大气生成宇宙成因核素^(10)Be在中国黄土中的应用研究进展

Review on the application of the atmospheric produced ^(10)Be in Chinese loess

中国黄土^(10)Be研究大体上经历了三个阶段:(1)地球化学行为研究:^(10)Be主要吸附于细颗粒及粘土颗粒,在黄土中保存性好,不会发生明显的化学迁移过程;(2)古气候代用指标应用:^(10)Be在黄土和古土壤层中的浓度变化与代表气候变化的深海氧同位素曲线变化一致,且可借此进行黄土年代标尺的建立;(3)地球环境示踪研究:示踪地磁场倒转及漂移事件,恢复古地磁场相对强度变化,以及定量重建黄土高原地区古降水变化历史等。由于近年来黄土^(10)Be环境示踪研究取得了可喜的成果,笔者认为有必要从以上三个方面对中国黄土^(10)Be研究历史进行较为系统的梳理回顾,总结当前最新研究进展,展望未来黄土^(10)Be在环境示踪中的研究方向,希望能使读者在短时间内了解中国黄土^(10)Be研究的发展脉络。

Background, aim, and scope The history of Chinese loess ^（10）Be studies can be recognized as three stages.（1） The geochemical behavior of ^（10）Be in Chinese loess： it is preferred to be absorbed on the small size particles and the clay minerals, it can be preserved well after it deposited and would not be leached by the raillfall;（2） as a paleoclimatic proxy： the variation of ^（10）Be concentration in the loess and paleosol correlated well with the marine isotope stages and with the correlation between these two proxies, it can be used to establish the chronology of Chinese loess-paleosol sequences;（3） tracing earth＇s environmental changes： recovering the variations of geomagnetic field intensity, tracing the geomagneticpolarity reversals/excursions and reconstructing the paleorainfall over the Chinese Loess Plateau. Since very significant progress has been made about the third one recently, it is necessary to make a comprehensive review on the application of cosmogenic^（10）Be in Chinese loess, and by doing this to discuss the future work about the Chinese loess ^（10）Be. Materials and Methods The publications of loess ^（10）Be studies are reviewed here. Among them, the papers published between 1980＇s and 2006 are used to discuss the sources, formation, chemical behavior and its climatic implications in Chinese loess, the others published after 2006 are used to show that the loess ^（10）Be are good proxy for paleogeomagnetic variations tracing and paleorainfall reconstruction. Results The ^（10）Be in Chinese loess is mainly from atmosphere where it is produced via cosmic-ray spallation. The chemical leaching experiments with acid solutions showed that the ^（10）Be in loess and paleosols is strongly bound to soil particles and the mobility due to dissolution is very little. The measurements of ^（10）Be concentrations in loess as a function of grain size indicated that the ^（10）Be is preferred to be adsorbed on small size mineral grains due to larger surface areas. The variation of loess ^（10）Be record is similar to the magnetic susceptibility which is well correlated with marine δ^（18）O, so it can be used to reconstruct the climatic variations. In addition, the ^（10）Be was also exploited to establish the chronology of loess-paleosol sequences based on the correlation between ^（10）Be and δ^（18）O. Because the production rate of ^（10）Be in atmosphere is regulated by the Earth＇s magnetic field intensity, i.e., when the geomagnetic field intensity is strong, the flux of galactic cosmic rays penetrating into the Earth＇s atmosphere is significantly attenuated by scattering off the magnetic field, resulting in decreased ^（10）Be production rate, and vice versa, ^（10）Be is a useful tool for the geomagnetic variations tracing studies. However, the efforts of using ^（10）Be to extract the geomagnetic field signal from Chinese loess was much more difficult than that in marine sediments. In Chinese loess, there are two sources of loess 10Be： some ^（10）Be comes from remobilized dust that fell to Earth at some time in the past, whereas a second component derives from atmospheric fallout of new ^（10）Be recently generated by cosmicray interactions in the atmosphere. In order to extract a geomagnetic strength signal from Chinese loess ^（10）Be, first, a correction must be made for ^（10）Be associated with recycled dust that fell to the earth some time in the past, but has since been remobilized by wind, and secondly, in addition to magnetic field strength, climate influencing the local flux of ^（10）Be needs to be eliminated. Zhou et al（2007a, 2007b） first proposed an idea of using striking correlation between measured ^（10）Be and magnetic susceptibility in Chinese loess to separate the climate signal（dust flux and precipitation） from the total ^（10）Be concentration. Based on this idea which named ＂Mean Value Concept＂, a series methods have been established for tracing geomagnetic field variation and reconstructing paleo-rainfall by using loess ^（10）Be. Discussion Applying these methods, two different loess sections from Xifeng and Luochuan to produce paleomagnetic records for the last 130 ka, two geomagnetic excursions-i.e. short-lived decrease infield intensity-known as the Laschamp and Blake events, are clearly seen by the increased geomagnetic modulated ^（10）Be signals in both Luochuan and Xifeng sections. Our reconstructed paleointensity record using this ^（10）Be signal can be well correlated to those from North Pacific, PISO-1500, and SINT 800. Using loess ^（10）Be, the Brunhes-Matuyama（B-M） reversal event was investigated because this reversal occurred much earlier in Chinese loess than that of marine sediments according to the paleomagnetic measurements. The ^（10）Be results from Xifeng and Luochuan sections show that the timing of B-M reversal is actually synchronous with that seen in marine records, verifying the standard loess time scale as correct. Except for the geomagnetic field tracing studies, a 130 ka paleorainfall record is also obtained using loess ^（10）Be. This record can be well correlated with the records of speleothem δ^（18）O which is regarded as monsoon a reliable record of Asian Monsoon intensity. Conclusions The reviews of previous work show that the loess ^（10）Be is a good tool for studies of geomagnetic field variations and paleorainfall reconstructions. These methods proposed by Zhou et al can not only be used in Chinese loess ^（10）Be but also suitable to the trace research for a multivariable geosystem where all variables are changeable and their distribution havebeen known except the one to be reconstructed. Recommendations and perspectives Based on the recent studies, it is recommended that the geomagnetic field intensity and paleorainfall of the whole Quaternary（~2.6 Ma） would be reconstructed by using loess ^（10）Be.