欧亚黄土中古地磁极性界线的地层学解释

STRATIGRAPHICAL INTERPRETATION OF GEOMAGNETIC POLARITY BOUNDARIES IN EURASIAN LOESS

本文以欧亚黄土中记录的布容一松山极性倒转界线为例，简要回顾近年来对黄土中 古地磁极性界线地层学解释的讨论。研究表明，由于在记录古地磁极性转换时存在不同程度 的“错位”，黄土中所测的极性界线的年代地层意义存在很大的不确定性。就中国黄土而言，第 ８层黄土（Ｌ_８）中下部或第８层古土壤（Ｓ_８）顶部测得的布容－松山界线系“错位”的产物，该层位 的年龄应老于 ０．７９ＭａＢ．Ｐ。对黄土中测得的Ｃｏｂｂ Ｍｏｕｎｔａｉｎ极性事件界线进行地层学解释， 推论蓝田猿人化石层位的年龄１．２ＭａＢ．Ｐ。

This brief review provides a summary of the recent discussions on the interpretation of the geomagnetic polarity boundaries recorded in Eurasian loess. The Brunhes / Matuyama boundary was recorded in many loess sections across Eurasia. Its value for establishing chronological framework for loess sequence has been widely recognised. One important observation was made in early palaeomagnetic investigations: there is a stratigraphical difference in the position of this boundary between loess and the deep sea sediments; one recorded in glacial loess and the other in interglacial Stage 19. This was initially explained as arising from the delayed acquisition of chemical remanent magnetization. More recent high resolution palaeomagnetic records suggest that postdepositional remanence acquisition plays an important role in loess magnetism. This raises a question as to the delay between the geomagnetic polarity change and the recording of the event. Some studies argue for a case of very limited delay (and displacement) and attribute the difference to a systematic lag in environmental response to climate change between continental and marine systems. Others consider the complicated processes involved in transforming aeolian dust to loess deposits and estimate more than lm downward displacement of the measured Brunhes / Matuyama boundary relative to the true level corresponding to the polarity change. Applying this latter model to the measured Cobb Mountain event in a loess section where the cranium of Lantian Homo erectus was found, we suggest that the age for the Lantian Man cranium is l.20MaB.P., i.e. 50000--70000 years older than previously dated. It is concluded that palaeomagnetism will continue to play an important role in loess research. However, as loess accumulation and magnetic recording in loess operate at different timescales, careful stratigraphical interpretation of measured geomagnetic polarity boundaries should be made taking into account fundamental aspects of loess formation.