中国黄土研究新进展(一)黄土地层

本文扼要介绍最近几年中国黄土研究中的土壤地层学、磁性地层学、气候地层学、第四纪下界及海陆古气候对比等方面的新进展。根据新发现的黄土剖面,中国黄土中共可区分出37个土壤地层单位。中国黄土底界的古地磁年龄为2.5MaB.P.。2.5Ma以来,中国北方共经历了37次大的冷暖交替。2.5MaB.P.前后,全球气候出现了一次大的转型事件,它可作为第四纪的开始。中国黄土-古土壤系列同深海氧同位素记录在1.7Ma至今的时段上可以很好的对比;2.5Ma至1.7MaB.P.的气候变化,以黄土记录较为明确。

PROGRESSES OF LOESS RESEARCH IN CHINA (PART 1) LOESS STRATIGRAPHY

The loess-paleosol sequences in central China with the continuous coverage of the Quaternary Period provide an independent record of the world-wide climatic variations. In recent years, quite a lot progresses of loess research in China have been made. The thick sandy loess L₁5 with the time roughly equivalent to 1.2 MaB. P. is a key marker layer in China loess sequence. Above L₁5, 19 paleo- sols have been recognized in Luochuan and Xifeng sections. However, the pedostratigraphie division below L₁5 still remains to be done. In the past two years, three new sections with clearly-defined paleosols in the entire loess column have been found in Baoji, Xi'an and Weinan in the southern most part of the Loess Plateau. Thirty-seven pedostratigraphic units are identifiable in the Loess Plateau. Each can be correlated at least in three sections over long distance, demonstrating the continuity of China loess in recording climatic oscillations. The bottom of loess deposits are paleomagnetically dated about 2.5 Ma B. P.（near the Matuyama/Guass boundary）in Baoji, Xi'an and Weinan sections. Other paleomagnetic reversals have been well defined in Baoji section: B/M boundary in the lower part of L₈, the Jaramillo Subchron between L₁0 and L₁2 and the Olduvai Subchron from the middle part of L₂5 to the upper part of L₂7. A new time scale of China loess named"periodic estimation time scale" has been established on the basis of orbital theory and paleomagnetic time controls. The climatostratigraphy of China loess may be divided into thirty-seven formations, each of which represents the first order fluctuation of climate from dry cold to humid warm. It is noted that there occurred an abrupt climatic shift from warm to cold in North China around 2.5 Ma B. P., indicated by the desert extension and loess deposition on large scale, coinciding with a global clima- tic deterioration recorded in various sediments throughout the world. From then on, climate has been characterized by frequent and wild fluctuations, strikingly contrasting with the continuons warming in the Guass Chron. No specific climatic event had been recorded in China loess around 1.65 Ma B. P., so it would be more reasonable to take the bottom of China loess, 2.5 Ma B. P. as the Plio/Pleistocene boundary than that proposed in Verica section near 1.65 Ma B. P.. A comparative study between oxygen-isotope records in deep-sea cores and the loess-paleosol sequences in central China shows the advantage of China loess over the deep-sea sediments in the recording of climatic changes in some aspects. The loess deposits are probably the most complete and reliable paleoclimatie record in the world for the Iast 2.5 Ma,