Reversals of the Earths Magnetic Field

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Reversals of the Earths Magnetic Field
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In this talk we use paleomagnetic observations
and geodynamo simulations to learn about
reversals.

• Paleomagnetic recording media
• Geomagnetic polarity time scale.
• Triggering of reversals.
• How does the field behave during reversal
  transitions?
• Geodynamo simulations and influence of lower
  mantle
• and inner core on the field.

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Paleomagnetic recording media
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Development of the geomagnetic polarity timescale
Allan Cox
0-5 Ma GPTS (1968)
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Ocean Sediment Cores
Marine Magnetic Anomalies
Joides Resolution Drill Ship
(Technique of transformation to the pole
enables stacking of profiles from all over
globe.)
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Extended GPTS (0-160) Ma, mainly from marine
magnetic anomalies.
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Cox (1968) model for triggering reversals
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Very early reversal records, from marine sediment
core
Field intensity decreases during reversals
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Average duration of polarity transition 7000 yr
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What can paleomagnetism tell us about the next
reversal?
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Yes, for sure the geomagnetic field has
reversed many hundreds of times during the past
160 million years.
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No. The field has been much lower in the recent
past without reversing.
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Low intensity and transitional directions during
excursions would be accompanied by similar
environmental effects as for reversals.
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What are the likely environmental effects?
? Expanded aurora borealis.

• Somewhat increased exposure to humans of
• charged particles at the surface.

? Increased cosmogenic nuclide production.
? Ozone holes and some power grid failures
during large solar events.
? Confused animals?
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Reversal records--from sedimentary rocks
Same early deep-sea record--avg. deposition rate
2 mm/kyr.
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Lake sediment record--avg. deposition rate 200
mm/kyr.
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Reversal records--from volcanic rocks
Steens Mountain, SE Oregon
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Steens Mountain basalt is the start of
Yellowstone hotspot volcanism
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Plume volcanism?
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Steens Reversal Records (Jarboe et al., 2007)
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Steens Reversal Composite Record (Jarboe et al.,
2007)
(Many duplicative flow directions omitted for
clarity.)
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Mono Lake Excursion
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Summary of what the field does during a reversal
from Steens and M-B (results not shown)
(1) Reversals often involve multiple excursions
in direction before establishing stable polarity.
(2) Reversals can take much longer than the
average 7,000 yr duration if all swings are
included.
(3) Non-dipole fields make reversal paths vary
in geometry and duration at different positions.
(4) Sedimentary rocks can potentially record
complete transitions, but commonly suffer from
artifacts that smooth and/or complicate the
record.
(5) Lava flow records are fragmentary and
therefore underestimate the complexity of
transition paths, but they provide accurate
instantaneous directions and sometimes
very-high resolution detail missed by
sedimentary records.
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Geodynamo Simulations that Reverse
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• Limited resolution spherical harmonic
• degree and order to 21, 21, Chebyshev
• polynomials to degree 48 in radius
• Hyperdiffusion damps energy in magnetic
• field terms noticeably beyond n7

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Pole paths of two relatively simple reversals
during geodynamo simulations transition
durations 5000-7000 simulated years
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Very long, complex reversal 28,000 simulated
years
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A magnetic excursion during a geodynamo simulation
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What effect does the mantle have on the field?
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Geographical Bias of Transitional Poles
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Geographical Bias of Transitional poles in
Tomographic Simulation
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VGPs from 14 North American Reversal and
Excursion Records (0.0316.6 Ma) (Glen et al.,
1999)
Geographic bias of transitional VGPs shifted
eastward
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Reversal Frequency
Influence of lateral temperature variation in
the lower mantle on heat conducted through the
CMB
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nm odd ? antisymmetric about equator nm even?
symmetric about equator
Spatial Energy Density at CMB for each n,m
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Spatial Energy Density at CMB
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Paleosecular Variation from 0-5 Ma Lava Flows

• Merrill McFadden, 1988 and
• McFadden Merrill, 1988
• showed that Sa2(b?)21/2 fits the PSV data,
  and the coefficient
• a derives only from the nm even
  (symmetric) terms
• b derives mainly from the nm odd
  (antisymmetric) terms

Sa2(b?)21/2
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(Data from McFadden et al., 1991)
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• PSV odd/even ratio
• Peaks during the Cretaceous Normal Superchron
• Is minimum during times of rapid reversal rate

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What effect does the inner core have on the field?
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Case S Small Inner Core Simulation
(homogeneous heat flux at the CMB Roberts
Glatzmaier, 2001)
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Odd/Even Energy Ratio up to 5,5
Coe Glatzmaier, 2006
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Were reversals much less frequent during
Precambrian Time?

• 2500-1000 Ma Statistical analysis of polarity in
  global paleomagnetic studies
• low percentage of studies with both polarities
  (Roberts and Piper, 1989).

• 2775-2715 Ma Australian flood basalts, tuffs,
  felsic volcanics, clastic sediments
• 2 reversals, minimum duration without
  reversals of 26 Ma (Strik et al., 2003).

• 2465 ? 15 Ma Matachewan dike swarm, fed flood
  basalt of 3-30(?) Myr duration
• 1 reversal (Halls, 1991).

• 2775-2465 Ma lower symmetric (even harmonic)
  contribution to secular variation,
• consistent with more stable Late Archean field
  (Smirnov Tarduno, 2004).

• 1468-1401 Ma North American Belt-Purcell
  Supergroup silt and sandstones
• 22 reversals in 67 Myr, 1 reversed chron of
  about 30 Myr (Elston et al., 2002).

• 1100-1050 Ma N. American Grand Canyon Supergroup
  and Keewanawan rocks
• 2 reversals (Elston et al., 2002), but see
  result below.

• 1100-1050 Ma Siberian 50-meter composite section
  micritic limestones and shale
• 16 reversals well recorded (Gallet et al.,
  2000) ? not coeval with result above?

• 1020-820 Ma North American Grenville orogen,
  uplifted slowly cooled rocks only
• 4 polarity periods (Dunlop and Yu, 2004), but
  would smooth away short chrons.

Avg. of 5 studies 0.17 rev/Myr in 225 Myr of
Precambrian time, compared to 1.7
rev/Myr from 0-150 Ma.
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Predicts extremely low average reversal frequency
in the middle Precambrian.
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• If these ideas are right, long-term average
  reversal frequency
• fluctuates on the time scale of
• mantle turnover
• (2) drifts upward on the time
• scale of inner-core growth.