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Paleo-intensity (PI)

Paleo-intensity (PI). Geomagnetic field: full vector field So far: declination & inclination Up next: intensity. Intensity Earth ’ s magnetic field. Unit: Tesla (T) 1 T = 1 N / (Am) ~ 20 to 60 μT (NL: 48.9 μT) Understanding geodynamo action

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Paleo-intensity (PI)

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  1. Paleo-intensity (PI) • Geomagnetic field: full vector field • So far: declination & inclination • Up next: intensity

  2. Intensity Earth’s magnetic field • Unit: Tesla (T) 1 T = 1 N / (Am) • ~ 20 to 60 μT (NL: 48.9 μT) • Understanding geodynamo action • Intensity dropped 12% since recording began: onset of a field reversal? • Interactions of inner & outer core, core & mantle: improving dynamo models • Atmospheric interaction: variable field shielding in relation to residence times and deposition of cosmogenic isotopes (radiometric dating!)

  3. Magnetosphere

  4. Enhanced solar activity Quebec blackout: 6 million people without power for 9 hrs. March 1989: solar storm

  5. Intensity Earth’s magnetic field + + _ + _

  6. Observations from space Hulot et al., 2002: Intensity dipole field decreases through growth of `reversed flux patch’ in Southern Atlantic Ocean. If continued at present pace then field would reverse in ~1500 years or so.

  7. PI in the “news” “The Observer” November 2002

  8. And of course … • Movie • Websites: • www.the-end.com • www.ijstijd.com • www.goto2012.nl • Etc.

  9. Paleo-intensity • Two `tastes’: • Absolute PI • Using `(p)TRMs’ (thermal: volcanics) • Spot-readings, dated • Relative PI • Using `DRMs’ (detrital: sediments) • Comparative in nature, time sequences

  10. Absolute PI: working with (p)TRMs pTRMs are independent and additive Tauxe, 2005

  11. Absolute PI – the principle Main assumption: TRM  HTRM field Y < field A = field X < field Z Partially replacing the NRM with a pTRM due to a known field.

  12. Absolute PI - different methods • Thellier style (Thellier, 1955) • Varying T in constant H • Using one specimen • Multi-specimen style (MSP) (Dekkers, 2006) • Varying H in constant T • Use more specimens

  13. Thellier-style - “ARAI-plot” HNRM = - slope * Hlab

  14. Multi-Specimen Style ( pTRM – NRM ) / NRM HNRM given by x-axis-intercept

  15. So we’re all set ... ? Both methods work perfectly fine for thermally stable SD grains, but… SD grains are very rare in nature! Some problems: • Chemical alteration • Alteration of the magnetic state • Unblocking T ? = blocking T • Magnetic treatment history (`tails’) `Multi-domain behaviour’

  16. What can we do to avoid these problems ? • Determine the maximum T to use in the experiment, before the onset of alteration • Test our results for MD-behaviour • Success rate of Thellier: 0 – 30% • Success rate of MSP: ? (too few data so far)

  17. Tests for chemical alteration Tests to determine the maximum T to use: Curie-balance measurements Susceptibility-vs-T experiments Day-plots before and after heating

  18. Curie-balance (the very basics…)

  19. Curie-balance

  20. Kappabridge (magnetic susceptibility)

  21. Susceptibility vs.T

  22. Day-plot before and after heating

  23. Chemical alteration tackled … Alteration T is now known, but… Changes in magnetic state and bias due to multiple heating steps are still there Two main problems left: Multi-domain behaviour (magnetic treatment) Changes in the way magnetization is acquired

  24. Magnetic Force Microscopy De Groot, Bakelaar, Dekkers, 2011/2012 Pristine Heated in 40 mT

  25. MD behaviour in Thellier-style experiments “Sagging” Dunlop et al., 2005 JGR

  26. How important is this “sagging”? Slope ≈ -2.7 Class exercise: Take two different slopes and calculate the corresponding PI. Hlab= 40 uT 2.7 x 40 = 108 uT 0.6 x 40 = 24 uT Dunlop et al., 2005 JGR Slope ≈ -0.6

