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Magnetic stratigraphy and the geological time scale
Secular variation, excursions and reversals
Constructing reference curves and the polarity time scale
Applications
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Secular variation
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Let’s go to France - the birthplace of the Thellier method
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Prediction for 47o, 2o
GAD, IGRF_2005, CALS3k_cst
CEBCE
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What is CALS3k_cst?An “IGRF-like” model for secular variation based on data from archeological artifacts, lava flows and sedimentary records spanning the last 3,000 years.
Latest version, Korte et al., 2009 (doi:10.1029/2008GC002297)
available from http://www.gfz-potsdam.de/portal/gfz/Struktur/Departments/Department+2/sec23/topics/models/CALS3k_ARCH3k_SED3k;jsessionid=3A2FC6F7136551879C4D0DE8B34FA22C
Based on data in Donadini et al., 2009 (doi:10.1029/2008GC002295)
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How well does it do?
It is only as good as the data that go into it.
Fits the data to within some specified tolerance to find a “smooth” model.
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Donadini et al. 2009
Here’s where the data are from
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Fit to the declination data
Korte et al., 2009
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Fit to the inclination data
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Fit to the intensity data
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BCE CE
Compare “Levantine curve” of Ben-Yosef et al. 2008) with CALS7k.2 (Korte and Constable, 2005)
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Excursions
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A classic example - the Mono Lake excursion
Wilson Creek Beds, Mono Lake
debate over exact age - could be ~30ka or even ~43ka
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what are “excursions”Definition varies:
original definition was a deviation in directions so - some use directional variation > 45o VGP latitude
some recognize lows in field strength
Are they global or only locally observed?
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Reversals
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a)
b)
Classic example from deep-sea sediment core (RC14-14)
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What happens during reversal?Huge arguments
duration (100s to 1000s of years)
rates of change
field structure (dipolar, multipolar, preferred VGP paths?)
cause (growth of “flux patches”...)
Substantial agreement over
when (the geomagnetic polarity time scale)
field strength is always low17
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What are they good for?
Magnetic stratigraphy
Calibration of the Geological Time Scale
Tracing of isochronous horizons in geological sections
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Mason & Raff, 1961
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bathymetry
profile
profile backwards
modelGPTS
10 my10 my
BM MGa GaGi Gi
0
0
1000
1000
!
0
!
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V16-134 (62oS,91oW)
Inclination
Dept
h in
Cor
e (c
m)
Brun
hes
Mat
uyam
a Ga
uss
Gilb
ert
Jaramillo
Olduvai
Mammoth
Pol Event Epoch
0.7 Ma
2.4
3.35
!
"
#
$
b) c) a) zones
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22240 270 300 330 0
-60
-30
0
30
60
13
13
13
13
25
25
M25
13
13
13
25
1325
34
25
13
13
34
13
M0
M0
5
13
1325
34
34
34M0
M0
M0
13
1325
34
34 M0
13 2525 34
3434
M0
13 25
-60
-30
0
30
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Basis of plate tectonic reconstructions
movie courtesy of Jean BesseWednesday, May 11, 2011
How to do magstratNeed to establish that magnetic record is of polarity history and not some overprint
Need to sample sufficiently long and densely to recover a unique period of geomagnetic polarity history (avoiding gaps and changes in sed rate)
Need some idea of age
Need to correlate to time scale
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Magnetostratigraphy example in Miocene east african rift deposits Tauxe et al. (1985)
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Magnetostratigraphy example in Miocene east african rift deposits Tauxe et al. (1985)
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Demonstrate stability
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Demonstrate two polarities
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Show stratigraphy
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Tracing isochrons, Siwaliks of Punjab Pakistan (Behrensmeyer and Tauxe, 1982)
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similarity to Indus/Soan River sstem today
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Take home message
Geomagnetic field constantly changing on time scales from years to millions of years
Can create predictive models of behavior
Can use the models for dating and correlating.
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