • Wind blown dust which can be linked to a period of aridity during glacials
        • 5. VOLCANIC ERUPTIONS
          • Revealed by high concentrations of CO2, SO2 + Volcanic dust
        • 4. SOLAR ACTIVITY
        • 2. RATES OF PRECIPITATION
          • Annual ice thickness - Warmer air causes greater snowfall - Interglacial
            • E.G. The sudden increase in thickness of annual layers in Greenland 11,500yrs/agowas caused by the Gulf Stream
        • 6. FOREST FIRES
          • Wind blown charcoal dust or soot
        • 3. LAND + OCEAN PRODUCTIVITY
          • The reduction in biogenic activity on land and sea is shown by a decrease in CO2
    • Pollen
  • STRENGTHS - Global        1. Eustatic rise in response to global warming and melting continues today at a rate of 1.8mm/day 2. Evidence of Eustatic seal level change can be corroborated with the oxygen Isotope Record          3. Direct evidence of minor Isostatic Rebound is in the number of minor EQ's  recorded in deglaciated areas
    • Sea Level Change Evaluation
      • WEAKNESS - 1.Changes in sea level can be caused by thermohaline changes. i.g. when salt water warms, it expands  and contracts when it cools. 2. The construction of sea level curves is done using accurate radiometric dating techniques, however the precision of radiometric dating is subject to a 10% uncertainty

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