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 Use and Implications of Magnetic Records Source:

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

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:

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:

Properties of Earth’s Magnetic Field Field Lines and Van Allen Radiation Belts Source:

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

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

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

Properties of Earth’s Magnetic Field Geodynamo: alpha-omega dynamo cycle Source:

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:

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

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. 14 C or dendrochronology VGP’s can be used to create a curve representing paleomagnetic variations

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

Sint-800: 800 kyr marine sediment record

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

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