全新世黄土剖面Li/Ba值变化及其古气候意义

对陕西华县老官台全新世黄土和古土壤剖面的化学分析表明:Li和Ba两种化学元素含量受后期风化成壤作用的控制而发生分离,Li在风化剖面中残留而相对富集,表现为残留富集的特点;而Ba在整个剖面中的变化相当显著,随成壤强度的增强而大量淋失,导致其含量急剧减少,表现为极强的活动性。所以Li/Ba值能较好地反映风尘沉积物在后期经历的风化成壤强度。由于控制风尘沉积物成壤过程的主导因素是降水量的多少,而影响黄土高原及周边地区降水量的主导因素是东亚季风环流,因此Li/Ba值变化实际上反映了东亚季风系统的变化。Li/Ba值是再现古气候环境变化的良好替代指标,这在其与磁化率和Rb/Sr值良好的对应关系中得到有力的证实。

VARIATIONS OF Li/Ba RATIOS AND ITS PALEOCLIMATIC SIGNIFICANCE IN THE HOLOCENE SOIL PROFILE

The measurement and analysis of elements in the Holocene loess-soil profile in the eastern Guanzhong Basin have shown that content distributions of Li and Ba show obvious differentiation following aeolian dust deposition. Li remains with relatively enriched content in the whole weathered profile, while the content reduction of Ba is quite remarkable along with the enhancement of intensity of soil formation, showing as a large amount of leaching lose. There are maximum and sub-maximum Li/Ba ratios in the periods of soil forming process of S02 and S01 in which the intensity of pedogenesis was stronger than other time spans during the Holocene. Contrary to this result, there are minimum and sub-minimum Li/Ba ratios in the periods of loess accumulating process of L1 and Lx in which the intensity of pedogenesis was fainter with increasing accumulation of aeolian dust. Therefore, the variations of Li/Ba ratios in the profile indicate changes of pedogenesis intensity after aeolian dust deposition and the Li/Ba ratio is thus a good proxy index for reconstructing paleoclimatic change. The internal cause of the differentiation of Li and Ba is mainly due to their chemical behavior under hypergene conditions. Li is a relatively stable element and thus remains in the paleosol layers while Ba is an activ element and leaches out from the paleosol layers. Further analysis indicates that there exists a steady negative correlation between the Li/Ba ratios and the contents of Ba. The primarily external factor causing relative enrichment of Li and stronger leaching loss of Ba in the profile is precipitation. As the east Asian monsoonal circulation is the controlling factor for the amount of precipitation in the Chinese Loess Plateau and its adjacent areas, the Li/Ba ratio thus records the evolutionary history of the East Asian monsoonal system. According to a detailed analysis of variations of the Li/Ba ratios in the Laoguantai loess profile, it reveals that the Holocene Megathermal (8 500～3 100aB.P.) period is not a constant warm-humid period because there existed a significant fluctuation in monsoon circulation during 6 000～5 000aB.P. which was characterized by weakened summer monsoon and /or reinforced winter monsoon. Such a result is affirmed well also by the curve of Rb/Sr ratio.