中国第四纪古地磁学研究进展

PROGRESS OF QUATERNARY PALEOMAGNETISM IN CHINA

本文扼要介绍了中国第四纪古地磁学研究的进展，其主要结论为：中国黄土－古土壤的主要磁性矿物是磁铁矿、赤铁矿和磁赤铁矿，特征剩磁载体为磁铁矿；Ｌ８和Ｌ３３分别记录了Ｍ／Ｂ和Ｇ／Ｍ地磁极性转换过程；Ｊａｒａｍｉｌｌｏ极性亚带的顶、底分别位于Ｌ１０和Ｌ１２；六盘山以东的黄土剖面，由于土壤化作用强烈，成土过程产生的次生剩余磁性已将持续时间只有５０００ａ左右的Ｂｌａｋｅ极性亚时的原始磁信息掩盖，无法分离出来；只有六盘山以西的黄土剖面才有可能记录Ｂｌａｋｅ极性亚时。黄土的剩余磁性受“Ｓｍｏｏｔｈｉｎｇ”和“Ｌｏｏｋ－ｉｎ”效应的影响很小，因此是研究极性转换期间地球磁场形态学和古风向的合适物质。

There are extremely thick loess and lacustrine sediments in the main land of China. Especially, some loess-paleosol sequences are of great environmental value as they provide the most complete continental records of variations in both magnetic field polarity and climate over Central Eastern Asia during Quaternary.Magnetostratigraphic studies have shown that the polarity of natural remanent magnetization obtained from the Chinese loess can be correlated with the geomagnetic polarity time scale[1,6-20] based on absolute dating of loess sediments. The locations of major magnetic polarity transitions in the loess-paleosol sequences are identical in those defined by paleomagnetic measurements. The Matuyama/Brunhes polarity reversal is located in the lower part of loess unit L8. The Gauss/Matuyama polarity transition is defined in the lower part of the oldest loess unit L33. It means that an age of about 2.5Ma can be estimated for the bottom of Chinese loess[25].The initiation and termination of the Jaramillo event are recorded in the upper Part of L12 and the middle of L10, respectively. The normally magnetized Olduvia sub-zone stretches from the top of L27 to the middle of L25. The post-and pre-Jaramillo events are also identified in loess unit L9 and L15, respectively[13,14,18,33]. The Blake geomagnetic polarity episode has been observed in the western part of the Loess Plateau, but it has never been documented in any loess section which lies to the east of the Liupan mountain. It is speculated that primary remanence, induced the Blake sub-polarity episode, could be overprinted completely by secondary remanent components formed during pedogenises.The primary remanent magnetization information of loess-paleosol sequences is mainly carried by magnetite and, perhaps, hematite. Magnetite may be transformed into a higher coercivity mineral during heating. But the stable part of natural remanent magnetization, which provides the signature of the earth's magnetic fields, is not affected by magnetic transformation. Although the low-field susceptibility as a paleoclimate indicator was identified by Heller and Liu[2,7], it is still argued that variations of magnetic susceptibility in loess-paleosol sequences are controlled by climate changes. Recently, magnetic parameters (including coercivity, remanent coercivity, saturation magnetization, low-field susceptibility, frequency-dependent susceptibility) have been determined in loess-paleosol sequences. The results show that coercivity, remanent coercivity and saturation magnetization are more sensitive indicator for climatic fluctuations and more suitable for studying the detailed climatic changes than frequency-dependent susceptibility. It can be inferred from relevant magnetic measurements that aeolian dust should be continuously deposited during soil-forming periods, but the dust influx could be different from that during glacial stages. Indeed, the magnetic assemblage in paleosol is different from that in loess.Although the polarity reversal sequence has been established in several representative sections so as to provide a suitable time scale over the entire Quaternary,the morphology of the earth's magnetic field during polarity transition is not well defined. Most of the records used to study its morphology have been obtained from marine or lacustrine sediments and lava flows. Each type of paleomagnetic recorders has its advantages and disadvantages, which may affect the interpretation of transition data. The Matuyama/Brunhes transition is defined over an interval of about 50cm in loess unit L8 at the Weinan section. The total duration of the transition is about 5 000 years. The transition zone display several directional fluctuations in both declination and inclination. These directional fluctuations have been named 'rapid polarity shift (RPS)' and the duration of each RPS is about several hundred years. The presence of short-lived directional fluctuations may be considered as evidence that large 'smoothing' did not occur in loess sediments formed during glacial periods