1. Magnetic Carriers in Rocks Dilute dispersions of magnetic minerals Concentrations several per cent down to several ppm Spinner magnetometers and cryogenic magnetometers must be sensitive Stepwise demagnetization yields the meaningful part of the signal

2. The magnetization of a pristine sample: TRM: thermoremanent magnetisation CRM: chemical remanent magnetisation DRM: depositional remanent magnetisation VRM: viscous remanent magnetisation Natural remanent magnetization (NRM)

3.  0 vH c M s = magnetic energy 2kT = thermal energy Viscous remanent magnetization

4. Time-temperature relation If M s and H c (H K ) do not vary over the temperature range under consideration:

5. Nomogram for magnetite Pullaiah et al., 1975

6. Stepwise progressive demagnetization Preferential removal of magnetic moments with short relaxation times No ambient magnetic field: - Alternating fields with increasing peak amplitude - Increasingly elevated T temperature The most stable part of the NRM is the oldest, often primary

7. Quality of NRM recording ✔ ✗

8. More components… JAJA JBJB JAJA JBJB

9. Secca Grande exercise Dec: 358; Inc: 58Dec: 18; Inc: 48

10. Secca Grande exercise Reversed: Clockwise Normal: Clockwise ??? How to establish the amount of rotation?

13. IRM acquisition curve fitting: Cumulative Log-Gaussian fitting Per component: three free parameters Concentration: SIRM Coercivity: B 1/2 (remanent acquisition coercive force) Spreading: DP (distribution of microcoercivities)

16. Assignment 7: Tectonics

18. Topical literature report 1)Select your topic of choice (from today) 2)Write a review report on a specific literature topic dealing with magnetic reversals (~1000 words; max 1500) 3) Present your results in class (March 18 th ) No grades (learn from comments/feed-back) 4) Finish and hand-in report at exam (March 23 rd ) in paper and digital version Report counts for 20% of exam grade 5) We will make an overview article of results Digital version will be distributed

19. Barcode: may give unique solutions - but of an essentially BINARY nature - and can be distorted in the rock record Sediments (DSDP) & biostratigraphy Marine magnetic anomalies Lavas & K/Ar dating “The Grey between Black and White”

21. a) Field reversals Mantle Convection time scale ~100 Myr Reversal frequency, superchrons Solid inner core Diffusion time scale 3-5 kyr Stabilises geodynamo process Kuang & Bloxham, 1997 Liquid outer core Convection time scale 300-500 yr Geodynamo action: Secular variation, excursions, reversals

22. b) Where does the pole go?

23. c) Intensity changes during a reversal

24. d) Storing the signal: lock-in depth Watkins, 1968

25. e) Delayed aquisition DRM vs. CRM:

26. Topical literature report a)Duration of geomagnetic reversals (how long does the geodynamo take to reverse?) b)Reversal paths (where does the pole go?) c)Intensity changes: (consequences for the paleomagnetic signal?) d)Storing the signal (how deep is the lock-in depth?) e)Delayed acquisition (do bio-chemical processes affect the paleomagnetic signal?) f)The Brunhes-Matuyama reversal (1:marine records; 2: continental records) (each project max two groups)