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Pedogenesis and its effects on natural remanent magnetization acquisition history of the Chinese loess/paleosol sequence PowerPoint Presentation
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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 Outline Introduction of the Chinese loess Magnetic carriers of loess ChRM

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slide1

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

slide2

Outline

  • Introduction of the Chinese loess
  • Magnetic carriers of loess ChRM
  • Low-T oxidation model of magnetite
  • Relative paleointensity
  • Conclusion
slide3

Winter monsoon

Summer monsoon

  • Introduction of the Chinese loess

Distribution of loess (wind-blown sediments) in central China (Kukla et al., 1988)

slide6

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

slide7

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

slide8

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

slide9

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)

slide10

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

slide11

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?
slide12

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.
slide13

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.

slide14

2. Magnetic carriers of loess ChRM

Aeolian magnetite (DRM)

Pedogenic maghemite (CRM)

Problem: it is a mixture

slide15

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

slide16

Question:

What is the carrier of ChRM? magnetite (aeolian) or maghemite (pedogenic) ?

slide17

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

slide18

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

slide21

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

slide23

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?

slide24

Temperature dependence of hysteresis parameters for a representative loess sample

Measured at room-T

slide26

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

slide27

4. Relative paleointensity

RPI=NRM/ (normalization parameter)

Normalization parameters: susceptibility, ARM and SIRM

Question:

Which one is a better parameter for RPI?

slide29

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

slide33

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
slide34

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.