Paleomagnetic Records: Discovery and Correlation

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Paleomagnetic RecordsDiscovery and Correlation

• Jeannie Bryson

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Outline

• Discovery of Magnetic Field
• Origin of Magnetic Field
• Properties of Magnetic Field
• Types of Paleomagnetic Records
• Volcanic
• Seafloor magnetic anomalies
• Archaeomagnetic
• Marine Sedimentary Sequences
• Use and Implications of Magnetic Records

Source http//geomag.usgs.gov/program.html
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Discovery of Magnetic Field

• Hans Christian Oersted-1821
• Accidentally discovered that passing an electric
  current over a compass caused the arrow to move
• Andre-Marie Ampere
• Further concluded that magnetism was a force
  between electric currents parallel currents
  attract and opposite currents repel

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Origin of Earths Magnetic Field

• Source motion of electrical charges
• Magnetic field produced by electrical currents
  but it is not fully understood how
• dynamo effect- One possibility
• electrical currents produced by the coupling of
  convective effects
• rotation in the spinning liquid metallic outer
  core of iron and nickel

Sourcehttp//csep10.phys.utk.edu/astr161/lect/ear
th/magnetic.html
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Properties of Earths Magnetic Field

• Magnetosphere
• Earths Surface Neutral
• 100km above surface Solar Rays cause charged
  ions
• Hinders the ability of solar rays to enter
  atmosphere, i.e. bow shock

Source http//csep10.phys.utk.edu/astr161/lect/ea
rth/magnetic.html
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Properties of Earths Magnetic Field

• Field Lines and Van Allen Radiation Belts

Sourcehttp//csep10.phys.utk.edu/astr161/lect/ear
th/magnetic.html
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Properties of Earths Magnetic Field

• Magnetic Field Components
• Declination angle between horizontal component
  of magnetic-field vector in relation to true
  north
• Inclination angle between horizontal plane and
  total field vector

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Properties of Earths Magnetic Field

• Dipole Component
• Magnetic field lines come out of south pole and
  converge at north pole
• Non-Dipole Component
• Poles are not antipodal-asymmetrical
• Axis is tilted 11 degrees relative to the
  rotational axis

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Properties of Earths Magnetic Field

• Geodynamo a dynamo in core generates
  electrical currents
• self-sustaining electrically-conducting fluid
  flow across magnetic-field lines and generate
  electrical current to support existing field
• Earths Magnetic Field 3.5 billion years old
• Rocks dated by paleomagnetic methods
• Proves that Earths magnetic field must be
  regenerative

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Properties of Earths Magnetic Field

• Geodynamo alpha-omega dynamo cycle

Sourcehttp//geomag.usgs.gov/intro.html
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Types of Paleomagnetic Records

• Magnetization
• Thermoremnant Magnetization
• Lava and clays contain small amounts of iron and
  when heated obtain magnetization that is parallel
  to Earths magnetic field after cooling (unless
  reheated)
• Depositional Remnant Magnetization
• Lake and ocean sediments are deposited and
  settle, iron particles record Earths magnetic
  field (unless disturbed)

Source http//www.archserve.id.ucsb.edu/anth3/Cou
rseware/Chronology/11_Paleomag_Archaeomag.html
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Types of Paleomagnetic Records

• Volcanic Records
• Thermoremnant magnetization
• 50 kyr record
• Lava cools and iron in lava obtains magnetization
  of the time and place of cooling
• Magnetization can then be measured and dated
• Lava records are not continuous

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Types of Paleomagnetic Records

• Archaeomagnetic Records
• Remnant magnetism heating above Curie point sets
  magnetization
• Virtual Geomagnetic Pole (VGP) measures location
  of magnetic north at a certain time
• Uses magnetization of object and another dating
  tool i.e. 14C or dendrochronology
• VGPs can be used to create a curve representing
  paleomagnetic variations

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Types of Paleomagnetic Records

• Marine Sediment Records
• Yohan Guyodo-1996,1999 200 kyr and 800 kyr
• 800 kyr 33 marine records stacked to create a
  composite curve
• Sediments provide continuous record with few
  disturbances
• Shows non-dipole moments
• Does not support periodicity thought to be caused
  by Earths orbital influence
• Data correlated with O18 reference curves

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Sint-800 800 kyr marine sediment record
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Types of Paleomagnetic Records

• Sea-Floor anomalies
• 780 kyr record
• East Pacific Rise
• Supported by seafloor glass paleomagnetic record
  for past 50 kyr
• Similar to marine sedimentary record
• Can be used to understand both ridge crest
  accretion and geomagnetic field behavior

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Concluding Thoughts

• Variations in geomagnetic intensity do not appear
  to show any signs of periodicity
• Due to non-dipole component
• Mean fluctuations in amplitude appear to be
  constant