Importance of Hematite Self-Reversal in Al-Rich Soils Magnetostratigraphy: Revisiting the Damei Red Soil Sequence in the Bose Basin, Southern China

Understanding the acquisition of chemical remagnetization that commonly takes place in sedimentary red beds is crucial not only to assess the stability of primary chemical remanent magnetization (CRM) but also to evaluate the impact of diagenesis on the paleomagnetic record. The inconsistency between the magnetostratigraphy and the 0.803 Ma age of tektites within the upper vermiculated unit of a red soil sequence (Damei) in southern China strongly suggests pervasive remagnetization. This remagnetization has previously been interpreted as a CRM lock-in. However, our recent study suggests that the upper soil units of this section above the tektite-bearing layer have experienced milder weathering than the underlying layer. It seems that CRM lock-in has not completely overprinted the primary remanence of this section. To investigate the exact remagnetization mechanism, the magnetostratigraphy of the Damei sequence was revisited and the oriented samples were subjected to progressive thermal demagnetization up to 680 degrees C (instead of 585 degrees C in our previous study) by using a newly designed oven with ultralow magnetic field noise. The new demagnetization results for vermiculated and red clay samples document a high-temperature (HT) remanence component above 630 degrees C with some above 525 degrees C, and a self-reversal medium temperature (MT) component between 300 and 585 degrees C. The magnetic polarities of most HT components are consistent with the tektite age. The self-reversal MT component is carried by Al-substituted hematite transformed from Al-substituted maghemite. Self-reversal likely occurred during the maghemite to hematite transformation process. Additional attention should be paid when using magnetostratigraphy to date highly weathered aluminum-rich red sediments.

Automated summary

Understanding the acquisition of chemical remagnetization is crucial for assessing the stability of primary chemical remanent magnetization and the impact of diagenesis on the paleomagnetic record. A study on a red soil sequence in southern China suggests pervasive remagnetization and challenges the previous interpretation of CRM lock-in. The study highlights the need for caution when using magnetostratigraphy to date highly weathered aluminum-rich red sediments.