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Paleomagnetic Records: Discovery and Correlation

Paleomagnetic Records: Discovery and Correlation. Jeannie Bryson. 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

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Paleomagnetic Records: Discovery and Correlation

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  1. Paleomagnetic Records:Discovery and Correlation Jeannie Bryson

  2. 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

  3. 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

  4. Origin of Earth’s 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 Source:http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html

  5. Properties of Earth’s Magnetic Field • Magnetosphere: • Earth’s 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/earth/magnetic.html

  6. Properties of Earth’s Magnetic Field • Field Lines and Van Allen Radiation Belts Source:http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html

  7. Properties of Earth’s 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

  8. Properties of Earth’s 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

  9. Properties of Earth’s 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 • Earth’s Magnetic Field ~ 3.5 billion years old • Rocks dated by paleomagnetic methods • Proves that Earth’s magnetic field must be regenerative

  10. Properties of Earth’s Magnetic Field • Geodynamo: alpha-omega dynamo cycle Source:http://geomag.usgs.gov/intro.html

  11. Types of Paleomagnetic Records • Magnetization • Thermoremnant Magnetization: • Lava and clays contain small amounts of iron and when heated obtain magnetization that is parallel to Earth’s magnetic field after cooling (unless reheated) • Depositional Remnant Magnetization: • Lake and ocean sediments are deposited and settle, iron particles record Earth’s magnetic field (unless disturbed) Source: http://www.archserve.id.ucsb.edu/anth3/Courseware/Chronology/11_Paleomag_Archaeomag.html

  12. 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

  13. 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 • VGP’s can be used to create a curve representing paleomagnetic variations

  14. 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 Earth’s orbital influence • Data correlated with O18 reference curves

  15. Sint-800: 800 kyr marine sediment record

  16. 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

  17. 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

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