The Red Earth is widely distributed to the south of the Qinling Mountains and Huaihe River. It contains a wealth of information on palaeoclimate, palaeogeography, and hominin evolution. However, because of the climatic conditions of high precipitation and warm temperature, which reduce the preservation of mammalian fossils and other dating materials and cause serious overprinting of the primary remanence, it is difficult to obtain precise stratigraphic ages in the red soil area.
To probe into the nature of palaeomagnetic signals of subtropical red soil sequences in southern China, Doctor Liu from the Division of the Earth's Deep Structure and Process of IGGCAS, has carried out detailed rock magnetic and palaeomagnetic study on the red soils. This work was supervised by Professor Deng. Three sequences with different origins and different climate conditions were selected. These include the Xuancheng and Qiliting sequences, which are located in the lower reaches of the Yangtze River and the Damei sequence in the Bose Basin near the Tropic of Cancer. Among them, the Xuancheng sequence is of aeolian origin and the Qiliting and Damei sequences are of fluvial origin.
Detailed rock magnetic investigations, such as, the high-temperature magnetic analyses and thermal demagnetization of three-component IRM, hysteresis loop, combined with X-ray diffraction (XRD) analyses, suggest that magnetite, maghemite, goethite and haematite are present in the sediments of these three sequences. However, the main remanent magnetization carriers of these three sequences are different. For the Xuancheng samples, the main remanent magnetization carriers are magnetite and haematite. For the Qiliting samples, the main remanent magnetization carriers are haematite and maghemite. Two kinds of haematite are present in the Damei sequence: one is detrital haematite with an unblocking temperature (TB) of about 680°C, the other is pedogenic haematite with TB of about 630–640°C and high coercivity. This pedogenic haematite is the main natural remanent magnetization carrier of the Damei samples, which carries secondary chemical remanent magnetization (CRM). This CRM is strong enough to overprint the primary remanence, and then lead to the remagnetization of the Damei sequence.
The seasonal dry-wet alternation, coupled with high mean annual temperature (MAT) and relatively low mean annual precipitation (MAP) in the Bose Basin near the Tropic of Cancer have greatly intensified chemical weathering, which has produced abundant secondary haematite and maghemite. These secondary magnetic minerals carried a strong CRM and then overprinted the primary remanence completely. However, the latitude of the lower reaches of the Yangtze River is 7° higher than the Bose Basin. The sediments there experienced a moderate degree of chemical weathering, and thus only produce a weak secondary CRM. That is not strong enough to overprint the signals of the primary detrital magnetic minerals. So, the sequences in the lower reaches of the Yangtze River have preserved the primary remanence, but the Damei sequence has been remagnetized. It is found that climate conditions, such as temperature and precipitation, have significant influence on the palaeomagnetic records in red soils. Our work is helpful for later paleomagnetic study on the red soils.
The above-mentioned findings were published in Geophysical Journal International recently (Liu et al. Mineral magnetism to probe into the nature of palaeomagnetic signals of subtropical red soil sequences in southern China. Geophysical Journal International, 181, 1395-1410, 2010).Download Here
