2. Methods of investigating glacial deposits Flashcards
What methods can be applied to the study of a glacial till and what are the purposes of these methods?
Fine gravel
- Composition. Århus 2-4 mm
Particle size
- Grain size distribution 0.002 - 63 mm
Clast Fabric
- Orientation
Stereonet
- Orientering af lag. Tektonik
Clast morphology
- indicator of transport processes
Micromorphology
- lodgement
- shearing
- flow
Magnetic susceptibility
- Good for till correlations
What is erratic boulders?
rock fragments that are not native to the area
What does class roundness tell us about where the material was transported?
The longer the transport = the more rounded are the grains
Materials inglacial - angularity is common
Materials at the ice bed - becomes more rounded
What are some extremely stable and very unstable minerals?
Extremely stable:
- zircon
- tourmaline
- rutile
- topaz
- corundium
Very unstable:
- apatite
- fayalite
What is a problem with dating by clay minerals?
post-sedimentary transitions of one clay mineral into another, such as illite to montmorillonite.
What is the difference between absolute and relative dating?
Absolute dating = result gives us a number (like 2 million years old)
Relative dating = do not give specific time but that this unit b) is older than a) but younger than c)
What can variations in O18 and O16 in the ocean tell us?
- Isotopes bound by marine organisms when creating shells.
- Then the organism dies, falling to the bottom - and is available for the study of the two isotopes.
- Isotopes evaporate (when glaciers form).
- O18 tend to return relatively quickly into the sea because it’s heavier - so in glacial times, there’s low component of O16 (trapped in ice)
The weight of them is different - important
How can magnetic field changes be used as a stratigraphical tool?
Magnetic minerals in lava: cooling → aligned according to Earth’s magnetic field.
Particle settling from suspension (water) or wind (loess) → aligned according to Earth’s magnetic field.
History of the Earth devided into
polarity epochs. For the Quaternary:
- Brunhes Epoch (+ normal polarization)
- Matuyama Epoch (- reverse polarization)
Each epoch has subunits and sub-subunits
How does C14 radiocarbon work?
C14, bindes til O og mixes med atmosfæren. Organismer stopper optag af C fra atmos, når de dør. C14 henfalder ved halflife 5730 y.
Ratio af C14/C12.
Range: 300-ca. 50.000 y
How does potassium/argon dating work?
K/Ar
40K decays to 40Ar
Applicable to volcanic deposits, e.g. tephra
Starting event required (setting the clock at 0) → volcanic erruption (release of 40Ar (gas) to atmosphere)
Volcanic rocks and minerals crystallize from molten lava → only 40K → decays to 40Ar
Range: up to 3-5 Ma
How does uranium/thorium dating work?
Used to determine the age of calcium carbonate materials
U decays to Th
Initially no Th (not soluble in water)
Ratio of uranium to thorium isotopes → time passed since formation
What is fission-track dating?
238U decay → α radiation + spontaneous nuclear fission
Physical scars created on the minerals - but they tend to ‘heal’/disappear during time
How does luminescence dating work?
Exposure of minerals (quartz, feldspar) to radiation from radioactive isotopes in soils and sediments increases with time after burial → dating of the burial event.
Intensity corresponds to the last heating or exposure to sunlight event (= deposition age)
How does beryllium dating work?
10Be – a cosmogenic isotope produced in the atmosphere by a reaction between cosmic radiation, nitrogen and oxygen.
Attached to aerosols → precipitation on earth surface and ocean floor.
- Exposure age dating → how long a surface has been exposed; e.g. after a glaciation → deglaciation age
Rapid sedimentation → low content (e.g. loess), time to accumulate is low
Slow sedimentation → high content (e.g. paleosols), exposion time is high
How does amino-acid dating of submarine sediments work?
Proteins with high molecular weight: only L-isomers
Death of organism
Gradual transformation into D-isomers, until equilibrium is reached
L/D ratio → Age determination
