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 RecordsSource: http://geomag.usgs.gov/program.html

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:http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html

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

Properties of Earth’s Magnetic Field

Field Lines and Van Allen Radiation Belts

Source:http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html

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:http://geomag.usgs.gov/intro.html

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

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. 14C 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