  27. Many Thellier-style protocols Per temperature step: Zero-field, In-field Per temperature step: In-field, Zero field Alternate these two: “IZZI” protocol Incorporate pTRM and pTRM tail checks Microwave excitation instead of thermal heating Include low-temperature demagnetization “Quasi-perpendicular” method Sample material: Plagioclase single crystals Sample material: Submarine basaltic glass

  28. Thellier examples (Biggin et al, 2007)

  29. Thellier examples (Biggin et al, 2007)

  30. MSP examples (Dekkers & Boehnel, 2006) Expected field: 50.6 uT pTRM acquisition temperature 400°C Measured field (uT): ( ± 1 st.dev.) PT: 48.4 ± 2.23 Placer: 47.6 ± 2.22 ST: 52.7 ± 2.06 ST demagnetized at 200°C before & after pTRM acq.

  31. MSP examples: Maui, Hawaii

  32. MSP examples: Maui, Hawaii Lava flow ages: 200 – 8200 years

  33. MSP examples: Maui, Hawaii 120 C 560 C 560 C 120 C 560 C 560 C

  34. MSP examples: Maui, Hawaii pTRMacquisitiontemperature below alteration

  35. Absolute PI: Hawaii Hawaii data compiled by Pressling et al., 2006

  36. Another MSP example: Mt. Etna 1971 1983 IGRF field: 44 uT

  37. MSP behavior Class exercise: What happens with the samples (in terms of acquisition) if an underestimate is observed? Would that behavior imply an over- or an underestimate in a Thellier-style experiment?

  38. MSP behavior (pTRM-NRM)/NRM (expected) < (pTRM-NRM)/NRM (altered) pTRM(expected)< pTRM(altered) Samples must have acquired MOREpTRM to yield anUNDERestimate

  39. Consequences for Thellier Samples that acquire MORE pTRM than expected, flatten the slope in a Thellier-style experiment, also yielding anUNDERestimate

  40. Check for magnetic alteration EGU 2011: De Groot, Dekkers, Mullender ARM-acquisition test: AF-demagnetization in the presence of a DC-field (compares with pTRM: thermal-demagnetization in the presence of a DC-field)

  41. ARM-experiments: results ARM-pristine > ARM-heated overestimated PI +50% ARM-pristine < ARM-heated underestimated PI -30% ARM-pristine ≈ ARM-heated ≈ correct PI

  42. Wrap-up absolute PI Laborious experiments Low success rates But… We’re getting there More checks… eventually maybe a correction ?

  43. Relative paleointensity • Detrital Remanent Magnetization (DRM) • Sediments records • Ocean drill cores • Compares intensities through continuous sections (thus through continuous time) Main assumption: MDRM HDRM

  44. Relative paleointensity

  45. Acquisition of DRMs

  46. Normalization of DRM • Relative PIs need to be normalised because of differences in: • Magnetic activity • Concentration of magnetic particles • Mass & shape of samples • Sedimentation rates • … • Normalized to ARM, IRM or susceptibility (c) Think of relative PIs as the percentage of the potential magnetisation of a sample that was magnetised by Earth’s field at time of sedimentation

  47. Normalizers: χ, ARM, or IRM c = magneticsusceptibility ARM = Anhysteretic Remanent Magnetization IRM = Isothermal Remanent Magnetization <100mT

  48. Normalised DRM

  49. Relative PI: considerations DRM: true detrital origin, high stability NRM: demagnetised (AF / TH) Concentration variation must be ‘small’ Magnetic mineralogy and grain sizemust be ~constant No coherence with bulk rock magnetic properties Spatially and temporally coherent, many records Independent time scale required

  50. Variability in magnetic properties ODP core Atlantic Ocean Normalize Oligocene and Eocene portions separately Tauxe, 2005 chapter 8

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