Pedogenesis and its effects on natural remanent magnetization acquisition history of the Chinese loess/paleosol sequence

1. Pedogenesis and its effects on natural remanent magnetization acquisition history of the Chinese loess/paleosol sequences Presenter: Qingsong Liu Supervisors: Subir K. Banerjee Michael J. Jackson

2. Outline • Introduction of the Chinese loess • Magnetic carriers of loess ChRM • Low-T oxidation model of magnetite • Relative paleointensity • Conclusion

3. Winter monsoon Summer monsoon • Introduction of the Chinese loess Distribution of loess (wind-blown sediments) in central China (Kukla et al., 1988)

4. Conceptual model for the formation of loess/paleosol cycles

5. Typical loess/paleosol sequences (LuoChuan, by Verosub et al., 1993)

6. SPECMAP oxygen-isotope record tuned chronology compared to magnetic susceptibility (Kukla et al., 1988)

7. Depth plots of magnetic susceptibility in Xifeng and Luochuan compared to SPECMAP oxygen-isotope record (Kukla et al., 1988)

8. Depth plots of paleomagnetic records of Matuyama-Brunhes (a) and Jaramillo (b) recorded at Weinan (Zhu et al., 1994)

9. Problems of paleoclimatic and paleomagnetic signals recorded by the Chinese loess 1. Ambiguities in interpreting paleoclimatic signals A conceptual model of the relationship between magnetic susceptibility and the amount of precipitation (after Liu et al., 1999)

10. 2. Puzzle of the Matuyama/Brunhes boundary recorded by loess and marine sediments (Zhou et al., 1999)

11. Summary of the current loess study • Chinese Loess is so far the best terrestrial material for recording long-term (<2.5 Ma) paleoclimatic and paleomagnetic signals. • Problems are that there exist barriers in accurately decoding the paleoclimatic-sensitive proxies (in terms of mineralogy, grain size, concentration of magnetic minerals?) • The primary detrital remanent magnetization (DRM) is overprinted by the chemical remanent magnetization (CRM) carried by the newly-formed pedogenic fine-grained (< 100nm) magnetic particles. How to separate DRM from the secondary CRM?

12. Focus of this study • We selected two loess profiles, Yuanbao (YB) and Jiuzhoutai (JZT) at the western loess plateau, characterized by high sedimentation rate and low effects of pedogenesis. • We focus on Marine Oxygen-Isotope Stage (MIS 5, between ~74-128 ka) and the top of MIS6 , which covers a complete glacial/interglacial cycles.

13. Site description The Jiuzhoutai (JZT) section is located near Lanzhou City (36oN/103o50E), and the Yuanbao (YB) section (35o38N/103o10E) is in the Linxia Basin The mean annual temperature at these two sites is similar, at about 6-7oC. In contrast, the mean annual precipitation at YB (~500 mm) is higher than at JZT (300-400 mm). Therefore, YB has higher degree of pedogenesis than JZT.

14. 2. Magnetic carriers of loess ChRM Aeolian magnetite (DRM) Pedogenic maghemite (CRM) Problem: it is a mixture

15. Traditional method for isolating the characteristic remanent magnetization (ChRM) **ChRM does not equal primary RM ! Thermal demagnetization has been widely used to get ChRM by heating samples to 300-350oC

16. Question: What is the carrier of ChRM? magnetite (aeolian) or maghemite (pedogenic) ?

17. Traditional methods can not solve this problem because they worked on bulk information, which reflect an assemblage of magnetic minerals. Our new approach We use low-temperature cycle (LTC) to directly measure the magnetic carrier of ChRM

18. J Coarse-grained pseudo single domain (PSD and multi-domain (MD) magnetite (> several hundreds nm) T 0 120 K 300 K Maghemite J Single-domain (SD) magnetite (~20-50 nm) T 0 120 K 300 K Behavior of remanences carried by magnetite and maghemite during LTC

19. Stratigraphy of the YB section

21. Conclusions • Thermal demagnetization can not remove remanences carried by maghemite, then can not separate remanence carried solely by magnetite. • Therefore, we suggest alternating field (AF) demagnetization More reasons

22. 3. Low-T oxidation model of magnetite Magnetite Maghemite

23. Effects of LTO One of the dramatic effects is that the magnetic particles become magnetically harder, namely increasing the remanence coercivity. Question: Is the aeolian magnetite partially oxidized?

24. Temperature dependence of hysteresis parameters for a representative loess sample Measured at room-T

25. Evidence from low-T experiments

26. Conclusions of section 3 Aeolian magnetites in loess samples are partially oxidized with much higher remanence coercivity than the fine-grained pedogenic maghemite particles. Therefore, the distinguished coercivity spectra between these two kinds of particles permit us to use AF demagnetization to separate their remanences

27. 4. Relative paleointensity RPI=NRM/ (normalization parameter) Normalization parameters: susceptibility, ARM and SIRM Question: Which one is a better parameter for RPI?

28. SIRM is better than ARM NRM60mT SIRM60mT

29. Comparison of depth plots of magnetic susceptibility of the JZT and YB profiles and the previous interpretation of pedostratigraphy

30. Comparison of RPI between the JZT and YB sections

31. Age models of the JZT and YB profiles

32. Effects of pedogenesis on magnetic properties

33. Conclusions of section 4 • Loess can record RPI • AF demagnetization is more efficiency to isolate remanence carried by the aeolian magnetite particles than thermal demagnetization

34. Acknowledgements I thank my supervisors Prof. Subir Banerjee and Dr. Michael Jackson. I also thank the the other IRMer for their helps during the past 5 years. They are Bruce Moskowitz, Peter Solheid, Jim Marvin, Thelma de Souza Berquo’. I thank the following persons for their helpful suggestions and co-operations on my research: David Dunlop, Ozden Ozdemir, Yongjae Yu, Lisa Tauxe, Barbara Maher, Andrew Roberts, Jose Torrent, Fahu Chen, Rixiang Zhu, Yongxin Pan and Chenglong Deng. I thank my friends at twin cities: Xianfeng Wang, Fu Qi, Qing Zhang, Jim Thill. My last thank is given to My wife, Qiong Lin.