Magnetic stratigraphy and the geological time scalemagician.ucsd.edu/SIO190/lectures/Archaeomag_L10.pdf · Magnetic stratigraphy and the geological time scale Secular variation, excursions

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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Wednesday, May 11, 2011

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


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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http://www.gfz-potsdam.de/portal/gfz/Struktur/Departments/Department+2/sec23/topics/models/CALS3k_ARCH3k_SED3k;jsessionid=3A2FC6F7136551879C4D0DE8B34FA22C










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)


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


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)


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


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

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Cor

e (c

m)

Brun

hes

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uyam

a Ga

uss

Gilb

ert

Jaramillo

Olduvai

Mammoth

Pol Event Epoch

0.7 Ma

2.4

3.35

!

"

#

$

b) c) a) zones


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

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34 M0

13 2525 34

3434

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-60

-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


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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Magnetic stratigraphy and the geological time scalemagician.ucsd.edu/SIO190/lectures/Archaeomag_L10.pdf · Magnetic stratigraphy and the geological time scale Secular variation, excursions

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