摘要
中国陆相风成沉积序列蕴含了晚新生代以来丰富的古地磁场和古气候信息。第四纪黄土-古土壤序列的岩石磁学性质可以揭示第四纪亚洲内陆地区干旱化过程和东亚古季风演化历史以及末次冰期-间冰期旋回的气候不稳定性特征。文章主要评述近年来中国黄土环境磁学研究所取得的一系列重要进展,同时提出目前需要深入研究的几个重要问题,如成土作用对黄土沉积物沉积剩磁的影响、微生物对磁性矿物的改造、亚洲内陆地区气候过程和环境演变的磁气候学记录等。
The Chinese loess-paleosol sequences contain continuous paleomagnetic and paleoclimate records during the Quaternary. Over the past two decades, magnetoclimatological studies of Chinese loess-paleosol sequences in terms of magnetic susceptibility and other mineral magnetic proxies provided a wealth of information on both regional and global paleoclimate changes. This review focuses on recent achievements in loess environmental magnetism, with special attention to the long-term magnetoclimatological records of Quaternary developments of Asian aridification and East Asian paleomonsoons, and to the short-term magnetoclimatological records of climate instability during the last glacial-interglacial cycle.
The magnetic properties of bulk loess sediments reflect the combined effects of mixed assemblages of multiple magnetic fractions with different mineralogy, grain-sizes and concentrations. The pedogenic increase in the concentrations of the single domain （SD） and small pseudo-single domain （PSD） magnetite/maghemite grains contributes greatly to the enhanced magnetic susceptibility. When loess underwent chemical weathering, some of the released iron was partitioned to ferrimagnetic grains. Continued pedogenesis causes the growth of SP grains into SD and PSD grains. However, magnetic phases of the citrate-bicarbonate-dithionite （CBD）-treated loess and paleosols are dominated by large PSD and multidomain （MD）-like magnetite grains of lithogenic origin, which are ideal carriers of paleoclimate signals from the dust source region. The long-term upwards variations in the pre- and post-CBD mineral magnetic properties of the Jiaodao section in the central Loess Plateau indicate a long-term decrease in summer monsoon intensity and/or a long-term increase in winter monsoon intensity over the Quaternary.
Variations in the ratios of SIRM 100mT/SIRM, SIRM100mT/SIRM30mT and SIRM100mT/SIRM60mT （ SIRMT represents the residual SIRM after an n mT alternating field demagnetization） of the Jingbian section at the northern extremity of the Chinese Loess Plateau have been used to document regional paleoclimate change in the Asian interior by correlating the mineral magnetic record with loess grain-size, geochemical proxies, and global δ^18O record of deepsea sediments. The long-term upwards decreasing trend in those ratios in both loess and paleosol units has been attributed to a long-term decrease in the relative contributions of eolian hematite during glacial extrema and of pedogenic hematite during interglacial extrema, respectively, which reveals a long-term decreasing trend in chemical weathering intensity in both glacial-stage source region （the Gohi and deserts in northwestern China） and interglacial-stage depositional area （the Loess Plateau region ）. We further relate this magnetic variation trend to long-term increasing aridification and cooling, during both glacial extrema in the dust source region and interglacial extrema in the depositional area. over the Quaternary.
In addition, mineral magnetic parameters have been successfully used to explore the climate instability during the last glacial-interglacial cycle. The new findings expand the scopes of paleoclimatic studies in the Chinese loess, and further support the presence of climatic teleconnections across the Northern Hemisphere during that period.
Finally, some outstanding issues concerning environmental magnetism of the Chinese loess were put forward, such as （1） pedogenic role in reworking the detrital remanent magnetizations, （2） microbial role in causing magnetic enhancement, and （3） magnetoclimatology of Quaternary paleoclimatic and paleoenvironmental processes. Further studies on these problems would lead to a better understanding of the relationships between the effects of paleoclimatic, paleoenvironmental and tectonic processes in Northwestern China and Asian interior.
出处
《第四纪研究》
CAS
CSCD
北大核心
2007年第2期193-209,共17页
Quaternary Sciences
基金
国家自然科学基金项目(批准号:40674032
40221402和40325011)
中国科学院知识创新工程项目(批准号:KZCX3-SW-150)资助
关键词
中国黄土
环境磁学
岩石磁学
磁气候学
Chinese loess,environmental magnetism, mineral magnetism, magnetoclimatology
